First Break August 2024 - Near Surface Geo & Mining

Near Surface Geo & Mining

EAGE NEWS Presidential interview

CROSSTALK

INDUSTRY

CONNECTING OUR HISTORY TO OUR FUTURE

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CHAIR EDITORIAL BOARD

Clément Kostov (cvkostov@icloud.com)

EDITOR Damian Arnold (arnolddamian@googlemail.com)

MEMBERS, EDITORIAL BOARD

• Lodve Berre, Norwegian University of Science and Technology (lodve.berre@ntnu.no) Philippe Caprioli, SLB (caprioli0@slb.com) Satinder Chopra, SamiGeo (satinder.chopra@samigeo.com)

• Anthony Day, PGS (anthony.day@pgs.com)

Applied shallow geophysics (seismic and electrical resistivity imaging) to geotechnical foundation design (Central Texas, USA) FIRST BREAK ® An EAGE Publication

• Peter Dromgoole, Retired Geophysicist (peterdromgoole@gmail.com)

• Kara English, University College Dublin (kara.english@ucd.ie)

• Stephen Hallinan, Viridien (Stephen.Hallinan@viridiengroup.com)

• Hamidreza Hamdi, University of Calgary (hhamdi@ucalgary.ca)

Gwenola Michaud, GM Consulting (gmichaud@gm-consult.it)

Fabio Marco Miotti, Baker Hughes (fabiomarco.miotti@bakerhughes.com)

• Martin Riviere, Retired Geophysicist (martinriviere@btinternet.com)

• Angelika-Maria Wulff, Consultant (gp.awulff@gmail.com)

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Andrew McBarnet (andrew@andrewmcbarnet.com)

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ISSN 0263-5046 (print) / ISSN 1365-2397 (online)

Editorial Contents

3

Sp ecial Topic: Near Surface Geo & Mining

39 Borehole GPR – Applications and advantages

Jaana Gustafsson, Paul Lehmann, Jesper Emilsson, Johan Friborg and Andreas Viberg

47 Integrated geophysical and geotechnical investigation for the rehabilitation of a water storage reservoir in Crete, Greece

Christos Orfanos, Konstantinos Leontarakis, George Apostolopoulos and Haralambos Gouvas

55 High-frequency FWI Imaging: repurposing seismic data for imaging shallow hazards Hung Dinh, Thomas Latter, Mike Townsend, Nils Grinde, Ståle Høgden, Nicholas Robb, Marte Aksland and Alexander Bertrand

65 Advanced seismic imaging solutions for hardrock site evaluation and characterisation across scales

Nicoleta Enescu and Calin Cosma

71 Insights gained from two decades of seismic reflection profiling for mineral exploration in Finland

Suvi Heinonen and Viveka Laakso

77 Applied shallow geophysics (seismic and electrical resistivity imaging) to geotechnical foundation design (Central Texas, USA)

Hector R. Hinojosa and Jorge E. Rangel

89 Smart Exploration Research Centre: Knowledge and innovation for exploration of critical raw materials

Alireza Malehmir, Magdalena Markovic, Myrto Papadopoulou, Karin Högdahl, Maria Ask, Maria Strømme, Iain Pitcairn, Tina Martin, Thomas Zack, Jaroslaw Majka, Mats Svensson and Ronne Hamerslag

95 Integrated structural health assessment of industrial buildings in areas of high seismic risk

Gwenola Michaud, Roberto Zamparo and Alessandro Brovelli

101 Data acquisition and lessons learnt from geophysical Remotely Piloted Aircraft System (RPAS) surveys in northern Canada

Irina Nizkous and Ross Penner

105 A joint analysis of Rayleigh and Love waves using MASW for site characterisation

Juan José Hellín-Rodríguez, Pedro Martínez-Pagán, Ignacio Valverde-Palacios, Antonio García-Jerez, Koya Suto, Marcos Antonio Martínez-Segura and Koichi Hayashi

111 Tailings pond outfiltration monitoring with electrical conductivity surveying Pauli J. Saksa

118 Calendar

cover: Ultrahigh-resolution seismic survey at a quick-clay landslide site in southwest Sweden to check retrieval of non-aliased shear-wave reflections. Photo courtesy of Alireza Malehmir.

European Association of Geoscientists & Engineers Board 2024-2025

Near Surface Geoscience Circle

Andreas Aspmo Pfaffhuber Chair

Florina Tuluca Vice-Chair

Esther Bloem Immediate Past Chair

Micki Allen Contact Officer EEGS/North America

Hongzhu Cai Liaison China

Deyan Draganov Technical Programme Officer

Eduardo Rodrigues Liaison First Break

Hamdan Ali Hamdan Liaison Middle East

Vladimir Ignatev Liaison CIS / North America

Musa Manzi Liaison Africa

Myrto Papadopoulou Young Professional Liaison

Catherine Truffert Industry Liaison

Mark Vardy Editor-in-Chief Near Surface Geophysics

Oil & Gas Geoscience Circle

Yohaney Gomez Galarza Chair

Johannes Wendebourg Vice-Chair

Lucy Slater Immediate Past Chair

Wiebke Athmer Member

Alireza Malehmir Editor-in-Chief Geophysical Prospecting

Adeline Parent Member

Matteo Ravasi YP Liaison

Jonathan Redfern Editor-in-Chief Petroleum Geoscience

Robert Tugume Member

Anke Wendt Member

Martin Widmaier Technical Programme Officer

Sustainable Energy Circle

Carla Martín-Clavé Chair

Giovanni Sosio Vice-Chair

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Sanjeev Rajput Vice-President

Laura Valentina Socco President

Martin Widmaier Technical Programme Officer

Andreas Aspmo Pfaffhuber Chair Near Surface Geoscience Circle

Maren Kleemeyer Education Officer

Yohaney Gomez Galarza Chair Oil & Gas Geoscience Circle

Carla Martín-Clavé Chair Sustainable Energy Circle

Diego Rovetta Membership and Cooperation Officer

Peter Rowbotham Publications Officer

Christian Henke Secretary-Treasurer

Geoscience’s responsibility in the energy transition era

Incoming EAGE president Valentina Socco shares her goals for her term in office.

What will be your priorities as EAGE president?

It is a great honour and a great responsibility to be president of EAGE. The Association has taken a clear path toward energy transition and this trend must be sustained together with the commitment toward ensuring energy security. Beside these two pillars, many other applications of geosciences are emerging in various fields of great importance for society ranging, among others, from climate change adaptation to resilient and sustainable infrastructure, from mineral exploration to groundwater and coastal environment characterisation and monitoring. Giving space to all this in an integrated framework is my major task. The task will become more meaningful if we are able to engage and involve in our work a growing number of students and young professionals that represent the future of the Association, industry and academia.

Italian-born Laura Valentina Socco is a full professor in applied geophysics at the Department of Engineering of Environment, Land and Infrastructures (DIATI) at the Politecnico di Torino, where she took her civil engineering MSc in 1992 and her PhD in Environmental Geo-engineering in 1996. In 2014 she received the EAGE’s Conrad Schlumberger Award. She has been an SEG honorary lecturer and in 2019 received the SEG outstanding educator award.

Prof Socco was editor-in-chief of Geophysics from 2017 to 2019.

Moreover, I think we could better value the huge knowledge repository represented by EarthDoc through the application of AI that may transform our publication archive into a modern research tool.

What led you into a life in geoscience?

When I graduated as a civil engineer I knew I wanted to be a researcher and that I wanted to bring my engineering competence into applied sciences. The opportunity of starting to work with the geophysics research team of my department with a short research scholarship came by chance, and it was the beginning of a passionate relationship with this discipline and its fascinating multi-scale, multi-physics world.

In which areas are near surface geoscience technologies relevant to the energy transition?

In spite of being always very active in the field that we call near surface geoscience, I have always perceived this name as a limitation more than as a clear definition. Near surface can be few metres, a few hundreds of metres or a few kilometres. One of the marvellous things of geoscience and geophysics in particular, is that many methods and techniques are easily scalable and can be equally useful in a multitude of problems and applications. In this sense, the methods which are traditionally associated with near surface can be of paramount importance for energy transition.

To mention just a few examples, shallow marine geophysics can provide essential information for the design of windfarms, the integration of multi-physics approaches can improve our comprehension and management of geothermal fields, improved mineral exploration can help ensure the supply of raw materials which are essential to green transition, emerging technologies that exploit existing or newly deployed fibre-optics networks can lead to the monitoring of dynamic properties of the subsurface that are

extremely relevant for gas storage and other energy transition-related activities. I think we are just at the beginning of the integration of our traditional fields of activities in a more comprehensive framework aimed at creating a more sustainable world.

Can more be done to make the geoscience community more divers/inclusive?

A lot has been done to make EAGE an inclusive and diverse community. But this objective is one of those that requires tireless effort and continuous commitment, because diversity and inclusion are concepts that evolve with society’s evolution. Our commitment to these values should always have a priority position in our agenda. I still see space for improvement in terms of gender balance, and feel that in times of geopolitical instability, like the present, the care and the attention toward our members wherever they live and operate becomes extremely important.

During your career what have been your most rewarding research projects?

Research is a fantastic job. I always say to students who are thinking of approaching a research career that they have to be ready to dig in the darkness and not feel uneasy by unanswered questions. So, every new project is a challenge and I tend to fall in love with new scientific and technical problems no matter what their size or budget. It is therefore hard to identify the most rewarding among all the projects I have carried out in a long career and in many different fields of application.

What certainly makes some projects special though are the people with whom we work. I could mention two projects of the more recent part of my career that have been particularly of interest. The first is Smart Exploration, a large consortium funded by the European Union to develop new and sustainable mineral exploration techniques. Among the 27 partners of the consortium, EAGE was in charge of dissemination and communication, and the project achievements are visible in the very nice project website that is kept alive beyond the life of the project. The cooperative environment, the continuous exchange and cross fertilisation among different expertise made this project special and extremely fruitful from the scientific and human point of view.

More recently, my research team had the privilege to be involved in one of the geophysical campaigns supported by the Odysseus Unbound foundation, that has the task of searching Homer’s Ithaca. The challenge of matching geological time with literature time is fascinating and the field work with a great international team from different research institutions was an extremely rewarding experience.

As a university teacher do you have any concerns about attracting a new generation of students into the geoscience and engineering disciplines?

Attracting talented young students into the world of applied geoscience and related engineering disciplines is s a challenge. As chair of my department education programmes for the last six years, I have spent a lot of time in communication and dissemination campaigns with this aim. I think one of our missions as EAGE is to facilitate the needed cooperation between industry and academia to convey to new generations the proper messages about the role of geoscience and engineering in ensuring energy transition and sustainability and about the opportunities they can have in this field. Communicating with new generations also means being ready to listen to their concerns and needs and seriously take them into account in defining our strategies. This is a very clear task for me during my presidential term.

New membership fees for 2024

As we chart a sustainable future for the Association and endeavour to continue offering members the same level of services, benefits, and support, it has become necessary to adjust the EAGE annual dues. This will be done gradually over the course of the next few years, subject to a yearly review by the EAGE Board.

The new fee structure, which will come into effect from 1 October 2024, will place the regular membership at

€85 per year, while the retired membership and the student membership will remain unchanged (€40 and €25 per year respectively).

All members (and prospective members) will have the opportunity of securing future years of membership at the current fees until 30 September 2024. In order to keep EAGE accessible to all, a number of support options are available for members who find it difficult to meet the fees but wish to

stay connected with the Association. These include a membership fee waiver and a 50% membership fee discount under the EAGE Economic Hardship Programme, as well as the opportunity for first-year sponsored membership for students.

If you have questions regarding EAGE membership or about any of the support programmes mentioned, please do not hesitate to get in touch via membership@eage.org.

Valentina Socco: active participant in Oslo.

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Opening the call for nominations for 2025 EAGE Achievement Awards

Everhard Muijzert, incoming chair of the EAGE Awards Committee, writes: The EAGE Awards are among the highest peer recognitions that geoscientists and engineers can achieve in their professional careers. These awards are especially meaningful because the nominations come from peers — educators, students, colleagues, co-workers, and other professionals. The EAGE Awards Committee invites all members to nominate the most deserving members for the 2025 Achievement Awards. The deadline for nominations is 31 October 2024.

This year, the EAGE is thrilled to include the Marie Tharp Sustainable Energy Young Professional Award for young scientists for the second time. The Marie Tharp Award is dedicated to promising and creative talents among the next generation of leaders who are committed to the idea of transforming energy systems and speed up the global energy transition. Competition for this recognition is fierce, with its first winner chosen from over 15 nominees from nine different countries, to be announced at the upcoming EAGE GET Conference in November in Rotterdam.

The EAGE Achievement Awards now feature two young scientist awards (the Marie Tharp and the established Arie van Weelden). Furthermore, there are three prestigious award categories for outstanding and lifetime contributions to the advancement of geoscience and engineering disciplines with specific focus on resource exploration and development, geophysics, petroleum geosciences and engineering (Desiderius Erasmus, Conrad Schlumberger and Alfred Wegener Awards). The year 2025 also marks a special milestone, celebrating 70 years since the first recipients of the Arie van Weelden and Conrad Schlumberger Awards. A full list of past winners can be found on the EAGE website.

Most awards are given to a single recipient each year. Finally, Honorary Membership can be awarded to multiple individuals for their exceptional service to the geoscience and engineering community and/or the Association.

All EAGE members are encouraged to nominate candidates who would be worthy recipients of these awards. When considering potential nominees, think about those who have made significant impacts on their geoscience and engineering disciplines and communities. Nominees are often still active contributors, but you

might also identify someone who made a significant early career contribution. Please make sure to comply with the specific nomination requirements and focus areas for each award.

A strong nomination package includes one or more supporting letters detailing the nominee’s professional impact and significance. These letters can be written by any peer, including past educators, tutors, students, colleagues and clients, and do not need to come from EAGE members.

rigour, the two most recent Desiderius Erasmus and two most recent Alfred Wegener awardees, as well as the past five EAGE presidents. Care is taken that the Committee reflects the diversity of EAGE and has expert knowledge in all of EAGE disciplines. We would like to thank outgoing committee members: Tiziana Vanorio (former chair), Gijs Vermeer, and Matthew David Jackson.

The EAGE Best Paper Awards are another important group of honours highlighting

Nominating nominees does require some serious input and the Awards Committee values your time and effort. The workload is often shared with multiple people contributing to the nomination letter and providing additional references.

The Awards Committee carefully considers all nominations and selects awardees for each category. Recommendations are then sent to the EAGE Board who may or may not approve them, but the Board does not select awardees themselves. The Awards Committee and Board especially welcome nominations reflecting the diversity of EAGE in terms of discipline, gender, and geographic distribution.

The EAGE Awards Committee comprises esteemed EAGE members with a long history of academic excellence and

excellence in publications and presentations. The selection for these is carried out by the Awards Committee later in the year, in co-ordination with the EAGE Publications Officer, the editors of EAGE journals and the EAGE Technical Programme Officer.

Finally, the EAGE Awards Committee wishes to emphasise the significant impact these awards can have on recipients’ careers. We encourage you to take the time to identify and nominate candidates who have positively impacted your work and career and whom you hold in high esteem. Detailed nomination requirements for all achievement awards can be found on our website eage.org/awards.

We look forward to receiving your nominations and celebrating the remarkable achievements within our community.

Amazing success of Oslo Annual leaves lasting legacy

Our 85th EAGE Annual Conference & Exhibition, held in Oslo from 10-13 June 2024, successfully united professionals, academia, and industry leaders under the theme ‘Technology and Talent for a Secure and Sustainable Energy Future’. The event was distinguished by its advanced technology, dynamic sessions, and strong industry participation. Norwegian companies, notably our host Equinor, and the Norwegian Continental Shelf provided an ideal backdrop for discussions.

The Opening Ceremony celebrated the Annual’s rich history and recognised outstanding achievements with the announcements of 2024 Award winners, Best Local Chapters of the Year and Laurie Dake winners. The conference continued to be a platform for sharing research intersecting geoscience, engineering, and energy.

For the first time, delegates used headsets for presentations and discussions, creating a technologically advanced, immersive environment. This innovation fostered a silent, attentive, and highly interactive atmosphere, and enhanced what was generally regarded by participants as an exceptional and comprehensive Technical Programme covering the latest developments relevant to our geoscience and engineering community.

Our expanded Strategic Programme comprised 11 curated sessions addressing all the hot topics of the day including leadership, exploration strategies, digitalisation, and talent development in the energy sector. Industry leaders from Equinor, Viridien, Wintershall Dea, Petronas, Petrobras, ENI, ExxonMobil, TotalEnergies, Vår Energi, BP, NZTC, SLB, TGS, Baker Hughes, Shearwater, PGS, CGG, CNPC, Chevron, IRIS Instruments, NTNU, and Aker BP all participated. Highlights included the Opening Session ‘Fireside Chat’ between Philippe Mathieu (EVP EPI, Equinor) and Andrew

speakers

McBarnet, EAGE editor emeritus, and a very well received discussion panel with young professionals from leading companies providing perspectives on the varying definitions of energy security and their career options in the energy transition era.

The Exhibition was exceptionally busy and appreciated by participating companies who reported numerous existing and potential clients visiting their stands. Of note was the launch of CGG’s new Viridien identity and the 20th anniversary of Geo Expro, first launched at the EAGE Annual 2004 in Paris, both occasions underlining the enduring value that the event represents.

On the floor, two dedicated Exhibition zones stood out. The Digital Transformation Area highlighted advancements in digital technologies revolutionising the energy sector. The Spotlight Series ‘An open-source regulator’, thanks to the Norwegian Offshore Directorate, showcased the impact of open data and success stories of its use. The Energy Transition Area sessions focused on balancing sustainability with energy affordability and the role of geoscience in these changes.

Memorable moments here included presentations by winners of the Hackathon ‘Coding to Net-Zero’ and the interactive tool for navigating careers in the evolving energy landscape newly developed by the EAGE Education Committee and the five Technical Communities on energy transition topics.

Workshops, short courses, and field trips offered participants deeper knowledge and hands-on experiences. Workshops covered data science in geosciences, advanced seismic interpretation, and reservoir engineering. Short courses addressed microseismic monitoring, natural geologic hydrogen exploration, and geothermal energy production. Field trips

Attendees were immersed in an engaging environment that highlighted innovation and collaboration, making this year’s conference unforgettable.

explored the Oslo Rift, the Norwegian Continental Shelf, and the Oslo region’s geology.

Once again, the Community Programme activities attracted significant interest, particularly the ‘Geo-secrets of Norway’ talk, CV advice and professional photography sessions, and the ever-popular GeoQuiz.

The Conference Evening at the amazing Bygdøy was the best attended in our history with partygoers enjoying traditional Norwegian music and cuisine while exploring the country’s maritime heritage through the Kon-Tiki Museum, Fram Museum, and Norwegian Maritime Museum.

For such a successful event we extend heartfelt thanks to all delegates, speakers, session chairs, exhibitors, Local Advisory Committee and, most important, our sponsors for their fantastic support.

Stay tuned for our forthcoming Annual in Toulouse, where we will continue to drive innovation and sustainability in the energy sector. Mark your calendar for 2-5 June 2025!

HIGHLIGHTS OF EAGE ANNUAL 2024

LOOKING BACK AT AN AMAZING WEEK IN OSLO

KICKING OFF WITH ENERGY

A spectacular start at the Opening Ceremony

HIGHLIGHTS OF EAGE LOOKING AMAZING

DYNAMIC TECHNICAL PRESENTATIONS

Sessions that spark engaging discussions, provide valuable insights, and inspire new ideas

EAGE ANNUAL 2024 BACK AT AN WEEK IN OSLO

STRATEGIC PROGRAMME HIGHLIGHTS

Expert leaders exploring key issues and driving progress in our industries

EXHIBITION BUZZ

A central point for strategic talks, major business news, product showcases, cooperation deals, and connecting with peers

Thank you for making EAGE Annual 2024 unforgettable. Here’s a look back at some of the event’s most engaging moments! Scan the QR code for the event report!

HONOURING THE BEST

Highlighting notable contributions and student accomplishments

INSPIRING EDUCATIONAL SESSIONS

Our workshops and short courses fostered fresh knowledge and honed existing skills

ADVANCING CAREERS

Innovative activities focused on nurturing members’ professional advancement and personal development

HIGHLIGHTS OF EAGE LOOKING BACK AT WEEK IN EAGE ANNUAL RELIVING THE WEEK IN

UNIQUE EXPERIENCES

Field trips that offer new perspectives and memorable experiences

EAGE ANNUAL 2024 AT AN AMAZING IN OSLO

CELEBRATION NIGHT

Relive the joy and camaraderie of the Conference Evening

THE INCREDIBLE IN OSLO

What they said about the Oslo Annual

We live in times of upheaval, and that focuses on the importance of energy for society. I think it was a great theme that EAGE chose for this Annual and I was happy to represent Equinor as host company. The people, the enthusiasm, the willingness to contribute and to find ways forward has been an inspiration. So it’s really something that energises me as I go back to my day job.

Erling Vågnes

SVP Subsurface Exploration Production International, Equinor

EAGE 2024 Local Advisory Committee Chair

We have a new challenge: to explore how our members can engage effectively to raise awareness about balancing energy sustainability and energy security, and how geoscience can contribute to the energy sector and other key societal advancements. I think about infrastructure, climate change, and natural risk protection. There are many emerging topics where we can provide answers. This is where our role lies.

Laura Valentina Socco

Full Professor Applied Geophysics, Politecnico di Torino

EAGE President (2024-2025)

It has been fantastic to see so many people attending, to be amongst so many geoscientists, and see the energy in the room. The theme of this year’s conference was really energising. It sets us on the way to that transition that we’re all in.

Andreas Aspmo Pfaffhuber

CEO & Founder, EMerald Geomodelling Chair EAGE NSG Circle (2024-2025)

It’s been a great experience. And for us, it’s been a special moment because we’ve introduced our new name, Viridien. It was well attended with good representation of industry players, which means a good moment to interact with our stakeholders, our clients, and our partners.

Sophie Zurquiyah CEO, Viridien

Geosciences are bigger than energy because they integrate all the information together. It’s not about oil and gas anymore. It’s about wind farms. It’s about solar panels. It’s about geothermal. It’s about nuclear fusion. It’s about hydrogen. So when we use our geoscience and engineering technologies to nurture these technologies, I think we are heading towards a sustainable future.

Sanjeev Rajput

General Manager & Global Head-Reservoir Geoscience/ Geophysics, Petronas

EAGE Vice President (2024-2025)

It was the first time in 20 years that we were welcoming the EAGE Annual in Norway. It was really important for us and I’m so glad to see that it was a real success. People were really eager to participate, to meet us, to come to Norway. So I would say that all our objectives have been met. We’re extremely satisfied and happy with this fantastic, positive atmosphere and collaboration. Very good technology was presented along with an extremely insightful strategic programme. I can’t wait to see the same positive energy next year.

Severine Pannetier-Lescoffit

Chief Geophysicist, Equinor

EAGE 2024 Local Advisory Committee Member

It was good to have the conference back in Norway after so many years. It was impressive to see a very professional and inspiring exhibition, and really well attended event. The technical programme was of very high quality, a lot of attention to the workshops, good technical sessions, and dedicated sessions. It was really good to see.

Martin Widmaier

Chief Geophysicist, Sales & Services, TGS

EAGE Technical Programme Officer (2024-2025)

The event offered a really great opportunity to network with people that you already know from the industry, but also get to know new people across different companies to see what kind of exciting work that they’re doing, especially with the perspective of a young professional in this industry moving at such a dynamic pace.

Mahad Nadeem Junjua

Data Scientist, Wintershall Dea

It has been a forum for great minds to come together, to think about the energy trilemma, the challenge we have as a global state, and how we can work together to resolve this. It’s also a place where we can encourage geoscientists and subsurface professionals to see there’s still a bright future and career ahead, beyond this decade and decades to come … a place for young individuals to be inspired and mentored.

Julya Bonkat

Subsurface Manager, Competence Centre, Equinor

We are very proud of the way the Oslo Annual turned out to be such a success, and it’s great to have such positive feedback. I therefore thank all our staff for making it possible. Looking forward already to Toulouse next year.

Marcel van Loon CEO, EAGE

Enhance your geological storage expertise with the EAGE Masterclass CO2 Storage 2024

EAGE’s Masterclass CO2 Storage 2024 event on 30 September to 3 October promises to be an unparalleled opportunity to deepen your understanding of geological CO2 storage through a comprehensive programme of short courses led by renowned experts.

Hosted at the BP Sunbury Learning Centre, just a short distance from the Heathrow Airport, the Masterclass offers three distinct courses, each designed to cover critical technical aspects of CO2 storage. Attendees can choose to focus on a single course or maximise their learning with an all access pass, ensuring a robust and multi-faceted educational experience.

The intensive two-day course Risk Assessment of CO2 Storage by Understanding Coupled Thermo-hydro-chemical-mechanical Processes by Andreas Busch and Eric Mackay (Heriot-Watt University) on 30 September and 1 October will explore the complexities of assessing CO2 storage risks by examining coupled phenomena, including reservoir conformance and storage

integrity. The instructors will give insights into the effects of pressure, temperature, and geochemistry on subsurface storage and sealing formations, equipping participants with the knowledge to evaluate the technical viability of CO2 storage projects.

On 2 October, Prof Philip Ringrose (NTNU) will present a course on CO2 Storage Project Design and Optimisation (Saline Aquifers). He will guide attendees through the intricacies of designing and optimising CO2 storage projects in sandstone saline aquifer systems. This course

‘We’re

according to Doster who will provide a comprehensive overview of flow mechanics, utilising principles such as Darcy’s law and mass conservation to explain fluid movement in reservoirs. Participants should leave with a solid foundation in flow mechanics applicable to geological CO2 storage.

The EAGE Masterclass CO2 Storage represents a unique opportunity for professionals to advance their expertise in this critical area of environmental science and engineering. Whether you’re a seasoned expert or new to the field, these courses

excited to present a Masterclass series featuring CO2 Storage experts. Embrace the unique opportunity for in-depth learning and direct engagement with specialists at these in-person courses.’

Maren Kleemeyer, EAGE Education Officer

will cover essential topics such as project timelines, site characterisation, trapping mechanisms, fluid dynamics, storage capacity estimation, well design, CO2 transport, geomechanical considerations, and long-term storage assurance methods.

Flow Mechanics for Geological CO2 Storage presented by Florian Doster (Heriot-Watt University) on 3 October closes the trio of outstanding courses being offered.

Understanding the fundamentals of flow in porous media is crucial for the successful planning and operation of CO2 storage,

offer valuable knowledge and practical skills to enhance your capabilities and contribute to the advancement of CO2 storage technologies.

Secure your place at the EAGE Masterclass CO2 Storage 2024 event and be part of the forefront of CO2 storage innovation.

For more information and to register

F. HASIUK & S. ISHUTOV

Visions of AI — past, present and future

EAGE Local Chapter Netherlands hosted an online event on 23 May featuring Marieke van Hout and Dr Paul de Groot from dGB Earth Sciences. The theme of the event was the past, present, and future of AI in seismic studies. Van Hout briefly introduced the history of dGB, the company behind the opensource software OpendTect. De Groot took over the wheel and continued discussing the development of AI in seismic studies.

De Groot reviewed AI definitions (e.g., neural networks, shallow networks, deep networks, etc.) and the corresponding history from Turing machine. He added that machine learning (ML) is famous for finding complex relationships and structures in data sets, however, ML has experienced peaks and troughs in popularity, a phenomenon known as the Gartner’s hype cycle. He believes we have to embrace the trend of AI with caution.

He reviewed the current progress of ML developments in the geoscience

world through various applications. For example, an unsupervised vector quantizer was used for 3D waveform segmentation to see hidden geology patterns. Meanwhile, popular shallow multi-layer perceptrons (MLP) can be applied to all kinds of seismic object detection tasks, e.g., faults, channels, salt, chimneys, etc. Rock porosity prediction and missing logs prediction were also achieved by quantitative MLP. As for deep networks, de Groot also demonstrated seismic classification by imageto-point workflows. Image-to-image workflows, e.g., U-Net, was discussed for 3D salt body prediction as well as for pseudo-3D volume conversion.

De Groot showed some pre-trained models and their huge potential inside OpendTect for cleaning up seismic data, removing migration smiles, and eliminating multiples. Thus, he believes that acceleration through new models could help people decrease time, efforts, and costs. Regarding the future of AI development, his conclusion was that both

interactive workflows and foundation models have the potential to play a significant role in the near future. He also highlighted that machine models cannot replace geoscientists; however, human interpreters must acquire new skills to use this technology effectively. Many questions were fired after the talk from the many audiences participating. One raised the possibility of digitalising vintage seismic documents using AI technology. Accuracy and repeatability of AI methods were also discussed. Some were wondering about the potential use in the field of civil engineering or construction. The role of synthetic/tank data and physics-informed elements in training AI models were raised as well by our audience. The recorded video can be found in the EAGE YouTube channel. Those interested in remaining updated on chapter initiatives are encouraged to follow the Chapter’s LinkedIn page and express their interest for joining by reaching out to eageLCNetherlands@gmail.com.

Best ‘Newcomer’ LC Kuwait reflects on journey so far

Ever since their grand relaunch ceremony in April 2023, Local Chapter Kuwait has been on a truly remarkable journey. Over 100 individuals from 16 diverse companies united to celebrate the restart of the local EAGE community. That first gathering wasn’t just an ordinary event: it served as a fantastic connecting opportunity, bringing together professionals from different disciplines of geoscience and engineering.

On that occasion the new LC team shared their vision for the coming years. It was just the beginning of a series of activities that led to winning the coveted title of ‘Best EAGE Local Chapter of the Year’ in the ‘Newcomer’ category, reserved to communities younger than two years.

Mohamed Dawwas Al-Ajmi, president of the Chapter, said the award ‘highlighted the exceptional efforts of our board members and their commitment to volunteer work. It inspires us to continue pushing boundaries and pursuing our ambitious vision for the future’.

One special achievement for LC Kuwait was the organisation of monthly webinars, featuring presenters from around the world who covered a wide range of topics, providing valuable insights and knowledge to professionals and students. The Local Chapter was very keen on making these technical sessions accessible anytime, anywhere and regularly published recordings.

The members also distinguished themselves for their outreach work to create awareness of geoscience and its applications among younger generations, for example, presenting a lecture and a tour in Ahmad Al–Jaber Oil and Gas Exhibition, enabling high school students to witness real-world applications of science.

‘Geoscience is often neglected in our educational system as many schools choose to give less priority to geoscience-related subjects in their curriculums, thus, students miss out on the opportunity to learn and discover its fascination’, says Hajar Al-Wazzan, vice president, LC Kuwait. ‘We believe that students should be more engaged by introducing Earth Science as an

enjoyable and interactive subject to help shape a brighter future for geoscience in Kuwait and gain it the recognition it deserves.’

The Chapter’s work included facilitating the creation of a new Student Chapter. ‘This opens a new gate for us to connect academia and industry’, says Fatemah Basha, communication officer. ‘Empowering students and young professionals is one of our main aims.’

A highlight of the past year was holding of a debate ‘Geologists vs. Engineers, friends or enemies’ aimed to bridge the gap between geoscientists and engineers by highlighting the roles of both professions, particularly in the context of drilling in the oil and gas industry. The conclusion was that we will need both the geologist’s brains and the engineer’s muscles!

Looking ahead, the team is brimming with fresh ideas and planning a wide variety of technical and social activities in the coming year. So there will be something for everyone to look forward to.

Underground sun shines in Prague

In May Local Chapter Czech Republic welcomed Markus Pichler, a senior reservoir engineer, subsurface energy storage development, RAG Austria, to tell the story of his company’s Underground Sun Storage project.

The meeting attracted a mixed audience including energy regulators, government advisors, researchers and gas storage geoscientist, all interested in hearing Pichler’s insights regarding his company’s development of underground hydrogen storage technology.

storage. The origin of the name of the project reflects the capacity to store solar energy underground when it is in over-supply and to use it when needed.

The initial project used a hydrogen admixture of 10% natural gas and addressed numerous scientific questions such as wellbore integrity, storage integrity, energy loss, mixing and dissolution issues associated with hydrogen/methane storage. To cut the long story short, despite numerous concerns, Pichler convinced the audi-

Pichler started his story describing the motivation for the use of hydrogen as an energy carrier which could potentially be transported and stored within existing energy infrastructure. The aim of the initial research was to find out if existing natural gas storage facilities were suitable for hydrogen

ence that no significant deterioration was observed when testing hydrogen storage in a depleted natural gas reservoir. The slightly more diffusive character of hydrogen compared to methane did not result in measurable hydrogen losses during the runtime of the project.

Another aspect of hydrogen storage is public acceptance for hydrogen storage projects. In general, they are regarded as positive compared to classic natural gas storage which in the light of lowering fossil dependence is seen as critical. As this aspect was only covered within the scope of a research project, the implications for an actual large scale commercial project are not yet completely understood. However, the project showed how important transparent communication and public engagement are if large scale energy infrastructure projects should be developed and built.

Final thoughts were devoted to a second option of renewable energy storage combining storage of hydrogen and CO2. When these two gasses are injected together into the reservoir naturally occurring microbes convert it to methane which is three times more storable than hydrogen. For this purpose, the underground sun conversion project and its follow-up projects have been conducted. Apart from using CO2, another benefit of this method is that renewable energy can be stored while no changes to the current gas infrastructure are necessary. Currently the process is not yet competitive with the direct storage of hydrogen or the alternative which is surface methanation.

LC Kuwait received the recognition at the 2024 Annual.

Markus Pichler makes the case for hydrogen.

Experience a rich combination of dedicated sessions at GET 2024

The EAGE Global Energy Transition Conference and Exhibition is coming up in just three months. Join us from 4-7 November 2024, in Rotterdam, Netherlands. This year’s event will feature dedicated sessions across four major technical conferences, each addressing critical aspects of the energy transition and associated technologies. Building on the success and positive feedback from last year’s sessions, these focused discussions will provide valuable insights into the latest advancements, challenges, and opportunities in CCS, geothermal energy, hydrogen, energy storage, and offshore wind energy. Attendees will benefit from the comprehensive knowledge shared by industry experts, gaining a deeper understanding of cutting-edge technologies and innovative solutions that contribute significantly to the global effort towards a sustainable energy future.

Here’s a detailed overview of what each conference will cover

Carbon Capture and Storage Conference

The dedicated sessions for the Carbon Capture and Storage (CCS) Conference focus on various facets of the CCS value chain and hub development. These sessions will explore the complexities and challenges of implementing CCS projects, emphasising the importance of technical, economic, environmental, safety, and societal considerations. Key discussions will include the advantages of CCS hubs in reducing costs and risks, the role of reservoir modelling in understanding CO2 plume behaviour, and the challenges of multizone evaluations. Additionally, the conference will explore synergies and barriers for CCS projects in future hubs, the current challenges and future of monitoring CCS fields, and the importance of well integrity during CO2 storage operations. Another session will address the opportunities and challenges of storing CO2 in depleted fields, focusing on injectivity, containment, and the integrity of existing facilities.

Geothermal Energy Conference

The Geothermal Energy Conference will highlight critical sessions on the opportunities and challenges associated with geothermal energy development. Topics will include the extraction of critical minerals from geothermal fluids, such as lithium, to support the energy transition. The conference will also review the past, present, and future of geothermal power in Europe, highlighting the lessons learned from previous projects and the potential for untapped geothermal resources. Another session will focus on geophysical data acquisition to accelerate geothermal development in urban environments, showcasing best practices and lessons learned from recent projects. The conference will also discuss new horizons in geothermal energy, investigating breakthrough solutions and novel technologies that could significantly lower costs and increase energy outputs, including offshore geothermal developments.

Hydrogen and Energy Storage Conference

Sessions at the Hydrogen & Energy Storage Conference will cover the pivotal role of underground hydrogen storage in providing flexibility and robustness to renewable energy systems. Attendees will learn from various pilot projects in different geological environments, gaining insights into the remaining uncertainties and de-risking requirements. Case studies on Compressed Air Energy Storage (CAES) and Underground Thermal Energy Storage (UTES) will highlight their scalability, efficiency, and environmental impacts. Another session will discuss strategies for minimising hydrogen emissions from storage operations, including innovative monitoring techniques. A panel discussion is planned on the hydrogen economy, exploring the challenges and opportunities from source to market while other sessions will cover the latest developments in natural hydrogen, from its source to market.

Offshore Wind Energy Conference

The Offshore Wind Energy Conference will offer sessions on optimising the decommissioning strategies for offshore wind farms, highlighting the importance of geoscience perspectives. These sessions will address the environmental sustainability and efficient resource utilisation during decommissioning. Another session will focus on wind and metocean measurements, emphasising the importance of accurate data for offshore wind farm design and operation. The alignment of offshore wind development with regulatory frameworks and geosciences will also be discussed, considering environmental and archaeological impacts. The conference will also explore how to bridge the gap between geophysical and geotechnical data for offshore wind engineering, aiming to support cost-efficient site characterisation strategies. Finally, the conference will address the challenges and opportunities in UXO (Unexploded Ordnance) risk management for offshore wind projects, where the need for improved detection capabilities and environmental impact mitigation is significant.

Visitor passes are still free and sign up with our early bird fee before 1 September to save significantly on access to the full GET 2024 conference and exhibition. Consider an all-access pass to benefit from all side activities, including workshops, field trips, and short courses. Learn more at eageget.org.

Dedicated session at GET 2023.

PRE- AND POST-CONFERENCE

ACTIVITIES

SHORT COURSES

– CO2 Storage Project Design and Optimisation (Saline Aquifers)

– Geophysical Monitoring of CO2 Storage

– Reservoir Engineering of Geothermal Energy Production

– Exploration of Subsurface Natural Geologic Hydrogen and Stimulation for Its Enhanced Production

– An Introduction to Offshore Wind

– Public Engagement for Geo-energy

WORKSHOPS

– Appropriation of the Subsurface and its Usage

– CCS and Geothermal at Its Core: Geological Risk Assessment for Geothermal and CCS on Core Material

FIELD TRIPS

– Porthos – First Large-Scale Project to store CO2 under the Sea

– The Rijswijk Centre For Sustainable Geo-Energy (Rcsg), Energy

Cave And Delft Campus Geothermal Project and Laboratories

– Hystock Hydrogen Storage of Gasunie in Zuidwending

– AYOP IJmuiden, Shell Noordzee Wind Farm, and TENNET’s visitor center

DIVERSE TECHNICAL PROGRAMMES 4

“The programme focuses on creating a coherent theme around hubs. In Rotterdam, projects are taking shape, scaling up operations. We’ve structured the programme to highlight the North Sea and projects in Norway, the UK, the Netherlands, and potentially Denmark, forming a regional hub for CO2 storage.”

BEN DEWEVER

Senior Geoscientist Carbon Capture and Storage Capability team, Shell

DEDICATED

SESSIONS TO BE A KEY HIGHLIGHT

• CCS value chain and hub development: perspective and challenge

• Synergies or barriers for CCS projects in future hubs

• Reservoir modelling in a CCS hub context

• Opportunities and challenges of CO2 storage in depleted Fields

• The future of monitoring and the current challenges

• The opportunities & risk of wells in CCS operations

“Attendees of the conference will be able to participate in sessions focused on finding different ways to overcome both technical and sociological obstacles. By sharing insights from different fields, we can identify common challenges, such as the social licence to operate in an area, which is not a technical issue but is very important for discussion.”

ALL ACCESS PASS

Gain full access to all events, including short courses, field trips, and workshops.

CHECK

“The programme and dedicated sessions will focus on hydrogen storage piloting, with industry representatives sharing insights, learnings, and highlighting knowledge gaps for commercial scale deployment. Another focus is natural hydrogen, gaining attention for its disruptive potential.

“With focus on the recent advancements and innovations in both qualitative and quantitative geo-modeling & characterization techniques along with the use of AI (and machine learning) in addressing the challenges in offshore wind, EAGE GET will play an important role in bringing minds together for achieving a unified global success.”

GEHRIG SCHULTZ COO | Geoscience EPI Group

• Opportunities and challenges of critical mineral extraction from geothermal fluids

• Unlocking European geothermal power: past, present and future

• Geophysical data-acquisition to accelerate geothermal development in the urban environment

• New horizons

KARIN DE BORST

Subsurface Specialist Shell

SANKET BHATTACHARYA Seismic Business Development Manager, Fugro

• Underground hydrogen storage pilot projects

• Case studies and pilot projects for storage

• Understanding hydrogen emissions: strategies for sustainable storage operations

• The hydrogen economy – from source to market

• Natural hydrogen: from source to market

• Optimizing decommissioning strategies for offshore wind farms

• Wind and metocean measurements for offshore wind farm design

• Aligning offshore wind development and regulatory frameworks

• Geophysical and geotechnical data for offshore wind engineering

• Uxo risk management: challenges & opportunities

SUPPORTING ORGANISATIONS

“GET2024 provides a unique platform to engage, interact, and exchange ideas. As we gather in Rotterdam, I look forward to engaging in lively discussions that will influence the field of geosciences in our dynamic and constantly changing world. Together, we will address challenges, capitalise on opportunities, and collaboratively build a lasting and sustainable future.”

YOLANDE VERBEEK

Chair, Executive Committee

COO, EBN

STRATEGIC PROGRAMME

RESILIENT VALUE CHAINS FOR THE ENERGY TRANSITION

4-7 NOVEMBER 2024

ROTTERDAM, THE NETHERLANDS

STUDENT PROGRAMME

A programme designed to engage students in energy transition through interactive learning, career development, and recognition.

ENERGY INNOVA TORS A four-day programme for students to explore the technical, economic, and societal facets of energy transition.

BRIDGING THE GAP: SOCIETAL ENGAGEMENT IN THE ENERGY TRANSITION

DIGITALIZING THE ENERGY TRANSITION: INNOVATIONS IN TECHNOLOGY, MARKETS, AND TALENT

Discover how to build resilient CCS, Hydrogen, Geothermal, and Wind value chains focusing on business models, partnerships, regulations, and risk management.

Gain insights on enhancing societal engagement and boosting energy literacy and lobbying skills from psychology, environmental management, and education perspectives.

Explore AI-driven digital innovations for decarbonizing systems, scaling technologies, unlocking markets, and the need for policy support and skilled workforce.

Three days, four technical programmes, an extensive exhibition, engaging pre- and post-conference activities like short courses, field trips and workshops. Ensuring a comprehensive programme for each key topic while maintaining GET’s unique focus on integration within energy transition.

SPONSORS AND EXHIBITORS

Sign up for a week-long free exhibition and meet key players from around the world, including major energy companies, service providers, consultancies, licensing bodies, government sectors, and innovative startups.

CHECK OUR SPONSORSHIP OPTIONS HERE

MARIE THARP AWARD

Field trips add to what makes GET 2024 special

We have not forgotten the value that field trips can bring to an event. This is why GET 2024 intends to enhance participants experience by providing three different exciting, informative, and enjoyable field trips. Each one will be associated with one of the four parallel technical conferences at the event, e.g., carbon capture and storage, geothermal energy, hydrogen and energy storage, and offshore wind energy.

Field Trip 1: HyStock hydrogen storage of Gasunie in Zuidwending

Explore the advancements in hydrogen storage with a visit to the EnergyStock installation of HyStock. The field trip offers a deep dive into the near future of hydrogen storage, showcasing the company’s plans and addressing the challenges faced. HyStock will share knowledge and results from the lifecycle hydrogen test of a cavern (200 bar) and demonstrate its hydrogen value chain installation, including a 3MW solar facility, 1 MW PEM electrolyser, compressor 300b, trailer load station, 100% H2 heater, and fleet owner HRS.

Field Trip 2: The Rijswijk Centre for Sustainable Geo-Energy (RCSG), Energy Cave, and Delft Campus Geothermal Project and Laboratories

This field trip enables you to find about the innovative world of geo-energy technologies with a visit to the Rijswijk Centre for Sustainable Geo-energy (RCSG). The Energy Cave in Rijswijk serves as a meeting point for discussing the energy transition. The Delft Subsurface Urban Energy Laboratory addresses critical questions regarding geothermal energy in urban environments, including project density, energy extraction longevity, efficient energy system design, seasonal thermal energy storage, and avoiding adverse impacts.

Field Trip 3: Porthos – First large-scale project to store CO2 under the sea

Witness the future of carbon capture and storage with a visit to the Porthos project, the first large-scale initiative to store CO2 beneath the North Sea. Porthos is developing a system where CO2 from industry in the Port of Rotterdam is transported and stored in empty gas fields offshore. The CO2 will be supplied to a collective pipeline, pressurised in a compressor station, and transported via an offshore pipeline to a platform in the North Sea, then pumped into empty gas fields situated more than 3 km beneath the sea. Porthos aims to store around 37 Mton of CO2, approximately 2.5 Mton per year for 15 years, with operations expected to start in 2026.

Field trip 4: AYOP IJmuiden, Shell Noordzee Wind Farm, and TENNET’s visitor centre

Join us for a full-day field trip exploring the Netherlands’ offshore wind industry. We’ll start at AYOP in IJmuiden, where you’ll learn about initiatives supporting offshore wind companies near the new North Sea wind farms. Next, enjoy a boat trip to the Shell Noordzee Wind Farm with an onboard presentation and lunch (weather permitting). This wind farm, operational since 2007, was the first 100+ MW wind farm in the Dutch North Sea. Finally, visit TENNET’s visitor centre for a presentation on connecting offshore wind farms to the Dutch grid, including a tour of the transformer station.

To participate in one of our field trips, you will need to register with our all-access pass, which also provides access to short courses, workshops, and the full conference and exhibition. Learn more at eageget.org/field-trips/.

‘Energy Transition Student Days’ at GET 2024 offers hands-on learning opportunity

What are the intricacies of developing a Carbon Capture & Storage (CCS) project or a geothermal development plan? How do these compare to traditional oil fields in terms of energy content, emissions, profitability, and stakeholder considerations?

The ‘Energy Transition Student Days’ during the EAGE GET 2024 presents a conference designed to provide students with a comprehensive understanding of the technical, economic, and societal aspects of the energy transition.

The student event will run from 4 to 7 November 2024, during the EAGE GET conference and will be led by two distinguished instructors: Dr Raymond

Franssen, and Dr Manuel Willemse. While a basic knowledge of earth sciences is preferred, it is not required. The learning process combines lectures, practical team exercises, and class discussions, ensuring a thorough understanding reinforced by hands-on experience.

Participants will be given a subsurface map, a well profile, and other key data. Working in teams, they will be tasked to create a development plan for underground resources. This learning process involves reading maps, creating production profiles, discussing how geology affects project feasibility, and tackling both technical and non-technical challenges. By examining

projects from multiple angles, participants will see how their work can contribute to achieving the Paris Climate goals and national targets, especially in the context of the Netherlands.

This is a unique opportunity to advance understanding and skills in this vital field. Whether you are a student of earth sciences, engineering, or policy, the course offers valuable insights and practical experience to prepare you for the challenges and opportunities of the energy transition.

For more information and to register, visit the EAGE GET 2024 website. Secure your place today and contribute to shaping a sustainable energy future.

Journal launches themed issue for early career geoenergy researchers

Geoenergy is inviting contributions to a thematic collection showcasing the breadth of innovative research in the field of geoenergy by emerging leaders in the field. In the race to realise the transition to a sustainable energy future, early career scientists are paving the way. By repurposing existing methodologies for novel applications and developing entirely new techniques, research led by early career scientists is driving change. Our hope is that this will offer an independent voice to early career researchers, stimulate and foster a network of these scientists, and further encourage multi-disciplinary research in the diverse field of geoenergy.

The collection is offering opportunities for PhD candidates, postdoctoral researchers, academic tenure track researchers (assistant professors, or equivalent occupying the position for less than eight years), and early career scientists in the industry sector (less than 10 years of experience), to submit novel research papers in the broad field of sustainable geoenergy. The thematic collection follows the scope of Geoenergy, hence we encourage submissions of geoscience articles with applications in geothermal energy, energy storage, hydrogen exploration, critical minerals and raw materials, CO2 sequestration and sustainability.

Besides the targeted group of scientists, the uniqueness of the proposed collection lies in the promotion of the scientific independence of early career researchers and hence their willingness to develop out-of-the-box fundamental and

this thematic collection suggest limiting the number of experienced co-authors to one professor and/or one associate professor (or equivalent).

We will give publication priority to original ideas developed by multi-disciplinary teams of early career researchers, which may also involve master’s or undergraduate students.

Submission

Expressions of interest to publish should be sent to the Geoenergy Editorial Office (geoenergy@geolsoc.org.uk) no later than 30 November 2024.

Please include the following information: title, list of authors, abstract, commitment to transform this intention of publishing into a paper. Authors of accepted intentions will then be invited to submit a full manuscript within six months, which

EAGE Student Calendar

will enter a standard peer review process (note that invitations to submit do not guarantee acceptance after peer review). We expect the full thematic collection to be complete by the end of 2025.

Guest editors for this special issue are Pierre-Olivier Bruna, TU Delft, Netherlands; Sian Evans, University of Oslo, Norway; and Guofeng Song, TU Delft, Netherlands.

Following publication, the guest editors are also motivated to organise a special workshop on this thematic collection, which would be preliminarily scheduled for 2026. For any queries please contact the Geoenergy Editorial Office.

Full details on this thematic collection

Enrich research in the diverse field of geoenergy led by early career scientists. (image credit: Gerd Altmann from Pixabay).

Near Surface Geoscience 2024 is nearly here

Just one month before the EAGE Near Surface Geoscience Conference and Exhibition 2024 takes place on 8-12 September in Helsinki. This event stands out as a premier gathering in the world for the near-surface geoscience community featuring the 30th European Meeting of Environmental and Engineering Geophysics (see accompanying article), the 5th Conference on Geophysics for Mineral Exploration and Mining, and the 4th Conference on Airborne, Drone, and Robotic Geophysics.

NSG2024 presents a rich technical programme with over 35 sessions across three parallel meetings, exploring a broad spectrum of geophysical topics including discussions on climate change impacts, geothermal energy, CO2 storage, and advanced monitoring for nuclear waste. Sessions will also cover geohazard studies, engineering geology, and the role of geophysics in achieving sustainable development goals. Special features include the latest in airborne, drone, and robotic geophysics, as well as new advances in rock physics and mineral exploration.

In the exhibition area you can expect a hub of activity, featuring more than 30 leading companies from around the world. Those attending will have the opportunity to engage first-hand with technologies and services aiming to further understanding

of the near surface. From state-of-the-art equipment to innovative software solutions, the exhibition will be a key resource for anyone looking to stay abreast of advancements in the field.

NSG2024 also offers specialised workshops Digital Outcrop Modelling, the Transient Electromagnetic Method, and Hard Rock Physics providing participants with hands-on experience and insights into integrating these methods into their own work, bridging the gap between theory and application.

An extra benefit of the event are the great field trips on offer, such as the Geowalk in the World Heritage site of Suomenlinna, the exploration of Salpausselkä Geopark, and the Tytyri Mine Experience. These trips are more than just educational - they are immersive experiences that highlight the geological splendour of Finland and provide contextual understanding that enhances the conference’s learning opportunities.

If you’re keen to explore the realm of near-surface geoscience, don’t miss NSG2024 in Helsinki. This year, we are excited to introduce the All Access Pass for the first time. To learn more about the conference, review all available pass options, and register, visit www.eagensg.org. Make sure to secure your spot by the regular registration deadline on 20 August.

Celebrating 30 years of near surface geoscience innovation

The European Meeting of Environmental and Engineering Geophysics at NSG2024 will be celebrating its 30th anniversary in Helsinki, celebrating three decades of innovation and fostering a global conversation on geophysical methods crucial for addressing environmental and engineering challenges.

NSG conferences have expanded to cover a wide range of geoscience topics, including environmental studies, archaeology, engineering, and mining, with investigation depths from 10 cm to 1000 m. New specialised conferences were introduced alongside the main event to attract diverse participants, starting successfully in Athens in 2014. These additions boosted EAGE member engagement and delegate participation, with notable success in shallow marine geophysics, MASW workshops, and near-surface seismic.

George Apostolopoulos , former NSG Division chair (20182020), said: ‘From its inception, the Near Surface Geoscience Division, now Circle, has benefited from the integration of EEGS and EAGE experiences, facilitating substantial growth in applied geophysics outside the traditional oil and gas sectors. This strategic integration has not only broadened the conference’s scope but also enriched its impact, paving the way for the initiation of specialised forums.

Alireza Malehmir, NSG Division chair from 2020 to 2022, pointed to the tangible impacts of the event’s broadened focus. ‘The shallow subsurface retains a vast history of what has happened above and below, offering insights into water and mineral searches and the study of active tectonics.’ His engagement with the field began through studies on quick-clay land-

slides, a journey that showcases how near-surface geophysics directly benefits the understanding and mitigation of geohazards.

Andi A. Pfaffhuber, the current NSG Circle chair, referred to changing dynamics within the geophysics community. ‘Traditionally, non-energy geophysics did not attract the same level of attention or resources as those in oil, gas, or mineral exploration. However, the recent shift towards sustainable energy exploration is merging these previously distinct areas.’

The EAGE NSG conferences continue to improve and expand. Although autonomous, they align with EAGE’s broader goals of energy diversity, green environment, mineral exploration, climate change mitigation, water resources management, and coastal engineering activities.

JOIN US!

• Hands-on Workshops

35+ Technical Programme Sessions

30+ Exhibitors

Dedicated Student Activities

• Special Talks

• Exciting Field Trips

“The shallow subsurface retains a vast history of what has happened above and below, offering insights into water and mineral searches and the study of active tectonics.”

- Alireza Malehmir

NSG Division Chair (2020-2022)

- George Apostolopoulos

NSG Division Chair (2018-2020)

- Esther Bloem

NSG Circle Chair (2022-2024)

“The field of Near Surface Geoscience is diverse, but the geoscientists and engineers working in this field share the same curiosity, trying to understand and predict subsurface processes in relation to human activities.”

“From its inception, the Near Surface Geoscience Division-Circle has benefited greatly from the integration of EEGS and EAGE experiences, facilitating substantial growth in applied geophysics outside the traditional oil and gas sectors.”

Find out more

WE ARE LOOKING FORWARD TO SEEING YOU IN HELSINKI

Sven Treitel (1929-2024)

Sven Treitel, one of the most significant geophysicists of his generation, died in April aged 95. He made enormous contributions to signal processing, finite-difference modelling, geophysical inverse theory and more. To those lucky enough to have worked with Treitel, his personal legacy is even more notable. He was that rare individual whose grace, wisdom and sense of fun touched practically everyone he met.

For three decades Treitel was the guiding spirit of geophysics at the Amoco Research Lab, having joined Pan American Petroleum Research Centre in Tulsa in 1960 before it was renamed by Amoco. He kept the same office until his retirement in the mid-1990s.

He was instrumental in bringing in talented scientists and creating a true research atmosphere – partly pure research, but also industrial research producing results useful in oil and gas exploration. Treitel did not accomplish this all by himself. The head of the Lab and the head of geophysical research were co-conspirators; to their credit, they made heavy use of his guidance. He also earned the respect of senior levels at Amoco management, not always fully reciprocated. He sometimes jokingly referred to the Lab’s large satellite dish as a monitoring instrument for signs of intelligence in upper management.

Treitel is probably best known as one of the early developers of digital signal processing. An entire generation of geophysicists learned from the Robinson-Treitel Reader, written with his fellow Massachusetts Institute of Technology (MIT) graduate Enders Robinson. The book introduced a broad audience to the possibilities of digital data analysis in geophysical applications.

One of his most important papers was Predictive Deconvolution: Theory and Practice by K. L. Peacock and Sven Treitel, published in Geophysics in 1969. The method described in the paper was immediately adopted by the industry for routine processing of seismic data and was sometimes known as ‘gap decon-

volution’. Given its significance, Treitel years later admitted to a colleague that he was surprised at getting permission from Amoco to publish. His self-deprecating theory was that the examples in the paper weren’t that impressive, so management did not really appreciate its importance.

Treitel was wont to say he experienced three dictatorships in his life before the age of 30. Born in Freiburg, Germany, in 1929 to a Jewish family, he moved with his parents to Spain in the mid-1930s to escape the Nazi oppression, finally settling in Buenos Aires, Argentina. After graduating from Lincoln School (the American International School of Buenos Aires), he worked for a year to earn his fare to the United States, and then enrolled as a college student at MIT. In 1958, he completed his PhD in geophysics while working with research colleagues inspired by mathematics professor Norbert Wiener. According to an MIT account, they theorised that a computer could process seismograms more accurately than grad students with magnifying glasses. To prove it, at night they employed MIT’s Whirlwind Computer, which the US Air Force commandeered during the day, until the sponsors closed the project down in 1957.

His first job, with Chevron, took him to Cuba until the Castro revolution in 1959. On learning that he spoke Spanish (he was also fluent in English, German,

French, Hebrew and Yiddish) the new regime offered him a post, which he declined. This led to his return to the US with his wife Renata, and the start of his long and distinguished career with Amoco.

During his lifetime, Treitel received numerous awards. From SEG, he was a four-time winner of the Best Paper in Geophysics, recipient of the Reginald Fessenden Award (1969), Honorary Membership (1983), and the society’s highest honour, the Maurice Ewing Medal, (1989). EAGE recognised his achievements with the Conrad Schlumberger Award in 1969 and Desiderius Erasmus Award in 2007, in combination with Honorary Membership. The German government’s Alexander von Humboldt Prize allowed him to spend a year as a Visiting Scholar at the Geophysical Institute in Karlsruhe. After its discovery in 2001, asteroid 54820 Svenders was named in honour of Sven Treitel and Enders Robinson, so they could continue their storied collaboration indefinitely in the heavens. In 2012 Treitel received the Marcus Milling Legendary Geoscientist Medal from the American Geosciences Institute.

Upon his retirement in 1994, a twoday ‘SvenFest’ symposium with speakers from all over the world, was held in his honour. In 2024, he was nominated to the US National Academy of Engineering.

Treitel remained active with SEG very late in life. For example, in 2022 he participated in the SEG initiative to help Ukrainian geophysicists and students who suffered after the Russian invasion of Ukraine.

Treitel is survived by his wife, Renata, his children, Geoffrey and Corinna, his granddaughter, Isabella, and his sonin-law, Krister.

Remembered by Joseph Dellinger, John Etgen, Sergey Fomel, Samuel Gray, Peter Hubral, Evgeny Landa, KurtMarfurt, Tijmen Jan Moser, Leon Thomsen, Ilya Tsvankin and Anton Ziolkowski, with additional material from SEG sources, gratefully acknowledged.

Sven Treitel (5 March 1929 – 8 April 2024).

Challenge ahead for our Critical Minerals Technical Community

Anna Lim, Pierre Josso, Chris Nind, Aline Melo, and Ebbe Hartz explain

the role of EAGE’s

recently created Critical Minerals Technical Commmunity.

The new Critical Minerals Technical Community (CMTC) is committed to a multi-disciplinary approach to advance the collective understanding of mining, past and present, and contribute to a better future. We intend to leverage diverse expertise in forging and disseminating new ideas and solutions that balance economic, environmental, and social interests with the goal of promoting sustainable mineral resource management. This means analysis of the current space of knowledge and ideas, proactive stakeholder engagement, and targeted communication efforts.

Mineral resources are indispensable to modern economies and are integral to the global push towards mass deployment of technologies capable of harnessing renewable sources of energy. However, the development of these resources often involves complex interactions between various stakeholders including scientists, industries, policymakers, and the public. We believe that enhancing collaboration across these groups can drive a more balanced and grounded understanding of minerals and their role in our society, which in turn can lead to effective, and sustainable mineral resource management.

Perspectives and misconceptions

We need to challenge entrenched practices and misconceptions, for example the belief that economic development is invariably at odds with environmental conservation. By fostering a culture that values and seeks out varied viewpoints, stakeholders can discover innovative solutions that might otherwise be overlooked.

Inclusive discussions require a proactive approach to include under-represented groups thereby ensuring that all stakeholder interests are considered. This not only enriches the dialogue but also strengthens the decision-making processes, making them more robust against biases and oversights.

Bridging disciplines

The complexity of modern mineral resource development transcends traditional boundaries. While geology, geophysics, and geochemistry are fundamental for understanding minerals, integrating fields like materials science and biology can provide insights into the environmental impacts, for instance, biotechnological opportunities in mining. Moreover, reaching outside of natural sciences, for example, incorporating economic and history considerations, ensures that decisions are not only economically viable but take account of past lessons and societal trends.

This expansive multi-disciplinary approach should not be seen as complicating the process. By challenging and examining the conventional wisdom and embracing a broader and more dynamic perspective, stakeholders can uncover innovative solutions that respect both the environment and economic realities. The goal is to forge pathways that are not only technically feasible but also socially and economically strategic, ensuring that mineral resource development is sustainable and environmentally responsible.

Communication

Effective communication of scientific facts and theories is essential to public understanding and acceptance of mineral resource projects. Strategies need to be developed to communicate complex scientific ideas in accessible language that can engage and educate a non-specialist audience, thereby bridging the gap between scientific research and public knowledge.

Public perception gaps

The public often perceives the push for renewable energy as a purely ‘green shift’, overlooking the role of mining and mineral

The EAGE Technical Community on Critical Minerals led a conversation on raw materials criticali assessment at the EAGE Annual in Oslo.

resources in enabling this transition. It is imperative to address these perception gaps by highlighting how responsible mineral development is crucial to sustainable energy initiatives. For the mining industry itself, filling this gap is essential in providing an acceptable pathway to deposit development. Engagement with the public, landowners and local communities should be an integral part of the exploration phase, in other words, co-design of the project opportunities rather than a fait-accompli once a mineral deposit has been found and an economic assessment developed.

The biggest challenge continues to be gaining public acceptance of new projects rather than the finding of new viable

resources. In the national push for security of supplies heavy industries such as mining are coming in contact with frontier environments and communities that have not experienced such exposure in generations. Breaking the legacy of poor mine management is therefore essential to achieve the scale of development required by the energy transition.

In this era of information overload and misinformation, it is crucial to consolidate and disseminate accurate, objective facts about mineral resource development. Researching, verifying, and sharing scientifically reviewed information will be vital in informing public debates and policy-making, something to which our Technical Community can contribute.

Caribbean energy opportunities in the spotlight

EAGE’s inaugural Caribbean Energy Opportunities Conference on 6-8 November 2024 in Port of Spain, Trinidad and Tobago will highlight the dynamic and complex energy landscape of the Caribbean region. It is intended to be a central gathering for industry professionals and academics alike to focus on the unique geological and tectonic characteristics that define the Caribbean and the Guyana-Suriname Basin.

Participants will explore a range of issues including prospectivity in complex geological environments, frontier exploration areas and the region’s increasingly productive E&P history. Complex exploration in foreland basins, carbonate reservoirs and hydrocarbon-rich turbidite reservoirs, all of which are essential to understanding the petroleum potential of these areas, will be discussed.

A highlight of the conference will be the in-depth basin analysis sessions. Experts will present research on basin modelling, source-to-sink dynamics and geochemical studies, providing insights into the petroleum systems of the Caribbean and northern South Atlantic basins. Part of this will be the importance of Cretaceous-Cenozoic source rocks, which are critical to future exploration and production efforts.

The conference will also address the role of energy transition in the region, exploring the synergies and potential of geothermal energy in the Caribbean and the Guyana-Suriname Basin. This focus underlines the conference’s commitment to promoting sustainable and innovative energy solutions for the future.

We invite researchers, industry professionals, and stakeholders to join us in this wide-ranging dialogue and shaping the future of energy in the Caribbean. Secure your registration by 30 September to take advantage from the discounted fee.

The EAGE Student Fund supports student activities that help students bridge the gap between university and professional environments. This is only possible with the support from the EAGE community. If you want to support the next generation of geoscientists and engineers, go to donate.eagestudentfund.org or simply scan the QR code. Many thanks for your donation in advance!

Caribbean tectonic map.

Jack wouldn’t trade his life of geoscience Personal Record Interview

Ian Jack refers to himself as ‘mostly retired’ after an eventful and influential career with seismic contractor GSI and as a technology adviser at bp. His visionary advocacy of 4D seismic was topic of the first SEG Distinguished Instructor Short Course (DISC). Ever the adventurer, student years included a trip with friends to Turkey on a double decker bus.

Ian Jack

Post-war upbringing

My father was killed in the 1939-45 war so my brother and I were brought up in Scotland in somewhat straitened circumstances during our early years. However, this made us independent and our schooling was assisted by scholarships thanks to funds set up through the generosity of former pupils.

University to seismic company

I was interested in audio technology and electronics, and while at school and university I built things like amplifiers (valves/ tubes of course at that time!) and progressed to a helium-neon laser and into holograms which was exciting. I enjoyed travel, and in my last year at university I purchased a double-decker bus and drove it to Turkey and the Balkans with some friends which was quite eventful. I graduated in physics and had a job offer from the BBC but also one from the seismic contractor GSI which paid much more, looked interesting, and so in 1968 I was an instrument engineer on a land seismic crew in the Middle East.

Learning on the job

The seismic method was interesting and was developing fast so it was quite a stimulating technological environment. One of the visitors to the crew had brought a copy of Geophysics and I was drawn to the 1964 paper by Robinson and Treitel. This led me to manoeuvre myself into seismic data processing and analysis which for GSI Middle East was in Beirut. I always find it embarrassing to be in a country without much knowledge of the language, so I enrolled in night school

for Arabic lessons for several semesters. Then I was sent off to Dallas to be part of a team working on GSI’s new processing and analysis system. There I enjoyed a desk adjacent to a fantastic young lady (Eleanor, my wife to be) who had joined the company the week after me. We moved with the first production computer to Holland, and of course the two of us attended night school to learn Dutch. However, the days of mainframe computers were numbered as emerging supercomputers could be anywhere. We moved to England where GSI was establishing itself in Bedford, and the mainframe was relocated to Austin, Texas.

Transfer to bp

BP advertised. It looked attractive. I liked the people who interviewed me, I thought the job was a good match, and they processed my application in just a few days. I also interviewed with BNOC in Glasgow who seemed to be in a complete shambles.

Highlights of your career

Marriage and children. I’ve had a fantastic time in geophysics with experience at the sharp end, in data processing, software development, and in R&D management. My year as the inaugural SEG DISC instructor was ‘full on’ as I had to do it plus my own job at bp, but very rewarding. I presented the course in about 25 locations worldwide, some twice. Being elected a vice-president of the SEG gave me a useful perspective on a professional society. At bp I became its inaugural ‘distinguished adviser’ and manoeuvred myself into the

Reservoir Engineering department, able to influence the integration of the geoscience disciplines and work with contractors to develop technologies the industry needed.

Technology vision realised?

Mostly. I pushed for improved land seismic cost/efficiency, adoption of 4D seismic, and integration of disciplines. In my study I have a Stryde node weighing 150 gm that can be deployed cheaply and efficiently by the thousand. 4D is now routinely adopted and old discipline silos have been largely swept away by modern asset team structure. Land seismic is still challenged by the very low frequencies. Ocean bottom seismic was another of my favourite subjects. At the EAGE Annual in Oslo I was impressed by the number of nodes on display.

Communication skills

These probably developed out of my year as DISC instructor. It was important to communicate the value of 4D to the senior managers and accountants.

Future for geoscientists?

We will need geoscience graduates for all sorts of reasons and over many disciplines. But universities are struggling to attract students without mentioning oil and gas exploration.

You are ‘mostly retired’

This means enjoying watching the industry change, progress and adapt. And meeting the very many friends I’ve made in the business over 56 years, and making new ones.

15

1 October 2024 23rd European IOR+ Symposium

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First EAGE Workshop on the Triassic and Jurassic Plays in Northwest Europe

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CROSSTALK

BY ANDREW M c BARNET

BUSINESS

Showing what geoscience can do

The energy and enthusiasm witnessed at our Annual Conference & Exhibition in Oslo could leave no doubt that meeting the challenge of energy transition was the main preoccupation and conversation point for participants. Few panels or technical presentations went without the issue of sustainability being raised in some form.

For EAGE as an organisation this has to be welcome vindication of its decision some years ago to promote the society as a leader in steering the geoscience and engineering community towards finding and realising solutions to the world’s climate change crisis. Yet the contradictions and frustrations of this mission were as evident as ever for anyone who cared to take a step back.

Commendable excitement generated at our society gatherings by what geoscience technology can contribute to moving the energy transition forward has to confront a stubborn reality.

We have to be honest with ourselves. Progress is still painfully slow, while the scope of geoscience disciplines equipped to contribute to the energy transition era is still unclear. This is not as unnerving as it may seem because – like it or not – anyone with an ounce of common sense can see that the oil and gas industry is set to be a major source of energy for decades to come. In other words adapting to new energy scenarios is not going to change the outlook for geoscientists overnight. What was remarkable about listening to the views of the young professionals featured in the Opening Day panel session at the Annual was how sanguine they all were about future energy scenarios, and not in the least worried about their career prospects.

money. Where companies choose to invest will determine the opportunities, hopefully through research and development that is something that geoscientists can influence. Also, before long, we may see governments taking a much firmer hand on the direction of climate change mitigation.

‘A highly appropriate topic for the Oslo meeting’

Our Annual in Oslo was probably as good an indication as any of the current technology focus in geoscience. A rough guide by the numbers shows that out of nearly 150 oral and poster sessions at the Annual this year, E&P oil and gas unsurprisingly continues to be the main focus. Fifteen were actually labelled Energy Transition, additionally, nine of the 13 Dedicated Sessions focused on a theme related to energy transition aspirations. Easily the most prominent topic in the energy transition discussions was carbon capture and storage (CCS), followed by geothermal with hydrogen, critical minerals and offshore wind receiving some attention along with decarbonisation issues (which were doubtless referred to in much of the Technical Programme). Short courses at the event also covered CCS, geothermal and deepsea mining indicating a need for education on developments in these fields. Highlighting CCS makes perfect sense because the expertise needed comes straight out of the oil and gas E&P playbook involving geological investigation, seismic surveying and monitoring techniques.

The tricky question for the geoscience community is to identify exactly where its expertise can best be employed to further the energy transition. The answer to that is not obvious. For a start too many technologies are still nascent, and their geoscientific component, if any, is still to be determined. More pragmatically, it will probably be a matter of following the

CCS was also a highly appropriate topic for the Oslo meeting given Norway’s pioneering role in trying to reduce the amount of CO2 released into the atmosphere. The discussion in the Strategic Programme on ‘Transformation from O&G province to broader energy super basin’ provided a great insight into government and industry forward thinking on what can be done in the energy transition area. The vital ingredients for such an energy hub are energy security guaranteed by plentiful oil and gas, emphasis on low carbon production enabled by ‘co-located’ access to renewable energy plus the possibility of CCS at scale. Norway has all these requirements – huge oil and gas reserves,

electrification of offshore installations powered by the country’s hydro and offshore wind power, and capacity for plentiful CO2 storage offshore.

Similar conditions exist in a number of other oil and gas provinces around the world, but Norway is the first nation to explicitly target an energy super basin with the UK following in its footsteps in all but name. In both cases the respective governments are pressing the case for renewable sources of energy and CCS deployment offshore, and it may be that to maximise the benefits of a super basin, government support and regulation (such as planned licensing sites) are going to be essential.

You have to assume that there is something in this for the oil and gas (energy) industry to stray outside its traditional E&P business model to pursue energy-related projects yielding a much smaller return on investment. It may be a misreading, but you could get the impression that companies were a little fuzzy on this score. Phillipe Mathieu, the senior executive who represented Equinor in the Opening session ‘fireside chat’, made it clear that the company’s energy transition initiatives were not ‘philanthropic’, i.e., were intended to provide a reasonable return as they gain traction. The energy super basin panellists concurred that integrating all the elements involved was a good economic proposition (not defined in detail), but also agreed that it had to be done, which bore something of a moral responsibility rather than a commercial vibe.

be removed from the atmosphere by mid-century if the world is to meet the 1.5°C Paris Agreement target. That is a big ask and CCS may not be the favoured option. On the plus side, according to a Wood Mackenzie estimate, planned global CCS capacity at the end of Q1 2024 stood at 1.5 billion tonnes, 50% of which is in the early stage of devlopment.

‘CCS initiatives need to be put into perspective’

In its 4th New Technologies Outlook Wood Mackenzie ranks technologies likely to make a low carbon impact. CCS is reported to be making steady progress with support from Big Oil. The analysis ordered 243 technologies across ten transition themes at various stages of development scored on six metrics: maturity, pace of change, cost of carbon abatement, carbon offset potential, policy support and dependency (the potential for other nascent technologies to piggy-back). Topping the rankings with technologies already disrupting the market are identified as transport (passenger and light commercial electric vehicles, electric buses and fast charging infrastructure); power generation from fixed offshore wind, and the crystalline silicon p-type cells (as well as the newer n-type) set to underpin solar’s future dominance. The value of lithium-ion batteries for short duration energy storage is also a contender.

So far Norway earns full marks for its vision and the support it has received from the energy industry. It is hard not to be impressed by the Northern Lights project as the first ever cross-border, open-source CO2 transport and storage infrastructure network. Heavy industry across Europe will be able to store their CO2 safely 2500 m under the seabed at the Aurora storage complex offshore Norway. Captured CO2 will be shipped to the onshore Øygarden receiving terminal, western Norway before transport to its subsea destination. Final commissioning of Phase One of the project is due any time now with a planned capacity of up to 1.5 million tonnes of CO2 per year with the hope of adding a further 3.5 million tonnes depending on market demand.

Northern Lights is a partnership between Equinor, Shell and Total, and is a key component of Longship, the Norwegian Government’s full-scale CCS project, with the hope of capturing and storing a possible 0.8 Mtpa of C02 by 2024 from a cement factory in Brevik and the Fortum Oslo Varme waste-to-energy district heating facility in Oslo (depending on funding). The Longship project boasts close cooperation between Gassnova, the state enterprise for CCS, and industrial partners.

Excitement surrounding Norwegian measures and indeed the pipeline of CCS initiatives around the world need to be put into perspective. They are a start, but a 2024 State of Carbon Dioxide Removal report co-led by researchers at the University of Oxford found that around 7-9 billion tonnes of CO2 per year will need to

More salutary is what this all means in the context of energy transition. Recent data from the International Energy Agency (IEA) estimates investment in ‘clean energy’ by both governments and private industry reached more than $1.7 trillion in 2023. This included spending not just the $659 billion on renewable power (wind, solar, etc.) but also electric vehicles, battery storage, nuclear, carbon capture and more. More generally governments and industry worldwide from 2015 to 2023 have spent $11.7 trillion (inflation-adjusted) on clean energy. This is basically the equivalent of all the goods and services produced in Germany, Japan and the UK combined in 2023, according to the Fraser Institute, a Canadian research organisation, which asks what was the return on investment.

One answer comes from the recently published Statistical Review of World Energy, previously a bp publication now published by the Energy Institute. It states that 2023 was a second consecutive record year for global primary energy consumption as it grew 2%, reaching 620 exajoules (EJ). Its growth rate was 0.6% above its ten-year average and over 5% above its 2019 pre-Covid level. Whilst a new record in the consumption of fossil fuels (in absolute terms) was recorded, in 2023 it fell to 81.5% compared to almost 81.9% in 2022. With demand for natural gas, a relatively low carbon-intensive fossil fuel, remaining flat, the increased use of more carbon-intensive oil and coal meant that energy-related greenhouse gas emissions also reached a record high, exceeding 40 GtCO2e for the very first time.

No escaping the feeling that we continue to live in the parallel universes of the constantly challenging shift to energy transition and our relentless consumption of fossil fuels.

Views expressed in Crosstalk are solely those of the author, who can be contacted at andrew@andrewmcbarnet.com.

INDUSTRY NEWS

Net zero by 2050 looks increasingly unlikely warns bp in Energy Outlook

The Paris climate agreement of limiting global temperature rises to below 2 centigrade will be difficult to achieve because of growing emissions and poor energy efficiency, says bp in its 2024 Energy Outlook.

reduce emissions on a rapid and sustained basis, the greater the risk of a costly and disruptive adjustment pathway later,’ says the report. ‘Government ambitions and provisions in support of the energy transition have grown in recent years, but further

The energy giant said that carbon emissions have grown by 0.8% a year for the past four years (2019-2023). If emissions were maintained at close to recent levels the carbon budget estimated by the Intergovernmental Panel on Climate Change to limit global temperature rises to 2% would be exhausted by the early 2040s.

‘The carbon budget is running out. The longer the delay in taking decisive action to

global policy action is needed to achieve a Paris-consistent pathway.’

The target has been further undermined because progress on improving energy efficiency has been ‘disappointing’, improving at around a quarter of the rate currently being targeted by global leaders, said bp.

Energy efficiency, measured as the amount of energy used per unit of

economic activity, has improved by an annual average of just over 1 per cent over the past four years, BP estimates, slower than the previous decade and much weaker than the 4% annual rate targeted in the energy efficiency pledge at the last United Nations Climate Change Conference, more commonly known as COP.

Investment in low carbon energy has risen by 50% since 2019, rising to $1.9 trillion in 2023, although investment has lagged in emerging markets because of the cost of financing. ‘The energy additions from low carbon sources have not, however, been sufficient to meet the growth in energy demand, meaning the use of fossil fuels has risen’. However, bp predicts that energy efficiency will improve under both its Current Trajectory and Net zero scenario, increasing by an average of 2.1% and 3.4%, respectively, which would also be better than a typical rate of 1.6% over the past 25 years. This will be driven by significant global increases in wind and solar power generation.

Fossil fuel consumption reached a new high in 2023, driven by primarily by rising oil consumption. Oil demand has been 0.5 million barrels a day higher since 2019, driven by demand from emerging economies. Demand in developed economies is 2 million barrels a day lower than before the Covid-19 pandemic and

Sercel sells software to ExxonMobil

enters OBN market

Searcher starts survey offshore Brazil

bp said that oil demand would remain high for the next 10 to 15 years.

5.5 million barrels a day below its historic peak in 2005.

Oil and gas investment totalled $550 billion in 2023, ‘Although upstream investment remains below its peak in the early 2010s, production has continued to grow steadily, supported by improving productivity of investment,’ says the report.

‘Oil demand declines over the outlook but continues to play a significant role in the global energy system for the next 10-15 years. This requires continuing investment in upstream oil (and natural gas).’

Under BP’s current trajectory, carbon emissions will peak around the middle of this decade but decline by only 25% by 2050. Under the Net Zero scenario CO2 emissions would decline by around 95% by 2050.

‘The single biggest element accounting for the faster transition pathway in Net Zero is the quicker and more comprehensive decarbonisation of the global power system. The average carbon intensity of global power falls by a little over 60% over the first half of the outlook in Net Zero compared with a fall of a little over 35% in Current Trajectory. By 2050, the

TGS launches 3D onshore survey in the US

TGS is preparing a multi-client onshore 3D seismic survey on the western flank of the Appalachian Basin in the US, aligning with the most prospective trend of the Utica-Point Pleasant formation and Clinton sands.

The 206 km2 Birmingham 3D survey will target multiple exploration zones, including the Ordovician Trenton and Black River, Cambrian reservoirs, Ordovician Utica/Point Pleasant and Silurian Clinton sands. ‘This area is characterised by world-class source rocks in the Devonian Marcellus and Ordovician Utica formations,’ said TGS.

Birmingham sits just up-dip of the Point Pleasant-Utica condensate and light oil trend imaged by TGS’ Utica Merge 3D and will target the underexplored oil window of the Point Pleasant. Deep-rooted structures will be mapped, which are crucial for understanding hydrocarbon trapping, facies changes, basin heat flow and accurately landing horizontals, the company added.

TGS will leverage its data on 73,857 wells and 144,597 logs in the Ohio portion of the Appalachian Basin. Kristian Johansen, CEO of TGS, said: ‘The Birmingham 3D survey represents a significant investment towards understanding the full potential of the Appalachian Basin. Alongside

our other supporting products in the region, such as our extensive well log library and interpretive products, we are confident this survey will provide valuable insights for our clients.’

The acquisition of Birmingham 3D is scheduled to commence in early Q4 2024. Preliminary products are expected to be available by Q1 2025, and final data by Q3 2025.

Meanwhile, TGS has released the Dynamic Matching FWI (DM FWI) volume for West Kermit 3D in the Delaware Basin. The final new data volumes include time reprocessing and a fully integrated depth model, featuring diving wave and reflection DM FWI.

The West Kermit 3D seismic survey spans 1054 km2 across Loving and Winkler Counties, Texas, and aids in evaluating and developing prolific multi-target Delaware Basin acreage. The FWI product enhances the structural image of the main reservoir zones, the Wolfcamp and Bone Spring intervals. TGS will provide imaging below the problematic dissolution fill zone and interpretation of the revised Rustler and Salado horizons. The enhanced image now shows visible depositional transitions from the Capitan Reef to the basin clastic deposits of the Delaware Mountain Group.

global power sector in aggregate in Net Zero is entirely decarbonised, whereas in Current Trajectory it reaches only the same reduction in emissions as achieved in Net Zero by the mid-2030s. This faster and more complete decarbonisation of the power sector accounts for between 40-45% of the difference between Current Trajectory and Net Zero over the outlook’.

The remaining differences between Current Trajectory and Net Zero can be accounted for by the different pace of the sectors in which energy is ultimately used – industry, transport, and buildings –reducing their carbon emissions.

Fugro wins wind site geodata contract offshore Japan

Fugro has won a site characterisation contract for a wind farm development off the coast of the Japanese cities of Murakami and Tainai, Niigate Prefecture, Japan.

The offshore wind farm off the country’s west coast will be developed by a consortium of RWE, Mitsui & Co. and Osaka Gas. Fugro’s Geo-data will contribute to the detailed design of future turbine foundations and cable routes.

The fieldwork started in May 2024 and is being performed from the Fugro jack-up platform, the Amberjack and two of Fugro’s vessels The Equator and The Mariner. All are equipped with Fugroowned geophysical, geotechnical and positioning systems, which will acquire geo-data to enable the detailed design of turbine foundations and cable routes.

Fugro vessel The Equator.

Sercel sells Marlin software to ExxonMobil

Sercel has sold its Marlin vessel monitoring and alert system to ExxonMobil to support its offshore operations in Guyana.

As part of the wider Sercel Marlin software suite for optimising offshore operations, the Marlin system represents ‘a significant advance in offshore operational safety’, claimed Sercel. The system is designed to protect floating production storage and offloading platforms (FPSOs)

and other high-value assets against a wide range of marine risks, even in the most challenging marine environments. The solution’s real-time monitoring and proactive intelligence capabilities deliver insights into vessel movements, operational parameters, and potential risks, enabling operators to make more informed decisions and enhance operational safety, said Sercel.

Meanwhile, Sercel has also sold and delivered its GPR300 ocean bottom nodal solution for $20 million. The solution will be deployed by a major customer on an upcoming seismic survey project in the North Sea.

Sercel GPR nodes feature the QuietSeis MEMS technology, which provides broadband signal sensing capability, fidelity, and ultra-quiet performance. ‘The GPR300 excels in shallow water depths down to 300 m and has a compact, lightweight design for easy manual handling and simplified deployment and retrieval,’ said Sercel.

Shearwater signs seismic research agreement with Petrobras

Shearwater GeoServices and Brazilian oil giant Petrobras have signed an agreement for joint development and execution of scientific research and technology innovation within seismic processing and acquisition focused on data quality, value and acquisition efficiency.

The five-year agreement comes in addition to previously announced co-operation between the two companies on Marine Vibroseis and Reveal software licensing. The deal establishes a governance platform supporting Petrobras’ and Shearwater’s joint effort to innovate technologies that improve insights to the subsurface and enhance data value, while also reducing the time, cost and environmental impact of seismic acquisition projects.

‘This co-operation seeks to bolster our engagement in geophysical R&D and innovation, thereby accelerating the

exploration and development of energy resources. By leveraging advanced technologies together, we aim to significantly enhance geophysical technologies and achieve superior operational efficiency, ultimately driving innovation and sustainability within the industry,’ said Roberta Mendes, general manager of R&D&I at Petrobras.

Company executives seal the deal.

Marlin software protects FPSOs.

BRIEFS

Based on preliminary reporting from operating units, TGS is expecting IFRS revenues for the second quarter of 2024 to be $224 million, compared to $206 million in Q2 2023. POC revenues are expected to be $215 million, compared to $241 million in Q2 2023.

Zanzibar has extended the deadline for bids for the country’s first licensing round for eight offshore oil and gas blocks. The deadline has been extended from September to December 2024 for interested parties to submit their proposals.

Deltic Energy has accepted one of the two licences that were provisionally awarded in Tranche 3 of the UK’s 33rd Offshore Licensing Round. Licence P2672 is located to the west of the West Sole gas field and covers blocks 47/5e, 47/10c and 48/6c and contains the Pharos and Teviot discoveries. Deltic’s preliminary evaluation suggests that the Pharos discovery and the Blackadder prospect are in fact a single Leman sandstone structure.

CNOOC has started production on the first ‘green design’ oilfield offshore China – Wushi 23-5 Oilfields Development Project. Located in the Beibu Gulf of the South China Sea, the project is expected to achieve a peak production of 18,100 barrels of light crude per day in 2026. It is the first oilfield in the South China Sea powered from shore. The project will realise full-process recovery and utilisation of the associated gas through integrated natural gas treatment.

Spanish energy company Repsol is reported to be in talks to merge its UK North Sea oil and gas business with private equity-backed NEO Energy. It would be the latest consolidation effort by companies that operate in the basin amid greater tax pressures. The company would have output of more than 110,000 barrels of oil equivalent a day, making it one of the largest producers in the UK North Sea.

EMGS enters OBN market with Barents Sea survey

EMGS has entered the ocean bottom node (OBN) market and is carrying out a fully pre-funded OBN seismic survey in the Barents Sea. The survey, in collaboration with Velocitas Geo Solutions, marks EMGS’ first venture into seismic node projects. The survey, set to commence in the third quarter of 2024, will be conducted using the vessel Atlantic Guardian and is expected to take two weeks to complete. The total contract value is $1 million.

‘EMGS aims to leverage this project to gain experience and evaluate the potential of incorporating seismic node services into its portfolio. This move aligns with the company’s strategy to diversify its offerings and enhance vessel utilisation,’ said a company statement.

Meanwhile, EMGS has entered into a $1.4 million prefunding agreement

related to a multi-client acquisition in the North Sea.

Finally, EMGS has entered into a prefunding agreement related to multi-client acquisition in the Barents Sea. The scope of the survey has not yet been defined but represents a minimum commitment of $2 million. The survey is expected to commence in the third quarter of 2024.

PGS shoots 3D survey off Uruguay

PGS is acquiring a large 3D multi-client survey in Uruguay. The company, which became part of TGS on 1 July, is shooting a 3D GeoStreamer broadband seismic acquisition to enable a comprehensive assessment of the exploration potential offshore Uruguay.

The survey, covering both shallow and deepwater blocks, is poised to provide the industry with the subsurface insight necessary to unlock the hydrocarbon potential within the Cretaceous and Tertiary formations. The area has been attracting a lot of interest recently driven by significant

discoveries, such as Graff and Venus, in the Namibia Orange Basin, which is geologically very similar to its conjugate margin of Uruguay.

Offshore Uruguay offers extensive exploration opportunities, evidenced by the regional presence of oil seeps and slicks, gas chimneys, and bottom-simulating reflectors (BSRs), all of which indicate an active petroleum system, said PGS. Many of these geological plays remain untested, presenting substantial potential for future exploration and investment.

Meanwhile PGS is offering regional coverage and high-grade GeoStreamer 3D data offshore New Zealand after the country’s government removed the ban on offshore exploration that has been in place since 2018. PGS has regional coverage and high-grade GeoStreamer 3D data ready for evaluation.

The Taranaki Basin is the only producing petroleum basin in New Zealand. PGS completed a contiguous regional 3D data product in 2016, complemented by GeoStreamer 3D datasets for broadband insights with Taranaki South (2016) and Taranaki West (2017).

Vessel Atlantic Guardian.

Shallow and deepwater blocks are covered.

Companies bid to perform seismic studies offshore Guyana

Eight companies have submitted bids to conduct a 3D seismic survey of oil blocks offshore Guyana.

Last month, the South American country’s Ministry of Natural Resources extended a request for expressions of interest for a ‘reputable and experienced’ company to conduct 3D multi-client seismic surveys offshore Guyana.

According to the notice, Guyana is looking for companies process and interpret high-quality 3D seismic data to facilitate the exploration and potential development of hydrocarbon resources offshore Guyana, and to ensure that this data is available for effective evaluation during future bidding and licensing rounds.

Dr Bharrat Jagdeo, vice-president of Guyana, said that seismic surveys were not performed before the previous licensing round in 2023. ‘We went out for the last auction without any 3D Seismic study, so we didn’t have much data for the areas. When you have less data, people don’t

put in great bids, because they don’t know what is there.

‘We are hoping that [for] all of the unallocated areas, we may have the 3D seismic studies. They do it, we don’t have to pay for it because it’s a very costly exercise, and they can share the data with us, and sell it to the clients,’ he added.

Last year’s auction concluded with 14 oil blocks on offer within the country’s shallow and deep-water areas. Eight blocks were shortlisted based on the bidders’ ability to meet the criteria of the expected work programme and the required financial commitments.

Companies awarded oil blocks included a female-owned Guyanese company, Sispro, which secured two blocks. Other blocks were awarded to Total Energies, Qatar Energy, Petronas, International Group Investment, Montego Energy, Liberty Petroleum Corporation, Cybele Energy, ExxonMobil, Hess New Ventures, CNOOC Petroleum and Delcorp.

There is a minimum signature bonus requirement of $10 million for shallow water and $20 million for deepwater blocks in the Production Sharing Agreement (PSA).

Dr Jagdeo said that negotiations are continuing, and agreements on the non-fiscal terms of the PSA are coming to a close. ‘We had some pushback on the non-fiscal elements – that they were too harsh, and that is what needs to be finalised,’ said Dr Jadego.

Viridien images basin in Algeria

Viridien has won a contract to carry out seismic imaging of a 3400 km2 high-density onshore data set currently being acquired over blocks B404a and B208 of the Berkine Basin in eastern Algeria.

Having been selected by Groupement Berkine – a joint venture between Sonatrach, Occidental Petroleum, and other international partners – Viridien will deliver subsurface insight and sharpen the resolution of the target area’s thin and faulted geology. A team of scientists at

Viridien’s advanced subsurface imaging centre in France will apply an advanced imaging workflow to the dataset, drawing on their experience of imaging similar large and ultra-dense land seismic surveys in the Middle East.

Peter Whiting, EVP, Geoscience, Viridien, said: ‘Our long track record in Algeria dates back to the 1950s. We have in-depth geological knowledge of the Hassi Messaoud Basin, and have recently successfully completed a series of reimaging projects in the country.’

General Secretary of the People’s Progressive Party, Dr Bharrat Jagdeo.

ENERGY TRANSITION BRIEFS

Six companies have been offered exploration licences to store CO2 in four areas in the North Sea. They are: Aker BP, Equinor, Lime Petroleum, OMV, PGNiG, and Vår Energi. The authorities have reviewed applications from eight companies after announcing two areas in March 2024. ‘This is the highest number of offers that have been sent out simultaneously,’ said Hilde Braut, assistant director for new industries.

The US Bureau of Ocean Energy Management (BOEM) is seeking ideas for environmental and socioeconomic studies to inform its decisions on potential offshore wind energy activities in the US territories.

SLB is collaborating with John Cockerill Hydrogen to create a strategic partnership to accelerate the development and deployment of pressurised alkaline electrolysers. The partnership will combine John Cockerill Hydrogen’s commercial portfolio of pressurised alkaline electrolysers and technology development expertise with SLB’s technology industrialisation expertise and global footprint.

The US Department of Interior has approved the Atlantic Shores South wind project consisting of two wind energy facilities — Atlantic Shores Offshore Wind Project 1 and 2 — which are expected to generate up to 2800 megawatts of electricity. The project is approximately 8.7 miles offshore New Jersey at its closest point. Atlantic Shores South proposed up to 200 total wind turbine generators and up to ten offshore substations with subsea transmission cables making landfall in Atlantic City. BOEM has approved construction of up to 195 wind turbine generators.

Wood has won a contract from Centrica Energy Storage (CES) to redevelop the UK’s Rough field in readiness for future hydrogen storage. The Rough reservoir, located in the Southern North Sea, has been used to store natural gas safely for more than 30 years and has the potential to provide over half of the UK’s hydrogen storage requirements.

Searcher Seismic starts 3D survey offshore Brazil

Searcher Seismic has started acquiring the MC3D survey in the Pelotas Basin, offshore Brazil.

The 12,000 km2 3D survey will be acquired over two phases in 2024 and early 2025. The survey is being conducted by its partner Shearwater GeoServices and represents the fifth multi-client 3D survey project for the joint venture and the sixth by Searcher over the Atlantic Conjugate Margin.

The acquisition area covers a ‘significant amount’ of open acreage which is expected to be available in the 5th cycle of the Brazilian offshore Open Acreage Release and over existing acreage recently awarded in the 4th cycle of the same Open Acreage Release. The resulting data will aid explorers in understanding the geological setting of the Pelotas Basin and reveal the relationship between the conjugate margin pairs of the Pelotas Basin and the Orange Basin in Namibia and South Africa, which is increasingly being recognised as an emerging super basin of global interest and significance.

‘It is great to be kicking off exploration in the least explored tertiary-cretaceous

delta on earth, the Pelotas Basin,’ said Alan Hopping, Searcher managing director. ‘The shear size of our survey, acquiring such a large amount of open acreage in such an unexplored area is really exciting and we are driven by the possibilities that this survey will open up for further exploration opportunities in the near future.’

PostSTM Fast Track Stacks are expected to be available in Q3 2024 and preSDM Stacks will be available in Q3 2025.

National Geothermal Centre is launched in the UK

The UK National Geothermal Centre (NGC) has been launched to accelerate the UK geothermal sector.

The centre, a joint venture of the Net Zero Technology Centre (NZTC), Durham University, SHIFT Geothermal, and the Reece Foundation, will support research as well as help to create a policy, regulation and investment framework.

It will drive collaboration between government, industry, and academia, championing the integration of geothermal energy into the future renewable energy mix, as a low carbon option for heating homes and industries, and power generation.

The geothermal sector has the potential to meet 10GW of the UK’s heating demand and 1.5GW of electricity demand by 2050, the centre said in a statement. Geothermal

expansion could create 50,000 jobs and result in an annual reduction of 10 million tonnes of CO2 emissions, it added.

Anne Murrell, a director of the NGC, said: ‘Geothermal energy is an inexhaustible source of clean heat and power beneath our feet. The new UK National Geothermal Centre will work to unearth geothermal energy. With its expert stakeholders from industry, academia, finance and government, the NGC will expand geothermal development, at speed and at scale.’

Dr Charlotte Adams, another director of the NGC, said: ‘There is growing interest in UK geothermal and significant progress has been achieved in recent years. The timing is perfect. The National Geothermal Centre will shape and accelerate our growing geothermal sector through collaborative cross-sector working.’

Some 12,000 km2 of 3D data will be acquired.

TGS to begin wind ocean measurement campaign off US west coast

TGS is launching an offshore wind and metocean measurement campaign in Morro Bay, off the US West Coast.

The campaign will enhance understanding of offshore conditions across three wind energy lease areas in Morro Bay by calibrating TGS proprietary wind models with observational data. This three-year deployment in an area with an average depth of 1000 m will be the first by TGS on the US West Coast and will add to its library of wind and metocean data collected on the US East Coast, Norway and Germany.

The data gathered will offer insights throughout the floating wind farm development lifecycle. This includes environmental impact assessments and technical decisions such as turbine selection, layout optimisation, foundation design, and operations and maintenance planning. Additionally, it will enable more accurate modelling of capital expenditures, operational expenditures, potential energy production, and grid requirements. High-quality ocean current measurements and tidal information collected over this period will be valuable for grid connection planning, while accurate atmospheric turbulence intensity observations will provide key inputs for wind farm energy yield.

Supplied by EOLOS, the buoy is equipped with advanced sensors designed to capture detailed measurements of wind, metocean, and environmental data. Key metrics include wind speed and direction at turbine hub height, wave heights, ocean current data across the full water column, and monitoring of birds, bats and fish. Data will be continuously streamed, quality-controlled,

and made available daily to customers via Wind AXIOM, TGS’ site evaluation and wind data analytics platform. Wind AXIOM enables wind developers and stakeholders to improve the quality and speed of their decision-making processes by identifying the most critical factors affecting the viability of offshore wind projects, said TGS.

Kristian Johansen, CEO of TGS, said, ‘We are excited to bring our solutions to California, where floating wind technology is gaining momentum. With our multi-client floating LiDAR campaign, we make high-quality wind and metocean data more accessible, enhancing investment and planning decisions and contributing to the success of US offshore wind development. This ninth LiDAR deployment within the last two years demonstrates our commitment and the multi-client model’s continued value to the offshore wind industry. It provides superior data earlier to developers worldwide, helping them reduce risk throughout the project lifecycle.’

This deployment is expected to be launched in Q3 2024.

Meanwhile, TGS has established a dedicated team in its office in Houston, Texas, to enhance the company’s 4C Offshore wind solution across the Atlantic and is recruiting ‘top talent’ in the region.

Subscribers to 4C Offshore can access hundreds of thousands of data points, including details on more than 32 offshore wind farms in 60 countries, specifications for nearly 300 wind turbine models, information on over 46,000 SL contracts, and an extensive database of offshore wind vessels and electricity interconnectors.

IEC commissions large 3D survey in Indonesia

Indonesia Energy Corporation’s (IEC) joint operation contract with Pertamina covering the Kruh Block has been extended by five years from May 2030 to September 2035. Credit: AntonKl/Shutterstock.

Indonesia Energy Corp has launched a 3D seismic survey at its 63,000-acre Kruh Block in Indonesia ahead of the drilling of production wells by the end of 2024.

IEC’s 3D seismic program will cover the Kruh, North Kruh and West Kruh Fields, focusing on existing proved reservoirs of the

Talangakar and Lemat formations, as well as the very large and promising shallow oil/ gas zones in the K-28 well discovered by IEC’s work in 2022.

The 3D seismic data will enable the identification of additional locations of proved undeveloped reserves and resources. This in turn will pave the way to prioritise the sequence of upcoming drilling locations as IEC recommences drilling operations at Kruh

IEC announced in September 2023 that its joint operation contract with Pertamina, the Indonesian state-owned oil and gas company, covering the Kruh Block, was extended by five years from May 2030 to September 2035. Kruh Block covers approximately 63,000 acres and is located onshore on the Island of South Sumatra in Indonesia.

The amended joint operation contract increases IEC’s after-tax profit split from the current 15% to 35%, for an increase of more than 100%.

Frank Ingriselli, IEC president, said: ‘We are very excited about the commencement of new seismic operations on our Kruh Block enhanced by the significant improvements in our economics from the 2023 contract extension with the Indonesian government. We continue to believe that Kruh Block is a worldclass asset and, in order to maximise future production capability, the seismic operations planned across the entire Kruh Block should positively leverage what we have learnt from our previous discoveries, including our 2022 gas discovery, and determine the best locations to restart our continuous drilling campaign.’

Special Topic

NEAR SURFACE GEO & MINING

Submit an article

This month we feature some of the papers that will be presented at the EAGE Near Surface Geoscience event in Helsinki on September 8-12.

Jaana Gustafsson et al tell the story of borehole GPR, give examples of applications and predict a bright future.

George Apostolpoulos et al present an integrated geophysical and geotechnical investigation to assess the geotechnical properties of the subsurface for better understanding the reservoir’s failure.

Hung Dinh et al demonstrate that high-frequency FWI can identify shallow anomalies and fault systems more effectively than standard imaging methods.

Nicoleta Enescu et al demonstrate how advancements in seismic imaging built around the 3D IP transform have impacted on the characterisation of fractured hardrock masses.

Suvi Heinonen et al provide an overview of hardrock seismic mineral exploration in Finland.

Hector R. Hinojosa et al demonstrate how combining near-surface seismic and electrical resistivity imaging with geotechnical drilling allowed a more proactive and rapid site assessment approach.

Alireza Malehmir et al present the Smart Exploration Research Centre in Sweden.

Gwenola Michaud et al present a study that aims to enhance understanding of structural behaviour under potential seismic loading conditions.

Irina Nizkous et al discuss data acquisition and lessons learnt during geophysical Remotely Piloted Aircraft System (RPAS) surveys in northern Canada.

Ignacio Valverde-Palacios et al present the joint analysis of multi-component data based on Rayleigh and Love waves to obtain 2D Vs sections for site characterisation.

Pauli J. Saksa depicts the baseline setting from one mine site and one monitoring line time-lapse result.

First Break Special Topics are covered by a mix of original articles dealing with case studies and the latest technology. Contributions to a Special Topic in First Break can be sent directly to the editorial office (firstbreak@eage.org). Submissions will be considered for publication by the editor.

It is also possible to submit a Technical Article to First Break. Technical Articles are subject to a peer review process and should be submitted via EAGE’s ScholarOne website: http://mc.manuscriptcentral.com/fb

You can find the First Break author guidelines online at www.firstbreak.org/guidelines.

Special Topic overview

January Land Seismic

February Digitalization / Machine Learning

March Reservoir Monitoring

April Underground Storage and Passive Seismic

May Global Exploration

June Technology and Talent for a Secure and Sustainable Energy Future

July Modelling / Interpretation

August Near Surface Geo & Mining

September Reservoir Engineering & Geoscience

October Energy Transition

November Marine Acquisition

December Data Management and Processing

More Special Topics may be added during the course of the year.

Photo courtesy of Alireza Malehmir

Borehole GPR – Applications and advantages

Jaana Gustafsson1*, Paul Lehmann2, Jesper Emilsson1, Johan Friborg1 and Andreas Viberg1 tell the story of borehole GPR, discuss advantages and disadvantages, give examples of applications and predict a bright future.

Abstract

Borehole ground penetrating radar (GPR) has been in commercial use since the 1980s, for many different applications and around the world. However, the knowledge of the benefits of borehole GPR has varied as well as the extent of its use. Borehole GPR is in many places the only way to achieve knowledge of our subsurface. In this article we will present the development and measurement techniques of borehole GPR, discuss advantages and disadvantages, and give some examples of applications.

Introduction

Ground penetrating radar (GPR) is a well-established near-surface geophysical method used in a wide variety of applications. While surface measurements are commonplace, borehole GPR is a technique not as widely used. Users may not even know good borehole radar equipment exists or believe the technique is complicated to apply.

Development of ground penetrating radar started already in the 1920s and was initially mainly applied to ice and snow applications (Daniels, 2004). As instruments improved, GPR became more useful for soil and rock investigations (Daniels, 2004). The first commercially available GPR solutions were introduced in the early 1970s (Olsson, 1990; Olsson et.al., 1991). Even though the early analogue GPR solutions were used and provided good information, the true breakthrough for the GPR method happened as digital GPR equipment became readily available in the early 1990s.

In Sweden, borehole radar development begun in the late 1970s (Olsson, 1990). Due to the specific technical challenges with working in boreholes, this development helped to push the envelope for ground penetrating radar technologies in general. Within the international Stripa Project (funded by concerns regarding nuclear waste repositories), digital GPR and the use of fibre-optic communication were all first developed for borehole radar. The Stripa Project resulted in RAMAC, the world’s first digital borehole ground penetrating radar solution (Olsson, 1990). In 1985 RAMAC was put into operation, primarily aimed at characterising rock volumes at deep depths. Solutions using different antenna centre frequencies, as well as directional antennas, were built for borehole surveys down to 2000 m. The continuation of the initial RAMAC development has since then carried on through MALÅ Geoscience (now part of Guideline Geo AB) in Sweden.

Pros and cons

The major benefit of borehole GPR measurements is the possibility of collecting high-resolution data from areas non-accessible from the ground (see Figure 1). Also, and in most cases, borehole radar investigates the bedrock that is more homogenous and normally less conductive than the topsoil commonly investigated using surface GPR. Therefore, the borehole GPR signal tends to penetrate further than comparable surface GPR antennas and clutter in data is less prominent.

Through the application of optical fibre cables and connections, the maximum depths that borehole radar can reach are really only limited by the hydrostatic pressure in greater depths of vertical boreholes.

Currently, conventional borehole GPR antennas are omnidirectional, meaning that radar signals are emitted and received in 360 degrees around the borehole (see Figure 2). Therefore, the direction to an identified feature can be hard to establish unless

1 Guidelinegeo | 2 Bo-ra-tech

* Corresponding author, E-mail: jaana.gustafsson@guidelinegeo.com DOI: 10.3997/1365-2397.fb2024062

Figure 1 The main benefit of using borehole GPR investigations is that areas, inaccessible from the ground surface, can be reached, with boreholes, both vertical, inclined, or horizontal.

collecting data from several (at least three) nearby boreholes, which will allow triangulation of local or plane reflectors, if the penetration range sufficiently overlaps. Alternatively, there are specially developed borehole radar antennas that achieve a certain directionality through either shielding or specific crossed antenna designs. These systems are, however, much more complicated in design and handling, bigger in their dimensions and thus more limited in their application range, compared to the conventional omnidirectional borehole radar. Borehole GPR cannot be used

the borehole.

in boreholes with metallic casing or in a borehole filled with a conductive fluid (as brine), which will significantly reduce or completely eliminate the penetration range. Furthermore, the borehole layout, together with the geometry of fractures or geological or anthropogenic investigation targets needs to be considered to achieve a clear reflection from them.

Measurement modes

In Figures 3 to 5 three different borehole GPR measurement modes are explained. Note that borehole GPR measurements can be made both vertically, horizontally, or inclined.

Applications

The most common borehole GPR applications are:

• Karst, void, mining shafts etc and tunnel mapping

• Fracture mapping, quantification and orientation in rock

• Mapping of structures as concrete and steel piles

• Mapping of larger subsurface objects such as utilities, tanks

• Mapping and monitoring of leakage, groundwater, saline traces, ground frost etc

• Mapping of rock type

• Soil profile mapping

In the following section we will look closer at fracture mapping, salt mines, shaft congestion and karstification.

Fracture mapping

Borehole GPR is very useful for mapping fractures in e.g. hard crystalline bedrock (Hansen and Lane, 1995; Gustafsson, 2008; Serzu et.al, 2004). Even thin fractures, filled with water, other minerals or air will give rise to quite clear reflections in the GPR data. The amplitude of the first arrival also indicates zones of higher conductivity, such as water seepage. Most often these types of investigations are carried out in single-hole mode. From 2002 to 2007 the Swedish Nuclear Fuel and Waste Management Company (SKB) carried out an extensive investigation campaign with core boreholes down to approximately 1000 m and hammered drill holes down to approximately 200 m to establish

Figure 2 Omnidirectional borehole measurement. The radar waves are emitted in 360 o and the reflections gathered from 360 o around

Figure 3 Single-hole measurements. The transmitter (Tx) and receiver (Rx) antennas are placed in the same borehole and lowered down at a constant offset. This set up can be compared to ground investigations with unshielded antennas.

Figure 4 Different cross-hole measurements. The Tx and Rx antennas are placed in two separate boreholes and the investigated volume is between these two boreholes. Either as fast parallel cross-hole measurement (left measurement technique) or as tomography (with varying Tx/Rx-depth combinations, right measurement technique).

the location for a future atomic waste deposit. All boreholes were investigated with borehole GPR. The investigations were made with 250 MHz, 100 MHz, 20 MHz dipole antennas and a 60 MHz directional antenna, to get both high resolution with the higher frequencies and a wide investigation radius with the lower frequencies. In Figure 6 one example of a 100-m borehole section is shown, with the amplitude of the first arrival and marked structures for both 250 and 100 MHz antennas (Gustafsson and Gustafsson, 2007).

Structural and resource assessment of salt mine deposits

The efficiency and safety of salt mining depends on comprehensive knowledge about the geologic setting of the salt deposit including its stratigraphic boundaries and its structural features.

Borehole GPR can be used as an efficient tool to investigate salt deposits, especially in halite, but also in potash deposits. In Germany GPR and borehole GPR have become standard methods of investigation in salt mines over the last decades, for e.g. the assessment of the remaining deposit thickness over large areas. Measurements have been carried out both from the floor and roof of existing drifts and from vertical or (sub-)horizontal boreholes into the undeveloped rock mass. (Thierbach, 1994).

In Figure 7 and 8 an example of an investigation in the salt mine of Heilbronn, Germany, is shown (Lehmann, et.al, 2020). To be able to estimate the thickness of the salt deposits, in front of an active mine gallery, horizontal investigations were made as the salt layer thickness was expected to decrease to only some metres. The horizontal drillings had to be kept within the salt and not diverting into the surrounding conductive anhydrite, which fully attenuates the radar signal.

The boreholes were slightly inclined upwards (3° to 5°), so the probes were moved in the borehole using a custom-built anchored pulley system with a rope to pull the probes in and the cable winch to pull it back out during measurement, as seen in Figure 7.

The results, in Figure 8, show clear reflections from the Base Anhydrite from below, in a somewhat irregular, not continuous reflection signal close to the borehole, as well as the clearer, more significant and continuous reflection from the Anhydrite Bank at a greater distance from above the borehole. These differences in the reflection signal characteristics can be explained with the different properties of the lower Base Anhydrite, which has more brittle, broken-up composition whereas the Anhydrite Bank features a solid slab-like contiguous body. This known difference allows the more certain interpretation and differentiation between

Figure 5 Surface to borehole measurements. The Tx is left on the ground and the Rx antenna is placed and lowered in one borehole.

Figure 6 Example of borehole GPR investigations in hard crystalline bedrock. The graph on the left-hand side represents the variation in amplitude of the first arrival. This is correlating with the zone of more water seepage. In the radargrams for the three antennas (250 MHz, 100 MHz and 20 MHz), some of the identified structures are marked. The marked structures correspond to a high degree to fractures or rock type changes also observed in BIPS (Borehole Imaging Processing System) logs.

reflections coming from below and above in the otherwise omnidirectional radargram.

Detection of shaft congestion

In an Austrian quarry limestone is excavated from the side of a mountain and transported down to the valley via long vertical shafts. One of these shafts got repeatedly congested at an unknown depth, especially between seasons, possibly related to thawing and freezing along water-filled fractures crossing the shaft. When withdrawal of material at the bottom of the shaft continued but the filling level at the top remained, a cavity of significant size could form depending on the time between noticing the stable filling level and stopping the withdrawal at the bottom (see Figure 9). An uncontrolled sagging of hundreds of tons of material would cause an enormous risk to the withdrawing mechanism. Furthermore, production was significantly delayed for days or weeks when the quarry had to wait for the congestion to resolve by itself. For targeted measures to resolve the congestion the exact depth and dimension of the congestion were needed.

Cross-hole radar measurements from two boreholes placed around the shaft could provide that information due to the faster wave velocity when travelling through air. Two vertical boreholes placed across each other at a distance of 3 to 4 m from the shaft walls were drilled to the full 120 m length. A ‘zero’ measurement to assess the undisturbed state of the shaft and surrounding rock mass before the next congestion was conducted using both reflection measurements (single-hole) for the structural analysis of shaft contours and fault systems crossing the shaft, and crosshole tomography for the identification of air-filled voids via velocity analysis. Over a period of seven months three separate congestions occurred and could subsequently be surveyed with borehole radar (Lehmann, et.al, 2020).

The cross-hole measurements showed that the congestion occurred at approx. 80 m depth causing a void of up to 9 m vertical extent. Through the use of tomographic measurements, the rough contour of the upper and lower edge could be determined as shown in Figure 10. The comparison to the second and third occurrence showed that an earlier notice of the congestion – which led to an earlier stop of the withdrawal at the bottom of the shaft –produced a shorter vertical extent of the congestion. Furthermore, the reflection measurement showed a network of high reflectivity faults crossing the shaft at the exact depth of the congestions indicating a correlation to freezing in the water-bearing fractures. Especially, the additional measurements with a higher frequency borehole radar system (250 MHz) showed that the reflection of the shaft walls featured irregularities in the shaft’s contours in the respective depths.

The radar data provided critical information and the necessary detail on depth and dimensions of the occurring congestions for the planning of a targeted injection drilling that could henceforth be used to resolve future congestions. Furthermore, the data delivered background for possible causes of the congestions.

Figure 8 Radargram of the 100 MHz borehole radar antenna with marked reflections from Base Anhydrite (blue) from below and Anhydrite Bank (red) from above. The borehole section (green) is 170 m long and shows a salt layer of approximately 16 m thickness.

Figure 7 Rope and cable winches to move the radar probes inside a deep horizontal borehole during measurement.

\Assessment of karstification

During the excavation of several tunnel tubes for the railway projects both in Germany (Swabian Alb — Stuttgart 21) and Slovenia (Divaca-Koper) limestone strata were crossed that were prone to high degrees of karstification, endangering the safety of the excavation and tunnel stability in the later operation phase. Borehole radar was applied to detect and determine locations and approximate dimensions of fault and karst zones as well as open or sediment-filled karstic cavities ahead of the tunnel face and in the vicinity of up to 15 m around the tunnel tubes.

In the Swabian Alb a combination of single-hole (Figure 11a) and cross-hole measurements (Figure 11b) was applied from a star-shaped pattern of six boreholes arranged radially every 15 m

over a length of over 9 km (Figure 11c) following the tunnel excavation. The goal was to detect reflections from faults and karst structures around the tubes from up to 20m-deep boreholes and assess their potential filling (air or sediment) from the analysis of the wave velocities and amplitudes of the transmission between the different borehole combinations. This setup guaranteed a comprehensive coverage of the entire area around the tubes, facilitating the detection of cavities down to a size of 1 m³.

In total more than 500 karstic structures were identified from the radar data, 200 of them were explored with verification drillings due to their dimensions and location (Figure 11d) and over 90% were found at their predicted locations and could successfully be backfilled.

Figure 9 Principle of the shaft congestion and placement of the investigation boreholes around the shaft for single-hole and cross-hole measurements.

Figure 10 Radargrams from parallel cross-hole measurements (above) and tomograms (below) of all three investigated shaft congestions (in August 2018, December 2018 and April 2019), showing the same depths but different vertical extents (right).

11 Radargrams from single-hole and cross-hole measurements (a and b), resulting radar wave velocity plot with anomalous velocity (c) and drilling results from verification drill pattern confirming karstic cavity.

In the Divaca-Koper tunnelling project the goal was to determine the karstification degree of the Dinaric Karst limestone ahead of the tunnel excavation of two almost 7km-long tunnels (T1 and T2) with two tubes each (main and service tube). Borehole radar was applied in a single 100 m long horizontal borehole drilled from each tunnel face at roughly 90 m intervals (Figure 12). The reflection data (single-hole) of two different applied radar frequencies (100 and 250 MHz) enabled the prediction of the location of karstic structures and resulting karstification degree for the top-heading excavation and influence zone through the interpretation of reflectors in

a radius of more than 12 m around the central borehole. The challenge of not being able to determine relative direction and size from only one borehole was a compromise of cost and benefit.

Over 80 accessible caves were encountered by the direct tunnel excavation, many of which were predicted correctly ahead of the excavation from the radar data. In some situations, karstic caves that were located in the pillar between main and service tube could be detected from the individual single-hole measurements from the two neighbouring tubes and thus permitted a triangulation and size estimation (Figure 13).

Figure

Figure 12 Principle of the borehole radar technology as a reflection measurement from a single horizontal borehole from the tunnel face to investigate the rock mass ahead of the excavation.

Conclusion

The shown examples prove how borehole radar has been successfully applied in various branches for investigations of the subsurface beyond depths that can usually be reached from the Earth’s surface. Borehole GPR should continue to hold a significant place among geophysical methods used to investigate the subsurface as the need for more investigations will continue to be high with more infrastructure projects and mining endeavours in Europe and worldwide.

Today, there are several manufacturers of borehole GPR systems, and the variety of available antenna frequencies is increasing. The antennas are becoming smaller and more user-friendly. As the electronic components becomes faster, the recording speed and data storage increases, more stacking can be applied to further improve the signal-to -noise ratio. With growing demand for applications such as geothermal energy and safe repositories for nuclear waste, research is continuing to develop borehole GPR systems that can reach several kilometres deep into the ground, to stand up to both high temperature and pressure. Both grant a bright future for borehole GPR.

References

Daniels, D (ed.) [2004]. Ground Penetrating Radar. 2nd edition, IEE Radar, Sonar and Navigation series 15. 726 pp. Gustafsson, J. [2008]. Borehole radar investigations for subsurface characterisation. First Break volume 26, November 2008. 93-97. Gustafsson, J. and Gustafsson, C. [2007]. Forsmark site investigation. RAMAC and BIPS logging in borehole KFM02B and KFM08D. SKB P-07-96. 123 pp.

Hansen, B.P. and Lane, J.W. [1995]. Use of surface and borehole geophysical surveys to determine fracture orientation and other site characteristics in crystalline bedrock terrain, Millville and Uxbridge, Massachusetts. US Geological Survey, Water-Resources Investigations Report 95-4121. 25 pp.

Lehmann, P., Schmidt, M., Richter, T. and Eidner, M. [2021]. Geophysical structural exploration in civil engineering, tunneling and mining. Conference proceedings of 6th International Conference On Geotechnical and Geophysical Site Characterisation (ISC’6). 5 pp. (https:// doi.org/10.53243/ISC2020-466)

Olsson, O. [1990]. Ground Penetrating Radar as an investigation tool in connection to construction work – Experience and Potential. ABEM AB Report ID-no 90002. 70 pp.

Olson, O., Falk, L., Forslund, O., Lundmark, L. and Sandberg, E. [1991]. Borehole radar applied to the characterization of hydraulically conductive fracture zones in crystalline bedrock. Geophysical Prospecting, 40(2), 109-142.

Schmidt, M., Richter, T. and Lehmann, P. [2017]. Innovative geophysical technologies for the exploration of faults, karstic structures and cavities in tunnelling, Geomechanics and Tunnelling 4, 10, pp. 380 - 394.

Serzu, M.H., Kozak, E.T., Lodha, G.S., Everitt, R.A., and Woodcock, D.R. [2004]. Use of borehole radar techniques to characterize fractured granitic bedrock at AECL’s Underground Research Laboratory. Journal of Applied Geophysics, 55, 137-150.

Thierbach, R. [1994]. Twenty years of ground-probing radar in salt and potash mines. In: Fifth International Conference on Ground Penetrating Radar. European Association of Geoscientists & Engineers, 1994. S. cp-300-00069.

Figure 13 Radargrams from single-hole radar measurements in horizontal boreholes of two neighbouring tunnel tubes ahead of the excavation detecting a karstic structure in the pillar between the tubes.

Integrated geophysical and geotechnical investigation for the rehabilitation of a water storage reservoir in Crete, Greece

Christos Orfanos1, Konstantinos Leontarakis1, George Apostolopoulos1* and Haralambos Gouvas2 present an integrated geophysical and geotechnical investigation to assess the geotechnical properties of the subsurface for better understanding the reservoir’s failure and to promote measures to reduce future subsidence, reduce water losses and improve soil behaviour.

Introduction

The main aim of the integrated geophysical and geotechnical investigation is the assessment of the soil condition of the broader area of an existing artificial water storage reservoir in Crete, Greece (Figure 1). It concerns the rehabilitation of the Agioi Theodoroi water storage reservoir located at the SW of Stomio bay (approximately 200 m from the shoreline) and near the Xiropotamos River. The name of the water reservoir comes from the church of the community of Vathis in the municipality of Kissamos in the Regional Unit of Chania, in Crete.

In 1995, during the construction phase, a subsidence was created in the central part of the reservoir bottom, in an area where the drainage channels end up. The sinkhole had a maximum depth of about 3.5 m and took the form of a cylindrical hole with peripheral cracks. The whole disturbed area was about 20 m in diameter. The main cause of the sinking was considered to be the entrainment of fine-grained material, by the flow of water, mainly caused by the rainfall, resulting in the creation of voids and subsequent subsidence of the overlying soil. This was followed by extensive rehabilitation measures of the damaged area until its completion and filling with water in March 1997. From this point until 2011, it was operating

normally without any apparent loss of water and without any problem.

On July 2011, while the reservoir was filled with more than 500,000 m3 of water, a vortex appeared on the water’s surface and while the water level decreased simultaneously. This phenomenon continued for two days until the complete leakage of the water from the reservoir. Specifically, two large holes appeared on the bottom (about 3 m in diameter), one of medium size (about 2 m in diameter) and some smaller ones. Moreover, several cracks appeared at the crest of the reservoir, on the west side, where nearby several smaller holes existed and a failure of the membrane and a subsidence of the subgrade was observed.

Nowadays, after such a long period of time, the reservoir shows signs of abandonment and restoration interventions must be carried out to make it functional and beneficial to the local community. In this context, an integrated geophysical and geotechnical investigation program was carried out for the assessment of the geotechnical properties of the subsurface for better understanding the reasons of the reservoir’s failure as well as for being able to promote measures to reduce future subsidence, reduction of water losses and improving soil behaviour in respect to the earthquake.

1 National Technical University of Athens | 2 Geoper

* Corresponding author, E-mail: gapo@metal.ntua.gr

DOI: 10.3997/1365-2397.fb2024063

Figure 1 The water storage reservoir of Agioi Theodoroi in Crete, Greece and the disturbed zones that appeared in 2011 and resulted in the water losses.

Geological setting

In the wider study area, alpine and post-alpine formations have appeared. The alpine basement is composed of Triassic recrystallised limestones and dolomites of the Ionian Zone at which the metamorphosed ‘Fyllite-Quartzite’ series overlaps. Above these formations a heterogeneous Quaternary and Neogene clastic system is found with rocks originating from the erosion and

disintegration of the north-eastern mountains. In particular, the clastic materials consist of Pleistocene alluvial unconsolidated deposits from the river terrace of the Xiropotamos river, as well as consisting of lateral debris from cohesive conglomerates and sandstones. The materials of the Pleistocene deposits are mixed and in places slightly welded with weathering mantle (terrarossa) of the underlying alpine background (carbonate background). On the above formations, microcrystalline gypsum and anhydrites appear in the form of diapyrites penetrations of significant dimensions. The main stratigraphy of the wider area of the reservoir is presented in Figure 3.

The geophysical survey

For better understanding the subsurface conditions under the water storage reservoir, a geophysical investigation was carried out using the Ground Penetrating Radar (GPR) and seismic methods.

The layout of the GPR and seismic profiles are presented in Figure 4.

GPR Method

The Ground Penetrating Radar method was applied in the survey area to identify discontinuity interfaces or finite-dimensional structures of interest. Specifically, 24 transects were acquired, with a total length of approximately 5.5 km, 10 of which had a north-south orientation and 14 west-east (Figure 5). The MALA geoscience ‘RAMAC’ georadar was used, with a 100 MHz frequency shielded antenna. Raw data were processed using the Reflex-W software package (Sandmeier Geophysical Research). A series of processing steps were carried out on the georadar

Figure 2 Photograph of the problematic areas of reservoir.

Figure 3 The main stratigraphy in the survey area.

Figure 4 The geophysical acquisition layout for the Georadar and seismic methods.

recordings (raw data), to highlight the possible targets and discontinuities. Specifically, the following filters were used: 1) Dewow filter, 2) Deconvolution, 3) Backround removal, 4) Energy decay, 5) Band pass filter and 7) Kirchoff migration. A velocity of 0.1 ns/m was used for the migration and the depth conversion.

The utilisation of the 100 MHz georadar antenna, enabled the survey with a sufficient resolution up to about 11 m, so that it was possible to map and evaluate the subsurface structures and discontinuities down to this depth. In addition, the dense coverage of the georadar cross-sections allows for better spatial correlation and interpretation of discontinuities and structures of interest observed in the various radargrams.

For optimal interpretation of the processed radar plots, a correlation of the results with existing information was performed for each of the 24 georadar profiles. The existing information mainly relates to the location of surface subsidence and fractures. In the profiles that pass close to the sinkholes, characteristic diffractions and hyperbolas are observed that could correspond to disturbed parts of the subsurface, extending in some cases to greater depths. By identifying similar characteristic anomalies in the radar plots of each individual profile, a spatial assessment of potential problem zones was carried out over the entire area of the reservoir. It should be noted that these diffractions and hyperbolas could also correspond to artificial structures or the presence of

Figure 5 Processed GPR profile 2 depicting characteristic anomalies in respect to the existing surface ones (upper panel). The final interpreted 2D map depicting spatial distribution of the zones of interest by taking into account all the GPR profiles, in respect to the observed problematic areas in the surface.

more coarse materials, without being able to directly separate them from the potential problematic zones.

In Figures 5, an example of one of the processed radargrams is presented (profile 2), in respect to the existing surface subsidence locations that are marked in blue. Zones of characteristic anomalies and structures of interest are depicted and shown in yellow rectangles. In general, the depth of the problematic zones reaches approximately 4 to 6 m and in some cases approaches 8 to 10 m, where they coincide with the surface occurrences of sinkholes. After pointing out the areas of interest on each radar plot, a final 2D map was created and overlaid on the topography map of the survey area. Figure 5 depicts the spatial distribution of the characteristic anomalies categorised in three different depth ranges, in respect to the existing problematic areas and the geophysical layout.

Seismic method

The seismic data acquisition was carried out following a special design in order to allow the simultaneous application of the methods of seismic refraction and multi-channel analysis of surface waves, for a total length of 355 m seismic section. Thus, for the 2D acquisition setup, a curved line of 72 vertical 4.5Hz geophones was deployed, with inter-station distance of 5 m, as illustrated in Figure 6. Specifically, three individual seismic lines of 24 geophones, each 115 m long, were deployed in series,

using a 24-channel seismograph (Gea24 of PASI instruments) for acquiring data which was generated by repeated seismic shots (at least 2 shots at each position for each individual seismic line). The shots were performed using a 6 kg sledge hammer. Sampling rate of 8000 sps with record length at 1024 ms was applied for all shot gathers. The specific setup concluded finally to a total of 13 different shot positions along the whole seismic section. The exact coordinates of the shots and geophones positions were measured using an RTK GPS Leica system.

Refraction tomography

The 2D seismic refraction tomography was applied to calculate the subsurface distribution of P-wave velocity, exploiting the principle of refraction and the first-arrival times of the P-waves in the acquired seismograms (Figure 7).

The first arrivals were used to construct the travel-time curves, i.e. the diagram of the seismic arrivals as a function of the distance of the geophones from the seismic source. In this diagram (Figure 7), the direct and refracted waves from each interface can be distinguished and a first estimation of the apparent velocities of the different layers can be made.

The construction of the final tomographic P-wave velocity model of the total seismic section (Figure 10) was based on the inversion of the apparent velocities of the travel-time curves, using an initial model of 40 layers, 1m thick each, with increasing velocity from 500m/s to 3000m/s. For the inversion, the smoothed nonlinear least squares technique was applied to minimise the objective function of the difference between measured and calculated travel-times.

2D surface wave tomography

For the current study, in order to determine the S-wave velocity distribution and generate the two-dimensional Vs section of the

area under investigation, the 2D approach of the Common-Midpoint Cross-Correlation Stacking Tomography technique (Leontarakis et al., 2022) was used. The recordings obtained from each seismic shot (raw data sequence) were checked for integrity and a standard preprocessing was performed (data decimation to 2000 sps, bandpass filter of 5-100 Hz, signal muting) in order to better isolate and highlight the surface wave energy.

Then, for each shot sequence, the cross-correlations (CC) of the recordings of all possible geophone combinations were calculated, so that all common shot-point gathers were used to create a single database of all cross-correlations. Next, a frequency-velocity (f-c) analysis was initially performed on the totality of the calculated CCs in order to produce both the average group and phase velocity dispersion images, on which reference velocity curves and frequency dependent velocity limits for the final automatic velocity picking, were manually defined, once for the whole seismic section (Figure 8).

The following CCs were grouped according to their mid-point at 5 m intervals along the seismic line and frequency-velocity (f-c) analysis was performed on each individual CC to produce

Figure 6 The seismic acquisition layout and an example of a sequence of recordings.

Figure 7 Example of first arrival picking and travel times curves for the seismic refraction method.

a dispersion image of group and phase velocity. Afterwards, all dispersion images that belonged to the same CC group were stacked to produce the final group and the final phase velocity dispersion image for each different location. Finally, the local maxima within the pre-defined velocity limits were automatically selected in all stacked dispersion images. In this way, the local dispersion curves were generated for each defined point on the seismic line. The group and phase local dispersion curves along the seismic line are plotted as velocity sections in Figure 9.

The calculation of the S-wave velocity distribution below the overall seismic section in the survey area (Figure 10) was performed by inverting individually the velocity dispersion curves corresponding to each point on the seismic line (pseudo-2D Vs model). For this purpose, a joint 1D group and phase velocity

inversion was applied to each pair of the estimated dispersion curves (local DCs). More specifically, a nonlinear least-squares algorithm, which combines the use of damping and smoothness constraints following Constable, 1987; Meju, 1994; and Lai, 1998, was used to invert our results in the frequency band between 10Hz and 80Hz.

Interpretation of seismic results

Figure 10 shows the velocity models of P & S waves in the study area. As can be observed, the propagation velocities of both P and S seismic waves, in general, increase with depth and decrease slightly towards the south, but show a characteristic increase at about the middle of the total section. The high velocity zone (Vp > 1600m/s and Vs > 800m/s), which could be characterised as a rocky basement, typically follows this trend. Also, in the southern part of the section, there is a decrease in the seismic velocity values corresponding to the surface layers (up to 5 m depth), compared to the northern part of the section.

To determine the average value of the S-wave velocity for the top 30 m of the subsoil (parameter Vs30) along the seismic line, we used the formula:

Vs30 = ∑di / ∑ti = 30 / ∑(di/Vsi)

where di is the thickness of each individual layer, ti is the time it takes for a seismic S-wave to travel through it and Vsi is the corresponding velocity of S-waves for each individual layer. As can be observed in the lower panel of Figure 10, Vs30 varies at about 600m/s along the seismic section, classifying the soil according to Eurocode 8 in class B. In fact, there is an increase in Vs30 at about the middle of the section, indicating soil tending to approach class A, which is apparently due to the characteristic uplift of the rocky bedrock in this area.

In addition, it is worth noting that in the P-wave velocity model (Vp), at about 190 m from the beginning of the section, high velocity values are observed almost to the ground surface. This point corresponds to the location of the main subsidence where surface gypsum outcrops have been identified. At the same time, a characteristic velocity reversal (from high to low velocity) is observed a few metres before (about 170 to 185 m from the beginning of the section) at a depth of about 12 m which

group and phase local

Figure 8 Average phase velocity dispersion image corresponding to the totality of the seismic data, along with the manually defined velocity limits for the automatic local dispersion curves picking.

Figure 9 The calculated

dispersion curves along the seismic line using the Common-Mid-Point Cross-Correlation Stacking Tomography technique.

could correspond to a possible loose zone of finite dimensions within the subsurface. This zone, more restricted (175 to 180 m), is also represented as a sharp lateral change in velocity at the corresponding depth in the S-wave velocity model (Vs), as well as in the Vs30 distribution as a local decrease in values. Of course, due to the uncertainty of geophysical methods in general, the correlation of geophysical observations with possible drilling data is necessary to confirm the observations. Finally, surface zones of low velocities are observed in both Vp & Vs velocity models at distances of about 40 to 80 m from the beginning of the section, 120 to 195 m and finally 225 to 330 m, which could correspond to problematic and disturbed zones near the ground surface. Of course, it should be emphasised here that, according to the acquisition setup followed in this survey, the final velocity models are expected to be more reliable from 2-3 m depth to nearly 30-40 m.

The geotechnical survey

In the context of the rehabilitation of the water storage reservoir, a geotechnical investigation was carried out after the completion of the geophysical survey.

The geotechnical survey included the drilling of three boreholes, marked as Borehole 1 (B1), Borehole 2 (B2) & Borehole 3 (B3) in Figure 4, of a total drilling length of 118.25m in the area of the reservoir. During the progress of the drilling (per 2 m progress

Figure 10 The P-wave velocity model (Vp) is shown on the upper panel, the S-wave velocity model (Vs) on the middle panel and the Vs30 distribution along the seismic profile on the lower panel.

Figure 11 An example of mineralogical analysis using the XRD method for a gypsum specimen found in Borehole 1.

1a ‘Sand gravel with gypsum gravels, loose structure, to strongly disintegrated Gypsum (IV), fragmented, off-white’

1β ‘Microcrystalline Gypsum, healthy to moderate disintegrated (I-III), off-white colour with spots spots of oxides - hydroxides.’

1γ ‘Alterations of whitish colour Microcrystalline Gypsum with gray color Microcrystalline Anhydrite’

2 ‘Clay Sandstone with cobbles, brownish colour’

3 Dense Silty Sandstones of brown-red colour

4 ‘Cohesive conglomerates and Sandstones with uncohesive zones of Silty Sandstones’

Ι ‘Brown to red Brown Sandy and gravelly clay’

ΙΙ ‘Very dense Silty Sand gravels of brown and brownish yellow colour’

of each drilling), standard penetration in situ tests (SPT) were performed. The number of blows of the SPT test (N-SPT) is an indication of the in-situ cohesion of soil formations. In addition, during the construction of the wells, 25 MAAG and LEFRANC water permeability tests were carried out. Characteristic samples from the boreholes were subjected to soil classification tests and determination of their physical properties as well as mineralogical analysis using the X-ray-Diffraction (XRD) method for specific specimens (Figure 11).

During and after the execution of the geotechnical survey, measurements of the groundwater level were taken in the boreholes B1, B2 and B3. Based on the geotechnical formations encountered in the tree boreholes and the results of field and laboratory tests, a geotechnical model was compiled and proposed by the geotechnical experts and is presented in Table 1.

Integrated analysis of geotechnical and geophysical survey

According to the results of the geotechnical investigation, the alluvial materials are loose depositions and agree with the results of the geophysical surveys according to which low velocity values are observed both on Vp and Vs model, until a depth approximately of 5 m.

In particular, the geophysical survey with the GPR method in the area of borehole B1 estimates that the thickness

of the loose materials extends to a depth of 6.0 to 9.0 m (Figure 5), which is also confirmed by the borehole, in which the loose zone was encountered at the surface and down to a depth of 8.0 m. In borehole B2, which was drilled in the crest of the western dyke of the reservoir, no loose formations were encountered. In borehole B3 according to the geotechnical survey the thickness of the loose formations extends to a depth of about 5 m, which agrees with the results of the GPR method.

In addition, the seismic survey that was carried out on its western side of the reservoir contributed significantly to the better delineation of the deeper structure of the subsurface as well as the better understanding of the mechanism of the reservoir’s failure. Both the seismic velocity models Vp and Vs revealed the underlying morphology formation of gypsum – anhydrite and also delineated a specific area where a characteristic increase on velocities exists. According to the seismic profiles of both Vp and Vs models, gypsum rises in the area where the largest sinkholes exist and depict a loose structure up to a depth of approximately 8.0 m. The borehole B1 that was drilled in the same area absolutely confirmed the results of the seismic survey.

Figure 12 shows the proposed geotechnical section along the seismic line that derived by taking into account both the results of the geophysical (Vp model) and geotechnical investigation. This is a classical approach that was followed by geotechnical/ geological experts in order to jointly interpret geophysical and

Figure 12 Proposed geotechnical section along the seismic line

Table 1 Geotechnical design parameters proposed by the geotechnical experts.

geotechnical information, providing the geotechnical engineers with a specific model for further analysis.

Going a step forward, we applied to this data the Integrated Spatial Geophysical and Geotechnical Evaluation tool (I.S.G.E) that has been proposed by Orfanos et. al (2021) for the automatic conversion of geophysical inverted models to geotechnical parameters based on Fuzzy logic. The I.S.G.E. algorithm is fully automated, easy to use and it can convert geophysical interpreted models to unbiased statistical driven geotechnical models, by exploiting the drilling information. The emerged models can be more easily understood by engineers and can give more insight in the spatial assessment of geotechnical parameters in a survey area. The main advantage is the incorporation of the possible available geotechnical information (borehole data, soil or rock in situ tests and laboratory measurements) in the multiparameter analysis. In this way, the propagation of the sparse, or even point, geotechnical information, from 1D to 2D or even 3D space, can be achieved without the need of predefined equations linking the measured geophysical to the desired geotechnical parameter.

As a first step in the ISGE algorithm, in order to reduce the uncertainty of the geophysical interpretation, a multiparameter analysis using the Fuzzy C-means Clustering (FCM) technique was initially performed following specific processing steps for the creation of a Unified Geophysical Model (U.G.M) (Orfanos

and Apostolopoulos 2013). The automatic derived U.G.M. model for four clusters that combines the information of both Vp & Vs seismic models, is presented in upper panel of Figure 13. As we can observe, the hard cluster model is adequate correlated to the respective geotechnical cross-section of Figure 12, proposed by the geotechnical experts. For the incorporation of the geotechnical information in the I.S.G.E. algorithm, a data-driven approach is followed, based on a modified version of the methodology proposed by Paashe (2017). Using the data of the current study, the I.S.G.E. output depth sections for SPT blow counts and Unit weight are presented in the lower panel of Figure 13.

Conclusion

The integrated approach using geophysical and geotechnical means of exploring the subsurface contributed significantly to the better understanding of the failure’s mechanism of the existing sinkholes. The application of the GPR method delineated the depth of the problematic areas that appears in the surface of the reservoir, as well as highlighted specific regions of possible loose zones. The seismic survey revealed the underlying morphology of the basement and delineated a specific area of high velocities that is related to the presence of gypsum formation up to the ground surface, as it is absolutely confirmed by the borehole data. Transforming geophysical models to geotechnical information promoted the geophysical research to an integral part of the geotechnical investigation programs. Both the manual conversion approach and the automatic output of I.S.G.E algorithm provided engineers with an additional interpretation tool for better understanding the subsurface conditions and reducing the spatial uncertainty at the area under investigation.

References

Constable, S.C., Parker, R.L. and Constable, G.G. [1987]. Occam’s Inversion: A Practical Algorithm for Generating Smooth Models from Electromagnetic Sounding Data: Geophysics, 52, 289-300.

Lai, C.G. [1998]. Simultaneous Inversion of Rayleigh Phase Velocity and Attenuation for Near-Surface Site Characterization: PhD dissertation, Georgia Institute of Technology.

Leontarakis, K., Orfanos, C. and Apostolopoulos, G. [2022]. Common-midpoint cross-correlation stacking tomography: A 3D approach for frequency-dependent mapping of Rayleigh waves, group and phase velocity throughout an active seismic network. Near Surface Geophysics, Vol. 21, Issue 1.

Meju, A. [1994]. Geophysical data analysis: understanding inverse problem theory and practice Tulsa, OK: Society of Exploration Geophysicists, 296.

Orfanos, C., Leontarakis, K. and Apostolopoulos, G. Zevgolis, I. [2021]. Integrated spatial geotechnical and geophysical evaluation tool for engineering projects: A 3D example at a challenging urban environment in the city of Athens, Greece. Sixth International Conference on Engineering Geophysics, Virtual, 25-28 October 2021.

Orfanos, C. and Apostolopoulos, G. [2013]. Multiparameter analysis of geophysical methods for target detection: The unified geophysical model approach», Geophysics, 78(6), p. IM1–IM13.2013.

Paasche, H. [2017] Translating tomographic ambiguity into the probabilistic inference of hydrologic and engineering target parameter. Geophysics, 82, EN67-EN79.

Figure 13 Unified Geophysical Model of 4 clusters of the seismic profile (upper panel). I.S.G.E. output for SPT N-values (middle panel) and Unit weight (lower panel).

High-frequency FWI Imaging: repurposing seismic data for imaging shallow hazards

Hung Dinh1*, Thomas Latter1, Mike Townsend2, Nils Grinde2, Ståle Høgden2, Nicholas Robb2, Marte Aksland2 and Alexander Bertrand2 demonstrate that high-frequency FWI and associated attributes can help to identify shallow anomalies and fault systems more effectively than standard imaging methods, with a significant increase in resolution.

Abstract

Dedicated high-resolution site surveys are used to identify potential hazards prior to the placement of infrastructure. Apart from the additional acquisition expense, site surveys are often acquired as a series of 2D lines, which limits their spatial resolution. We describe how FWI Imaging results from existing, raw, conventional 3D seismic data, can be used as a rapidly available 3D alternative or supplementary dataset to help improve understanding of the shallow subsurface. Two case studies demonstrate the potential of this approach. Specifically, they show that high-frequency FWI and associated attributes can help to identify shallow anomalies and fault systems more effectively than standard imaging methods, with a significant increase in resolution. Furthermore, FWI has proven its ability to provide shallow image quality comparable to that of multibeam echo sounder measurements, even with less densely acquired data.

Introduction

Understanding shallow geohazards is a critical consideration due to their potential impact on infrastructure placement. In oil and gas applications, challenges ranging from gas seepages, shallow water flow, or unstable soil conditions pose risks to drilling operations. However, achieving the necessary resolution in the near surface can present a challenge even to 3D seismic data, which has been acquired specifically for shallow imaging. Conventional 3D towed-streamer data for exploration purposes (referred to here as ‘conventional 3D’ data), acquired for deeper (>500 m) targets, is more readily available. Conventional 3D acquisition geometries (narrow azimuth, multiple streamers) imaged with traditional processing flows (applying standard migration algorithms imaging only primary reflections) are unlikely to provide sufficient resolution for shallow hazard analysis. This is the result of poor crossline sampling and limited near-offset coverage with large azimuthal variations.

A longstanding solution for shallow hazard imaging has been the acquisition of 2D high-resolution (2DHR) site surveys (Douglas, 2011). These employ specialised acquisition equipment, yielding high sampling rates along the survey line. However, 2DHR site surveys usually lack the necessary spatial

1 Viridien | 2 Vår Energi

* Corresponding author, E-mail: Hung.Dinh@viridiengroup.com

DOI: 10.3997/1365-2397.fb2024064

resolution between the survey lines (typically spaced 150-300 m apart) and incorrectly image features with a degree of geometric heterogeneity owing to erroneous imaging of reflection points outside the 2D plane. Geological details can therefore be missed that would help operators distinguish between sites for safe infrastructure placement from those harbouring unforeseen hazards. Dedicated 3D high-resolution (3DHR) site surveys can overcome these limitations (Kassarie et al., 2017), but these come with substantially higher financial investment, limiting their widespread application.

In some cases, a hybrid approach has been adopted, which combines a 2DHR site survey with a conventionally imaged 3D seismic survey. This approach aims to combine the advantages of the high temporal resolution of the 2DHR survey with the 3D imaging benefits of a conventional 3D survey. However, case studies using this approach result in a conventional 3D survey suffering from suboptimal resolution, as well as delivering an inconsistent interpretation with 2DHR site surveys. This leaves a gap between localised high-resolution information and the broader regional context (Sharp and Samuel, 2004; Selvage et al., 2012).

Alternatives without new data acquisition

Reprocessing of conventional 3D towed-streamer data with workflows to recover more near-surface resolution is not new (Kanrar et al., 2019). However, the traditional Kirchhoff algorithms utilised are limited by technical constraints, such as ray tracing and velocity approximations, sampling constraints including the near-offset sampling gap, or illumination imbalances. These constraints chiefly originate from the use of only primary reflection energy. Two recent alternatives are the use of near-field hydrophone (NFH) data, which is readily available for routine monitoring of the source and designing the source signature, and multiple imaging techniques. NFH data may provide high-resolution imaging results owing to their dense sub-millisecond sampling (Tyagi et al., 2021). Meanwhile, multiple migration improves image quality by using recorded data as secondary sources (Poole, 2021). These two methods have been compared by various authors as approaches to enhance shallow imaging

(Lim et al., 2022; Tang et al., 2023). However, some pitfalls of NFH imaging lie in the low-fold nature of the data, difficulty in removing the direct arrival, the requirement for a full processing flow, and the challenge of addressing the sampling gap between source lines. Multiple imaging, on the other hand, is sometimes limited by its Born-approximated wave-equation that does not always perform accurately in complex shallow intervals (Lu, 2021).

Given the limitations above, Full-waveform inversion (FWI) offers an alternative approach utilising a two-way wave equation. Leveraging the full wavefield, including multiples, ghost energy and diving waves, from raw seismic data results in unparalleled subsurface sampling and illumination. FWI can deliver accurate models and quantitative estimates of subsurface properties rapidly through an iterative inversion process. Although significant computational power is required, advances in algorithms and high-performance computing have accelerated rollout. For example, Time-lag FWI (TLFWI; Zhang et al., 2018; Wei et al., 2023) has facilitated high-resolution models at more than 100 Hz. Having the potential to naturally compensate for illumination and transmission loss (Cooper and Ratcliffe, 2023), FWI can provide more accurate subsurface information and has become essential for imaging complex structures where conventional approaches struggle (Zhang et al., 2020). This helps compensate for the imaging limitations of 2DHR surveys and conventional 3D towed-streamer imaging, especially in the near surface.

Previous FWI work on shallow hazards

As early as ten years ago, FWI was applied to invert for shallow overburden velocities up to 9 Hz using reflections, diving waves, and refracted head waves (Wiarda et al., 2014). Small-scale gas bodies were identified and correlated to 2DHR site survey data for the first time by Bright et al. (2015) in a 7 Hz FWI anomaly

volume, which was built using narrow-azimuth towed-streamer data. Reaching mid-teen frequencies, while using conventional wide-azimuth towed-streamer data, Li et al. (2015) showed that FWI can detect gas bodies more confidently and effectively than conventional tomography. However, the resolution of the anomalies from these studies was still subpar compared to standard 2DHR data. Recently, FWI technology has advanced in assessing shallow hazards. With sparse OBC acquisition, a 20 Hz FWI study demonstrated superior resolution on shallow gas pockets, outperforming a 25 Hz RTM image despite acquisition sparsity (Vandrasi et al., 2020).

The most recent FWI studies by Latter et al. (2022), Espin et al. (2023) and Dinh et al. (2023, 2024) demonstrate such applications by pushing the inversion maximum frequency limit, in some cases above 100 Hz. Here, we investigate the feasibility of FWI for shallow hazard analysis, aided by attribute analysis on the FWI velocities and their associated derivatives from two recent studies (Figure 1). Examples include bathymetry maps, attribute extractions for low-velocity anomalies, and fault systems, which are then compared against 2DHR and conventionally imaged towed-streamer datasets.

Methodology

Seismic attribute analyses have often been used to assess geological properties, as they can reveal hidden responses from hazard features in regular amplitude sections (Sukmono et al., 2017; Tokarev et al., 2018).

For shallow hazards, attribute analysis can be performed directly on the FWI impedance model, I p, or from the FWI Image derived as its spatial derivative: (1)

given dip θ and azimuth φ of the normal vector n to the subsurface reflector (Zhang et al., 2020). Here, for simplicity, we consider the velocity-only version of this derivative.

Shallow geohazard attributes

Most common subsurface hazards have distinct velocity contrasts from their surroundings. Based on Bright et al. (2015), we propose a stable and accurate velocity anomaly extraction A by subtracting the median background velocity from the high-resolution FWI model v: (2)

The use of the median can help to detect small-scale anomalies more effectively, whereas using the mean or smoothing can suffer from anomaly outliers. This approach can detect a wide variety of overburden shallow anomalies, likely associated with gas pockets, shallow water-flow sands, boulders, pressure zones and pipelines.

Bathymetry map extraction

To detect the seabed or seabed features, a surface can be extracted based on the distinct velocity differences between sediment and seawater. Alternatively, this can be estimated by the depth at which the maximum derivative of the FWI Image is found:

Figure 1 Basemap showing two case study areas in the Northern Viking Graben area, and the Barents Sea.

(3)

This is expected to identify the water bottom as the strongest contrast between the water layer and sediment. However, with an unconsolidated seabed including mixed layers of soft and hard clay, the maximum derivative may represent the boundary between clay layers. To help minimise this, prior information about the water column, such as temperature salinity data, can be useful.

From the bathymetry map, two attributes can be calculated:

• The seabed surface gradient can reveal natural hazards, such as pockmarks, iceberg scours, unstable slopes, sink holes, or surface channels, present between two points A and B with their X and Y positions respectively, and water bottom depth Z as follows:

(4)

• An amplitude map extracted from the FWI velocity within a small interval below the seabed surface can reveal man-made seabed features such as cables, shipwrecks and debris from previous exploration or production operations.

Fault detection

Standard spatial coherency is an effective attribute to detect faults (Sukmono et al., 2017). A modified dip-coherency was introduced to aid interpretation by taking the root-mean-square (RMS) of the spatial derivatives of the velocity with respect to the plane of the reflector along and perpendicular to the local azimuth direction, nAz and nAz+90, respectively, as described by Kerrison et al. (2021):

(5)

A similar approach can be extended to the FWI Image (following Equation 1) to highlight different features in the FWI results.

Example 1

Data description

The Dugong area, located in the Northern Viking Graben area with a 330 m water depth, was surveyed by two 3D towed-streamer narrow-azimuth acquisitions: a 2016 north-south oriented variable-depth towed-streamer dataset and a 2020-2021 east-west multi-sensor dataset. Both deployed 8 km streamers with a 75 m separation and were processed using advanced Q-honouring dual-azimuth (DAZ) processing and multi-parameter velocity model building flow (Latter et al., 2022).

A 2020 2D high-resolution site survey with line spacing of 100 m provided bathymetric, seabed and sub-seabed data, focusing on shallow gas hazards within an interval at a 430465 m depth (550-590 ms TWT). The 2DHR site survey included multibeam echo sounder (MBES) data, from which a three-cell smoothed 3 m × 3 m bathymetry map was created. The 2DHR site survey also featured high-resolution 96-channel seismic acquisition at a sample rate of 1 ms with a 12.5 m channel interval, resulting in a 2D CMP spacing of 6.25 m. From this, a shallow gas hazard map was generated in the gas warning interval by combining amplitude anomalies from the 2DHR seismic lines and extrapolating their spatial extent using amplitudes picked from the existing 2016 north-south 3D seismic data (Figure 4a). This approach aimed to improve spatial accuracy and delineate shallow gas hazards that may have been missed by the 2DHR data.

A 40 km2 area covering the 2DHR survey was selected for this DAZ case study. The raw 3D DAZ seismic data was utilised to generate a high-resolution 100 Hz visco-acoustic TLFWI Image. Subsequently, bathymetry and gas hazard maps were calculated from the FWI Image and velocity model, respectively.

Figure 2 3D DAZ Q-Kirchhoff migration (a, b) compared to the 100 Hz TLFWI Image (c, d). Spatial resolution is significantly enhanced with FWI. Zoomed images reveal finer seabed details (e) and gas hazards (f) in the FWI Image as shown by Dinh et al., 2023.

FWI Imaging results

Figure 2 from Dinh et al. (2023) compares images from the 3D DAZ Q-Kirchhoff migration and 100 Hz TLFWI (calculated from Equation 1) using the same acquisition. Although it uses advanced DAZ processing techniques, the conventional imaging (Figure 2a, 2b) has lower lateral resolution compared to the FWI Image from the same acquisition (Figure 2c, 2d). The full-wavefield illumination and iterative least-squares data fitting process in FWI reveals fine features. For example, it shows

buried iceberg scour marks and small-scale low impedance accumulations likely to be associated with gas (Figure 2e, 2f) that are less than 10 m wide, which are indistinguishable in the conventional imaging.

Site survey comparison

Bathymetry map

The bathymetry map derived from the MBES (Figure 3a) and the FWI-derived map (Equation 3, Figure 3b) show similar character-

Figure 3 Site survey bathymetry map (a) shows the seabed with iceberg plough marks (green arrows) and pockmarks (yellow arrows). TLFWI maximum derivative map (b) shares similar features but adds spatial resolution (red arrows), as shown in Dinh et al. (2023).

Figure 4 Site survey amplitude anomaly map (a), combining 2DHR amplitude anomalies and 3D Q-PSDM amplitude anomalies, shows a poor correlation. The corresponding FWI shallow gas amplitude map (b) shows better spatial resolution and resolves missing features.

istics, including multidirectional elongated iceberg plough marks (green arrows) and numerous depressed pockmarks (yellow arrows). Despite the high temporal resolution of the MBES, the FWI-derived map from the conventional 3D towed-streamer data provided significant uplift in lateral resolution, resolving small-scale features not detected by the site survey acquisition grid (red arrows).

Gas hazard map

The shallow gas map derived from the site survey data within the gas warning interval of 550-590 ms (Figure 4a) combines prioritised interpretations from 2DHR data (cyan colour) with 3D anomalies (red features) inferred from 3D Q-PSDM of conventional towed-streamer data. These show a low correlation, especially in the central area where numerous small-scale 2D anomalies are not confirmed by the 3D data. This mismatch may result from the limitations of 2D sampling, erroneous 2D imaging of out-of-plane subsurface reflectors, or the low spatial resolution of conventional 3D imaging. In contrast, FWI-derived anomalies extracted from within the corresponding depth interval (Equation 2, Figure 4b) show improved lateral resolution of features that could be inferred from the 3D data, enhancing geohazard interpretation potential.

Example 2

Data

description

In this example, high-resolution FWI Imaging was applied in the Barents Sea, known for its rugged seafloor topography. In 2019, a seismic survey was conducted in water depths of 400 m, with a source-over-spread configuration using five 75 m-separated airgun arrays, positioned halfway along the 16 8 km-long, 62.5 m-spaced cables (top sources). The five sources were supplemented by an additional source array placed in front of the cables (front source) for longer offset coverage. This design provided dense spatial sampling and full-azimuth coverage at near offsets.

In 2023, a 2D high-resolution site survey was conducted over an area of prospective wells. Seismic data was acquired with a 400 Hz maximum source frequency, 96 channels, 1 ms

sample interval and 6.25 m CMP spacing. It also featured an ultra-high-resolution (UHR) data acquisition using a 1.6 kHz maximum source frequency, 32 channels, 0.25 ms data sampling, and a 1.5625 m bin size. A bathymetry map was derived from the site survey MBES measurements.

FWI Imaging was run up to 100 Hz on a 50 km2 subset of the source-over-spread configured data, covering the 2D UHR site survey. From this dataset, a bathymetry map and geobody anomalies were extracted (Equations 2 and 3).

FWI Imaging results

The 100 Hz FWI image significantly improved the image quality compared to conventional Kirchhoff migration (Figure 5). The improved illumination and signal-to-noise ratio (SNR) allowed for the detection of multiple small-scale geobodies with clearer fault imaging. Shallow geology was better resolved, and evidence of fluid conduits was indicated by brighter amplitudes. Unlike the Kirchhoff migration, FWI Imaging did not struggle with structural imaging below shallow gas. This is likely due to the inherent illumination balancing and transmission loss compensating characteristics of the FWI algorithm.

Highlighting improvements in spatial resolution, Figures 6a and 6b show a coherence attribute comparison at 450 m depth. This was calculated from the Kirchhoff image and FWI Image (Equation 5) and is overlaid by the legacy velocity and FWI velocity model, respectively. This comparison demonstrates the significant uplift in detail of the FWI Image, featuring sharply defined, high-resolution overburden iceberg plough marks and small-scale velocity anomalies (sub-15 m wide). Figures 6c and 6d show this coherency comparison at 520 m depth, without the velocity overlays. At this depth, Figure 6e provides the addition of the FWI Image. The FWI Image coherency shows a complex network of regional faults, the most apparent of which is highlighted by the arrow in Figure 6d, and a highly illuminated local fractured zone, most visible in the FWI Image (Figure 6e). In contrast, the Kirchhoff image using the legacy velocity field did not detect fault activity at this depth due to lower SNR (Figure 6c).

Once these features were identified from corresponding attributes, they were converted and displayed as 3D geobodies to

Figure 5 3D Q-Kirchhoff migration (a) compared to the FWI Image (b), both displayed to 100 Hz. Note improved SNR, illumination and structural imaging compared to the conventional image underneath the seabed and mid-section fault imaging.

obtain a more comprehensive understanding of shallow hazards (Figure 7). Such visualisation can be integrated with information from other sources to reduce uncertainty and improve ground model accuracy.

Site survey comparison

The water bottom surface extracted from the FWI Image result exhibited a significant improvement in detail, resolving smallscale (sub-10 m wide) iceberg keels and pockmarks that were not detectable in the legacy Kirchhoff image (Figure 8). Compared with the site survey bathymetry map, all details were consistently captured in the FWI Image result. This observation demonstrated that high-frequency FWI Imaging for spatial resolution can provide comparable results to the MBES at the water bottom level, despite the data not being designed for this purpose.

Discussion

In both examples, the FWI-derived bathymetry map exhibited a high degree of correlation with the MBES-derived map. The hazard map, produced from conventional 3D data in the first exam-

Figure 6 Coherence attribute and corresponding velocities at a 450 m depth from 3D Q-Kirchhoff migration (a) compared to the 100 Hz FWI Image (b) show uplift in spatial resolution in the FWI result, revealing a variety of potential hazards (white arrows). Example at 520 m shows fault imaging in the Q-PSDM coherency (c) has been improved in the FWI velocity coherency (d) and FWI Image (e). Crossed circles indicate well locations.

Figure 7 3D visualisation of low-amplitude anomalies as a tool for data integration and hazard analysis. Black lines indicate prospective well paths.

ple, demonstrated the ability to enhance the spatial resolution and reduce uncertainty for shallow hazard identification compared to a benchmark site survey supplemented with conventional 3D imaging. The second example expanded the attribute technique to other potential hazards, such as boulders and fault systems, suggesting an effective visualisation tool to provide valuable insights into shallow geological features.

FWI will be influenced by the acquisition geometry. To validate the FWI Imaging in a coarser acquisition environment, the source-over-spread data from the second example was decimated to match a more conventional acquisition configuration (two 150 m-separated top sources with a 7.5 m actuation spacing every 30 m, and 62.5 m streamer separation with 12.5 m receiver spacing). A similar FWI workflow was then applied to this decimated data using the same starting model derived from a smoothed 13 Hz FWI velocity model. The results were then compared with the undecimated source-over-spread FWI results. Figure 9 is extracted from the centre of the test area. In the decimated data, we do notice some limitations in resolving steeply dipping events, limited illumination between sail lines, and some degradation in SNR (red arrows). Yet the 60 Hz FWI Images from both scenarios present comparable resolution at and beneath the seabed level, thanks to iterative data fitting and the full-wavefield sampling including primaries, ghosts, multiples, and diving waves. The use of high-order narrower-angle multiple ray paths helps to increase near-offset illumination. This is beneficial in shallow water areas where events may otherwise not be imaged owing to the post-critical refraction threshold. Combined with the first example, this observation has demonstrated that a conventional acquisition configuration can still be suitable for FWI Imaging,

and the technology still has potential even when dense data coverage is not always available.

In both examples, the crosstalk between velocity and other parameters, such as viscosity (attenuation) or density, has not been fully considered beyond a low-frequency update. Extending the FWI to either a multi-parameter visco-acoustic (described by Latter et al., 2023) or even visco-elastic workflow (Masmoudi et al., 2023) could reduce residual inversion error here. Once these crosstalk effects are properly accounted for, interpretable resolution is the next consideration. Regarding spatial resolution, the use of the full wavefield, including high-order multiples, can resolve finer details than conventional Kirchhoff migration at the same frequency, which relies on primary-only inputs. The FWI models show that the technology can resolve geobodies sub-10 m wide observed in both examples. Figures 10b and 10e also show that the FWI Image from conventional data has richer low frequencies compared to a dedicated site survey, in addition to the 3D sampling, recording aperture, and the lateral resolution accuracy improvements mentioned throughout this article. There are reasons, however, why high-frequency FWI Imaging from conventional towed-streamer 3D seismic may not replace dedicated HR site surveys. Site surveys offer superior vertical resolution and often provide supplementary information, including geotechnical data that conventional 3D data often does not provide. Also, site survey acquisition timing may be more favourable relative to infrastructure placement decisions as well as other potential insurance considerations. Nevertheless, the ability to leverage existing raw 3D exploration seismic data with minimal image processing to provide high-resolution data in a short timeframe is a significant advantage of the FWI Imaging

Figure 9 Comparison of the 60 Hz FWI results between full source-over-spread data (a and b) versus decimated source-over-spread data (c and d) in horizontal view (top row) and vertical view (bottom row). In the top view, red dots are top source shots and green dots are front source shots. In (c), only two top sources are used for FWI Imaging. The observations suggest the resilience of the technology to sparser data.

Figure 8 Seabed surface from Kirchhoff migration (a), 100 Hz FWI Image (b) and MBES site survey (c). Note that the FWI Image resolves consistent features to the dedicated MBES, such as multi-keel iceberg ploughmarks and pockmarks.

technology, bringing rapidly available supplementary information to guide or potentially reduce the scope of subsequent 2D or 3DHR site surveys.

Conclusions

We have presented an approach for shallow hazard analysis by repurposing conventional 3D towed-streamer seismic data as well as the application of high-frequency FWI Imaging techniques. Through additional attribute analysis on the FWI products, we have generated meaningful insights to identify potential anomalies, as demonstrated in two case studies which surpassed what could be achieved by conventional primary imaging approaches alone. High-frequency FWI Imaging can serve as a valuable complementary tool to dedicated HR data. The potential impact of this technology for shallow hazard analysis is significant. Where existing conventional 3D data is already present over the target area, it can provide cost-effective, rapid turnaround 3D results, to help directly identify risks or support better planning of dedicated surveys.

Acknowledgements

The authors would like to thank Vår Energi and partners (Petrolia NOCO, INPEX Idemitsu Norge, Concedo, Petoro and Equinor), Viridien Earth Data and TGS for permission to use and present the data, our colleagues in Viridien Geoscience for their support and comments during the technical review.

References

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Figure 10 Bandwidth split indicates richer low-frequency content in the FWI Image (e and f) compared to the 2DUHR site survey (b and c) despite the limited upper bandwidth (d vs. a).

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Advanced seismic imaging solutions for hardrock site evaluation and characterisation across scales

Nicoleta Enescu1* and Calin Cosma1 demonstrate how advancements in seismic imaging built around the 3D IP transform have had a notable impact on the detailed characterisation of fractured hardrock masses.

Introduction

Seismic imaging techniques for characterising fractured hardrock environments have evolved significantly over the past four decades, driven by the introduction of seismic processing techniques specifically adapted to hardrock geological contexts. Integration of surface and borehole seismic data has also been a key element for reaching the accuracy and reliability levels required for applications such as nuclear waste disposal and deep mining exploration. The 3D Image Point (IP) transform and migration contributed to resolving the challenges related to the hardrock complex geological and structural settings. Joint surface and borehole seismic studies using Image Point processing have been used for nuclear waste disposal studies in Finland from the early site selection stage, to site characterisation, to detailed site characterisation, to validation and cross-validation across scales during the construction of ONKALO, the Finnish final repository for spent nuclear fuel. Coming full-circle, the 3D IP migration proved recently its worth in an integrated surface and borehole seismic study conducted for preliminary characterisation of a site assessed as a potential Deep Geological Repository (DGR) for Canada’s used nuclear fuel. Examples evocative of the methodological development and refinement are showcased.

Methodological evolution

In crystalline rock, essentially all groundwater transport takes place in fractures and fracture zones, hence mapping the fracture zones is essential for assessing the suitability of sites for nuclear waste disposal. The evolution of the hardrock seismic imaging technology for nuclear waste applications began more than four decades ago, triggered by the need of resolving complex geological structures encountered in hardrock environments, while limiting the amount of drilling for probing the subsurface. Multi-offset, multi-azimuth Vertical Seismic Profiling (VSP) conducted in multiple boreholes has been found to be economically effective for identifying and locating those features that control the groundwater flow and for predicting the distribution of the groundwater salinity (Cosma, 1986; 1995; Olsson, 1992).

At Olkiluoto, the geometrical model derived from VSP includes directly imaged deep steeply dipping features which are generally difficult to capture by surface seismic. Data collected in several boreholes was merged into a comprehensive site model. The

development of the 3D IP transform marked a change in hardrock seismic data processing (Cosma, 1996a). This transform method redefined how seismic data (from multi-offset, multi-azimuth VSP surveys, integrated with surface and tunnel 2D and 3D seismic data) were processed, interpreted and integrated. The hardrock nuclear waste candidate repository sites that needed to be characterised had relatively small surface expressions, compared with traditional seismic exploration sites, and, as opposed to most contemporary mining development sites, could not be densely drilled. The focus on small-scale, deep 3D rock structure was novel. 3D IP has been introduced in response to these requirements, as an intrinsically 3D approach that could process jointly diverse acquisition geometries: surface, borehole and tunnel-based and accurately image seismic reflectors of any orientation.

The IP transform converts reflection events from intermingling curved reflection paths in time-domain profiles to distinct spots in the 3D IP domain. True reflectors are preserved and enhanced while multiples, other wave types, coherent and incoherent noise are suppressed. By its ability of turning complex patterns of criss-crossing wavefields into isolated regions of the IP space, the 3D IP technique simplifies the implementation of neural networks-based optimisation. Figure 1 illustrates the effect of IP processing on problematic data sets. It shows a VSP profile (zero-offset of Olkiluoto borehole KR8), where the conventionally processed section (left) shows faint and unresolved traces of possible reflectors. After denoising and polarisation filtering in Image Space, the same section is shown on the right. As customary at that time (1996), the middle plot displays a corridor stack of the latter, with limited relevance because of the diverse dips of the reflectors.

Technical advancements in data processing

Several advanced processing techniques can be applied with great efficacy in the IP domain (Cosma, 2003; Lee, 2008). Among these, the selective enhancement of signals based on the true dip and strike of the originating reflectors is instrumental for characterising fractured rock masses where reflectors are diversely oriented. By improving the coherency of seismic data and, in particular by filling data gaps caused by structural complexity, this method facilitated clearer and more interpretable seismic images, essential for accurate fracture network, geological and hydrogeological

1 Vibrometric

* Corresponding author, E-mail: nicoleta.enescu@vibrometric.com

DOI: 10.3997/1365-2397.fb2024065

modelling. The technique operates with reflection wave fields propagating through a constrained velocity field, produced by planar reflective elements with defined 3D orientations and sizes that can be linked to the dimensions of the Fresnel zones. This enhances true reflectors while suppressing direct, multiple and reflected wave types propagating with other velocities, regardless of the reflector angular disposition. Effective model-driven noise reduction in the IP domain ensures that the data integrity is maintained, enhancing the quality of the final images.

Methodology for 3D IP migration

The complete methodology for 3D IP migration (Cosma, 2010) involves several key steps:

1. Application of the IP Transform: This initial step transforms the data into the IP domain, using the acquisition geometry and a velocity model appropriate for the site. The position, orientation and reflectivity of each possible elemental reflector are conserved in the transform.

2. Filtering and Stacking: True dip filtering, polarisation filtering and, occasionally, statistical non-linear enhancement are applied, before profile stacking in IP domain, to refine the data further, emphasising the geological features of interest.

3. Creation of Migrated Image Volumes: The final step involved transforming the processed data directly to a depth migrated volume, where detailed representations of the subsurface structures are imaged in true 3D.

Figure 1 Zero-offset VSP shot point of borehole Olkiluoto KR8. (Cosma, 1996b). Conventionally processed section (left) and the same section after polarisation filtering in the Image Space (right) identifies several gently dipping and a multitude of steeply dipping reflectors, later interpreted and incorporated to the site model. The middle section represents a standard corridor stack.

Figure 2 Example from Olkiluoto site: Migration of integrated surface 3D and VSP data. 3D cube inline / crossline and VSP profile view (left), Site features interpreted from multi-borehole and surface combined analysis (right). The joint analysis permits the 3D direct imaging of both gently and steeply dipping features. (Ohman, 2008; Cosma, 2007)

Examples of integrated surface 3D seismic and VSP are shown in Figure 2. At the time (2000-2007), VSP studies were conducted with few sparse offsets, insufficient for building a VSP migration volume, hence oriented VSP migrations were instead inserted into the surface 3D cube (left). Features interpreted from multi-borehole and surface joint analysis (right) were subsequently validated by their intersections with the ONKALO access tunnel and included in the site model (Cosma, 2007), as exmplified in Figure 5. The integration of oriented VSP migrations into a 3D seismic cube remains useful, especially in the early characterisation stages, when less data is collected. The Crux Point method (Cosma, 2002) allows the transport of interpreted features from one survey to another (it is applicable for all seismic acquisition geometries, from surface, boreholes and tunnels) and can be used for determining the 3D positions and orientations of rock features, including ripples and discontinuities.

Case studies and practical applications

Multidisciplinary studies were conducted for the identification of bedrock volumes suited to final disposal, aiming at a more detailed definition of the properties of these volumes. Geophysics was mainly used for safety assessment purposes, but also addressed engineering and construction work topics, and had a significant contribution to both the preliminary and detailed characterisation of the planned repository host rock.

Case studies from various geological settings confirmed the practical efficiency of the 3D IP migration method in diverse conditions (Cosma, 2004; 2007; Enescu, 2022) and demonstrated the merits of the 3D IP migration method over traditional approaches, particularly for imaging steeply-dipping reflectors and delineating complex fracture zones (Enescu, 2022; 2023). Synthetic data were occasionally used to highlight in controlled environments the specific capabilities of the IP transform of enhancing image clarity and detail (Cosma, 2010).

Various techniques of seismic site characterisation produce information at largely diverse scales (regional, site specific and detailed characterisation, i.e. from kilometre to metre and sub-metre). Bedrock models integrate data at different scales, to meet the

needs of specific phases of the project: licensing and permitting, design, safety assessment. The objective of the site characterisation is to correlate and combine data and interpretations across all scales into a cohesive geophysical site model. At Olkiluoto, sub-vertical features interpreted from the magnetic and EM surface data and features interpreted from the VSP show a good match (Figure 3). Joint analysis of events from detailed borehole logging and 3D VSP imaging also show notable consistency (Figure 4). The investigations conducted for high level nuclear waste repositories are focused on deep, relatively small volumes of rock, which limits the relevance of the studies conducted from ground surface. With VSP, higher resolution and more detailed directional information are obtained than with surface seismic

Figure 3 Magnetic map of the ONKALO region, Olkiluoto site (left), with (near) linear discontinuity indications, gathered on regional and site scales, together with subvertical interfaces interpreted at site scale from VSP data in boreholes KR4, KR10 and KR14 (on the right, view from above).

Figure 4 Example of lithology, sonic, density and fracture frequency logs from Olkiluoto, borehole KR4 depth level 700-780 m (left) together with a site scale presentation of the VSP reflectors from boreholes KR4, KR10 and KR14 (right, 3D view from NE of the same reflectors as in Figure 2 and Figure 3).

Figure 5 Examples from Olkiluoto: (a) View from the SW slightly below ground level of the geological (yellow), hydrological and electrical (brown) and geophysical (VSP, black) models of a brittle fault zone. Also shown are tunnels and vertical shafts (yellow) and borehole paths (grey) with disks representing the fracture zone intersections and (b) View from above, SW, of an altered and deformed shear zone shown in green, and corresponding VSP reflector elements in black.

and rock structures found in several boreholes can be linked and combined into a detailed site model. Direct observations made during the excavation of the ONKALO tunnels closely matched and confirmed the seismic predictions. Figure 5 shows examples of both gently and steeply dipping site features interpreted by VSP and validated by multidisciplinary integration.

The match between petrophysical and geological features interpreted at different scales and rock features predicted by VSP is exemplified in Figure 6. Due to the weakly populated statistics, local distributions of fracture orientation observed over ~100-m borehole segments (Figure 6 b,c,d) are widespread, while the subhorizontal VSP features are well represented. Conversely, the geometrically imprecise definition of the intersection depths of steeply dipping features with vertical boreholes leads to a poorer local match. Notably, the site-scale average trends observed from detailed borehole logging and 3D VSP imaging display remarkable consistency (Figure 6 a).

A more recent case study integrates surface and borehole seismic imaging to characterise shallow structures within the Revell batholith, at Ignace, ON, Canada, assessing its suitability for a deep geological repository for nuclear waste. The results include the detailed mapping of subsurface structures, both steeply (Figure 7) and gently dipping (Figure 8). The effective 3D positioning and characterisations of structures was facilitated by 3D IP imaging and by an integrated approach. Nineteen kilometres of multi-line 2D data (Enescu, 2022) and VSP data in three boreholes from a total of 90 shot points (Enescu, 2023) have been collected and analysed. 3D-IP VSP migration volumes were produced, as shown in Figure 7. One notes sub-horizontal features as well as well-defined steeply inclined features, some of which do not intersect the borehole. Twenty seven lineaments mapped on surface were successfully interpreted as a subset of the subvertical features imaged in depth (Villamizar, 2023).

A limited attempt to more quantitative geological interpretation focused on sub-horizontal mafic intrusions within the granodiorite-tonalite host rock has been conducted independently (Villamizar, 2023) on seven 2D lines processed using the 3D IP technique (Enescu, 2022). More than 30 mafic structures were identified and characterised across the site (Figure 8), by integrating all the 2D, VSP and borehole logging data available at the site.

Conclusions

The body of work in seismic imaging for hardrock environments developed and documented over the past four decades offers a valuable insight into both technological advancements and methodological strategies. The advancements in seismic imaging built around the 3D IP transform have had a notable impact on the detailed characterisation of fractured hardrock masses and contributed to the decision-making process in site selection and development for nuclear waste disposal and other critical geological engineering projects.

The completeness of the measuring layouts in terms of view angle diversity over the rock site volume and station density bear a major influence on the performance of advanced seismic imaging techniques. The current developments with optical fibre data acquisition techniques like Distributed Acoustic Sensing (DAS) are bringing a significant increase of data volume, efficiency and capability, waiting to be exploited. Current research includes integrating 3D-IP and similar special Radon transform techniques with DAS and emerging ML tools. We have learnt that integration of technologies boosts results and that in rockmass characterisation efforts collaboration between disciplines should be the modus operandi from the start. Furthermore, adaptability to geological variability is a must and continuous innovation is crucial.

Nuclear waste repository site characterisation recognised from its beginnings more than four decades ago that compliance with environmental and regulatory considerations should be the norm that guides both the short and the long-term vision. A corollary of this principle points to the reduction of the footprint of the site investigations, which in turn asks for increasingly performant remote rockmass imaging methodology. This seems highly relevant also nowadays, for deep mineral exploration and delineation in hard rocks. We have also learnt that geologic nuclear waste disposal is a long process, striding over generations. Education and training of the next generation of specialists should be a day-to-day routine.

Acknowledgements

The methodological evolution and refinement of hardrock seismic imaging technology, as well as the accumulation of a vast body of knowledge deeply rooted in practical experience across scales, from kilometre to metre and beyond would have not been

Figure 6 Lower hemisphere, equal-area stereo projection at Olkiluoto. VSP reflectors (in red) and fracture (and foliation) orientation distributions (grey) from several boreholes KR4, KR10 and KR14 (a) and over depth ranges in one borehole, KR4, (b, c, d).

Figure 7 Example of 3D IP VSP migrations at Ignace, borehole IG-BH06: Eastward line (left), Northward line (middle), top view of the cylindrically parameterised 3D migration volume (right). (Enescu, 2023).

possible without the long-term commitment and vision of several groups of specialists from Posiva, SKB, Andra, NAGRA and NWMO. We are grateful for having the opportunity to contribute to their programmes and collaborate with all of them.

References

Cosma C., Heikkinen, P. and Konuainaho, K. [1986]. Overview of borehole seismic methods and their application to the characterization of fractures. Rock mechanics symposium.

Cosma, C. [1995]. Characterization of subsurface structures by remote sensing. Proceedings of the International Congress on Rock Mechanics, ISRM, 3, 1013-1021.

Cosma, C. and Heikkinen, P. [1996a]. Seismic Investigations for the final disposal of spent nuclear fuel in Finland. Journal of Applied Geophysics 35, 151-157.

Cosma, C., Heikkinen, P., Honkanen, S. and Keskinen, J. [1996b]. VSP-survey at Olkiluoto in Eurajoki, borehole OL-KR8 and extended parts of boreholes OL-KR2 and OL-KR4. Posiva Work report 96-11e

Cosma, C. and Enescu, N. [2002]. Multi-azimuth VSP methods for fractured rock characterization: North American Rock Mechanics Symposium, 5th Workshop, International Society for Rock Mechanics, Expanded Abstracts, 54-60.

Cosma, C., Heikkinen, P. and Keskinen, J. [2003]. Multi-azimuth VSP for rock characterisation of deep nuclear waste disposal sites in Finland. SEG Hardrock Seismic Exploration, edited by D. W. Eaton, B. Milkereit and M. H. Salisbury, ISBN 0-931830-41-9.

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Cosma, C., Enescu, N., Kemppainen, K., Ahokas, T. and Heikkinen, E. [2007]. Seismic prediction of hardrock fault zones and confirmation by

Figure 8 Multi-lines migrations of the 2D seismic data at Ignace (top: Enescu, 2022); View of seismic reflection images and borehole traces through the Revell site. Interpolated upper and lower domain boundaries of the larger rock unit domain (RU3b) containing mafic dykes, as observed in drill core and seismic reflection data (middle: Villamizar, 2023); Interpreted surfaces derived from surface seismic data (green surfaces), and VSP reflection surfaces (blue surfaces). Surfaces are interpreted to represent individual or clusters of mafic dykes. NW view (bottom: Villamizar, 2023).

tunnel observations. Proc. 7th International Workshop on the Application of Geophysics to Rock Engineering.

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Insights gained from two decades of seismic reflection profiling for mineral exploration in Finland

Suvi Heinonen1* and Viveka Laakso2 provide an overview of hardrock seismic mineral exploration in Finland, where the potential of new critical raw material discoveries is high and several mines are located, including the only operating chromium and phosphorus mines in the European Union.

Abstract

In Fennoscandia, the era of easy-to-find outcropping ore deposits discoveries has ended, while enhanced exploration success is required to meet the climate and digital objectives of modern society. This prompts the geoscientific community to develop novel technologies for deep subsurface imaging. Seismic reflection methods, adapted from hydrocarbon exploration to crystalline bedrock, offer superior resolution at depth for imaging complex geology typical for hardrock formations hosting mineralisation. In Finland, crustal scale seismic images provided new insights into the Precambrian bedrock, laying a foundation for mineral system approach in exploration. High-resolution seismic reflection profiles have demonstrated their utility in identifying rock interfaces and subsurface structures in important mining and mineral exploration districts including volcanogenic multimetal massive sulphides as well as mafic intrusions hosting Ni-Cu-PGE and Chromium deposits in Fennoscandia. Drill hole data and petrophysical measurements complement seismic surveys by offering a solid basis for interpretation. Seismic data integrated with geological information enable the creation of 3D geological models that improve the understanding of mineral systems, thus enabling efficient exploration strategies.

Introduction

Critical raw materials are essential for modern societies, particularly for renewable energy production, defence, and strategically important technologies. In Europe, heavy reliance on imports persists due to a lack of domestically producing mines and a poor success rate in mineral exploration. Fennoscandia, especially northern Finland, is renowned for its high mineral potential and operational mines. For instance, Kemi and Siilinjärvi Mines are the only chromium and phosphorus producers, respectively, within the European Union. The multimetal Kevitsa Mine supplies copper, nickel, cobalt, platinum, palladium, and gold to meet European needs. These deposits were discovered with the assistance of laymen, who submitted samples for detailed analysis to the Geological Survey of Finland (GTK). However, it is widely acknowledged that the era of easily discoverable outcropping ore deposits has ended, necessitating the development of novel

technologies for deep subsurface exploration to meet modern society’s climate and digital objectives. Moreover, discovering new deposits at depth near existing mines promotes responsible and sustainable mining by leveraging existing infrastructure and avoiding the need to develop areas with pristine nature. The seismic reflection method, adapted from hydrocarbon exploration, offers superior depth resolution to image the complex geology typical of hardrock formations hosting mineralisation.

1 University of Helsinki | 2 Geological Survey of Finland (GTK)

* Corresponding author, E-mail: suvi.heinonen@helsinki.fi

DOI: 10.3997/1365-2397.fb2024066

Figure 1 Hard rock seismic reflection surveys conducted over the past two decades overlaid on metallogenic areas in Finland. © Geological Survey of Finland.

From crustal scale to high-resolution seismic imaging

In the late 20th century and early 2000s, several crustal-scale seismic reflection experiments were conducted globally over crystalline bedrock areas, particularly in traditional mining countries such as Canada (Clowes, 1993) and Australia (Goleby et al., 1989). The FIRE (FInnish Reflection Experiment 2001-2005) seismic sections, depicted in Figure 1 with over 2000 line-kilometers plotted with metallogenic areas, provided a groundbreaking insight into the depths of Precambrian bedrock (Kukkonen and Lahtinen, 2006). The FIRE data were acquired with 50 m geophone and 100 m vibroseis source point spacing. The transects explored crustal structures down to a depth of 80 km, imaging the Moho boundary. The large-scale crustal architecture revealed by these deep seismic sections forms a robust foundation for a mineral system approach in exploration (e.g., McCuaig et al., 2010). In this approach, the origin of an ore deposit is considered within the framework of lithospheric-scale processes manifested by faults and different geological domains. The alteration processes leading to mineral deposition often change the physical properties of rocks, making them detectable by geophysical methods. Crustal-scale seismic reflection data can delineate potential source areas for mineral-carrying fluids, as well as fluid pathways and traps where mineralisation occurs through the chemical extraction of metals or other materials from the fluid.

Figure 2 illustrates a FIRE 3 transect extending over 130 km from the historical mining district of Outokumpu towards the East. Changes in the surface geology are distinctly observed west of the section, showing prominent reflections within the Outokumpu region characterised by mica schists and migmatites. In contrast, the upper crust to the east appears nearly transparent due to the similar acoustic impedances of granitoids and gneisses typical for that area. The Outokumpu district also exhibits reflectors terminating abruptly to a fault zone (marked with red arrows in Figure 2), which likely played a significant role in the deposition of the multimetal ores found in the mining district.

While the FIRE project was still ongoing, high-resolution seismic reflection profiles using a 12.5 m geophone spacing

were acquired in the Outokumpu mining district, famous for its multimetal sulphide deposits, and in the Suhanko layered mafic intrusion containing platinum group metals. These test surveys demonstrated that rock interfaces of interest for mineral exploration can be detected using the seismic reflection method. In the Outokumpu region, prominent reflectivity was attributed to ophiolite-derived rocks of the Outokumpu nappe, namely serpentinite, skarn rock, quartz rock, and black schist (Kukkonen et al., 2012). Strong reflectivity also originates from contacts between mafic sills within the felsic Archaean basement rocks. Later petrophysical studies, such as those from deep drill hole in Outokumpu (Kukkonen, 2011), further supported the use of seismic methods for mineral exploration. Inspired by results from FIRE, the HIRE (HIgh Resolution Reflection Seismics for Ore Exploration 2007-2010) program was launched, resulting in the acquisition of high-resolution seismic reflection data from 15 mineral exploration and mining areas, and providing reflection images of the geology at the Olkiluoto nuclear waste disposal site (Kukkonen et al., 2010). Since the HIRE experiment, more seismic data has been acquired for mineral potential mapping in multiple projects, including COGITO-MIN in Kylylahti in 2016 (Heinonen et al., 2019), XSoDEx in Sodankylä in 2017 (Niiranen et al., 2020), Smart Exploration in Siilinjärvi in 2018 (Malehmir et al., 2019), KOSE in Kuusamo in 2018 (Gislason et al., 2018), the Kevitsa Seismic survey in 2022 (Laakso et al., 2022), and SEEMS DEEP in Kuusamo in 2023 (Heinonen et al., 2023), with COGITO-MIN and SEEM DEEP also including sparse 3D surveys. Additionally, GTK has conducted hardrock seismic reflection surveys in Helsinki Central Park in 2019 (Cyz et al., 2022) and Pirkkala in 2023 (Koskela et al., 2024) to study subsurface structures for geoenergy development. The survey in Helsinki Central Park highlighted the challenges of urban data acquisition due to high noise levels from traffic and restrictions on receiver deployment and seismic source use near infrastructure. However, the survey was technically successful, and seismic data were utilised to identify large fracture zones and potential faults that affect drilling and influence heat production from wells.

Figure 2 The seismic reflection transect FIRE3 provides information about the crustal architecture of the historical mining region of Outokumpu. This mining district is characterised by strong reflectivity in the upper 15 km of the crust, which is seemingly absent in the Eastern Finland Complex, primarily consisting of granitoids and gneisses. Figure modified from Kukkonen and Lahtinen, 2006.

Figure 3 Above: Seismic reflection profiles acquired from Kevitsa plotted on an aerial photo. HIRE profiles were acquired in 2007, prior to the commencement of mining operations. Location of example shot gathers are marked with violet circles. Below: Perspective of the seismic reflection profiles HIRE E4 and Kevitsa 2022 showing prominent reflectors continuing over large volumes of bedrock. While many reflections originate from the mafic intrusion hosting the ore deposit (marked with yellow arrows), some are related to sedimentary lithologies not associated with mineralisation (marked with red arrows). Disruption of reflectivity can be correlated with a fault zone (dashed line). Drill hole data is required to confirm the source of a reflection and for reliable mapping of the subsurface. Geological map from © Geological Survey of Finland (Bedrock of Finland 1:200 000).

Reflection seismic images from ore districts

During the HIRE project in 2007, four intersecting seismic reflection profiles (Figure 3) were acquired at the Kevitsa Ni-CuPGE deposit using dynamite shots as a seismic source and a wired geophone system with 402 channels and 12.5 m receiver spacing (Koivisto et al., 2012). Each channel effectively consisted of a linear group of 6 geophones or 3 swamp geophones at each point. The survey was conducted in December under arctic winter conditions, before the mining infrastructure was built. Additionally, prior to the commencement of mining activities in 2010, an approximately 9 km² 3D seismic survey was conducted to target the expected volume of mineralisation (Malehmir, 2018). The low noise levels in these data make them exceptionally valuable for both maximising exploration efficiency and optimising mine planning as operations continue at depth.

When new seismic reflection data were acquired in Kevitsa in March 2022 southwest of the operating mine, noise from

pumps in the tailing ponds area partially masked subtle reflection signals (Figure 3). Similar to previous surveys, the 2022 data were acquired under arctic winter conditions over frozen swamps and snowy forests using explosives drilled to approximately 2 m depth as a seismic source. While the HIRE 2D survey in 2007 used linear geophone groups and 402 channels, in 2022 a nodal geophone system with a thousand receivers was employed. Figure 4 shows an example shot gather from both surveys. Geophone groups are efficient at suppressing noise caused by the source, namely surface and sound waves, but the seismic response is smeared over the linear array dimension. Single-point geophones lack the benefit of source-related noise attenuation, which may reduce the signal-to-noise ratio (S/N ratio). However, deploying single-point geophones is substantially faster and easier, and a large number of nodes allow for long offsets to be recorded and the acquisition of long profiles without rolling.

Despite the noise related to the mine operations, the 2022 Kevitsa sections contain abundant reflectivity marking abrupt changes in subsurface physical properties, and these reflectors can be easily correlated with the HIRE profiles that intersect the new section (Figure 3). Continuous reflections were first interpreted to originate from the mafic intrusion hosting mineralisation, but later drillings revealed that sedimentary rocks and volcanite contacts also cause reflections. Thrusting, faulting, and shearing have moved crustal blocks relative to each other, dividing the subsurface into distinctive domains. The example from Kevitsa demonstrates the usability of the seismic reflection method at all stages of the mine’s life, even though ideally data should be acquired before mining operations commence to minimise the decrease in signal-to-noise ratio caused by those operations.

Petrophysical measurements conducted in the laboratory or using drill hole logging sondes provide important background information about the potentially reflective rock contacts in the area of interest. Typically, sulphide ores have a high acoustic impedance, especially when the content of pyrite (8.0 km/s, 5.0 g/ cm³) is high, compared to the typical host rocks (about 6.0 km/s, 2.75 g/cm³) (Salisbury et al., 2000). The contrast in the physical properties causes a prominent reflection, but the direct detection of ore bodies using seismic profiling is inherently difficult, as the seismic profile would need to be nearly perfectly aligned directly over the ore body. In addition to the petrophysical properties, the geometry of the ore body must be favourable for detection. High acoustic impedances and sharp contacts between host rocks and ore deposits favour direct detection, but complex geometries may hinder it, as is the case with the Pyhäsalmi volcanogenic massive sulphide (VMS) deposit (Figure 5). Ore deposits hosted by mafic layered intrusions have a favourable geometry for detection, but often, instead of sharp contacts, there are gradual changes in petrophysical properties, resulting in the refraction and bending of seismic rays rather than prominent reflections.

In many locations, not only is the ore itself reflective against the host rock, but typical host rock units in contact with the surroundings may also cause detectable reflections (e.g., Heinonen et al., 2012 and 2013; Koivisto et al., 2012; Malehmir et al., 2017). Furthermore, lithological contacts with no direct relation to the mineralisation can be imaged, along with faults and fracture zones that are of primary interest for studies related to

geoenergy or infrastructure construction. For mineral exploration, creating a subsurface map — a vertical geological cross-section — is as valuable as geological mapping at the surface. By integrating intersecting seismic reflection profiles and geological cross-sections, a 3D geomodel visualising the specific mineral system can be generated. A robust 3D geomodel provides testable hypotheses for drilling.

Additionally, to Pyhäsalmi from where previous example was shown, seismic reflection profiles acquired in VMS exploration and mining areas in Finland include those at Vihanti, Outokumpu-Polvijärvi region, and Mullikkoräme in the vicinity of Pyhäsalmi (Figure 1). All these historical mining areas belong

to the Raahe-Ladoga belt hosting 90% of the known massive sulphide deposits in Finland. The seismic profiles provide detailed information about the subsurface structural geometries down to ca 5 km depth and crossing profiles lay foundations for 3D modelling of the survey areas. Subsurface 3D mapping of the folded and faulted volcanic strata based on the reflectivity characteristics is useful for strategic planning of deep exploration and improving the mineral system concept of the particular area. In Figure 6, the seismic reflection profile acquired from Vihanti shows prominent reflections (Heinonen, 2013). Abundance of drill hole data in the vicinity of the profile facilitate the detailed interpretation. Based on the correlation between the geological model derived

Figure 4 Example shot gathers from parallel HIRE Kevitsa E2 (2007) and Kevitsa 2022 surveys (see Figure 3 for source locations). Automatic gain control has been applied to both gathers for visualisation. The HIRE survey has a higher signal-to-noise ratio compared to the Kevitsa 2022 survey, which was conducted during active mining operations. However, the nodal wireless receiver deployment in the Kevitsa 2022 survey allowed the recording of longer offsets.

Figure 5 Cross-section of the Pyhäsalmi massive sulphide ore deposit overlain on top of the seismic reflection profiles (a-d). The bottom of the ore deposit is causing prominent reflections, but it would be difficult to recognise the bottom without any geological information due to the complex geometry (e). The sharp contact between the ore and host rocks (f), as well as seismic P-wave velocity and density measurements from drill holes, support the seismic detection of massive sulphides. Additionally, interfaces with mafic and other rocks typical of the Pyhäsalmi mining district can be mapped using seismic reflection methods based on their distinct physical properites (g).

from drill hole lithologies and seismic data, it is apparent that reflectivity arises both from lithological contacts and fracture zones. Comparison of reflections with different origin shows that the dominant frequency of fracture-originated reflections is lower compared to lithological, and also observed reflectivity is more discontinuous, possibly inherited from the nature of fracturing itself (Heinonen et al., 2013). Detailed interpretation of the seismic data remains challenging due to the polyphase deformation that the Precambrian bedrock has undergone over millions of years. Additionally, without complementary information from drilling or other geophysical methods it might remain unclear if a reflection originates from lithological contacts or if it is attributed to faulted, sheared, or otherwise fractured zones in the bedrock.

Hardrock insights gained and outlook to the future

Based on experiences gained over 20 years of hardrock mineral exploration, we summarise our gained insights as follows:

1) Seismic data acquisition is best done before the commencement of mining activities. This ensures accessibility for source and receiver deployment, enhances the signal-to-noise ratio, and optimises utilisation of seismic data for both exploration and mine planning purposes. The data can be revisited for different purposes during the life cycle of a mine site, making seismic data an investment with long-term benefits.

2) Seismic reflection data forms the skeleton of a 3D geomodel that facilitates understanding of a mineral system. Drill hole information and/or other geoscientific data are necessary for reliable interpretation of reflectors. In particular, petrophysical measurements are valuable for estimating which lithological contacts are reflective

3) In addition to mapping lithological changes, seismic reflection profiling provides valuable information about largescale geological structures such as faults. Direct detection of an ore body with 2D seismic data is challenging, and the method is best suited for supporting mineral system modelling and strategic planning of drilling programs.

Clearly, 3D seismic surveys are in many ways preferable to 2D. A limiting factor for using the 3D methodology is not only the substantially higher cost but also terrain accessibility, especially for seismic sources. Dense forest and swamp areas typical of Fennoscandia are often inaccessible even for a small drilling machine needed to make the holes for explosives. Compared to 3D surveys, a clear benefit of seismic profiling is that large areas can be covered cost-efficiently by utilising the existing network of forest roads. Development within receiver and seismic source manufacturing has made the seismic reflection method more affordable and attractive to the mineral industry. Small wireless seismic nodes are lighter to transport in difficult terrain typical of mineral exploration sites, and instrument pools, such as FINNSIP (www.finnsip.fi), enable development and testing of the methodology within the academic community. While the history of hardrock seismic investigations is built on crustal-scale transects, development continues towards less penetration depth but within more favourable survey locations and higher resolution by utilising small seismic sources and dense arrays. Nodal receivers and accelerated weight drops or electric vibroseis systems could be key to successfully mapping

Figure 6 Seismic reflection profile acquired in Vihanti shows prominent reflections. Based on drill core and logging data, the reflection dipping towards right between CMP 1100 and 1250 is caused by lithological changes while reflections dipping towards left at CMP 1300-1450 are caused by fracturing in the bedrock.

mineral-prone areas down to depths of 800-1500 m, providing sufficient data for detailed 3D geomodelling of the subsurface down to depths most relevant for mineral exploration and mining.

Acknowledgements

Acquisition of seismic reflection data is a collaborative effort that relies on diverse expertise. We extend our sincere thanks to everyone who have contributed to the seismic projects in Finland, whether through permitting, instrument deployment, first break picking, or scientific publishing. We acknowledge the funding support provided for the COGITO-MIN project through The Third ERA-MIN Joint Call 2015, SEEMS DEEP through ERAMIN3 Joint Call 2021, and Smart Exploration through H2020 (SC5-13c-2016-2017). The GLOBE Claritas and Geoscience Analyst software were used for data visualisation.

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Applied shallow geophysics (seismic and electrical resistivity imaging) to geotechnical foundation design (Central Texas, USA)

Hector R. Hinojosa1* and Jorge E. Rangel1 demonstrate how combining near-surface seismic and electrical resistivity imaging with geotechnical drilling allowed a more proactive and rapid site assessment approach that optimises foundation conditions and reduces geotechnical risks.

Introduction

Karst terrain is a natural landscape where either surface or groundwater slowly dissolves the underground or exposed soluble rocks like carbonates (limestone, dolomite) and evaporites (gypsum, halite, anhydrite), leaving karstic features prone to becoming geohazards as well as water reservoirs. Karst features critical to urbanisation and planning include sinkholes, cavities, caves, and opened-dissolution fractures. Underexplored or unexplored karst landscapes may hide unknown geohazards often discovered during construction or utility work, leading to unexpected setbacks, extra costs, or even loss of lives worldwide (Gutiérrez et al., 2014). Karst systems can be highly complex, heterogeneous, and erratic due to the range of geological and hydrological controlling factors. Thereafter, engineering works may find competent, solid karst rock for founding a structure at a site but challenging and hidden shallow conditions in an immediate location within the same geological setting (Parise et al., 2015). Residential and commercial urbanisation in central Texas has developed in ground conditions dominated by karstic carbonate rocks of the Edwards Plateau (EP) and expansive clays. Near-surface cavities or weak

ground may pose severe hazards to human safety and urban infrastructure, especially in highly urbanised population centres. In some urban areas of the EP, sinkholes, cavities, and caves have been encountered during early and advanced construction. Hence, in such areas, the physical and mechanical characterisation of the surface and subsurface geology is imperative before designing and constructing new buildings or structures. The location and depth of subsurface voids, the estimation of their size, and the evaluation of the overburden thickness are necessary to assessing the risk of collapse. The integration of seismic refraction tomography (SRT), electrical resistivity imaging (ERI), and geotechnical drilling was pivotal for the site characterisation of a lot awaited residential development in the city of Lago Vista, Texas, US (Figure 1).

Regional and site geology

The project site is located immediately west of the southern edge of the northern segment of the Edwards Aquifer Recharge and Transition Zones (Jones, 2019), comprised of carbonate rocks of the eastern EP and affected by the seismically inactive

1 Cordillera Geo-Services

* Corresponding author, E-mail: hector@cordillerageo.com

DOI: 10.3997/1365-2397.fb2024067

Figure 1 The project site (yellow triangle) is in Lago Vista, Texas, northwest of Austin (aerial photograph from Google Earth).

Balcones Fault Zone (BFZ). Capped by Cretaceous limestones, the EP is one of the largest continuous karst areas in the United States, according to the Texas Speleological Society (TSS, 2020). The EP is a broad, gently rolling upland locally incised by moderate-size streams and a few other perennial streams (TSS, 2020). Local streams entrench the plateau to ~565 m in 24 km. The upper drainages of streams are waterless draws that open into box canyons where springs provide permanently flowing water. Stream erosion of the fault escarpment sculpts the Hill Country from the cities of Waco to Del Rio. Sinkholes commonly dot the limestone terrain and connect with a network of caverns, according to the Texas Bureau of Economic Geology (BEG, 1996). The resistant rocks at the plateau’s surface are thin- to thick-bedded Cretaceous limestone with some dolomite and dip very gently to the south and southeast. Fractures are likely related to regional uplift rather than local faulting and folding. Many of the caves, some quite extensive, are confined to small vertical intervals, either by lithology of the confining beds or by still-stands of nearby rivers (TSS, 2020).

The BFS is a curved belt of major SW-NE-trending normal faults bounding the eastern and southern EP (BEG, 1996; Ferrill et al., 2004, 2019). The BFZ is an arc-shaped succession of high-angle, en-echelon normal faults that spans much of central Texas from Del Rio to Dallas. Its width ranges between 10 and 60 km and trends eastwards in Uvalde County to Bexar County and San Antonio. From there, it trends northwards to Travis County and northeastward in Williamson and Bell Counties. It is a dominant structural feature controlling the topography of the region and the Edwards Aquifer’s hydrogeology, including the San Antonio, Barton Springs, and Northern segments

(Ferrill et al., 2004, 2019; Schindel, 2019; Green et al., 2019; Jones, 2019).

Across the northern segment of the Edwards Aquifer, normal faults of the BFZ displace Cretaceous limestone, dolomite, marl, and shale that represent more than 610 m of a shelf and shelf-margin deposition (Collins and Raney, 2002). Most of the normal faults in this structural system strike N-NE and are high-angle (40°-80°), dip-slip normal faults generally downthrown to the east and southeast and have throws between ~244 m to < 0.30 m (Collins et al., 2002; Collins and Raney, 2002). The faults control the structural position of the porous limestone units of the Edwards Aquifer, and they bound much of the recharge zone/ outcrop band. Normal faults serve as conduits for groundwater flow, and at some locations, faults may displace porous beds against relatively less porous beds, causing abrupt changes in groundwater flow paths (Collins et al., 2002; Collins and Raney, 2002). In this aquifer segment, joints and fractures are parallel and linked to the BFZ system, increasing the permeability and dissolution of the underlying limestone strata and, thus, local and regional groundwater flow patterns (Collins, 1987; Jones, 2003, 2019). In addition, karstification has produced highly permeable pathways for groundwater flow and the formation of many caves and springs (Jones, 2019) and shallow sinkholes throughout the region. Figure 2 shows the regional geology of the area.

The site’s geology consists of a thin soil veneer deposited over exposed, weathered bedrock. The bedrock comprises Upper Glen Rose Limestone rocks of Cretaceous age. These rocks are in the Trinity Group, part of the Comanchean Series. According to Barnes (1981), the Glen Rose Formation comprises limestone, dolomite, and marlstones in alternating resistant and recessive

Figure 2 Regional geology around the project site (yellow pin) (after Stoeser et al., 2005). In the city of Lago Vista, the site is located west of the Balcones Fault Zone (black solid lines). Carbonate rocks dominate the rocks (blue) in Lago Vista.

beds, forming stairstep topography, and is divided into lower and upper members. The limestone is aphanitic to fine-grained, hard to soft and marly, light-grey to yellowish-grey. The dolomite is fine-grained, porous, yellowish-brown. Marine fossils include molluscs, rudists, oysters, and echinoids. The upper part thickens about 67 m, is relatively thinner bedded, more dolomitic, and less fossiliferous than the lower part, which is ~50 m thick and more massive.

The stratigraphic thickness of the Glen Rose Formation in the east reaches about 268 m and feathers out westwards towards the igneous-metamorphic Llano Uplift region. Figure 3A-3D illustrates a typical road outcrop of the Upper Glen Rose Formation in Lago Vista and the geologic map of the project site and vicinity. The outcrops in Figure 3A-3B are located about 3 and 12 km east of the site, respectively. The outcrops shown in Figure 3C-3D are only about 300 m west of the project site. One NS−trending, E−dipping normal fault runs about 1.6 km west of the project site. Field observations along road cuts and outcrops reveal the presence of shallow sinkholes and erosion of the intercalating marl units of the Glen Rose Formation. Rocks of the Travis Peak Formation and the Glen Rose Formation occur within Lago Vista. However, the Upper Member of the Glen Rose Formation rocks are the most dominant ones and occur in the inner parts of the city. Rocks of the Lower Member of the Glen Rose Formation and the Hensell Sand and Cow Creek Limestone

of the Travis Peak Formation occur outwards of the city, as shown in Figure 3C. Figure 4 depicts a geological cross-section made from publicly available water well reports from the Texas Water Development Board (https://www3.twdb.texas.gov/apps/ waterdatainteractive/groundwaterdataviewer).

Geophyiscal-geotechnical site characterisastion campaign

Two-dimensional (2D) seismic imaging with compressional P-waves (Vp) and horizontally polarised shear-waves (SH or Vs), as well as electrical resistivity imaging, are commonly used for subsurface site characterisation. In particular, Vp and Vs seismic velocities are quantitative parameters that describe the dynamic elastic properties of subsurface earth materials (e.g., soils and rocks). For the case of electrical resistivity applied to geotechnical parameter characterisation, it can also be done on the surface in 2D (Sudha et al., 2009) or three-dimensions (3D) (Osinowo and Falufosi, 2018), between boreholes (e.g., electrical resistivity tomography), or in a surface-to-borehole setting. In any of these instances, basic geotechnical parameters, such as soil moisture content, grain size of geomaterial, density (bulk and dry), porosity, void ratio, and Atterberg limit, hold a relationship to the electrical resistivity value measured in the field, following Abidin et al. (2014). However, Vp and Vs provide a better means of estimating rock strength. In most cases, the use of geological

Figure 3 Field photographs of the Upper Glen Rose Formation near the project site in Lago Vista, Texas, show the alternating pattern of resistive limestone and recessive marls (A-B) and within 150 m from the site showing thickly-bedded limestone (C) and with karstification (D). The geological map of the study area and vicinity (E). An NS−trending, E−dipping normal fault occurs about 1.6 km west of the project site. Map source: https://txpub.usgs.gov/txgeology/.

logs from geotechnical drilling is paramount for calibrating the seismic velocities and resistivity values.

There are several land-based and borehole-based methods for estimating 2D seismic velocity models from Vp and Vs seismic waves, though land-based methods are non-invasive, cheaper, and faster compared to borehole methods. Conventional land-based seismic refraction methods, including the intercept time method (Gurvich, 1972), the generalised reciprocal method (Palmer, 1981), and the plus-minus method (Hagedoorn, 1959), provide a simplifying rough estimate of the subsurface velocity and structure. However, they fail in the presence of substantial lateral velocity variations. On the other hand, land-based Vp and Vs seismic refraction tomography may give more detailed depth models even in the presence of complex velocity structures. In particular, Vs measured with land-based Vs refraction and Vs reflection surveys and with both down-hole and cross-hole Vs surveys tend to be challenging and costly compared to carrying a seismic surface wave survey, which is regarded as easy, cheaper, and practical, as a surrogate of an SH-wave velocity survey.

While Vs surveys yield a lesser weak body wave content (low signal-to-noise ratio), a highly sensitive data acquisition, and a lengthy and complicated data analysis, surface-wave surveys yield a stronger surface wave content (high signal-to-noise ratio), highly tolerant data acquisition, and a simple and effective data analysis. A common and successful surface-wave method is the Multi-Analysis of Surface Waves (MASW). However, for 2D surveys it requires plenty of space so that the geophone spread can be deployed and then displaced laterally to achieve its profiling capability. Due to the inherent strong signals of surface waves in shot records and providing a fast and convenient way to evaluate soil stiffness even in urban environments, MASW has been increasingly used (Yordkayhun et al., 2014; Abdel-Gowad et al., 2018; Abbas and Abdelgowad, 2024). According to Yordkayhun et al. (2014), pitfalls in Vs determination from SH-refraction data are hidden layers and statics problems, mode conversion of waves, the accuracy of picking first arrivals, setting up a reasonable initial model, and stability of inversion. The pitfalls in Vs determination from MASW data are interference of random noise, lack of the low frequencies surface wave, accuracy of picking dispersion curve, setting up a reasonable initial model, and

Figure 4 Geological cross-section based on selected publicly available water well reports from the Texas Water Development Board. The water wells are shown above the geological cross-section.

stability of inversion. Nonetheless, when the site conditions are critical and demand state-of-the-science measurements or when monetary budgets are not an issue, more sophisticated geophysical-geotechnical site characterisation can be implemented using a combination of surface-to-borehole methods, including surface SH-wave refraction tomography or 2D MASW, coupled with crosshole P-wave and SH-wave seismic and geotechnical drilling data. The works of Whiteley (2012) in Australia, Yordkayhun et al. (2014) in Thailand, Intriago-Álvarez et al. (2022) in Ecuador are true-case examples of these multi-method surface-to-borehole surveys used worldwide.

Field observations and previous geological studies show that the project area may yield lateral velocity variations due to fracturing, lateral facies change, karstification, and geotechnical characteristics variation in the subsurface. Because space was the major limitation within this project site due to an adjacent house and two roads, MASW was not the seismic method of choice for determining Vs. In addition, any borehole-based seismic or electrical resistivity deployment was rejected due to its higher cost. In a 1320 m2 surface area (40 m long × 33 m wide), the deployed multi-method shallow subsurface site characterisation comprised a 2D Vp and Vs seismic refraction tomography (SRT), 2D electrical resistivity imaging, geotechnical drilling, with standard penetration tests (SPT) carried out at 30 cm interval with rock core recuperation and one very shallow hand-dug excavation pit, all shown in Figure 5. The position and orientation of the ERI and SRT profiles were different in sampling as many areas as possible. A topographic correction was required for both ERI and SRT data inversion due to the uneven topography and the northward slope gradient within the property. This multi-method approach was adopted because it satisfies several site-specific needs, including assistance of geological mapping, top-to-bedrock mapping, detection of faults and karst conditions, evaluating subsurface conditions around sinkholes, cavity detection, and providing soil-strength estimates for building foundations (cf. Steeples, 2001). Similar multi-method approaches have been implemented successfully in karstic terrains worldwide (Nordiana et al., 2012; Abdel-Aati and Shabaan, 2013; Hinojosa-Prieto and Hinzen, 2015). Nordiana et al. (2012) used traditional 2D seismic refraction, 2D

ERI, and geotechnical borehole logs for site characterisation in Malaysia. In Egypt, shallow traditional seismic refraction was used to identify karstic features below the overburden, structures such as sinkholes, cavities, faults, and pinnacled rock heads, and depth to bedrock in a mature karstic limestone terrain (Abdel-Aati and Shabaan, 2013). However, these workers used traditional seismic refraction of P-waves instead of SRT of both P-waves and SH-waves. Hinojosa-Prieto and Hinzen (2015) used Vp and Vs SRT with geological bore logs for site characterisation in the Argive Basin of Peloponnese, Greece, and a follow-up study with Transient-Electromagnetic (TEM) soundings with the same geological bore logs database (Hinojosa-Prieto e t al., 2021).

Geophysical data acquisition and inversion

2D Seismic Refraction Tomography (SRT)

Vp and Vs data were measured along two perpendicular SRT transects, namely SRT-L1 and SRT-L2 (Figure 5). Due to space limitations, each seismic transect used 12 vertical and 12 horizontal 4.5-Hertz geophones for Vp and Vs data acquisition, respectively, spaced constantly at 1.5 m intervals, resulting in 16.5 m-long transects. P-waves were generated by vertically striking a metal plate with a sledgehammer. SH waves were generated by striking the ends of an SH-wave aluminum source (for SRT-L1) and the ends of a wooden beam (for SRT-L2) laid perpendicular to the horizontal geophone spread with the same hammer. Five vertical stacks (or hammer blows) were done at each shot point location to enhance the signal-to-noise ratio.

SRT-L1 and SRT-L2 used five and four seismic shots spaced at 6 m, respectively. For both datasets, the record length was 500 ms, with a sampling interval of 0.25 ms. The Vp and Vs SRT data were measured with the SUMMIT X One seismograph by DMT. The geographic coordinates and elevation of the geophones along each profile were measured with the high-precision Reach RS2+ RTK (Real-Time Kinematic) GPS (global positioning system)

Figure 5 Map of the geophysical survey deployment and geotechnical drillhole and exploratory pit. The SRT lines are SRT-L1 and SRT-L2. The ERI lines are ERI-L1 and ERI-L2. The site’s surface area is 1320 m2 (40 m long × 33 m wide). The geotechnical drillhole was drilled over the SRT-L1 transect. The asphalt roads bound the northern and southern sides of the site.

instrument by EMLID. Figure 6 shows field photographs of these instruments. The seismic records were processed by manually picking the first arrival travel times using the Rayfract® software (v. 4.02) by Intelligent Resources Inc. The Vp and Vs models were generated using the wavepath eikonal tomography (WET) method with a smoothed initial model with Rayfract®

2D Electrical Resistivity Imaging (ERI)

2D ERI measurements were performed with two oblique transects, namely ERI-L1 and ERI-L2 (Figure 5). Each transect was deployed with 28 stainless-steel electrodes spaced constantly at 2.0 m intervals, yielding a total profile length of 54.0 m. ERI data were acquired in resistivity mode using multiple electrode arrays, including the dipole-dipole, dipole-gradient, edge-gradient, and Wenner-Schlumberger. The ERI data was measured with the SuperSting R8/IP/SP resistivity meter by AGI (Advanced Geosciences, Inc.). The geographic coordinates and elevation of the electrodes along each profile were measured with the Reach RS2+ RTK GPS. Figure 6 shows field photographs of these instruments. ERI data processing included merging the electrode array raw data files into a single raw data file and its corresponding terrain file for 2D inversion with the Earthlmager2D® software (v. 2.4.4) by AGI. Both resistivity profiles’ inversion-stopping criteria were based on a root-mean-square (RMS) percent error of 3%. The smooth inversion algorithm was used. The finite element method was chosen for the forward modelling with two mesh divisions (cells) between a pair of electrodes, translating into a 1 m element size. Topographic correction with the damped mesh transforms the method with a constant slope boundary topography applied for the 2D inversion.

Results and interpretation

Results of the Vp and Vs SRT and ERI inversions and the geologic cross-sections constructed from the available geological logs were constrained by our field observations at the outcrops

and exposures in the area and ground truthing by geotechnical drilling and the excavation pit.

2D Seismic Refraction Tomography (SRT)

Figure 7 shows the results of the Vp and Vs SRT survey, with the same Vp and Vs velocity range and colour scheme used in both profiles. The inverted Vp SRT-L1 and SRT-L2 profiles were imaged down to 10.0 and 6.0 m, respectively. The inverted Vs SRT-L1 and SRT-L2 profiles were imaged down to 8.0 and 5.0 m, respectively. SRT-L1 and SRT-L2 profiles’ Vp varies from 200 to 5200 m/s and 200 to 3500 m/s, respectively. In general, the uppermost 5 m comprises shallow hard-to-very hard weathered-to-less weathered limestone bedrock. The Vp is

slower in the W-E−trending SRT-L2 profile, with a Vp between 200 to 1550 m/s in the uppermost 5.0 m. Hence, following Table 1, Vp ≥ 1550 m/s marks the start of competent limestone bedrock in both SRT profiles. The profiles SRT-L1 and SRT-L2 yield Vs velocities varying from 120 to 2125 m/s and 250 to 1500 m/s, respectively. Following Table 1, the Vs < 650 m/s is considered highly weathered limestone rock. Hence, competent limestone bedrock is reached at Vs ≥ 650 m/s. However, the depth of this Vs threshold is not the same in both profiles: 3.0 to 4.0 m in SRT-L1 and ~1.0 to 2.0 m in SRT-L2. This suggests that the depth to competent bedrock is somewhat irregular. The SRT results detect a continuous interval of limestone rock mass without underground cavities, sinkholes, or geological faults are

Figure 6 Field photographs of the seismic, electrical resistivity, and RTK GPS instrumentation used in the shallow geophysical engineering survey. All vertical and horizontal geophones, P-wave and SH-wave seismic shots, and electrodes were georeferenced.

Figure 7 2D seismic refraction tomography profiles of Vp and Vs data from SRT-L1 (upper row) and SRT-L2 (lower row). The increasingly downward seismic velocities suggest a ~10 m thick hard-to-very hard, weathered limestone of the Upper Glen Rose Formation, whose mechanical strength also increases downwards. The geotechnical drillhole is plotted on the Vp and Vs SRT-L1 profile.

Rock competency

Stiff to very stiff decomposed limestone 200-600 < 325 Soft, very porous, highly weathered limestone rock 600-1,550 325-650

Hard, weathered limestone rock. 1,550-3,000 650-1,500 Less weathered, very hard limestone > 3,000 > 1,500

Table 1 Seismic refraction tomography of Vp and Vs data interpretation and summary from seismic profiles SRT-L1 and SRT-L2. The local limestone bedrock is very shallow and presents an increasing downward competency.

not detected. Table 1 summarises the interpretation of the Vp and Vs SRT results.

2D Electrical Resistivity Imaging (ERI)

Figure 8 shows the resulting 2D inverted electrical resistivity models for ERI-L1 and ERI-L2 profiles. The same resistivity range and colour scheme are used in both profiles, yielding resistivities from 40 to ~350 Ω-m. The low-resistivity range suggests a ~16 m thick weathered limestone of the Upper Glen Rose Formation. The weathering is non-uniform, with less weathered hard rock sections generally surrounded by softer rock and saprolite (Figure 8), as shown by the shades of green-yellow-red, indicating relatively more competent or less weathered limestone bedrock. The shades of purple-to-blue highlight less competent limestone. A few sub-vertical narrow fractures are interpreted in both profiles at 40 to 41 m distance, suggesting that a fracture zone runs through the subsurface across the northern side of the property. The 2D inverted resistivity models do not yield evidence of either underground dissolution cavities, sinkholes, or geological faults within the surveyed site.

Geotechnical drilling and exploratory pit

After completing the geophysical survey, a vertical 4.6 m deep, 5.0 cm diameter exploratory geotechnical borehole sampled the thickest and most weathered rock for direct subsurface sampling and examination. In addition to the geotechnical borehole, a 0.50 m deep exploratory pit was excavated very close to the drill hole to analyse the presence of saprolites and regoliths associated with the weathered shallow limestone. Figure 5 illustrates the location of the geotechnical borehole and the exploratory pit relative to the geophysical survey layout. The borehole and the exploratory pit reveal the following subsurface conditions:

• 0.0 to 0.45 m depth: soft, highly weathered, fractured, moist limestone.

• 0.45 to 4.6 m depth: hard, highly weathered, fractured, moist limestone.

• The limestone corresponds to the Upper Glen Rose Formation.

Geotechnical foundation design from geophysical and drilling data

The use of P and SH (Vs) wave refraction techniques allows for the obtaining of many soil engineering properties at very low strains. In determining bearing capacity values, near-surface seismic methods consider the dynamic elastic behaviour of undisturbed soil (low strain), rendering higher values than those obtained by static invasive and destructive methods like drilling and lab tests. The different material mechanical properties associated with the local stratigraphy determined in this site investigation were calculated using the geophysical results. The measured Vp and Vs wave velocities are significantly related to earth material types, rock weathering conditions, excavation methods, and mechanical properties as, among others, dynamic elastic

Figure 8 ERI-L1 (upper) and ERI-L2 (lower) profiles correspond to the resulting 2D inverted electrical resistivity models. The low-resistivity range suggests a ~16 m thick weathered limestone of the Upper Glen Rose Formation. Shades of green-to-red (220-340 Ω-m) indicate hard weathered limestone. Shades of blue-topurple (40-200 Ω-m) correspond to highly weathered soft limestone. Both profiles yield a sub-vertical narrow fracture at a 40 to 41 m distance, suggesting that a fracture zone passes through the site.

properties (bulk density, Poisson’s ratio, Young’s modulus, shear modulus, and bulk modulus). Dynamic soil properties combined with measured Vp and Vs wave velocities can be used to calculate several geotechnical parameters as the number of corrected blows (N60) vs. depth, stiffness, and competence, bearing capacity, modulus of subgrade vertical reaction, unconfined compression (qc), ultimate shear (cu) pertaining to different identified earth materials shown in the SRT Vp and Vs profiles. For this geotechnical site investigation, the quantitative procedures shown

Figure 9 Field photographs of the 4.6m-deep geotechnical borehole and the exploratory pit relative to the geophysical deployment. The drill hole was positioned over the thickest and most weathered limestone detected with the geophysical survey. Dynamic Elastic Property Formula

Compressional wave velocity: Vp from Figure 7 (left side)

in Table 2 were used to calculate the different identified material dynamics and geotechnical properties. Table 3 shows the results of these calculations. The calculated dynamic elastic properties and geotechnical properties of subsurface materials allow for the recommendation of geotechnical design foundations. Based on the local stratigraphy’s geotechnical properties, stepped concrete slabs or mat foundations are recommended for residential home foundations. The foundation grades could be constructed as shown in Figure 10.

measured in the field

Shear wave velocity: Vs from Figure 7 (right side) measured in the field

Material density: ρ

Poisson’s ratio: υ υ = (Vs/Vp)2 = (1- υ)/(0.5- υ)

Shear Modulus: G G = ρ × Vs2

Young’s Modulus: E G = E/[2×(1+ υ)]

Standard Penetration Test (SPT) N60 = SPT corrected values

Relation between N60 and Vs Vs = 50×

Geotechnical Property

Modulus of vertical subgrade reaction: Ks

B = Foundation width

Ks = E/[B×(1- υ 2)]

Ks = 40×qu

Ks Units: KN/m3; Ks = 12×qu with Ks in Kip/ft3

Table 2 Summary of dynamic and geotechnical properties calculated from the measured Vp and Vs SRT survey.

Stokoe and Santamarina, 2000

Bowles, 1996

Braja, 1993

Anbazhagan and Sitharam, 2006

Sivakuvan and Braja, 2010

Bowels, 1996

g = earth gravity aceleration; r = material density; gd = Dry Unit Weight, w = Moisture Contet; Vp = Compressional Wave Velocity,Vs = Shear Wave Velocity, u = Poisson Ratio, G = Shear Modulus; E = Young’s Modulus, G = N60 = Corrected SPT Value

Table 3 Calculated dynamic and geotechnical properties of subsurface materials.

Table 4 indicates the corresponding allowable bearing capacities in lb/ft2 and KN/m2 units and corresponding Ks (Foundation Vertical Reaction Modulus) vs. Foundation Width. Additional values of allowable bearing capacities are included for foundation depths at 0.45 m, with the highest result of 2883 Kpa, well below the presumptive bearing capacity of hard-weathered limestone of 5000 Kpa, indicating conservative obtained values for bearing capacities. Considering the purposes of this geotechnical site investigation for foundation design for a home, having a total weight in the range of 200-350 tons, it is not necessary to consider higher bearing capacity values over 422 Kpa; hence, a safety factor of 7 is appropriate.

The site’s geology consists of a thin soil veneer deposited over exposed weathered limestone bedrock. Up to at least 4.6 m

Figure 10 A field photograph showing the site’s north-dppping ground surface (top). The generalised geological cross-section illustrates the areas of limestone removal for the construction of foundation grades (bottom). The uppermost 0.45 m of the ground surface is highly weathered limestone, and anything below is less weathered, slightly fractured, very hard limestone. The most economical way of constructing the foundation grades is to use stepped concrete slabs.

depth, sizeable underground dissolution cavities or sinkholes associated with karst areas in limestone rocks were not detected within the limits of the site. Based on the observed geotechnical properties of local stratigraphy, a shallow foundation consisting of a stepped concrete slab or mat is recommended for the future house’s foundation. It is recommended that a minimum foundation depth of no less than 0.30 m and, if possible, to use the indicated allowable bearing capacity values (qa) = 0.378 Kips/ft2 (18.09 KN/m2) for slabs foundations of B = 35 m (114.80 ft) to spread foundations of a minimum width of 0.45 m (1.5 ft) equal to 6.659 Kips/ft2 (311.08 KN/m2) since at that minimum depth any foundation grade would be on hard rock.

Rock cuts for the construction of the foundation grade must be sloped in the interval of 1H:1V to 1V:0.57H and heights great-

Table 4 Corresponding allowable bearing capacities in lb/ft 2 and corresponding Ks (Foundation Vertical Reaction Modulus) vs. Foundation Width.

er than 0.50 m. Also, the main home floor can be extended using reinforced concrete columns and beams or girders, supported on stepped strips or spread foundations, but this alternative could be a lot more expensive and represent longer construction times. Foundation grades must be very well horizontally levelled and should be adapted to the existing site’s natural relief to avoid excessive excavation volumes in hard rock. Concrete slabs or mat foundations must be supported on a granular subbase and bases constructed over a well-conformed subgrade free of loose soil. The recommended subbase should have a minimum thickness of 7.6 cm and be comprised of 2.0 cm crushed stone. It should be very well compacted with vibratory medium-weight smooth drums. The recommended base should have a minimum thickness equal to 5.0 cm of 2.0 cm crushed stone with 20-30% fines (passing No.100 sieve), very well compacted using vibratory medium-weight smooth drums. All the concrete slab foundations must be reinforced with top and bottom steel mats.

Conclusion

Combining near-surface seismic and electrical resistivity imaging with geotechnical drilling allowed for a more proactive and rapid site assessment approach and optimal foundation conditions, potentially reducing geotechnical risks and construction costs significantly. This combined approach revealed a shallow, hard-to-

very-hard weathered limestone site that is adequate and safe for a residential building foundation despite the site’s nearby karstic features. 2D ERI and Vp and Vs SRT were pivotal in exploring the presence of subsurface geohazards and in providing complete in-situ dynamic elastic testing on rock strength and quality. The 2D ERI survey was complementary to the seismic survey and together corroborated the absence of problematic karstic features.

From the Vp and Vs seismic velocities, dynamic elastic soil properties were extracted to compute the geotechnical parameters of subsurface materials effectively without the need for numerous rock coring sites and costly geotechnical sampling. Vp and Vs SRT provide higher resolution than traditional refraction surveys of subsurface structures, which were helpful in identifying horizontal and vertical variations in soil properties to determine conservative bearing capacities better, offering complementary advantages for depth penetration and ease of interpretation. Combining both seismic attributes enhanced the accuracy and reliability of this geotechnical site investigation, enabling the obtaining of several soil engineering properties at very low strains. In determining bearing capacity values, non-destructive land seismic methods rely on the dynamic behaviour of subsurface earth materials, rendering higher values than those obtained by static destructive methods like drilling and lab tests. In particular, Vs SRT can detect the anisotropic properties of materials, which

is essential for the accurate assessment of bearing capacities in layered or heterogeneous soils.

Acknowledgements

The work in this project was for the private sector, so the details of the geographic coordinates remain confidential. The authors thank the client for allowing the publication of this paper.

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TECHNICAL PROGRAMME Includes Oral and Poster Presentations

WORKSHOP AND PANEL DISCUSSION

SOCIAL PROGRAMME

2-5 SEPTEMBER 2024 I OSLO, NORWAY

NETWORKING OPPORTUNITIES

Smart Exploration Research Centre: Knowledge and innovation for exploration of critical raw materials

Alireza Malehmir1*, Magdalena Markovic1, Myrto Papadopoulou1, Karin Högdahl1, Maria Ask1, Maria Strømme1, Iain Pitcairn2, Tina Martin3, Thomas Zack4, Jaroslaw Majka1, Mats Svensson 5 and Ronne Hamerslag 6 demonstrate how the new centre in Sweden connects exploration with mineral processing and nanotechnology to enhance environmental studies and develop effective extraction and beneficiation methods.

Short summary

In response to the rising needs for long-term research and innovation in the field of critical raw material exploration, the Smart Exploration Research Centre was established in 2024 in Sweden. Funded by the Swedish Foundation for Strategic Research (SSF), this initiative involves collaboration among academic institutions, industry, and the public sector. Building on the H2020-funded Smart Exploration project, which involved 27 European organisations including the European Association of Geoscientists and Engineers (EAGE), the centre aims to advance the global standing of Sweden’s exploration. It seeks to gather skills and create a network that will leave a lasting legacy in the field of mineral exploration. The multidisciplinary centre aims to be a fast-track hub for addressing exploration challenges in the mining industry through synergistic efforts. It connects exploration with mineral processing and nanotechnology to enhance environmental studies and develop effective extraction and beneficiation methods.

Introduction

e.g., green steel industry, and the policies around CRMs (e.g., supply risks for car, battery, supplier, and energy companies) have forced many countries to reassess their resources and pay more attention to innovation and on addressing environmental-social-governance (ESG) issues already at the exploration stage. Therefore, ample opportunity exists for new players, companies, solutions, and competences for the exploration of CRMs in Sweden and worldwide.

Although we focus in Sweden in this article, the situation likely applies to a number of other European countries. Sweden has a suitable geology (pink regions in Figure 1), with several CRM deposits already identified from Archean to Phanerozoic rocks. Significant knowledge gaps, however, exist: (a) on fertility indicators, source rocks, carriers (e.g., Vanadium in black shales) and barriers, (b) emplacement of alkaline rocks and carbonatite systems, (c) limited knowledge on lithospheric structures (e.g., for magmatic Ni-Cu deposits), (d) link between green-tech solutions and society, (e) rapid response to immediate exploration challenges; e.g., what is critical today, might not be tomorrow and alternative sources might be in demand, (f) unpreparedness to educate the 21st century generation of exploration staff; new generations want to gain skills to become entrepreneurs, to generate wealth and make our society sustainable, (g) lack of global (international players and skills) and national competitiveness (incentives and broader competences).

Climate action requires an accelerated energy transition for decarbonisation and carbon neutrality, which in turn requires access to minerals (e.g., Ali et al., 2017), in particular the critical minerals and strategic metals, here collectively referred to as CRMs. Sweden was once world-leading in mineral exploration and innovation (e.g., frontier on geophysical systems; Malmqvist and Parasnis, 1972; Pedersen et al., 2009), but subsequently directed the focus towards exploitation and less on research and innovation in mineral exploration. As a result, Sweden, and to a greater extent Europe, is now a follower in exploration solutions rather than a leader. To ensure sustainable access to CRMs, focused knowledge centres involving broader expertise and institutions are needed. Governments and mining companies should invest in short and longer-term research, development and innovation work. The accelerated push for green technologies, fossil-free and/or carbon neutral extractive 1 Uppsala University | 2 Stockholm University | 3 Lund University | 4 University of Gothenburg | 5 Tyréns | 6 Nordic Iron Ore * Corresponding author, E-mail: alireza.malehmir@geo.uu.se DOI: 10.3997/1365-2397.fb2024068

Smart Exploration Research Centre (Figure 2) was established at Uppsala University following the success of the EU-wide H2020-funded Smart Exploration project (Malehmir et al., 2020a) in the beginning of 2024 in order to address longterm and continuous research filling these knowledge gaps by developing new science and technologies for innovative exploration of CRMs. It aims to serve Sweden with new exploration solutions and become a global centre, providing high-quality science and research, attracting mining companies, and drawing in young professionals. It will employ more than 30 MSc, PhD and post-doctoral researchers. To ensure long-term sustainability,

Figure 1 Commodities and regions targeted (A to D in the inset Figure) in the centre, compared with the EU CRMs 2020 list (an updated list slightly grown for more commodities from 2023 is now also available). Pink regions in the inset Figure are identified as prospective. Lithospheric transects are shown as black and orange lines (BABEL, Fennolora, Uppland and FIRE). BABEL offshore line 4 (marked as red) and a laser topographic map (LiDAR) of the Blötberget are shown later in the article.

tenure-track lecturer positions will also be created in exploration geophysics and economic geology.

Legacy from Smart Exploration and the near-surface geoscience community

Smart Exploration name and legacy comes from the H2020 initiative where 27 partners from nine European countries (Figure 3) took part in a research-innovation action project focusing on four key elements: (1) reviving and showcasing the value of legacy data, (2) developing and showcasing new geophysical prototypes, (3) developing new solutions for improved targeting, and finally (4) educating a new generation of young professionals with an entrepreneurship mindset (Malehmir et al., 2020b and references therein). Thanks to the Near-Surface Geoscience community, the project was put together through a number of European Association of Geoscientists and Engineers (EAGE) Near-Surface Geoscience (NSG) events and EAGE played a major role in reaching out to the stockholders and for the dissemination and exploitation of the results. The project resulted in five prototypes, six software solutions and educated more than 30 young professionals. As a highlight, the prototypes included: (1) a GPS-time transmitter for denied environment, (2) UAV-based magnetic and electromagnetic platform (Bastani et al., 2020), (3) deep-probing helicopter-based time-domain electromagnetic system, (4) electric-based broadband seismic vibrator (Brodic et al., 2021), (5) modular and digital-based slimhole geophysical system. At

the Ludvika Mines in central Sweden, the project findings let to the suggestion of an additional 10 Mt inferred resources at depth untapped for iron-oxide mineralisation and associated rare earth elements (REEs), as well as improved geological understating of the host rock and role of fault systems in the 3D configuration of the deposits (Markovic et al., 2020; Malehmir et al., 2021).

Given the success of the H2020-funded Smart Exploration project, it was logical to keep its legacy and continue with similar approaches to tackle the issue of CRM exploration through a dedicated and longer-term centre. A number of core partners of the centre took part in the European project, and the network was expanded in the centre to improve the expertise beyond geophysics and tech solutions involving also social sciences, IT, artificial intelligence, and nanotechnology.

From mineral systems to secondary resources

For many decades non-CRMs have been studied given their significance in the foundation of our past (and partly even today) industry. Thus it is of no surprise that there is a near-total lack of functional exploration vectors, for example for Lithium-Cesium-Tantalum (LCT) pegmatitic deposits, although Sweden has a suitable geology for them. Little is known about their source granites and why some granitic pegmatites are barren, while others are enriched. While Sweden was a pioneer in lithospheric-scale studies in the 1980s and 1990s (e.g., FENNOLORA and BABEL experiments, Figure 1) and high-profile science,

only sparse deep crustal-scale studies (e.g., Buntin et al., 2021) have since been conducted (compared with Canada, Australia, and even Finland) and those historical data have not even been properly revisited.

Some of these data (e.g., northern BABEL lines in Figure 1) cross major tectonic boundaries, like those between the Archean and Paleoproterozoic lithospheres of the Fennoscandian Shield. BABEL line 4 legacy data acquired in 1989 show a deep set of reflections interpreted as a remnant of oceanic crust (a fossil slab) extending down to 22-24 s below the Moho and this together with other evidence were used to suggest plate tectonic processes during the Paleoproterozoic time (BABEL Working Group, 1991). We have recovered these historical and legacy data for reprocessing and will use them to serve the centre for improved understating of mineral systems (combined with new deep-probing onshore data planned to be acquired).

Some of the major igneous provinces occur near the Paleoproterozoic and Archean boundary and show a good potential

for Ni-Cu mineralisation. Nordic countries are already a hub for mega-battery factories, hence attractive for major companies to invest in Cu and Ni, which are considered strategic metals for the entire EU. Tier 1 and 2 magmatic Ni-Cu deposits may be present in the so-called nickel line near this plate boundary. Exploration strategies, however, require a good understating of Li and Ni-fertility, the presence of pegmatitic and magmatic rocks, footprints on the lithospheric scale and, if one can fingerprint their origin at depth, and what sort of magma fractionation is required. Deep, high-resolution, long transects (+600 km) across these major plate boundaries onshore are required to map the Moho topography (e.g., up-doming or crustal keels) and lithospheric structures (velocity inversion, melt regions and any hint on a possible discontinuity and fluid migration path towards the surface) and are prioritised, as well as revisiting of the northern BABEL offshore seismic profiles. There is a great scope for cross-country collaboration and synergetic work among various fields of geosciences and the centre will serve as a platform for this cross fertilisation.

Figure 2 The overall structure of the Smart Exploration Research Centre includes four thematic hubs: green sensing technologies, mineral systems, data analytics, and geomodels. The green sensing technologies hub focuses on developing new prototypes and hardware solutions. The mineral systems hub studies deeper structures of mineral-endowed regions in search for regional footprints and connects them to depositscale ore genesis. The data analytics hub examines fingerprints and patterns of mineralisation, including target generation and ranking. The geomodels hub develops plugins and add-on software solutions. All hubs share a common goal of targeting across various scales. In addition to the thematic hubs, the centre will conduct ESG and social licence to operate (SLO) pilot studies. It will also provide seed funding through pitches and accelerator programs to ensure a dynamic working environment and staying at a frontier position.

Figure 3 The H2020-funded Smart Exploration project paved the way for the establishment of the research centre. The key ingredients of the project were giving a focus to legacy data, new prototypes and algorithms, and educating a new generation of young professionals with an entrepreneurial mindset working together with the mining companies and tech solution providers.

Europe and in particular Sweden has had many historical mines. According to the UNEXMiN website (accessed in 2024) over 30,000 abandoned mines are present in Europe of which many still contain CRMs. Some mines closed due to the technological limitations, others due to financial issues related to increased global iron ore production. In Sweden, the drop in iron ore price led to the closure of numerous iron ore mines, particularly in central Sweden, where iron making laid the foundation of today’s Swedish industry and wealth. Many of these iron-oxide deposits contain phosphorous (in the form of apatite and other phosphate minerals), which often contain some quantity of REEs. Phosphate minerals can be extracted in a reversed floatation procedure. However, they often end up in mine tailings. Consequently, some mine tailings in Sweden contain substantial amounts of iron oxide and phosphate min-

erals, both containing light and heavy REEs, which are crucial for high-tech applications, super magnets, and several other green technologies (e.g., wind mills, turbines, among others). In Sweden alone, over 30-50 historical mine tailings are estimated to be present. Given centuries of mining and mine tailings, not much is documented and conducted on these mine tailings. They can potentially be repurposed as secondary resources and a cost-efficient way of extracting CRMs since much of mineral processing work down to likely mineral liberation has already been done. Figure 5 shows a set of historical mine tailings observed on a LiDAR map of the Blötberget iron-oxide mine. There is a great scope to develop new geochemical, geomechanical, geophysical solutions for characterisation, separation and extraction of REEs from these tailing materials. The Blötberget mine tailings were partly covered by a sparse 3D seismic dataset

Figure 4 Recovered BABEL 4 offshore line (acquired in 1989) processed poststack for coherency enhancement showing a clear down-going set of reflectivity offsetting the Moho discontinuity for 5-7 km. This feature and similar ones observed along the line are interpreted as a remnant of oceanic crust (subduction) during the Paleoproterozoic time. Interestingly, in this context, the Skellefteå massive-sulphide-endowed district appears as a back-arc rift geological setting consistent with the petrogenesis of the volcanic rocks. A key problem of these legacy data is the lack of good images on the top 3-4 s, or approximately 10-12 km, implying that it is difficult to directly link uppercrustal geology with deeper features. Thanks to over 30 years of progress and seismic imaging capabilities, we are confident these parts of the section will be better imaged for improved mineral systems understanding and exploration decision making.

Figure 5 LiDAR elevation map of the Blötberget mine in Central Sweden, a wellstudied hard rock seismic test site where historic mine tailings are clearly observed. The mine tailing materials were intersected by a sparse 3D seismic dataset allowing us to partly characterise them in terms of thickness and its dynamic geomechanical properties. Modified from Malehmir et al. (2021).

and through the refraction static solution and surface-wave analysis, we estimated approximately 40-50 m thick tailing materials and extremely slow P- and S-wave velocities on the order of 500-800 m/s and 300 m/s, respectively (Alofe, 2021; Papadopoulou, 2021). It is possible to combine new geophysical solutions such as distributed acoustic sensing (DAS) with UAV-based methods and combine them with novel active- and passive-seismic processing solutions to better characterise these mine tailings in order to address our needs for CRMs with sustainable and socially acceptable practices.

Addressing ESGs, SLOs and synergetic aspects

No exploration work can be successful and sustainable if the idea, process and outcome are not transparently communicated with the public and stakeholders. Since exploration is the first step of a longer process, it has to cover ESG issues and receive the acceptance of the society and the immediate community to ensure a smooth development of the mining projects. Exploration technologies have to be socially acceptable, environmentally friendly and yet effective for the target CMs. Young generations are more considerate about the environment. They also accept innovation, synergetic work involving various disciplines, from social sciences to tech industry, as long as a common language is found and spoken and pros and cons are thoroughly discussed. The mineral exploration industry has realised this and has made good progress. However, there is a good scope to learn from other domains (e.g., the built environment industry) to ensure our environment and living conditions are not sacrificed while we transition towards a clean and CO2 neutral society.

Acknowledgements

Smart Exploration Research Centre (www.smartexploration. se) is based at Uppsala University and involves core partners from University of Gothenburg, Lund University and Stockholm University, Nordic Iron Ore, Inmet Mining Sweden, Eurobattery Minerals, Amkvo, BitSimNow, Epiroc, Tyréns, and Samarkand2015. The centre is also supported by the geological surveys of Sweden (SGU), Finland (GTK), Norway (NGU), Denmark and Greenland (GEUS), Anglo-American and BHP for pilot studies, mobility, knowledge and data exchanges. The Smart Exploration Research Centre has received funding from the Swedish Foundation for Strategic Research (SSF) under grant agreement no. CMM22-0003. This is publication SE24-002.

References

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BABEL Working Group [1990]. Evidence for Early Proterozoic plate tectonics from seismic reflection profiles in the Baltic shield, Nature, 348, 3438.

Brodic, B., Malehmir, A., Pecheco, N., Juhlin, C., Carvalho, J., Dynesius, L., van den Berg, J., de Kunder, R., Donoso, G., Sjölund, T. and Araujo, V. [2021]. Innovative seismic imaging of VMS deposits, Neves-Corvo, Portugal — Part I: In-mine array. Geophysics, 86, B165–B179.

Buntin, S., Artemieva, I.A., Malehmir, A., Thybo, H., Malinowski, M., Högdahl, K., Janik, T. and Buske, S. [2021]. Long-lived Paleoproterozoic eclogitic lower crust. Nature Communications, 12, 6553.

Malehmir, A., Markovic, M., Marsden, P., Gil, A., Buske, S., Sito, L., Bäckström, E., Sadeghi, M. and Luth, S. [2021]. Sparse 3D reflection seismic survey for deep-targeting iron oxide deposits and their host rocks, Ludvika Mines, Sweden. Solid Earth, 12, 483-502.

Malehmir, A., Gisselø, P., Socco, L.V., Carvalho, J., Marsden, P., Onar Verboon, A. and Loska, M. [2020a]. Smart Exploration inspires innovative geophysical solutions for mineral exploration in Europe. First Break, 38(12), 51-56.

Malehmir, A., Gisselø, P., Socco, L.V., Carvalho, J., Marsden, P., Onar Verboon, A. and Loska, M., [2020b]. New ways of exploring the subsurface with Smart Exploration solutions. First Break, 38(8), p.77.

Malmqvist, D. and Parasnis, D.S. [1972]. Aitik: Geophysical documentation of a third-generation copper deposit in North Sweden. Geoexploration, 10, 149-200. https://doi.org/10.1016/0016-7142(72)90030-0.

Markovic, M., Maries, G., Malehmir, A., von Ketelholdt, J., Bäckström, E., Schön, M. and Marsden, P. [2020]. Deep reflection seismic imaging of iron-oxide deposits in the Ludvika mining area of central Sweden. Geophysical Prospecting, 68, 7-23.

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EDUCATION PACKAGES

Integrated structural health assessment of industrial buildings in areas of high seismic risk

Gwenola Michaud1*, Roberto Zamparo1 and Alessandro Brovelli1 present a study that aims to enhance understanding of structural behaviour under potential seismic loading conditions and to help in maintenance planning of critical infrastructures.

Abstract

In high seismic risk zones, comprehensive structural health assessments are crucial for critical buildings, for example those in the energy distribution networks. Recently, a feasibility analysis was conducted for a pressurised spherical tank for liquified gas storage, demonstrating the validity of structural monitoring using ambient noise. This work presents an extension of the previous work, applied to a different type of industrial building, composed of a steel frame and reinforced concrete. The method was extended, including a pushover analysis to identify critical conditions that may lead to failure. The objective of the work is to identify vulnerable structural components and to predict collapse mechanisms of a structure located in a seismic prone area. The methodology is based on the characterisation of the structure of the shallow subsoil to evaluate the site-specific seismic response and on the identification of the natural resonance modes of the buildings to assess the state of the structure. A combination of passive and active surveys is used to this end. A finite element model is then built and calibrated. Finally, a push-over analysis is carried out to estimate the capacity curve for the structure. Comparison between capacity curves and design spectra would provide insights into displacement demands and elastic behaviour, giving information for enhancing the resilience of the structure and estimating safety thresholds of earthquake intensity, expressed in terms of Peak Ground Acceleration (PGA). Integrating geophysics and engineering knowledge, this feasibility study aims to enhance understanding of structural behaviour under potential seismic loading conditions and to help in after maintenance planning of critical infrastructures.

Introduction

Maintenance of structure and infrastructure systems alongside safety assessment for communities are topical and central aspects of a sustainable territorial management in general, and in high-seismic areas in particular (Cimellaro et al., 2016; Troisi et al., 2021). In seismic-prone regions, the resilience of critical infrastructures requires rigorous structural health assessment and mitigation plans, to ensure prompt reactivations of essential services (e.g. water, electricity and gas distribution) major earthquakes. Recently, a feasibility study was conducted to use an array of tools from the geoscience and civil engineering

disciplines to study and monitor the integrity and resilience of a pressurised spherical tank used to store liquified petroleum gas (Brovelli et al., 2024). There are examples of collapsed pressurised spherical vessels during strong earthquakes, and the resulting damages have been extremely severe (Zama et al., 2012; Hatayama, 2015). Building upon the work of Brovelli et al., 2024, an extension of the proposed methodology is presented, applied to an industrial building composed of two sections with very different mechanical characteristics (hence resilience to earthquakes): a steel frame on one side, and reinforced concrete on the other side. The seismic vulnerability of a structure is controlled by its design and the material properties but also on the site characteristics and soil-structure interactions. Any monitoring methodology should therefore consider these three aspects. In addition, in areas with many critical buildings (e.g. refineries, storage sites, chemical factories etc), it is important to be able to perform an initial rapid screening and focus the more detailed investigations only on the structures that have shown higher vulnerability.

This manuscript presents the methodology: from the geophysical characterisation of the near surface up to the push-over analysis to estimate the limit peak ground acceleration that the building under investigation can sustain before undergoing failure. The objective is to provide better characterisation of the structural response, and to facilitate effective risk mitigation strategies (e.g. structural retrofitting), enhancing seismic resilience of essential industrial infrastructures.

Methodology

The methodology for data analysis and modelling includes the following steps:

1. Geophysical characterisation of the soil to estimate the shear wave velocity structure of the near surface and its lateral variability.

2. Ambient noise analyses to assess noise levels, frequency content and data quality during the survey period, as well as their temporal evolution.

3. Power spectral density analysis to identify frequency components which could be associated to modes of the structure.

4. Operational modal analysis (OMA) using a Frequency Domain Decomposition (FDD) algorithm to refine modal estimation.

1 ISAMGEO

* Corresponding author, E-mail: gwenola.michaud@isamgeo.com

DOI: 10.3997/1365-2397.fb2024069

5. Modelling by finite elements to reproduce natural modes using building design and expected material properties.

6. Push-over analysis to determine the elements of the structure most at risk of collapse of damage during a severe earthquake.

Soil characterisation is performed using multichannel analysis of surface waves (MASW) (Park et al., 1999; Foti et al., 2014) and direction horizontal/vertical spectral ratio (DHVSR) analysis (Havenith, 2004; Chen et al., 2020). The analysis of surface waves using multi-offset acquisition (MASW) is achieved through the deployment of geophones regularly spaced along a line and an active seismic source (seismic hammer) along the same line. The analysis is focused on the propagation of surface waves (Rayleigh waves), particularly on the recording of their vertical component of motion. The propagation velocity of Rayleigh waves is said to be dispersive, i.e. it depends on frequency. Specifically, lower frequencies, and consequently longer wavelengths, penetrate a greater thickness of soil from the surface. Consequently, lower-frequency surface waves, which affect deeper and generally more compact layers, tend to have a higher propagation velocity along the surface compared to higher-frequency waves that affect shallower subsurface layers. The analysis of seismic data in the frequency phase velocity domain provides an estimation of the dispersion curve, from which a 1D S-wave velocity profile can be obtained at the near surface. Repeating the same measurement along different directions and positions within the same site gives also an information about the lateral variability of soil. This is important as in industrial sites the soil conditions often change rapidly as a result of repeated excavations/refilling, hence the geotechnical characteristics can drastically differ from point to point. MASW is an active geophysical method, that can be used in conjunction with passive methodologies, such as (D)HVSR or ReMi (Refraction Microtremor). Focusing on DHVSR, it has the significant advantage that both acquisition and processing are extremely lightweight, and the survey can be quickly repeated on multiple locations, to characterise lateral variability. DHVSR analyses are performed using a single-station, 3-component sensor, which is left on site to record the ambient noise. The ratio between the spectra of the horizonal and vertical components provides information about the subsoil structure and shear wave velocity. The presence of peaks in the H/V spectra can be used to estimate the resonance frequency of the site (Havenith, 2004) while azimuthal variations may provide indications of dipping subsurface layers (Cheng et al., 2020).

The characterisation of the buildings is instead performed using OMA, which is a passive method that relies on ambient noise as the source of energy. In engineering, it is called an Output-Only measure, as the source (input) is not controlled. Using OMA, one can analyse the dynamic response of structures and its temporal evolution, with the underlying assumption that the ambient noise has similar amplitudes and spectral characteristics: in particular it is assumed to be broadband and white (Rainieri and Fabbrocino, 2014). Clearly this is seldom the case, and it is even more unlikely in an industrial context, where the variation of noise levels can be abrupt and content spectral. Due to the presence of engines and other sources of

mechanical and electromagnetic noise, the measured spectra are extremely complex to interpret. To partly address this issue, as a part of the proposed methodology it is suggested recording the ambient noise at the base of the building, so that the spurious peaks can be later deconvolved from the measurements. In other words, it is important to be able to discriminate between peaks (in the frequency domain) that are due to the noise itself from the eigenfrequencies of the structure. By using a sensor placed on the soil at the base of the structure, it is therefore possible to study the soil-structure interaction and how the soil vibrations affect the movement of the building.

It is important, when comparing different measurements (in space and or time), to also quantify the noise levels. Due to the stiffness of the structure or the sensitivity of the sensor, certain behaviours may be visible only when the noise level exceeds certain thresholds. To measure the noise level, the Root Mean Squared amplitude (RMS) of each receiver stack is computed on a sliding time window. Each point represents the average noise level at the corresponding time, with higher values indicating stronger seismic or vibration noise while lower values indicate quieter settings. For computation efficiency, following Parseval’s theorem, the root mean squared amplitude is calculated in frequency domain.

For OMA, sensors are placed on the structure, possibly near joints and other structural elements. Sensors are typically accelerometers, although our experience has shown that often velocimeters should be preferred, given the higher sensitivity. On the recordings, frequency domain analyses are carried out to assess how the structures respond to dynamic loads or vibrations. This analysis helps in defining the natural frequencies of the structure. This measurement is to be compared with the spectra at the base of the building to identify the peaks that started to appear (or are amplified) in the spectrum, which are likely to be related to the modes of vibration of the structure.

The Frequency Domain Decomposition (FDD) is performed as singular value decomposition on the energy cross-spectra. The resulting singular values are used to refine modes, where peaks can be identified with sufficient accuracy under reduced noise conditions. Each frequency represents a natural frequency of the structure, while the associated mode shape can show how the structure deforms or vibrates at that frequency. For a detailed description of OMA, see Rainieri and Fabbrocino, (2014). The algorithm used for this analysis is documented by Pasca et al. (2020).

The structural elements for the finite element modelling of the building structure are represented as one-dimensional elements for beams and columns, and two-dimensional plate elements for the reinforced concrete wall. The type of structure and its behaviour are defined by its floor load geometry and its characteristics. The wall infills are included by defining linear loads applied to the beams.

Once the most representative model is created, a nonlinear push-over analysis is conducted to estimate failure mechanisms and thresholds. The goal is to correlate the push-over analysis with the maximum displacements that building elements can sustain, and relate it to a measure of earthquake intensity, such as Peak Ground Acceleration (PGA). In this way, the operator of the site or building can set up appropriate responses to take in case a

seismic event exceeds the PGA limit. For the push-over analysis, taking response spectra from a regional database, the intersection of a radial line on the elastic portion of the capacity curve with the design spectrum provides estimation of the displacement demand (Fajfar, 2021). In this way, modelling can be done to determine if displacements remain within the elastic range or exceed it, still satisfying the displacement demand without reaching collapse.

Data acquisition summary

• Data acquisition is organised in phases following survey design as shown in Figure 1.

• Multichannel analysis of surface wave data, with 2 perpendicular acquisition lines following the directions of the structures.

• Site and structure survey to collect the necessary information for model construction, with position and geometry of key structural elements.

• Passive monitoring of the structure with one velocimeter deployed near the building and 3 to 6 accelerometers,

installed on the walls/structural elements of the building for at least a few days and up to one week. The accelerometers can be installed on the reinforced-concrete walls or the steel pillars. This is quite different from what is typically done in traditional modal analysis, where data acquisition lasts from a few (15) minutes to a few (2-3) hours. Acquisition of longer time-series does not increase the installation costs and allows us to (1) improve the signal to noise ratio through data staking and (2) select appropriate time windows for signal extraction. Ultimately, a longer time series makes the processing robust and reliable. This is particularly important as changes in eigenfrequencies can be subtle.

Data analysis

The analysis of the surface wave data as MASW gives a velocity profile for S-waves from the picking and the inversion of the dispersion curves of phase velocity versus frequency (Figure 2). This profile provides information on the very near surface up to 20-30 m, and characterises the near-surface condition where the structures are. The Vs30 profile, i.e. shear velocity profile in the top 30 m is a typical proxy for geotechnical characteristics.

The ambient noise analyses are done on each receiver, being accelerometer or velocimeter with the objective to quality-control each instrument and the data completeness during the data acquisition. In Figure 3, the root mean squared of each receiver stack is computed on a sliding time window of 30 min, every 15 min for the entire data acquisition period. The ambient noise varies as a function of activity change on site. For instance, days and nights are well marked, with a constant minimum amplitude floor for the entire survey. In addition, the morning and afternoon time frames are also consistent. Finally, smaller operational noise is observed during Sundays with fewer activities compared to the rest of the week.

The analysis in the frequency domain is made on each component of the accelerometers deployed on the reinforced-concrete walls or steel pillars of the structure (Figure 4). Some peaks of frequency are visible around 2 Hz, 5 Hz, between 6 and 7 Hz as well as around 8 Hz, 9 Hz and 10 Hz. The largest amplitudes on the power spectral density are usually on the inside accelerometers, installed on the steel frame and on the

Figure 2 Illustration of MASW analysis on shot gather (a), with dispersion curve picking on phase velocity versus frequency plot (b) and its inversion to obtain S-wave velocity profile (c).

Figure 1 Example of data acquisition geometry for a rectangular cuboid structure composed of steel frame and reinforced concrete part.

component perpendicular to the structure in grey and pink solid lines on Figure 4 (left side).

These peaks are also visible on the FDD results obtained considering all the accelerometers at once. On Figure 4 (right side) are plotted the 1st single value for 3 cases, considering:

• All components regardless their directions in black,

• The components along the transverse direction of the building in red,

• The components along the longitudinal direction of the building in green.

The frequency domain decomposition provides a way to ease and refine the identification of the potential natural modes. For instance, the largest peaks observed on the power spectral density around 2 Hz are not any more the largest on the frequency domain decomposition (Figure 4). Instead, the largest peaks are around 7 Hz, 8 Hz and 9 Hz. In addition, it is easier to visualise the poten-

Figure 3 Example of ambient noise analysis as RMS values estimated over a sliding time window of 30 min, every 15 min.

Figure 4 Frequency Domain Analysis on each component (left) and Frequency Domain Decomposition (right).

Figure 5 Comparison between observed peak of frequencies in PSD and FDD (bottom left) and power spectral density as a function of receiver positions inside the building on steel frame (top left), outside along walls (top right) and at outside corners (bottom right).

tial natural modes on the frequency domain decomposition: all peaks observed on the power spectral density can be seen on the frequency domain decomposition, but are seen on the frequency domain decomposition more clearly. The peak of frequencies obtained by the frequency domain decomposition can be seen on the power spectral density displayed as a function of receiver positions (Figure 5).

For the frequency domain decomposition, comparing the result obtained with all components versus with separate directions, i.e. longitudinal or transverse directions of the building, the provided curve with all components is the envelop of the curves obtained for the 2 distinct directions of the building (Figure 4). Considering components laying on the same direction eases the comparison with the modelling results given as a function of the structure directions. The outside receivers, deployed on the reinforced concrete present similar spectrograms with respect to the inside receivers, deployed on the steel frame. The building

vibrates differently as a function of the composition and geometry of the structure parts.

Finite element models and pushover analysis

The modelling of a building begins with the structural design and an onsite survey to understand the main elements of the building, including their dimensions, geometry, properties, and connections. This step is crucial for assessing the rigidity and strength of the various structural elements. In the finite element method (FEM), structural elements such as beams and columns are modelled with one-dimensional elements, while walls are modelled with two-dimensional elements. For this project, most common materials were assumed based on European standards for steel and concrete.

From Earthquake Hazard Map, lateral elastic design spectra are estimated for various types of earthquakes. These spectra are functions of ground motion levels, the probability of exceedance within 50 years, and the corresponding return periods (Table 1).

For each type of earthquake, a displacement shape is assumed, and the vertical distribution of lateral forces is determined for the structure. The relationship between base shear and top displacement is then defined, using displacement of a representative control node, typically located at the centroid position of the roof. After this estimation, the bilinear capacity curve is plotted against the design spectra for this location (Figure 6). At the end, the Peak Ground Acceleration (PGA) for the building is estimated as 40% of the scaled elastic design spectrum plateau.

The push-over analysis was conducted for the building, considering its distinct structure parts: the inside steel frame and the outside reinforced concrete behind a shear wall (Figure 7). These two parts are structurally independent of each other, without any rigid connections between them. For the steel part, it was not necessary to scale the spectrum to find the PGA for the transition

from the elastic to plastic field, as the capacity curve is well above the design spectrum, even in the worst case scenario of a DD-1 type of earthquake (Figure 7 Top). This part of the structure is expected to remain largely intact, even under the largest possible seismic event. However, damage to secondary parts and infill walls can still be expected.

For the reinforced concrete part, after scaling the design spectrum for a DD-1 type scenario, its plateau is estimated at a value of 1.40 g. The PGA, calculated by considering 40% of this value, results in 0.56 g.

Discussion

During this analysis, frequency domain decomposition was key to refining the individual peaks observed on the power spectral density. Generally, frequency domain decomposition presents clearer peaks in terms of amplitude and shape than those observed in the power spectral density. Exploring the frequency domain decomposition using components pointing in the same direction helps to refine data comparison and analysis. Additionally, decomposing the frequency domain into longitudinal and transverse directions eases the comparison with the finite-element modelling results, provided along those directions.

During this study, the focus was on the identification of the natural modes. The analysis can be further expanded also considering modal shapes. These give additional insights into the dynamic behaviour of the buildings, in particular its intrinsic damping, to better understand how the structure deforms or vibrations at those frequencies.

Conclusion

This paper presents the application of a monitoring method to assess the structural health of mission-critical industrial buildings in high seismic risk zones. This study is based on a methodology for comprehensive assessment, combining geophysical and engineering knowledge to broadly understand how a given structure on a specific site can sustain major earthquake.

Through a combination of passive and active surveys, the study effectively characterises the shallow subsoil and identifies natural resonance modes of the building. It provides insights into the structural integrity and potential vulnerabilities for a given building at a specific site. This work is a groundwork for informed maintenance planning and risk mitigation strategies to enhance the resilience of critical infrastructures and to ensure their continued functionality and safety in the face of natural hazards.

The thresholds of potential damage during a major earthquake are key to planning a structural health monitoring system for potential post-event evaluation. In particular, the plateau values can be compared with the measured values on building elements during earthquakes and the present natural modes with post-event modes.

In the data analysis, the identification of the frequency and mode peaks was seen as crucial to calibrate the model. Model calibration may require adjusting material properties and checking element geometry and properties. In this analysis, the comparison between observations and modelling was good, validating the monitoring method application in structural health assessment for such buildings and sites.

Table

Figure 6 Design spectrum and capacity curves with indication of how PGA is estimated as 40% of design spectrum plateau (From Fajfar, 2021).

Figure 7 Lateral Elastic Design curves for DD-3 in green, DD-2 in red, and DD-1 in grey, along with the bilinear curves for the metal part of the building (top) and reinforcedconcrete part (bottom).

For future projects, it would be interesting to apply this monitoring method to tall structures and to assess how the structural health evaluation varies as a function of receiver coverage following recently published work such as Aytulun and Soyoz (2023).

Acknowledgements

We would like to thank ISAMGEO for permission to publish this work. We acknowledge the use of the PyOMA code by Pasca et al. 2022 and sismicad for finite element analysis.

References

Aytulun, E. and Soyöz, S. [2023]. Structural condition assessment with structural health monitoring systems and nonlinear simplified models, Earthquake Engineering Struct Dyn. 2024, 1-24, DOI: 10.1002/ eqe.4132.

Brovelli, A., Ugur, F., Ozyurt, S., Fanizzi, S., Tazkir, A. and Michaud, G. [2024]. Feasibility Study of Structural Health Monitoring for Pressurized Spherical Tank in Liquified Petroleum Gas Storage, EAGE 2024 Conference.

Cheng, T., Cox, B.R., Vantassel, J.P. and Manuel, L. [2020]. A statistical approach to account for azimuthal variability in single-station HVSR measurements. Geophysical Journal International, 223(2), 10401053. https://doi.org/10.1093/gji/ggaa342.

Cimellaro , G.P., Noori, A.Z., Kammouh, O, Terzic, V. and Mahin, S.A. [2016]. Resilience of Critical Structures, Infrastructure, PEER Report No. 2016/08, Pacific Earthquake Engineering Research Center.

Fajfar, P. [2021]. The Story of the N2 Method, University of Ljubljana, Slovenia, International Association for Earthquake Engineering (IAEE).

Foti, S., Lai, C.G., Rix, G.J. and Strobbia, C. [2014]. Surface wave methods for near-surface site characterization, CRC press.

Hatayama, K. [2015]. Damage to oil storage tanks from the 2011 Mw 9.0 Tohoku-Oki tsunami. Earthquake Spectra, 31(2), 1103-1124. https:// doi.org/10.1193/050713eqs120m.

Park, C.B., Miller, R.D. and Xia, J. [1999]. Multichannel analysis of surface waves: Geophysics, 64

Pasca, D. P., Aloisio, A., Rosso, M.M. and Sotiropoulos, S. [2022]. PyOMA and PyOMA_GUI: A Python module and software for operational modal analysis. SoftwareX, 20, 101216. https://doi. org/10.1016/j.softx.2022.101216.

Rainieri, C. and Fabbrocino, G. [2014]. Operational modal analysis of civil engineering structures Springer, New York, , https://doi. org/10.1007/978-1-4939-0767-0.

Havenith, H.B. [2004]. Guidelines for the implementation of the H/V spectral ratio technique on ambient vibrations measurements, processing and interpretation. European Commission - Research General Directorate, 62, European Commission - Research General Directorate. Troisi, R., Castaldo, P. and Arena, L. [2021]. Maintenance Management of Infrastructure Systems: Organizational Factors in Territorial Planning IOP Conf. Ser.: Mater. Sci. Eng. 1203 032098.

Zama, S., Nishi, H., Hatayama, K., Yamada, M., Yoshihara, H. and Ogawa, Y. [2012]. On Damage of Oil Storage Tanks due to the 2011 off the Pacific Coast of Tohoku Earthquake (Mw9.0), Japan, Proceedings of the 15th World Conference on Earthquake Engineering (WCEE), Lisbon (PT).

Data acquisition and lessons learnt from geophysical Remotely Piloted Aircraft System (RPAS) surveys in northern Canada

Irina Nizkous1* and Ross Penner1 discuss data acquisition and lessons learnt during geophysical Remotely Piloted Aircraft System (RPAS) surveys in northern Canada, which include 12 major and 12 minor anomalies identified in the magnetometer data.

Abstract

This paper discusses data acquisition and lessons learnt during a geophysical Remotely Piloted Aircraft System (RPAS) survey in Northern Canada. The goal of the project was to identify areas that may have buried waste materials using a magnetometer attached to an RPAS. RPAS aeromagnetic surveys have a good coverage (and coverage rate) and high resolution compared to conventional walking terrestrial surveys (Everett 2007, Nieldzielski 2018 and Walter at al. 2019) especially in remote locations with variable terrains. The RPAS was able to cover an area of 55 hectares over two days of surveying. Twelve major and twelve minor anomalies were identified in the magnetometer data. Photogrammetry was also collected over a 315-hectare area. This included a high resolution ortho-mosaic as well as a digital terrain model and a digital surface model.

The RPAS magnetometer survey was highly successful at identifying areas with strong magnetic signatures as well as areas with weaker signals. The major anomalies identified all have very strong signals with the clear high and low pattern that is expected. The photogrammetry provided high-quality imagery of the area as well as surface models and greatly assisted in the interpretation of the magnetic signatures. RPAS surveys in northern parts of Canada have specific logistic and acquisition challenges that

affect the operation of the survey but do not affect the quality of the data.

Introduction

Geophysical methods and photogrammetry information were collected to help identify regions within three main sites (Figure 1) in Canada’s Northwest Territories that may have been used for waste disposal. The area of investigation is large as well as overgrown with vegetation, making it difficult to traverse by foot. For the data collection, a magnetometer was attached to a Remotely Piloted Aircraft System (RPAS). This method enabled data to be collected rapidly over a large area while also avoiding ground obstructions. All three sites have a complex topography with large changes in elevation and dense vegetation. This paper outlines data collection challenges as well as providing a discussion and interpretation of the results for a subsection of one of the main sites (Figure 1, the site located on the left).

Magnetic survey with RPAS platform

A standard magnetic survey will measure the magnitude of the magnetic field at the sensor. This will be a combination of the earth’s naturally generated magnetic field as well as any fields

1 DMT Geosciences Ltd

* Corresponding author, E-mail: Irina.Nizkous@DMT-Group.com

DOI: 10.3997/1365-2397.fb2024070

Figure 1 Satellite map of the area of investigation with the three major sites highlighted in blue.

induced in materials in the subsurface. Geophysical magnetic surveys look at the local variations in the magnetic field as these are the result of the fields induced in subsurface materials. These are used to identify and locate these materials.

There are two properties of materials that can cause local variations in the total magnetic field, remnant magnetism and the magnetic susceptibility. Materials with a high magnetic susceptibility will create their own significant magnetic fields when in the presence of other strong magnetic fields such as the earth’s magnetic field. Materials with remnant magnetism have strong magnetic fields even without the presence of external magnetic fields.

For landfill investigations, the high magnetic susceptibility of the ferrous materials that are often buried in landfills makes a magnetic survey an effective tool. Unfortunately, materials that can’t magnetise, like aluminum or concrete, are invisible to a magnetometer survey.

The data for these surveys is often collected by foot but by using a RPAS the data collection is faster while also avoiding obstacles on the ground. Figure 2 is a photo of a magnetic sensor attached to a RPAS while taking off.

The magnetometer is suspended from a cable that hangs 5 m below the RPAS. The RPAS must fly at an altitude to ensure that it doesn’t collide with any vegetation or structures. It also must fly high enough that it is visible to the operator at all times. For this survey, the RPAS was flown at an altitude of approximately 25 m and the magnetometer sensor was approximately 20 m off the ground.

The RPAS system will log the recorded magnetic field, the coordinates of the reading, and the altitude of the UAV sensor. A reference base station magnetometer was deployed at a fixed location on the ground to correct for natural diurnal changes in the earth’s magnetic field.

When magnetic fields are induced in ferrous materials, they create a dipole signal that has a rise in the field strength paired with a dip. The object or objects that create this magnetic field variation are found between the two poles. For objects with simple geometry, the direction of the dipole will align with the direction of the earth’s magnetic field. The strength of the field variation will increase with sensor proximity. The size, or

width of the field variation will decrease with sensor proximity. Because our sensor is being flown above the ground on a RPAS, we expect the magnitude of the signal to be weaker but broader, compared to a similar ground-based survey.

Photogrammetry

Photogrammetry is the collection and compilation of information about the environment by using a combination of photographs and videos. A RPAS mounted with a camera is flown along a preprogrammed flight path. Photos and videos are collected and their precise GPS position is logged.

After data collection is complete, the photos, videos and flight logs are combined to create multiple products. The first is an ortho-mosaic image, which is the combination of multiple photographs to create one combined georeferenced image. The second and third are a Digital Terrain Model (DTM) and a Digital Surface Model (DSM). The DSM is a model of the elevation in which all the surface features, including vegetation, are portrayed. The DTM is a model of the elevation of just the ground surface over the area of investigation.

Data survey challenges

1. Terrain mapping is critical for data acquisition success. The survey was completed by collecting data at a constant elevation but a flight plan that keeps a constant altitude about the ground is ideal. We did not have a digital terrain model at the time of surveying. We did develop a digital terrain model from the photogrammetry, but this is time consuming, and cannot be completed on site.

2. Geophysics RPAS is tricky as its payload must be slung below the RPAS to avoid interference from the drone itself. Because of this, the RPAS’s regular collision avoidance technologies don’t always work. The magnetometer needs to be as close to the ground as possible in order to get higher quality data therefore creating some risk of collision of the magnetic sensor with trees. This unfortunately occurred during our survey, resulting in an accident during data acquisition – the RPAS sensor caught a tree causing damage to both the magnetometer and the RPAS. For future surveys,

Figure 2 RPAS taking off with suspended magnetics sensor.

we will be collecting LiDAR data with RPAS to create an accurate Digital Terrain Model (DTM) prior to flying the magnetometer survey. Using this DTM, the RPAS operator can program in a terrain draping flight pattern that avoids trees or other obstacles while assuring the magnetometer can be flow as close to the ground as possible.

3. The data acquisition site is in northern Canada where magnetic inclination is very steep, so the position of the sensor hanging on the RPAS has to be adjusted, hanging at 70 degrees to the vertical. This ensures the maximum field amplitude and better signal to noise ratio. Figure 3 is showing magnetic inclination for North America in 2020.

4. Another challenge for northern work is battery issues. As the payload is heavy, large batteries are needed for the RPAS. For safety reasons these batteries can’t be shipped on standard passenger aircraft. For cargo air shipment the batteries have to be drained to 20% of capacity, and at the same time because of the remote site location the data acquisition requires multiple sets of batteries that have to be changed in sequence in the field. The need to use generator and frequently recharge batteries (as one surveying RPAS flight takes about 30 minutes) in the field make survey execution more time consuming and difficult.

5. Throughout the project, the weather provided another logistical challenge. The high winds limited the ability to fly the RPAS and collect data. In addition, the cold weather lowered the RPAS’s battery capacity and shortened the length of the flight times. Winds are particularly critical because of the payload hanging 5m below the drone. Although the drone itself may be rated for high winds, the actual safe flying wind velocity with the magnetometer payload is much lower.

6. Data acquisition in northern remote parts of Canada require wildlife monitor at all times. The sole job of the wildlife monitor is to watch for bears, cougars and other wildlife and protect the field crew from danger. This position requires a specially trained person with an understanding of wildlife behaviour.

7. The location of the magnetometer, 5m below the RPAS sometimes caused the sensor to sway when the RPAS veloc-

ity changed quickly. At these times the magnetometer would lose lock and there would be spikes in the data that needed to be removed. There was also position synchronisation issues because the magnetometer trailed behind the drone.

Survey results

Figures 4 and 5 show an example of the survey results and interpretation.

Figure 4 shows magnetometer data with anomaly interpretation and the colour grid. The choice of range for the colour grid affects what magnetic signatures are visible. A tighter range can make weaker signals more apparent whereas a wider range avoids the stronger signals clipping. This Figure shows the colour grid with a 100 nT range. Strong dipole features can be seen in this magnetometer data example, which indicates ferrous materials. The object or objects that create this magnetic field variation are found between the two poles. For objects with simple geometry, the direction of the dipole aligns with the direction of the Earth’s magnetic field. The strength of the field variation usually increases with sensor proximity. The size, or width of the field variation decreases with sensor proximity. Because our sensor is being flown above the ground on a RPAS, the magnitude of the signal is weaker but broader, compared to a similar ground-based survey.

Figure 5 shows the ortho-mosaic image of the area with overlaying magnetic field anomalies interpretation.

Interpretation

The magnetic field anomalies have been broken up into two categories: major anomalies and minor anomalies. Major anomalies have a strong response with a clear dipole feature. Minor anomalies will either have a weaker response or the dipole structure isn’t present or as clear. While both indicate the likely presence of magnetic material the major anomalies will be a more reliable target than the minor anomalies. Minor anomalies may also be related to geologic variations rather than debris.

All the anomalies are numbered and marked with an ellipse. The ellipse indicates uncertainty in location of the material suspected of causing the magnetic signal.

Figure 3 Magnetic inclination for North America 2020.

Figure 4 shows an example of major anomalies. There are three major anomalies with Anomaly 2 and Anomaly 3 have very clear highs and lows with the signal ranging between 300 nT and -125 nT. Anomaly 1 has a strong high, 100 nT, but the low did not have enough data coverage to be well defined.

The advantages of combining photogrammetry with the magnetometer data are seen in Figure 5, where the remains of a truck can be seen in the ortho-mosaic to the south east of Anomaly 2. This will have contributed to the magnetic signal at Anomaly 2 but there may be more unidentified materials.

Conclusions

This paper discusses the RPAS geophysical and photogrammetry surveys. The RPAS magnetometer survey was highly successful at identifying areas with strong magnetic signatures as well as areas with weaker signals. The major anomalies identified all have very strong signals with the clear high and low pattern that is expected. These areas are the most promising areas for investigation.

Figure 4 Magnetometer data: variation of the magnetic field with interpretation.

Figure 5 Photogrammetry data with overlaying magnetic field anomaly interpretation.

The photogrammetry provided high quality imagery of the area as well as surface models allowing better identification of the site buried materials.

RPAS surveying in northern Canada has specific data acquisition and logistic challenges. However, with care and understanding, these challenges can be mitigated and excellent quality data acquired.

References

Everett, M. [2013]. Near-Surface Applied Geophysics. Cambridge University Press, Cambridge.

Niedzielski, T. [2018]. Applications of Unmanned Aerial Vehicles in Geosciences: Introduction. Pure and Applied Geophysics, 175, 3141-3144.

Walter, C., Braun, A. and Fotopoulos, G. [2019]. High-resolution unmanned aerial vehicle aeromagnetic surveys for mineral exploration targets. Geophysical Prospecting , 68(1), 334349.

A joint analysis of Rayleigh and Love waves using MASW for site characterisation

Juan José Hellín-Rodríguez1, Pedro Martínez-Pagán2, Ignacio Valverde-Palacios1*, Antonio García-Jerez3, Koya Suto 4, Marcos Antonio Martínez-Segura2 and Koichi Hayashi 5 present the joint analysis of different and independent multi-component data based on Rayleigh and Love waves to obtain 1D Vs sections for site characterisation.

Abstract

The determination of reliable shear-wave velocity models using Multichannel Analysis of Surface Waves (MASW) has become more important for site characterisation studies due to their use in geotechnical studies and regulations. The standard MASW approach is commonly based on the analysis of vertical components of Rayleigh waves, which can result in inaccurate and potentially erroneous interpretations by personal bias. Thus, we present the joint analysis of different and independent multi-component data based on Rayleigh and Love waves to obtain 2D Vs sections for site characterisation. Those seismic data were recorded using a landstreamer consisting of 8 triaxial 4.5Hz geophones. To generate Rayleigh waves, the blows were given vertically on a plate, and for the Love waves the blows were given laterally on a horizontal wooden beam. A joint analysis of Rayleigh and Love waves data was conducted on seismic data recorded from the

metropolitan area of the city of Granada (Spain) to generate their dispersion curves. This new approach enabled a proper identification of fundamental- and higher-mode surface waves facilitating the reliable reconstruction of subsurface Vs profiles through a robust joint inversion process. The MASW 1D Vs versus depth models were corroborated at several test sites by the information obtained from boreholes. Thus, the main geological formations could be inferred from MASW 2D Vs sections down to a depth of 30 m, as well as the Vs30 parameter to perform a reliable seismic microsonation of the study area. This methodology provides a very well constrained inversion procedure capable of providing a robust subsurface Vs model for site characterisation.

Introduction

Site investigations necessitate the collaboration of various disciplines such as geologists, seismologists, and geotechnical and

Figure 1 Workflow for a geotechnical site investigation study (modified from Yilmaz, 2015).

1 University of Granada | 2 Universidad Politécnica de Cartagena | 3 Universidad de Almería

4 Terra Australis Geophysica Pty Ltd | 5 Kyoto University

* Corresponding author, E-mail: nachoval@ugr.es

DOI: 10.3997/1365-2397.fb2024071

earthquake engineers. The typical workflow for geotechnical site investigation studies is illustrated in Figure 1, aiming towards a geotechnical design (Yilmaz 2015). Figure 1 emphasises the importance of understanding the soil-column shear-wave velocity (Vs) in the design of civil engineering structures. Vs plays a crucial role in determining the mechanical properties of subsurface materials, ground amplification, and liquefaction potential during earthquake events (Martínez-Pagán et al. 2018; Duan et al. 2019; Aas and Sinha 2023; Suto 2023). Vs models are also utilised in seismic hazard assessment due to the varying ground amplification with shallow ground stiffness (Martínez-Pagán et al. 2018, Valverde-Palacios et al., 2014). International building committees like the European Committee for Standardization and the National Earthquake Hazards Reduction Program (USA) have adopted the Vs parameter, particularly the average Vs in the top 30 m (Vs30), as a fundamental ground parameter for structural design against earthquakes (López et al. 2022).

Therefore, it is essential to employ reliable approaches for developing Vs models. Currently, the primary methodologies for determining the subsurface Vs profile from surface seismic data include MASW (Multichannel Analysis of Surface Waves), ReMi (Refraction Microtremor), SPAC (Spatial Autocorrelation), MAAM (Miniature Array Analysis of Microtremors), and HVSR (Horizontal-to-Vertical Spectral Ratio) (Giancarlo Dal Moro 2020; Kumar, Satyannarayana, and Rajesh 2022). These methods typically involve recording seismic data using a set of vertical geophones

as a standard practice (Dal Moro 2020). However, these conventional methods only address a single observable, specifically the dispersion of the vertical Rayleigh wave component.

The objective of this study is to explore the simultaneous analysis of Rayleigh and Love waves through the application of the MASW method in order to address the issues of ambiguity and non-uniqueness in seismic data pertaining to subsurface shear-velocity models. By combining these two wave types, the research aims to improve the reliability of the inversion process and derive a more robust subsurface model essential for site characterisation.

Study area

The study was carried out in the towns of Fuente Vaqueros, Atarfe, and Santa Fe, which are part of the Metropolitan Area of Granada (MAG), Spain (Figure 2). These towns are situated on alluvial (Quaternary) soils consisting of clay, silt, and sand with some gravel, where the water table depth ranges from 2 to 8 m. The MAG is primarily a Quaternary plain located in the northeastern part of the Granada basin. The majority of the active faults in the Granada basin are concentrated in the eastern region between Sierra Elvira and Padul, passing through the Granada area. The MAG is recognised as the most seismically active region in Spain, with occurrences of liquefaction and ground settlement reported in specific areas during moderate (1806) and strong (1431) historical local earthquakes. Both historical

Figure 2 (A) General tectonic sketch of the Central and Eastern Betic Cordillera; (B) Geological and tectonic sketch of the Granada basin showing the main active faults, geological units, and epicentres of shallow instrumental (circles) and relevant historic earthquakes (stars); (C) Spatial location of soil units (zones and sub-zones) of the study area. Town boundaries are shown with a thin polygonal line; (D) Spatial distribution of water table depth.

and instrumental seismic data confirm its status as the most seismically active zone in Spain, leading to its classification as the most hazardous seismic zone in the Spanish Building Code.

The flat metropolitan area, covering a surface of nearly 900 km2, encompasses 32 small municipalities surrounding Granada city with a population of 525,000 inhabitants. Over the past five decades, the population of Granada and its metropolitan region has doubled. The urbanised land area has expanded by around 4650 hectares, with the most significant population and construction growth occurring within a 15 km radius of the city. Many of these towns are situated on thick, and soft soils, some of which with shallow groundwater tables. Moreover, this region is intersected by active seismic faults in which various phenomena associated with seismically induced ground liquefaction have been documented following intense and moderate historical earthquakes, such as ground settlement, lateral spreading, or foundation support failure leading to building damage (e.g., the 1431 and 1806 events). Accordingly, the assessment of these earthquake-induced hazards becomes crucial for earthquake risk mitigation measurements (Figure 3).

Methods

A multichannel analysis of surface waves (MASW) technique, which was developed and discussed by Park, Miller, and Xia (1999), to obtain 1D S-wave velocity profiles at various depths has been used. The MASW input data were gathered using a towed land-streamer equipped with a total of 8 4.5Hz triaxial geophones spaced 7 m apart (Figure 4a). This acquisition system, measuring 49 m in length, was moved 14 m between each shot. The recording device used was a 24-channel SUMMIT II Compact Seismograph manufactured by DMT, Germany.

After same preliminary tests the following acquisition parameters were chosen: a 14-m offset (distance between the seismic source impact point and the first geophone) to minimise nearsource effects, a 3-shot stacking, a distance of 14 m between recording stations, 1 ms of sampling interval, and 1 s of recording window. A 9 kg sledgehammer was used to generate both Rayleigh waves, blowing vertically on a plate; and Love waves, blowing laterally on a wooden beam (Figure 4b).

Seismic data joint analysis was carried out with open source Geopsy software for seismic component grouping of Rayleigh

Figure 3 Urban expansion of Granada city and its metropolitan area between 1956 and 2007.

Figure 4 Land-streamer, with 4.5Hz triaxial geophones, being towed through the Fuente Vaqueros streets (A); Sledgehammer-based seismic source for generating Rayleigh (plate), and Love (wooden beam) waves (B).

Example of a seismic dataset obtained at the Metropolitan area of Granada, corresponding to vertical component. Traces put in evidence the Rayleigh waves. The sampling interval was 1ms, with 1024 samples per trace, a source offset of 14 m, and 7 m of geophone spacing.

waves (vertical component or ZVF, which stands for vertical component and vertical force) and Love waves (horizontal component or THF, which stands for transversal component and horizontal force) (Figure 4); and winMASW® for data processing consisting of data filtering, and computing of the phase velocity spectra (i.e. the frequency-velocity matrix computed according to the phase-shift method discussed by Park et al. 1998; Dal Moro et al. 2003)). Finally, 1D/2D shear-wave velocity (Vs) models were obtained through mathematical joint inversion of dispersion curves retrieved from phase velocity spectra.

Seismic data were analysed jointly using the open-source Geopsy software to group the seismic components of Rayleigh waves (vertical component or ZVF, representing the vertical component and vertical force) and Love waves (horizontal component or THF, representing the transversal component and horizontal force) as shown in Figure 5. . The phase velocity spectra were computed based on the frequency-velocity matrix using the phase-shift method outlined by Park et al. (1998) and Dal Moro et al. (2003). Subsequently, 1D/2D shear-wave velocity (Vs)

models were derived through the mathematical joint inversion of dispersion curves obtained from the phase velocity spectra.

Results and discussion

Figure 6A depicts the phase-velocity spectrum obtained from a dataset of Rayleigh waves recorded in the vertical component of triaxial geophones and generated through vertical blows. It shows the picking undertaken (pink-colour dots) on the fundamental mode, and on the first higher mode. The retrieved portion of the dispersion curve of the fundamental mode starts at the point with phase velocity of 600 m/s and frequency of 8 Hz, which provides a depth of investigation of about 30 m (Martínez-Pagán et al. 2018), and the picking finishes at around 30 Hz, associated with a phase velocity of 200 m/s. Moreover, figure 5a provides the curve associated with the best (fittest) model shown by a blue line, and the curve associated with the mean model characterised by a green colour dashed line.

Figure 6B depicts the phase-velocity spectrum obtained from a dataset of Love waves recorded in the horizontal component of triaxial geophones and generated through horizontal blows. In the same way as previously discussed, it shows the picking undertaken (pink-colour dots) on the fundamental model, and on the first higher mode, which is the initial tentative dispersion curves to be used to generate the 1D shear-wave velocity for the horizontal component. The fundamental mode dispersion curve could be retrieved from the point with a phase velocity of 290 m/s and a frequency of 5 Hz, where lower frequencies are dominated by aliasing. In this example, the fundamental mode picking is extended to a frequency of 32 Hz.

The most suitable 1D shear-wave profiles were derived following an independent inversion process of the dispersion curves of the Rayleigh and Love waves. Each of the 1D shear-wave velocity profiles is characterised by a 8-layer model (Figure 7), which effectively represents the geological sequence identified in the study area through boreholes. Nevertheless, there is a noticeable discrepancy in the positioning of the layers between the two profiles. For instance, in Figure 7a, an intermediate layer with a lower Vs velocity of approximately 226 m/s is detected at a depth of 15 m, whereas in Figure 7b, the same layer is situated at a depth of 12 m. Furthermore, variations are observed at a depth of 30 m, where Figure 7a indicates a Vs value exceeding 700 m/s, while Figure 7b shows a Vs value below 500 m/s. It is important to highlight that the 1D shear-wave profile derived

Figure 5

Figure 6 Phase velocity spectrum of the vertical component (Rayleigh waves) with the best calculated dispersion curve (6A); Phase velocity spectrum of the horizontal component (Love waves) with the best calculated dispersion curve (6B).

and mean 1D

from the Rayleigh wave component serves as the primary outcome obtained from the MASW survey for site characterisation. Nonetheless, relying solely on this single observable may lead to concerns, as indicated by previous studies (Dal Moro and Keller 2013; Dal Moro 2020). In order to address the non-uniqueness of the solution and potential interpretative challenges, we have also taken into account the 1D shear-wave velocity profile obtained from the independent analysis of multi-component data, including Rayleigh and Love waves (Figure 8).

In this way, Figure 8 depicts the best model obtained using Rayleigh and Love waves, shown by a blue line, in which a

Figure 7 (a) Mean and the best 1D shear-wave velocity profile for the vertical component (Rayleigh waves); (b) Mean and the best 1D shear-wave velocity profile for the horizontal component (Love waves).

Figure 9 A comparative analysis was conducted on 1D shear-wave velocity models using joint inversion (Rayleigh + Love waves) and without joint inversion (only Rayleigh waves) in relation to the stratigraphic column from borehole FV-2, as well as its corrected SPT-N values.

depth of investigation of 30 m has been achieved. This joint 1D Vs model exhibits the intermediate layer of lower velocity at a depth of around 13 m. This layer is characterised by Vs values of 267 m/s. Then, the material is characterised by an increase in the Vs value of about 411 m/s associated with a horizon of gravels and sands.

The Vs30 value obtained from the 1D shear-wave velocity model using Rayleigh waves alone was 304 m/s, while the Vs30 value derived from the joint inversion model was slightly higher at 317 m/s. Despite this difference, both methods categorised the stratigraphic profile similarly as deposits of very dense or medium-dense sand, gravel, or stiff clay, in accordance with Eurocode 8 (Martínez-Pagán et al., 2014). The enhancement of the 1D shear-wave velocity profile through joint inversion of Rayleigh and Love waves was evident when compared to a nearby geological column and its corresponding corrected SPT N-value (Figure 9).

Figure 8 Minimum distance

shear-wave velocity profile obtained from joint inversion of the two observables/objects: the ZVF (Rayleigh waves), and THF components (Love waves).

Figure 9 illustrates the comparison between the two 1D shearwave velocity models and a stratigraphic column acquired from a nearby borehole, Borehole FV-2, along with its corresponding corrected SPT-N values. The data indicates a strong correlation between the 1D Vs model from joint inversion and the N-values. Specifically, as the N-values increase, the shear-wave velocity values also exhibit a similar upward trend, and vice-versa. For instance, the intermediate hard layer composed of gravels and sands with an SPT-N of 20 corresponds to an increase in Vs up to 411 m/s. Conversely, the adjacent layers consisting of lightbrown clay with or without gravels, which display decreasing SPT-N values of 14 and 11, respectively, are associated with a decline in Vs to 380 m/s and 328 m/s, respectively.

Conclusions

The comparison of the stratigraphic column and SPT-N values with the outcomes derived from the analysis of surface waves using multicomponent data (Rayleigh and Love waves) indicates an enhancement in the correlation when the velocity profile is acquired through joint inversion.

These findings propose a re-evaluation of the prevalent practice of conducting MASW surveys solely with the Rayleigh vertical component to mitigate potential uncertainties arising from the non-uniqueness of the final solution.

Consequently, we have implemented a methodology for acquiring 1D shear-wave velocity profiles through joint inversion for the entire project, which is currently in progress, to achieve a more dependable site characterisation of the Metropolitan area of Granada city, Spain.

Acknowledgements

The authors are grateful for the financial support provided by Spanish Research Agency (AEI) from Ministerio de Ciencia e Innovación (MCIN), and FEDER (MCIN/ AEI/10.13039/501100011033). The project presented in this article is supported by two project grants with identification codes PID2021-124701NB-C22 and PID2021-124701NB-C21.

References

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Beyond the Clichés, edited by Giancarlo Dal Moro, 1-53. Cham: Springer International Publishing.

Duan, W., Cai, G., Liu, S.and Puppala, A.J. [2019]. “Correlations between Shear Wave Velocity and Geotechnical Parameters for Jiangsu Clays of China.” Pure and Applied Geophysics 176 (2): 669-684. https:// doi.org/10.1007/s00024-018-2011-x.

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Kumar, A., Satyannarayana, R.and Giridhar Rajesh, B. [2022]. “Correlation between SPT-N and shear wave velocity (VS) and seismic site classification for Amaravati city, India.” Journal of Applied Geophysics, 205, 104757. https://doi.org/10.1016/j.jappgeo.2022.104757.

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López, F., Navarro, M., Martínez-Pagán, P, García-Jerez, A., Pérez-Cuevas, J.and Enomoto, T. [2022]. Vs30 Structure of Almeria City (SE Spain) Using SPAC and MASW Methods and Proxy Correlations. Geosciences, 12(11). https://doi.org/10.3390/geosciences12110403.

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Martínez-Pagán, P., Navarro, M., Pérez-Cuevas, J., Alcalá, F.J, García-Jerez, A. and Vidal, F. [2018]. Shear-wave velocity structure from MASW and SPAC methods: The case of Adra town, SE Spain. Near Surface Geophysics, 16(3), 356-371. https://doi.org/10.3997/1873-0604.2018012.

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Tailings pond outfiltration monitoring with electrical conductivity surveying

Pauli J. Saksa1* depicts the baseline setting from one mine site and one monitoring line timelapse result. He presents an example of how the water electrolyte content has evolved along the exemplified monitoring line and assesses the modelling potential.

Abstract

A new, shallow penetration electromagnetic (EM) surveying method for the detection and monitoring of contaminated mining water, called NOVEL-EM, was developed between 2013 and 2015. It has been used at several mine sites in Finland and abroad. The method is comprised of instrumentation, systematised on-site measurements, processing, and modelling procedures which aim to achieve high accuracy and repeatable data and the results of water chemistry changes in the surface or groundwater layer within a depth range of 0-10 m. This article depicts the baseline setting one mine site, one monitoring line time-lapse result, and an example of how the water electrolyte content has evolved along the exemplified monitoring line.

The second part of the article discusses modelling potential, although monitoring itself does not require numerical modelling. Layer-based modelling provides several supplementary uses. Constrained modelling can more accurately focus resistivity changes on layers of primary interest. Water chemistry calculations are possible with ground models. Finally, modelling can help to develop data processing as scoping calculations show the influence and significance of various physical conditions that are encountered, such as the variability of ground temperature.

It has been verified that detected anomalies are related to changes in water chemistry, and the monitoring line network has been expanded over the course of the years.

Introduction

Geosto Oy developed an electromagnetic surveying method for the detection and monitoring of contaminated mining water at shallow depths between 2013 and 2015 in the Finnish Green Mining programme. The system consists of hand-held electromagnetic (EM) frequency domain measurements for establishing a baseline, and subsequent monitoring surveys. The EM method and hand-held instrumentation were selected for this purpose due to the applicable depth range, cost-efficiency in field work, and high accuracy in measured responses and time-lapse differentiation.

The system included a measurement technique, processing, and software development. The system is called by name NOVEL-EM and the concept is straightforward; in the first phase,

1 Geosto Oy

* Corresponding author, E-mail: pauli.saksa@geosto.fi

DOI: 10.3997/1365-2397.fb2024072

permanent lines are marked and baseline EM measurements are taken. During the subsequent monitoring phase, these same lines are re-measured and the EM results are compared. Changes in groundwater chemistry are reflected in changes occurring in the calculated differences in electrical conductivity (EC) after processing.

In addition, a ground-penetrating radar (GPR) survey is run, other electrical conductivity measurements are taken (DC soundings, borehole logging) and geological and borehole data along with supplementary hydrogeological and infrastructure data are collected in a database. A special processing methodology was developed. The EM main instrument used was the GSSI EMP400 Profiler, which provides data from three frequencies at a time within a range of 1-16 kHz at a time and records secondary field real (Re) and imaginary (Im) values (in ppm).

The main focus of the system is on accurate, high-quality and comparable measurements and on data processing which compensates for changes in soil moisture, temperature and groundwater levels, and also levels the EM data sets well. All this has been implemented in customised MS Excel spreadsheets to which input EM data is imported. Processing produces results from data levelling to resistivity cross-sections and profile plots, quality checks and EC change logs. Finally, the length and magnitude of EC change are calculated for each EC change zone and then listed and transferred to the client’s reporting and GIS system.

The EM-based monitoring system has been used now at several large Finnish mining sites with more than ten years of results gained (Saksa, 2023a). The first part of the article includes the findings and experience gained during the ten-year period from 2013 to 2022 and from surveys conducted at several operating mining sites. Monitoring can be done without any numerical EM modelling software, but the second part of this article discusses how numerical modelling can further supplement the investigations, solve water-related parameters, and provide additional insight (Saksa, 2023b).

Monitoring method and theory

The hand-held short coil spacing instrument typically has coil spacing between 1 and 4 metres. It operates either in geometric

or frequency sounding modes and within a frequency range of 1-50 kHz. The depth range covered depends on the electrical conductivity of the ground, but usually stays between 1 and 10 m. The situation is very suitable for shallow groundwater layer observations (when the water level is situated within reach of the system and less than 5 m from the ground surface). The main idea is to first measure the baseline response at several frequencies and within a depth range of 0-10 m, and after each time-lapse survey, solve the differences at the same points. For surveying, we used the GSSI EMP-400 Profiler instrument, which can simultaneously record three frequencies in the 1-16 kHz range, either in HCP (Horizontal Coil Profiling) or VCP (Vertical …) modes. The surveyed lines were at staked points close to the edges of the waste area, and placed in natural, non-disturbed ground. The surveyed point locations are within ±1m of subsequent line point recordings.

Figure 1 depicts the apparent depth ranges Da surveyed in the frequency domain, calculated as the square-root of skin-depth values from EM theory (Saksa 2014) and for the EMP-400 system. Various soil types and resistive rock are also marked on corresponding curves. However, depth penetration in itself does not guarantee resistivity mapping from an embedded groundwater layer. The layer or volume must be situated well within the maximum depth penetration, the resistivity difference must create a measurable signal, and the EM instrument and site conditions must enable low noise levels in data. We evaluated the stability and accuracy of the EMP-400 unit at a system level and found that in the 1 – 16 kHz band, an accuracy of ±10 ppm can be achieved for the secondary field imaginary (Im) component (Saksa and Sorsa, 2017). The real component is used as a relative value, an indicator of noise field, and for magnetic responses.

Baseline data holds crucial importance as all later data are compared against it. Preferably, the baseline should be recorded before or in the very early stage of waste disposal activities. Regarding mining that has taken place over a period of years or decades, or for closure stage conditions, Figure 1 also shows the EM response variability with geology, which is then present

and makes the detection of small resistivity changes resulting from water chemistry that is particularly difficult and uncertain to infer.

During data processing, there are several factors which influence the recorded EM baseline-monitoring data and which has to be compensated before the differences can be evaluated. They can be divided into instrument and survey site, hydrological, and meteorological categories. In the first category, EM system calibration and stability have to be controlled (pre-survey site calibration, on-site point measurements, frequency cross-correlations), measurement points have to be the same and changes in surface conditions have to be recorded and treated. In-house EMDC1D, PLOT, CONVERTER and GROUNDMODEL software modules were developed for various modelling modes, water chemistry calculations and presentations.

Hydrological changes also influence the measured EM signals. Groundwater level, water infiltration conditions, and the degree of saturation in unsaturated soils can change. In the meteorological category, precipitation over time changes continuously, as does soil temperature (depth profile). Developed processing routines remove and compensate for all these as much as possible. In anomaly pick-up, it is noted if any conditions along the profile section have changed. It is also important to collect associated background data from the site, such as geology-hydrogeology, groundwater standpipe and drilling data, water levels, water drainage arrangements, and changes in infrastructure and land conditions.

Processing of EM Im-components included component static levelling per frequency, corrective-predictive filtering using the developed OXZM-method (outlier, x-z directional and median filtering), final level adjustments, and electrical conductivity (EC) change zone calculations. Change zone detection applies threshold values regarding zone length, change magnitude, and presence at the used frequencies to avoid false or uncertain zone identifications. Each zone is labelled, and a mass index (average conductivity increase in groundwater multiplied by zone length, in mS units) is calculated. Typically, an increase of 50-100 ppm is required per frequency in the Im-component for change zone

Figure 1 Apparent depth ranges Da in small coilspacing EM frequency domain surveying (Saksa 2014).

detection, but this depends on the overall apparent resistivity level, noise signal, and the variability of hydrogeological conditions.

Finally, all line data is presented as electrical conductivity change maps, as binary change zone maps, in profile presentations, and as a list of zones. Every monitoring data point has a coordinate, and data tables calculated in this way are transferred and documented in the client’s GIS-system, for example, to assess the location for a new groundwater sampling point.

Monitoring examples

At one large Finnish metal mining site, the baseline survey was done in 2013. During the first stage, 10 lines were staked around two tailings ponds at a total length of 7.2 km. The baseline survey formed the basis for all future evaluations. The baseline was also interpreted against geology-hydrology main setting and normally provides some new geological-hydrological information, too. Figure 2 shows the location of the lines around the tailings pond areas. Monitoring first took place in 2015 and has since been carried out roughly once per year. Many new lines have been added since 2015 based on water management interests and observations of changes in water chemistry. One particular addition was the

establishment of lines along certain service roads along the dam perimeter. These can show outfiltration points closest to the wastewater reservoir and therefore enable rapid operational actions like drilling, wells, and water sampling. Figure 3 displays Im-component data from one line, originating from the baseline and from monitoring in 2020.

Together, time-lapse measurements show how distinct zones related to changes in water chemistry have developed over the years, Figure 4. The main ones are zones A) and B), and their magnitudes reduced in 2022 due to the establishment of nearby pumping wells. Mass index values for the change zones are shown. The line network has been expanded at the mining site to cover the dam perimeter lines (along the service roads) and to cover monitoring of the rock disposal area at one side. The total line network currently in operation at the mining site is about 23 km. At certain locations, more detailed EM studies and monitoring have been conducted, for example, to characterise dam structures.

Modelling tools and approaches

Rather early on, it was noted that no flexible and customised numerical processing software was available. Therefore, the

Figure 2 Baseline line network around a mine site and its major tailings ponds.

Figure 3 Monitoring line data from 2020 and baseline data from 2013, showing Im-components.

main 1D modelling package EMDC1D was programmed to be able to calculate forward and inversion results for EM short-coil spacing systems in HCP and VCP modes, Slingram HCP, VLF-R, audio- and radiomagnetotellurics (AMT and RMT), and for several direct current (DC) resistivity measurement configurations (Wenner etc.). We also implemented a text-based general input data format, called .dat-files. The plotting module presents model and calculated data graphics.

In addition, two auxiliary software modules were developed for monitoring application. The following assistive functions were programmed into a converter module: resistivity pseudosection calculation, 2-layer magnetic susceptibility calculation, formation analysis and calibration with water EC, water chemistry calculation with soil-rock data, and model transfer as an XYZG point cloud for 3-D visualisation and volume modelling (Rockworks 2022). The fourth component is the GROUNDMODEL module, wherein the ground model can be defined in text input file format and visualised. The ground model can be linked to an input EM&DC data file and used to constrain the inversion process. The graphics use the DISLIN library (Michels, 2017).

In a subproject, 2D and 3D modelling tools were also developed in 2017 based on the ArjunAir and SamAir codes developed and published earlier in the AMIRA project. Development, re-coding and the GUI were carried out by PhD Markku Pirttijärvi, Radai Oy.

Modelling usage

There are several situations in which 1D modelling is helpful and can improve processing quality. Model calculation can identify static offset errors, show outliers well, and indicate in the form of a frequency effect if the line data covers a ground section that has several electrically varying layers, and therefore changes with depth. This helps us to decide what kind of layer structure is to be used. Forward modelling is one use and is mostly applied to study in the shallow EM context if the depth or layer of interest can be

Figure 4 Figure 3 monitoring of line change zone development through the years, EC changes shown as log-values.

mapped, and if a certain resistivity change in the groundwater layer is detectable. This leads to the selection of instrumentation and frequencies applied. However, the site noise level is usually not known but average noise levels recorded in comparable conditions can be used.

The easiest use is direct layer model inversion without constraints (automatic model) or applying a user-constrained model. This can function well for data lines where soil structure does not change much. It can also be used for solving variation in EC when there is a single dominating layer of groundwater. A more detailed model can be calculated by using constraining ground model geometry. Each layer can also have resistivity limits and changes in inversion which are regulated by setting fix-free parameters. Resistivities for main groundwater layers can be solved using this method.

Modelling can also sometimes cover bedrock and its groundwater variations. Water in bedrock is demanding to solve because porosity is very low in crystalline metamorphosed rock conditions. At mining sites, mineralised bedrock may also occur, which makes determining water chemistry even more demanding. Electrically conductive bedrock also sometimes manifests in Re-components, and magnetic susceptibility modelling can help to identify the influencing lithology.

Water chemistry calculation requires well-solved resistivities for hydraulically conductive groundwater layers. Typically, the EC, TDS or eNaCl of the water is calculated with the help of soil and temperature parameters. The main soil model applied in our approach is Waxman-Smits (Schön, 2004), along with Archie’s law for the electrolytic part. On many occasions, soil parameters are not known, so the converter module can use groundwater sample data linked to model layer data sections and calculate the formation factor, which is used as calibration in further water chemistry calculations.

The use of 2D and 3D modelling has been very limited. The reason for this is that there are very seldom several parallel lines

forming a dataset which enables 3D modelling and inversion. However, 2D modelling can solve narrow zones crossing EM data lines well, such as a shallow bedrock fracture zone, or a defect observed in an embankment dam structure.

Examples

Model calculation types are exemplified in one 565 m line data example. Only Im-component values were used in inversion (Re-comp weight = 0.0). Three different model types are used:

automatic, user-constrained and ground model-constrained. The automatic and user-constrained models consistently have three layers. The ground model was created with the help of GPR and soil drilling data. The Im-component profiles and the ground model applied are shown in Figure 5.

Automatic inversion uses internally determined layer thicknesses and resistivities as starting values. The user-constrained model had layers for peat, moraine and bedrock. Peat and moraine in 2-metre-thick layers had fix-free values of 0.9. The

Figure 5 Example line data Im-component profiles are shown above and a ground model constructed for the line is shown below. Soils TV = peat (grey), MR = moraine (light brown), RK = broken rock (brown), KA = intact bedrock (red). Groundwater level is shown as a dashed blue line.

Figure 6 Example line modelling results with three alternative inversion settings: above is the automatic 3-layer model, in the middle is the user-constrained 3-layer model, and at the bottom is the ground modelconstrained geometry and resistivities constrained per layer in inversion settings.

layers had initial resistivities of 200, 200 and 500 Ωm (fix-free 0.5, 0.0 and 0.3), respectively. The fix-free stiffness parameter is 0.0 when it can change freely, and 1.0 when fixed completely. The ground model has stratigraphy of four layers and layer geometry is fixed in inversion. Resistivity limits and stiffness per layer were set so that major changes can occur in moraine and broken rock layers. The inversion run results are shown in Figure 6. The automatic model varies the most widely. In the user-constrained model, resistivity variations concentrate on the middle moraine layer as pre-conditioned in the inversion by fix-free parameters set. In the ground model-constrained inversion, variations in resistivity results occur mostly in moraine and broken rock layers, and topographical variation is also depicted.

Further derivation and an example of water EC calculation are shown in Figure 7, with focus and parameters set on the moraine

Figure 7 Figure 2 ground model inversion-based water EC calculation values, focus on moraine (MR) layer (porosity 30%, temperature +5°C, clay content 5% and CEC = 5 meq/100g).

Figure 8 Average half-space ground temperatures at 60° latitude. July temperature values are used to set resistivity sections to 100 Ωm level.

Figure 9 Calculated Im-component values for 100 Ωm ground (set for July) at other times during a year.

layer. Three major water chemistry flow zones intersect the line between 80 and 150 m, 190-255 m and 270-275 m. A groundwater standpipe at 200 m had water EC of around 150 mS/m in 2020 and derivative modelling yielded 260-360 mS/m for the location. A section of broken rock was also found there exhibiting higher values.

An example of scoping calculation modelling

Ground temperature influences ground resistivity, increasing the electrical conductivity by approximately 2.2% per centigrade. In addition, there can be a frozen layer at the surface. Monitoring surveys at the site can be done at various times due to access restrictions and whenever needs arise, for example. Annual heat waves penetrate down to 5-10 m and change the measured resistivity response. Figure 8 shows typical half-space ground

temperature changes at the Helsinki level (60° latitude). To calculate variations in resistivity, the ground response in July is normalised to 100 Ωm at varying depths. This means in practice that the structural parameters of the earth change with depth as the temperature changes. For other months, resistivities are calculated with 2 8-layer models using temperature-based differences compared against the resistivity normalisation for July. In addition, a maximum ground frost depth of 0.5 m is considered, but the influence of snow cover is not accounted for in this case.

It is obvious that surveying at other times than in July would yield different results even in this simple case. Using calculated layer resistivities, the Im-component responses are calculated for the EM system used and presented in Figure 9. Annual resistivity changes are significant and need to be accounted for in data levelling for monitoring purposes. There are also periods like February, April and July, October when site temperature conditions are quite stable and only minor levelling is required.

Conclusions

During 2018, independent evaluation indicated that geophysics-based EC change zones and samples correlate well with direct groundwater sampling electrolyte results. Monitoring has also helped in positioning new groundwater standpipes and other environmental management actions that have been taken. A small number of lines have turned non-measurable due to earthworks at the site and the construction of new power lines. It has been noted that water chemistry change zones can also disappear or change form. The reasons for this include construction activities on the land, tailings pond operations, or other temporal sources of electrolytes. Pumping-related changes in the electrolyte content were also observed.

It can be also concluded that layer modelling can provide insight and allow more detailed parameter calculations in the context of monitoring effluent water from mining. Dam integrity

and similar outflow zones can also be inspected in the same way. The ground model-based modelling approach enables more detailed control of the inversion process behaviour and focuses on key layers and parameters of interest. Solving hydrological soil parameters adds new challenges for input data. Modelling is also needed in scoping calculations for variations occurring in site physical conditions, such as the example shown for temperature.

Acknowledgements

We would like to thank the Boliden company and its EHSQ manager Johanna Holm and head of section environment Auri Koivuhuhta at the mine for permitting the presentation of monitoring data. Thanks to Stacy Blyth at Verbum Kielipalvelut for revising the English of the full paper and extended abstracts, which were used as the basis for this article.

References

Michels, H. [2017]. The Data Plotting Software DISLIN, Version 11. Shaker Media, Aachen. 345 p.

Saksa, P., [2014]. Monitoring of Groundwater Quality and Temporal Changes with Shallow High-Resolution Electromagnetic Methods. International Association of Hydrogeologists IAH, 41st IAH International Congress, Marrakech, Abstract and presentation, 2014.

Saksa, P. and Sorsa, J. [2017]. System Stability and Calibrations for Hand-held Electromagnetic Frequency Domain Instruments. Journal of Applied Geophysics, 140, p. 84-92.

Saksa, P. [2023a]. Tailings pond outfiltration monitoring with electrical conductivity surveying. Oral presentation in EAGE Annual 2023, Vienna. Extended abstract 457. 4 p.

Saksa, P. [2023b]. Modelling approaches in frequency domain electromagnetics and applied in shallow groundwater investigations. Oral presentation in Near Surface Geophysics 2023, Edinburg. Extended abstract 103. 4 p. Schön, J.H. [2004]. Physical Properties of Rocks, Fundamentals and Principles of Petrophysics, Vol. 18. Elsevier. 583 p.

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