First Break July 2024 - Modelling / Interpretation

SPECIAL TOPIC

Modelling / Interpretation

EAGE NEWS 2024 award winners

CROSSTALK Why CGG is now Viridien

TECHNICAL ARTICLE Noise attenuation on arid model data

CONNECTING OUR HISTORY TO OUR FUTURE

With our 90+ year track record of innovation we continue to open up possibilities, from natural resource and energy transition discoveries to new advances in HPC and infrastructure monitoring. We are Viridien. viridiengroup.com

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)

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

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

• Stephen Hallinan, Viridien (Stephen.Hallinan@CGG.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)

EAGE EDITOR EMERITUS Andrew McBarnet (andrew@andrewmcbarnet.com)

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Hang Pham (publications@eage.org)

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Geophysical description of a groundwater aquifer using the combination of geoelectric measurements and sub-bottom profiling.

Editorial Contents

(print) /

29 Noise Attenuation on SEAM002 Arid Model data Mamadou S. Diallo and Nacim Brika

37 A novel simulator for probing water infiltration in rain-triggered landslides Cassiano Antonio Bortolozo, Tatiana Sussel Gonçalves Mendes, Daniel Metodiev, Maiconn Vinicius de Moraes, Harideva Marturano Egas, Marcio Roberto Magalhães de Andrade, Tristan Pryer and Luana Albertani Pampuch

Sp ecial Topic: Modelling / Interpretation

45 Revisiting Baranov’s thoughts on mathematics and geophysical interpretation Brian Russell

49 Geophysical description of a groundwater aquifer using the combination of geoelectric measurements and sub-bottom profiling Christoph Georg Eichkitz, Marcellus Gregor Schreilechner and Erwin Heine

55 Delivering sands to Venus and all the traps between: Orange Basin, Namibia Neil Hodgson, Lauren Found and Karyna Rodriguez

59 Integrating regional 2D seismic mapping and 3D seismic spectral decomposition to understand the fairway evolution of offshore Benin Pauline Rovira

65 Deep learning-based Low Frequency Extrapolation: Its implication in 2D Full Waveform Imaging for marine seismic data in the Sadewa Field, Indonesia

Sonny Winardhi, Asido S. Sigalingging, Wahyu Triyoso, Sigit Sukmono, Ekkal Dinanto, Andri Hendriyana, Pongga D. Wardaya, Erlangga Septama and Rusalida Raguwanti

73 Revealing the hidden paleomagnetic information from the airborne total magnetic intensity (TMI) data

Michael S. Zhdanov, Michael Jorgensen and John Keating

79 Application of simultaneous inversion of velocity and angle-dependent reflectivity in frontier exploration

Nizar Chemingui, Sriram Arasanipalai, Cyrille Reiser, Sean Crawley, Mariana Gherasim, Jaime Ramos-Martinez and Guanghui Huang

85 Why numbers matter

Doug Crice

86 Calendar

cover: A 3D spectral decomposition image of the Upper Miocene offshore Benin, revealing a channelised fairway originating from a Niger Delta source to the east. (photo courtesy of TGS)

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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Andreas Aspmo Pfaffhuber Chair Near Surface Geoscience Circle

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Congratulations to our 2024 Award winners

Every year at the EAGE Annual Conference, the EAGE Awards are presented to honour individuals who have demonstrated outstanding dedication to geoscience, engineering, and the EAGE community. Candidates for the 2024 Awards were nominated by colleagues like you. These nominations were submitted to an international Awards Committee, which diligently worked to select the winners. EAGE is delighted to announce the following exceptional recipients for the 2024 Award winners.

Desiderius Erasmus Award

For lifetime contributions in the field of resource exploration and development.

Presented to Philip Ringrose

For his contributions ranging from characterising reservoir structure to developments in CCS and large-scale CO2 storage projects.

Honorary Membership Award

For highly significant and distinguished contributions to the geoscience community at large or to the Association in particular.

Presented to Leo Eisner

For transferring technology from global seismology to passive seismic applications, for pioneering microseismicity for reservoir monitoirng, and for his continuous support of the EAGE community.

Alfred Wegener Award

For outstanding contribution to the scientific and technical advancement of EAGE’s disciplines, particularly petroleum geoscience and engineering.

Presented to Baojun Bai

For his contributions to petroleum geoscience and engineering, particularly for the development of preformed particle gel for conform ance control and many research activities.

Conrad Schlumberger Award

For outstanding contributions to the sci entific and technical advancement of the geosciences, particularly geophysics.

Presented to Alireza Malehmir

For his contributions to near surface geoscience, particularly to address critical challenges in mineral exploration and CCUS geophysics.

Arie van Weelden Award

For Young Professionals who have made highly significant contributions to one or more of the disciplines in our Association.

Presented to Grazia de Landro

For her work on induced seismicity related to underground industrial activities, critical for energy security and decarbonisation.

EAGE Awards stand at the core of the Association’s mission, promoting innovation and technical progress for inclusive and sustainable development.

We congratulate Prof Mike Stephenson for winning our Best Instructor Recognition 2023 bestowed by the EAGE Education Committee for his outstanding delivery of his course Basics of Carbon Capture. By filling a crucial gap in the education of carbon capture and storage (CCS), Stephenson’s course extends beyond the technical elements to encompass wider scientific, financial, planning, and social aspects. His integral approach addresses the critical issues that can significantly impact CCS projects.

Maren Kleemeyer, EAGE Education Officer, comments: ‘CCS is essential for achieving a low-carbon future. Providing upskilling in this area equips EAGE members with vital knowledge for a sustainable future. We’re delighted to offer attractive courses like Mike Stephenson’s in our short course portfolio.’

Stephenson identified a gap for comprehensive CCS education. ‘There are few courses on the wider science, risks, financing, planning, and social licence

Prof Mike Stephenson wins EAGE Best Instructor Recognition for 2023

aspects of CCS.’ His course aims to provide geologists, technical civil servants, planners, and investment organisations with a broad understanding of CCS, emphasising the importance of a multi-disciplinary approach.

In his course, participants learn about the global geological carbon cycle and the potential of CCS to contribute to carbon reduction. Through practical exercises and detailed discussions, the course covers the basics of CO2 transport, capture, and storage, as well as the challenges and solutions associated with these processes. Additionally, the course explores the policy and financial aspects of CCS projects, highlighting the importance of social licence including real-world examples of failed projects due to lack of public support.

Having taught the course globally, Stephenson addresses regional misconceptions and challenges. He emphasises the different perspectives on CCS across the world, from the ‘just green transition’ in Africa to the integration with enhanced oil recovery in the USA. By showcasing these diverse approaches, Stephenson helps participants understand the multi-faceted nature of CCS.

To keep his course materials current, Stephenson immerses himself in a wide

range of sources, including peer-reviewed papers, policy documents, regional assessments, and social media posts.

Looking ahead, Stephenson plans to expand his offerings to include courses on the development of decarbonisation clusters. These clusters, comprising groups of industries working together to decarbonise, represent a shift from traditional oil and gas projects to more collaborative and complex ventures. In his view, ‘The stakeholder range and complexity is very different from a normal oil and gas project’. His future courses will explore how these clusters are organised, funded, promoted by governments, and structured to succeed.

Prof Stephenson, now with his own consultancy, was a long time scientist with the British Geology Society (BGS) where he was executive chief scientist (2019-2021) and leader of the BGS ‘Decarbonisation and Resource Management’ Challenge. He has served as an adviser on energy topics to governments in the UK and worldwide.

Find out more about the course at learninggeoscience.org.

Delve into our education portfolio!

BY G. WACH AND M. DUSSEAULT

Prof Mike Stephenson taught the course Basics of Carbon Capture at GET2023 in Paris.

What the MP-FWI Imaging

See the latest jaw dropping DUG MP-FWI IMAGING results for yourself at EAGE 2024.

DUG MP-FWI Imaging completely replaces the conventional processing and imaging workflow, delivering high-resolution reflectivity images for both structural and quantitative interpretation using field-data input. Superior physics. Faster turnaround. Unsurpassed imaging. Now that’s game changing! We’ll be showcasing all the latest results at EAGE 2024 from booth #3200. Reach out to sales@dug.com to connect with our team or drop by to see the daily presentation schedule.

dug.com

Preview our latest showcase for yourself here or visit dug.com

Big opportunity for geoscience students at GET 2024

Addressing all students! The 5th EAGE Global Energy Transition Conference and Exhibition (GET 2024) in Rotterdam brings together a global community of geoscientists, engineers, researchers, and industry professionals. This year’s conference is packed with a diverse range of activities, including presentations, workshops, and interactive sessions that are designed to expand your knowledge and inspire innovative thinking.

You will have the chance to attend talks by industry leaders and renowned experts, offering you a unique perspective on the energy transition challenges and opportunities within the geoscience field. Hands-on workshops will provide practical experience with the newest tools and techniques, helping you to bridge the gap between theoretical knowledge and realworld application.

connect with professionals, researchers, and fellow students from around the world. The conference also offers numerous activities specifically designed for students such as the ‘The Geosciences and the Energy Transition Challenge’ course that provides a comprehensive understanding of the Energy Transition’s technical, economic, and societal aspects. Students will develop plans for carbon capture and storage and

geothermal projects, comparing them to oil fields. The course combines lectures, teamwork, and discussions focusing on practical examples. Scheduled during the conference, it will provide valuable insights for earth scientists, engineers, and policymakers. You can also present your research and receive constructive feedback from experienced professionals during dedicated student sessions.

Participate in the student networking session, where you can build meaningful connections in a casual and supportive environment. Engage with industry leaders during interactive Q&A panels, and gain insights that can guide your academic and career choices.

Our student community gains access to all sessions in the registration area, including the Poster Session and Mineral Exploration Symposium, plus entry to the GET Exhibition floor - all for free! We encourage you to secure your spot early, as spaces are limited and highly sought after. To make the most of this incredible opportunity, visit www.eageget.org to register and access detailed information about the event schedule, speakers, and more.

Students get the chance to mix with professional community.

Popular drone and robotic geophysics conference back for fourth time at NSG2024

Our 4th Conference on Airborne, Drone and Robotic Geophysics will again be a highlight at the Near Surface Geoscience Conference & Exhibition 2024 (NSG2024)

taking place from September 8-12 in Helsinki. The conference will be one of the three distinct parallel programmes, offering a unique opportunity to explore various areas of geoscience in one place.

The 4th Conference on Airborne, Drone and Robotic Geophysics is dedicated to exploring the latest innovations and advancements in geophysical acquisition technologies that go beyond traditional ground-based methods. With the rapid development of very-near-surface systems and the introduction of drone- and robotics-based acquisition methods, the gap between ground-based and airborne geophysical techniques is closing. This conference serves as a platform to present success stories, discuss current limitations, and explore future directions for modern geophysics.

Participants can expect to be immersed in cutting-edge technology, with presentations on the latest innovations in hardware and algorithms for airborne and robotic geophysics. Additionally, complex case studies will be discussed, showcasing how these technologies are being applied to overcome the limitations of traditional methods. The conference will cover a

wide range of industry topics, from rock physics of mineral deposits to seismic methods in mineral exploration.

Attending this event offers a unique opportunity for both professionals and students, an excellent occasion for networking and connecting with industry leaders, academics, and peers to share knowledge and experiences.

This is your opportunity to join the vibrant and innovative NSG2024 community in Helsinki. Registration is now open for the conference with an all-access pass. For more information and registration, visit www.eagensg.org.

Learn about future of energy excellence at upcoming Kuala Lumpur conference

Are you driven by the possibilities of digital transformation, predictive analytics, and the integration of digital twins in the energy sector? If so, make sure to save the dates of 15-16 October 2024 for the EAGE Conference in Kuala Lumpur on Energy Excellence: Digital Twins and Predictive Analytics.

Spanning a wide array of topics, the conference will immerse you in the latest advancements and best practices in digital excellence. Whether you’re a veteran professional aiming to keep ahead of industry shifts or an emerging researcher looking for inspiration, this event promises to deepen your knowledge and expand your connections within the global energy community.

Seize this unique chance to elevate your professional journey while enjoying the vibrant atmosphere of Kuala Lumpur. Join us in spearheading the digital transformation of the energy sector and positioning yourself at the forefront of innovation

and sustainability. Don’t miss out on this opportunity to make a significant impact in your field.

Learn more about this event and abstract submission information by scanning the QR code.

Scan the QR code!

Geodetic measurements of a mining site using drone.

Airborne diamond with drone.

WORKSHOP REPORT

Record attendance for forum highlighting Sub-Sahara opportunities

Report on our biannual EAGE Sub-Saharan Africa Energy Forum held on 4-6 March in Windhoek, Namibia.

Namibia’s capital city Windhoek was chosen as a dynamic and exciting location to host our forum. Reflecting the high industry interest, sponsorship was generously provided by several key companies active in the region: SLB, Shell, TotalEnergies, Woodside and CGG. Furthermore, the 96 local and international delegates in attendance from a broad range of IOC, NOC, technical contractors, regulatory, consultancy, media and academic backgrounds was a record for this event.

In addition to a memorable Icebreaker involving a safari game drive and sundowners on the plains outside

the capital, the delegates enjoyed and actively participated in the three-day technical programme, held in the centrally located Hilton Hotel, covering the extensive Sub-Saharan region. Technical sessions were split geographically into Namibia and related basins, and the wider margins of West and East Africa, with each comprising keynotes, presentations, posters, and moderated panel discussions covering a broad range of exploration and developments topics. These were predominantly subsurface, geological and geophysical focused, including petroleum systems and play evaluation, key wells and projects, latest data, digitalisation and workflows, but also expanded into other non-technical aspects of the energy transition, the future role of frontier exploration in the region, the significant above ground challenges facing some projects, and the wider role of geosciences in the future. These all ensured for a wide range of positive, open debates and discussions throughout the event proceedings.

The event was opened by Victoria Sibeya (executive upstream exploration at Namcor) with a summary of Namibia oil and gas exploration history highlighted by recent discoveries. This opening talk introduced the first session of the forum on ‘Revival of Orange Basin and Beyond’ which lasted the entire day.

The session started with a keynote from Benjamin Gatignol (exploration manager, TotalEnergies) on the recent successes in Namibia including the Venus discovery and upcoming operations. This talk was followed by eight presentations covering the basin history, geologic evolution, and extension of successful plays. A presentation from Westwood entitled ‘From Tupi to Liza and Beyond’ covering significant global exploration drilling results since 2007

gave good context and comparison to the recent Namibian discoveries. According to operators, seismic contractors and academics, similar prospects can be mapped to the north in the Luderitz and Walvis basins as well as across the border in South Africa. It suggests that exploration along the Southern African margin is just getting started.

A second keynote from Adeleye Falade (Shell country chair Namibia) highlighted the recent discoveries and continued impressive operational results of Shell’s exploration and appraisal drilling campaign since the Graff discovery. The session ended with a keynote from Guido Paparoni (Kudu asset manager, BW Energy) on recent learnings and revisions on the geological model of the Kudu field based on a newly acquired 3D survey.

To finish the day Bryan Gill moderated an insightful panel discussion on the Orange Basin. The panellists were Benjamin Gatignol (TotalENergies), Guido Paparoni (BW Energy), Anthony Fieles (PASA), and Aune Amutenya (Namibian Ministry of Mines and Energy). The conversations addressed the specifics of exploring the Orange basin and the companies’ future projects. This proved to be a very well-received format which stimulated more interaction, provided a longer time to explore and link the topics and allowed for a deeper and more meaningful learning experience.

The second day consisted of two sessions covering western Africa: 1) Exploration in mature and frontier West African basins and 2) Digitalisation, New Technology and Modelling. The morning started with a keynote from Lises Salles (TotalEnergies) on a recent Nigerian case study of Ntokon discoveries on OML 102. The talk on Ntokon was a positive example of fast-paced exploration in a mature basin and a great

Record breaking attendance at EAGE’s Sub-Saharan Africa Energy Forum.

introduction for Session 2 on ‘Exploration in mature and frontier West African Basins’. The session consisted of eight talks which provided a great update on the most recent operational and technical projects in the region. Exploration driven seismic campaigns in Benin, Nigeria and Angola reveal the extension of deepwater prospectivity into the most outboard domains. There were good examples of stitching 3Ds and integrating with other datasets to build regional play and prospect understanding, and PASA’s presentation on contourite influenced systems in Mozambique demonstrated the use of analogues in understanding prospective reservoirs in South Africa.

The third session was dedicated to Digitalisation, New technologies and Modelling. Several outstanding case studies were presented including CSEM, seismic interpretation using machine learning, 4D processing workflows, and seismic characterisation demonstrating latest technology development to support geoscience projects and decision-makers.

The technical sessions ended with a panel discussion moderated by Peter Elliott (NVentures) on Near Field Exploration and Production in Western Africa. The panellists were Vincent Curinier (TotalEnergies), Musa Jato (NUPRC) and Meciano Lorenzo (Eni).

The last day was dedicated to exploration and development in East Africa (Session 4) and was introduced by a

keynote from Peter Elliott on the oil and gas activities in this region. Six papers addressed the variety of geological settings and their associated challenges. These included onshore, shallow fields such as Tilenga, that require creative use of seismic processing to improve imaging, and examples from deepwater where seismic suggests the existence of untapped reservoirs, with reworking by contour currents and complex tectonic history challenging the interpreters. A review by S&P Global of six ongoing developments highlighted the above ground challenges that remain in the region, and the session ended with an overview by Westwood of key upcoming wells as a view going forward for the wider Africa region

and its clear exploration significance globally.

The event concluded with group discussions on the main themes and points made during the three days, in which delegates grouped according to which topic interested them most, discussed what their key takeaways were, and summarised them to the wider audience. This proved a good way for everyone to gather their thoughts at the end, and effectively distil and collectively encapsulate the diverse, wide-ranging topics covered. Judging by the survey feedback, the event was well received and enjoyed, being in Namibia was a great experience for many, and you could sense the excitement in all going on currently in the region.

Roof of Africa dinner was a great success with delegates enjoying the opportunity and delegates taking in the proceedings.

One-of-a-kind Icebreaker – a game drive set against the beauty of the Namibian wildlife.

EAGE’s flagship Digital Event in Paris a resounding success

EAGE Digital, our flagship event, addresses the transformative impact of digitalisation, technology, and innovation in the energy sector. The fourth edition of EAGE Digital was successfully held on 25-27 March 2024 in Paris, France, under the theme ‘Delivering energy in a transforming world’.

The event witnessed rapid growth, strong enthusiasm, and ambition for scaling the benefits of digitalisation. Attendees from around the globe gathered to explore, discuss, and collaborate on the latest advancements and challenges in the digital transformation of the energy industry. The vibrant atmosphere was a testament to the collective commitment and excitement within the community. We extend our heartfelt thanks to all participants, sponsors, exhibitors, speakers, and committees for contributing to this year’s success.

Gautier Baudot, vice president exploration excellence & transformation, TotalEnergies, and chair of EAGE Digital 2024, highlighted the importance of the event: ‘EAGE Digital is very important because it’s one of the unique occasions where we can gather all the industry actors, including industry operators and service providers. We also included universities and schools to encompass the whole ecosystem, which is essential for delivering solutions to our challenges. Participating in the digital transformation helps us face these challenges.’

Edward Wiarda, EAGE president 2023-24, said: ‘My key takeaway, something that transpired through all the technical sessions and also the strategic programme was that there is a clear urgency from both the digital realm, the energy sector and all its players to accelerate digital transformation.’

The conference explored digitalisation as a crucial catalyst for innovation and the necessary transformation of the industry through the energy transition. It featured an array of sessions, including keynotes, case stories, panels, and roundtables. Kaveh Dehghan, digital and innovation manager, TotalEner-

gies noted: ‘I see three different topics. The first one is the deployment of low-code platforms for the whole the industry. Integration of data scientists inside geoscience and reservoir teams is also very interesting. The second is the rise of large language models (LLMs) since last year. This is amazing and we can see how this technology can be used to bring in new use cases for our industry. Thirdly, somehow correlated with LLMs, is the very marked accelerated pace of deployment of these solutions. We can see the need to accelerate and this becomes more and more clear.’

Participants engage with immersive virtual reality experiences.

Experts shared their insights on leveraging digital tools and technologies to enhance efficiency, sustainability, and profitability in energy operations. The exhibition served as a specialised marketplace where companies could display their latest technologies, innovative services, and solutions to challenges posed by digital transformation. The bustling exhibition floor was filled with demonstrations and interactive displays, allowing attendees to experience first-hand the innovations driving the industry forward.

Cerys James, VP transformation & change, PGS, remarked that conferences like EAGE Digital are crucial for sharing information, networking, collaborating, and learning what works well and, importantly, what doesn’t, to avoid repetitive mistakes in our industry.’ Over 100 technical presentations provided insights into various digitalisation initiatives at the crossroads of geoscience, engineering, data science, and energy transition. Key topics included applied analytics, machine learning, digital subsurface, carbon capture and storage (CCS), and overall energy transition efforts. Discussions underscored the benefits and challenges associated with integrating digital technologies in these areas, and the importance of partnerships like OSDU data platform.

The Strategic Programme addressed a broad range of organisational and leadership issues. Embracing new technologies in geoscience meant inventing new ways of working and collaborating closely with non-traditional partners to drive innovation and create resilient companies. Discussions were driven by key daily themes, such as new ways of working, data and tools, and empowering people through digital. Leaders from various sectors shared their strategies for fostering a culture of innovation and resilience within their organisations, highlighting the role of leadership in navigating the digital era.

We are already excited for the 5th EAGE Digitalization Conference and Exhibition taking place from 24-26 March 2025 in London, UK, which promises to be yet another extraordinary event that will continue the momentum.

What participants had to say?

‘The conference was the realisation of just how much is happening in digitalisation that I wasn’t aware of. It’s a massive effort spanning from data preparation and collation to the development of innovative technologies aimed at solving end-user problems efficiently. This process helps us focus more on problem-solving rather than daily data management. This conference covers a broad range of digitalisation topics, from data management to applications for end-users. It’s particularly beneficial for the younger members of our industry who are the ultimate end-users.’

Lindsey Smith Geophysicist, BP

‘My key takeaway for the conference is very much the importance of data foundation. We talked a lot about the power of GenAI, the power of integration, the power of visualisation of data, to really get key insights, to underpin the investment decisions underpin the way we do our reservoir development. But fundamentally, if the data is not in the right shape or of the right quality. Actually GenAI rapidly becomes garbage in, garbage out. So actually really getting this data foundation solid was for me a key takeaway.’

Herlinde Mannaerts-Drew VP subsurface transformation and CCUS, BP

‘This EAGE Digital edition has been very interesting on the strategy side to see what the different companies were thinking, both from the operator side but also from the independent software vendors or even from the cloud providers. This has been a great opportunity to see where all the ecosystem is going and the evolutions through the different years of this conference.’

Camille Msika Principal Solution Consultant, AspenTech

‘I was excited this year to hear a lot about AI and what’s going on in that space. I think as an industry we always need to work faster and collaboration is really the only answer to get us there.’

Patrick Kelly

Product line manager – subsurface data & insights, Chevron

Navigating the future of carbonates and mixed systems at AAPG/EAGE cross-regional symposium

Maria Mutti (University of Potsdam) and Aus Al-Tawil (Aramco) report on the first cross-regional Europe and Middle East Symposium on Carbonates and Mixed Systems held in Palermo, Italy, a collaboration between EAGE and AAPG.

The recent AAPG Europe and Middle East first-of-its-kind cross-regional symposium held in Palermo (Italy), 22-24 April, in collaboration with EAGE, brought together experts from around the globe to discuss carbonates and mixed systems.

The symposium was co-chaired by Maria Mutti and Aus Al-Tawil, supported by a technical committee of experts and industry leaders from both regions. With a thematic focus on modern understanding and frontier applications, the symposium served as a nexus for both industry leaders like Aramco, Shell, ENI and MOL and global experts from academia to converge, discuss and chart pathways, while emphasising the critical role of fundamental concepts, technology, talent in different generations and AI applications in shaping a secure and sustainable energy future.

The event provided a platform for a deep dive into the fundamentals and the applications of carbonates and mixed carbonate systems with a rich and diverse range of presentations from around the world. This included seven plenary talks, 74 oral presentations in three parallel sessions, and 44 poster presentations. Andrea Cozzi (ENI) offered insights into carbonates of the Eastern Mediterranean: from a new hydrocarbon play to the energy transition, while Aus Al-Tawil (Aramco) discussed the future role of stratigraphy in realising subsurface energy resources in the age of sustainability. Cedric John (Queen Mary University of London) addressed how we can leverage computer vision and AI to enhance geological interpretations. Erwin Adams (Shell) discussed deployment and integration of advanced technologies in subsurface carbonate systems. Rick Sarg (Colorado School of Mines) offered unique insights on organic-rich lacustrine rocks encountered in the Green River formation, Western USA, and the pre-salt of Brazil; and Sam

Purkis (University of Miami) discussed recent data acquisition and climate modelling in the Red Sea rift. Christian Wilms (MOL) presented key subsurface engineering insights into carbonate reservoirs, and Attilio Sulli (University of Palermo) gave a full overview of the tectonic history and structural geology of Sicily.

Sessions included carbonate sedimentary systems as archives of earth system processes, carbonate lacustrine systems, organic and inorganic geochemistry, diagenesis, basin analysis, application of cross-disciplinary techniques and new technologies for carbonate fundamentals, integrated workflows, digitalisation and machine learning applications, virtual field trips, 3D visualisation and databases, reservoir management, pore-network characterisation and petrophysics and exploration/characterisation/development case studies and success stories.

A one-day trip to the Jurassic of Sicily preceded the Symposium, emphasising the drowning event in spectacular quarry scale outcrops, while showing textbook examples of Neptunian dikes and large Jurassic Ammonites in these cyclic carbonates. The trip finished with an overview of the geothermal Sciacca field. A half-day trip took participants to the unique Sicilian underground Messinian salt mines during the afternoon of the second symposium day.

A virtual field trip to Malaysia was also presented by Shell and transmitted from its offices to the symposium participants who chose to stay in Palermo. During the same afternoon, a primer course on carbonates was presented by Maria Mutti and Aus Al-Tawil and a workshop on leadership by Anita Csoma. Finally, a one-day field trip took place to the superb Messinian gypsum outcrops and to the Pliocene deep water car-

Pre-symposium one-day trip to the Jurassic of Sicily, emphasising the drowning event and Neptunian dikes in these cyclic carbonates. Photo courtesy of Nilo Matsuda (Petrobras).

bonates with a complete record of Milankovitch orbital forcing controls.

Discussions highlighted challenges and opportunities, emphasising fundamental concepts and technological advancements as a central theme. Participants showcased innovative approaches from imaging techniques to AI-driven interpretation in basin analysis, diagenesis, and petrophysics. Delegates presented case studies demonstrating the successful application of cutting-edge technologies from a palm-size drone at the outcrop to advanced machine learning, while participants envisioned AI as a bridge between traditional geological fundamentals and emerging technologies, driving innovation towards efficiency, present and future.

The symposium also highlighted the differing applications of subsurface knowledge to the industry in Europe and the Middle East. The discussion highlighted the need for integrating renewable energy solutions to reduce the carbon footprint and promote sustainable development while stressing the continued importance of a broad energy mix to meet growing energy demands and support economic development.

The symposium underscored the critical importance of talent development and knowledge exchange in ensuring a sustainable future for the discipline signalling the need for collaborative efforts between academia and industry to attract students to the geosciences and to nurture a skilled workforce capable of addressing the evolving needs of the energy sector. Mentorship programmes, inter-disciplinary collaborations, and academia-industry partnerships were identified as key mechanisms for attracting and fostering talent development and retention.

A critical takeaway was to continue building a robust community that fosters joint participation from academic and industry experts. Such collaborations are essential for driving forward innovative research and practical applications that can address the multifaceted challenges of this sector. By maintaining a dialogue between academia and industry, it is possible to align research priorities with practical needs. This collaborative community can serve as a breeding ground for new ideas, facilitate the transfer of knowledge, and create opportunities for joint projects that

push the boundaries of current capabilities of the science and applications of carbonates and mixed carbonate systems.

The momentum developed by this first cross regional symposium should become a catalyst for a wider expansion of the community and to achieve all these objectives. Please look for the announcements for the second symposium later this year.

H1/2024

IN REVIEW

1400+

new event proceedings from 9 Events, including the 2024 EAGE Annual and EAGE Digital

250+

new journal articles from Basin Research, First Break, Geoenergy, Geophyscial Prospecting, Near Surface Geophysics, and Petroleum Geoscience

Pliocene cyclic deep water carbonates. Sicily hosts for the first of its kind cross regional carbonates and mixed carbonates systems symposium, a chance to review possibilities in the era of energy sustainability.

OUR JOURNALS’

UPDATES

Geophysical Prospecting (GP) publishes primary research on the science of geophysics as it applies to the exploration, evaluation and extraction of earth resources. Drawing heavily on contributions from researchers in the oil and mineral exploration industries, the journal has a very practical slant. A new edition (Volume 72, Issue 6) will be published in July.

Editor’s Choice article:

Diving waves in acoustic factorized orthorhombic media — Kristoffer Tesdal Galtung and Alexey Stovas

Comparison of elastic anisotropy in the Middle and Upper Wolfcamp Shale, Midland Basin — Colin M. Sayers and Sagnik Dasgupta

Near Surface Geophysics (NSG) is an international journal for the publication of research and developments in geophysics applied to the near surface. The emphasis lies on shallow land and marine geophysical investigations addressing challenges in various geoscientific fields. A new edition (Volume 22, Issue 3) has beeen published in June, featuring 9 articles.

Editor’s Choice article:

Tunnel resistivity deep learning inversion method based on physics-driven and signal interpretability — Benchao Liu et al.

Authors seek recognition for Dutch scientist Christiaan Hugyens

A new book by Tijmen Jan Moser, Geophysical Prospecting immediate past editor-in-chief, and the late Enders Robinson (1930-2022), one of the most distinguished geoscientists of his generation, explores the genius of 17th century Dutch scientist Christiaan Huygens.

In Walking with Christian, From Archimedes’ Influence to Unsung Contributions in Modern Science (History of Physics) published by Springer, Moser and Robinson claim that history has overlooked the groundbreaking contributions made by Hugyens (1629-1695) in the fields of mathematics, physics, astronomy, and geophysics. While two of the most famous achievements in physics are Newton’s theory of gravity and Einstein’s general theory of relativity, the authors argue that insufficient credit has been given to Huygens who provided central elements to these theories. The book sets out to correct that deficit. For example, they show how Huygens used symmetry arguments to derive conservation laws for momentum and for energy, and what Einstein later called the principle of equivalence to derive the formula for centrifugal force.

In 1689, Huygens visited Newton. Together, they walked the streets of London. Newton had recently finished his masterpiece, Principia, expounding his laws of motion and the law of universal gravitation. Huygens had essentially completed his life’s work by then, building on

Archimedes, Leonardo da Vinci, Galileo, Descartes, Fermat, Pascal and his own ingenuity. He had established fame as an instrument maker (telescope, pendulum clock, planetarium). He also independently invented the 31 tone system, pioneered the first principles of remote sensing, discovered the rings of Saturn, and formulated the wave theory of light.

What would walking mentally with Christiaan reveal? The book provides some answers in nine chapters covering spontaneous order, the speed of light, Huygens’ principle, the telescope, the pendulum clock, Huygens-Fresnel principle, special relativity, centrifugal force, and curvature. In addition, there is a chapter titled ‘What Huygens could have written on diffraction’ and a chapter on ‘Huygens and Geophysics’. Mentally walking with Christiaan, browsing his collected works – a true treasure trove for puzzle enthusiasts – and rethinking his ideas creates a vivid impression of scientific life in the 17th century, an appreciation that is remarkably similar to ours, in the process offering an understanding of Huygens’ significant and lasting contributions to science.

Enders Robinson.

Tijmen Jan Moser.

Hungary meeting recognises young professional talents

EAGE Local Chapter Hungary, in collaboration with the Association of Hungarian Geophysicists (AHG) and the Hungarian Geological Society (HGS), organised in April the 54th Meeting of Young Geoscientists held in Eger, Hungary – with prizes for the best presentations.

The event attended by some 50 participants set the spotlight on the latest research developed by university students and newly graduated specialists, geologists, and geophysicists (under 35 years of age), through a rich technical programme featuring 28 oral and 14 poster presentations ranging from the possibilities of hydrogen reservoir monitoring and geothermal projects, to rare elements and their mineralogy.

The lectures were evaluated by a six-member jury of recognised specialists. Prize winners were Péter Ábel Polyák (University of Szeged) - first prize oral presentation - theory; Ali Shebl (University of Debrecen), first prize oral presentation - practical, and Bálint Bodor (University of Szeged), first prize poster presentation. Thirteen additional special prizes were awarded by the corporate and non-profit sponsors to outstanding speakers.

The participants had the opportunity to get acquainted with the latest trends in earth sciences, exchange knowledge, and network with fellow colleagues. Thank you all for your participation! Stay tuned as next year’s edition will be announced soon!

What went down when Czech LC went underground

Our Local Chapter Czech Republic organised a special visit to the medieval mine Bílý Kůň located in the Hloubětín district of Prague. Some 20 excited participants gathered with flashlights and helmets in the evening of 3 April, together with a supervisor from the local mining authority.

The cycle helmets visible in the picture do not reflect the nature of this professional outing! But helmets were required by the mining authority to keep safe during the viewing of the abandoned mine.

MSc. Jeroným Lešner (Geotechnik, CZ) shared his personal research of the underground area and explained both the geological as well as the anthropogenic origin of this medieval site. The mine was started probably around the 18th century by sand diggers who found nice clean sand suitable for wood polishing.

The sand was initially mined in large pits but, when it became apparent that significant layers were present, the pits were extended sideways and approximately 12 m in depth. The supporting columns are irregularly spaced as the mining was an unorganised activity, e.g., the underground spaces were dug irregularly resulting in a beautiful underground maze carved in white soft sandstone. In the 20th century the space was partly extended and used as vegetable

storage space. However, the vegetable spiced with sand grains was not tasty enough and so it was shortly abandoned again. Various larger spaces of the cave have their own names and the participants were given a map of the maze and asked to find a way between Tomato and Chamber halls. After successfully completing their tasks, participants were keen to continue the exploration. Unfortunately, due to safety reasons, some of the halls, such as ‘Archive’ and ‘Beers’, were not accessible.

The future of this cave is probably more practical: the maze might be used as a training ground for safety teams.

Next on the agenda for our Chapter is the Vlastislav Červený Student Prize for the best Master or Bachelor thesis in applied geophysics: applications for this competition are currently open until 11 October 2024.

The 54th Meeting of Young Geoscientists participants.

Local Chapters join forces to host seismic signal denoising event

Seismic signal denoising was the topic on 18-19 April when EAGE Local Chapters London, Paris and Pau hosted a joint hybrid event. The speaker was Dr Claudio Strobbia (Realtimeseismic) who provided

who would only have either evening or lunch time slots free. Collective efforts of the three Local Chapters in promoting this event helped to draw the attention of a broad geoscience audience.

a detailed walk-through focused on ‘true-events and true-amplitudes preservation’ in two sessions, one on Thursday evening, the other on Friday lunchtime.

The local geoscience community in Pau enjoyed the talk in person, whilst the global audience participated online. The idea behind splitting the lecture was to attract people in different time zones and to include those

Dr Strobbia, as a global expert in land seismic and signal processing, presented his options for increasing signal to noise ratio with challenging datasets that are lacking continuous reflections, suffering from poor illumination or from many other types of ordered and un-ordered distortions.

He showed what can possibly go wrong with an established processing

workflow in the presence of coherent or incoherent noise and illustrated the fundamental difference between noise attenuation and signal enhancement in real data examples. There is no silver bullet noise attenuation strategy. Data analysis includes identification of noise types, characterisation of noise properties, its spatial variation, and statistical distribution. Parameter driven signal enhancement and model-based noise attenuation may be appropriate in specific domains.

Altogether, over 70 people attended both sessions. For those who missed the talks or would like a recap, the recording is available on the EAGE’s YouTube channel.

EAGE Local Chapter London acknowledges Celina Giersz, Joseph Sutcliffe, Katie Hubbard, Tiexing Wang and Artem Kashubin. Local Chapter Paris acknowledges Laura Mozga and Emilio Darriba. Local Chapter Pau acknowledges Laure Dissez for organising this double-session event. We are very grateful to Dr Claudio Strobbia for his insightful lecture and detailed answers during the Q&A.

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!

Dr Claudio Strobbia (RTS) presented his options for increasing signal to noise ratio with challenging datasets at a hybrid session organised by the Local Chapters London, Paris, and Pau.

Personal Record Interview

Proving that anything is possible

Today Phuong-Thu Trinh is a youthful senior geophysicist at TotalEnergies Angola. How she got there so early in her career, starting from a humble background in Vietnam, is a remarkable and inspiring story.

Beginning in Vietnam

I was born in Vietnam at the early time in the renewal period, when the country changed from buying everything with coupons to money. My parents lost their jobs from a cooperative association, and we moved to a new town when I was three years old. I grew up seeing my parents struggling with jobs and money problems, but they never gave up. My mom did multiple jobs as tailor and rice seller. I helped her with sewing in my free time. I think I inherited from them resilience and hardworking traits. I did not have regular internet until university. Fortunately, the Vietnamese school system allowed me to progress.

University in France

I was expected to be a mathematician from primary school through the first years of Hanoi University of Science, participating in a number of national and international competitions. Success at these events led me to the ParisTech examination for an engineering degree in France. My English and French were limited, but a positive attitude and a smile won me a place at ESPCI Paris with a scholarship from TotalEnergies. I decided on physics and chemistry as an opportunity to explore new fields while retaining my passion for logic and complex problem solving. Moving to France was a luxury upgrade with a single room for myself. In Vietnam, I had shared a room with 11 girls in the university dormitory.

I found the engineering programme difficult due to my limited French, as well as the new focus on physics and chemistry. Every day I asked for extra time with the

professors. In the end I graduated with fluent French in several technical projects with high grades.

Why did you switch to geoscience?

My interest in geosciences sprang from different internships and industrial visits. Contact with mathematical models revealing the Earth’s structure and behaviour, as well as the effort required to use these models, struck both my inner mathematician and engineer as both beautiful and complex. I found seismic data is very colourful and powerful.

Studying in London

I was conditionally accepted to the MSc petroleum geophysics degree at Imperial College London (ICL) but was required to improve my English. The language barrier was manageable compared with the expensive living cost in London, beyond my savings and sponsorship. I rented a small room with a single bed in a shared flat, close to ICL to avoid the underground cost and travel time. I minimised my expenses by buying £2-vegetable packages from Sainsbury. I was grateful for financial help from the family of a good friend at ESPCI, paying them back later.

Internship experience

I did several academic projects such as photochemical grafting, probabilistic visual learning, experimental soundwave study, industrial internships in 4D seismic acquisition, geostatistics, and a PhD project in full waveform inversion with SEISCOPE consortium and TotalEnergies.

I always enjoyed the problem solving and logical thinking to put different puzzles together to solve a global integrated problem, which is close to my work today.

Your job in Angola

I am now a senior reservoir geophysicist for the Block 32 Kaombo deep-water offshore project. This combines all my previous experiences and includes well placement, reservoir monitoring and management. I am also learning the reservoir engineering job in parallel with my current geophysicist role. I truly enjoy the dynamic environment with daily operational and technical challenges, which require rapid action, integrated competences and strong teamwork.

View of energy transition

Energy transition is urgently needed, especially a sustainable transition, but fossil fuels will still be needed in the next 20-30 years. The O&G industry is carbon intensive, but it also has room for improvement. Many ongoing projects undertaken today might save significant amounts of GHG in the future. Geosciences are crucial to decarbonise energies in the coming years. Geothermal energy, CO2 storage and hydrogen production seem to be promising leads to decarbonise our society. I am trying to play a small part, by helping young people through the EAGE Student Committee.

Future career possibilities

I am interested in becoming an energy architect, who can design smart energy plans to participate in the future energy transition.

MORE TO EXPLORE

CROSSTALK

BY ANDREW M c BARNET

Many shades of green

Goodbye CGG, Hello Viridien. Just like that, a name that has stood the test of time (over 90 years) was last month consigned to the history books, with the new company look on display for the first time at the EAGE Annual Meeting in Oslo.

This is a big, bold decision. No one understands that better than Richard Branson, who can legitimately claim the Virgin brand launched in 1972 to be one of the most distinctive trading names ever invented. It may be slightly over the top (part of the brand), but he has said: ‘The way a company brands itself is everything. Beyond your products or services, it will ultimately decide whether or not a business survives.’ Branson should know as he has had a few brushes with insolvency over the years.

CGG certainly approached the challenge with enviable bravado. It has effectively appropriated the colour of the moment, green, with a made-up adjective, or at least not one found in standard French or English dictionaries. Artists in Francophone and English-speaking countries know all about viridian green spelt with an ‘a’. It originates from the Latin viridis. The smart trick of Viridien has been to give the ending, ‘-ien’, a French intonation when in fact viridian is a legitimate word with the same meaning in both languages.

In the geoscience business Polarcus, the marine geophysical contractor, launched the company with a series of vessels in a striking green claiming ‘a pioneering environmental agenda’, albeit in the context of finding hydrocarbons. The fleet is now turning blue since acquisition by new owner Shearwater Geoservices. Interestingly, green and blue are next to each other on the spectrum, and the two colours in the English language have at times been colexified, i.e., expressed using a single umbrella term. Wikipedia among other sources lists languages which do not distinguish between the green and blue, for example old Chinese, Thai, old Japanese, and Vietnamese.

‘Approached the challenge with enviable bravado’

From a branding perspective, coming up with invented words is nothing new. In 2018 Norway’s national oil company Statoil changed to Equinor most obviously to rid itself of the ‘oil’ label. There has never been a particularly convincing explanation of the ‘Equi’ part of the new name, but it has a good ring to it.

Viridien is right on the money in expressing an intention to be environmentally conscious. The logo is blue on one side and shades to green on the other. BP started the trend in the oil and gas industry more than 20 years with its green and yellow Helios sun symbol to represent the brand’s renewed environmental awareness and green growth strategy. Unfortunately, the 2010 Deepwater Horizon disaster in the Gulf of Mexico on BP’s watch put a big dent in the company’s green aspirations.

CGG’s new image is said to reflect the company’s transition since 2018. That’s when it decided to go ‘asset light’ and began divesting itself of all data acquisition services especially its operation of marine seismic vesssels, a process due to conclude in 2025. Sophie Zurquiyah, now CEO of Viridien, states that the rebranding ‘connects our company’s history to our future, confidently positioning us for accelerated growth as an advanced technology, digital and earth data company’. The company’s industry-leading high-performance computing, the Sercel seismic acquisition equipment arm, geoscience and earth data services, and sensing and monitoring are key segments plus ‘beyond the core’ new businesses’.

Viridien is of course still wedded to the oil and gas business for the bulk of its revenue, which arguably makes an association with green a stretch. However, its intended focus on supporting the energy transition sphere with its planned growth in low carbon markets, such as carbon capture and storage and minerals and mining, does signal a change of direction. Green does imply fresh and new, among other things

In history, green has been a fickle colour, by no means always carrying positive connotations. Our understanding of green as shorthand for environmentally beneficial policies is of very recent vintage. Unlikely as it may seem, a number of sources suggest

that the modern usage may derive partly from a movement by builders’ labourers around Sydney, Australia in 1970. Known as ‘Green bans’, they refused to work on what was regarded as the socially undesirable spread of office-block skyscrapers, shopping precincts and luxury apartments at the expense of green spaces and affordable housing.

Commentators believe this example of environmental activism influenced the emergence of the first political parties anywhere to run on an environmental platform, namely the United Tasmania Group in an Australian state election and the Values Party in a New Zealand general election, both in 1972. Like-minded political groups sprang up in a number of European countries, but the Green Party of Germany (Die Grünen) was the first specifically called Green to win representation at national level, in the 1983 federal election. From 1998 to 2005 it formed a coalition with the governing Social Democratic Party.

Very much in the same period, a small group of anti-nuclear activists in Vancouver, Canada came up with a crackpot plan to sail an old fishing boat to a proposed US military testing site at Amchitka, a tiny volcanic island off western Alaska. They christened themselves and the boat Greenpeace. During the voyage, a founding member Bob Metcalfe told CBC radio, ‘We call our ship the Greenpeace because that’s the best name we can think of to join the two great issues of our times, the survival of our environment and the peace of the world […] We do not consider ourselves to be radicals. We are conservatives, who insist upon conserving the environment for our children and future generations.’ Inevitably the ship was turned back but a movement was born.

there are plenty of references like the traditional ‘Greensleeves’; Tom Jones warbling about ‘the green, green grass of home’, and Donovan’s lyric ‘Green is the color of the sparkling corn, in the morning, when we rise …’

There is further amusement to be had in identifying and naming the many shades of green beyond your familiar greens such as emerald, olive, hunter, sage, mint, forest, etc. The actual number of shades is almost limitless because in nature greens vary with the light and the changing seasons not to mention how our eyes perceive colours, a science in itself. A question for Viridien is exactly which shade of green has the company adopted.

In art and decoration mixing shades of green has a long history of being problematic. In ancient Egypt, copper mineral malachite was used to paint green on tomb walls, but it was expensive and easily turned black over time. The Romans soaked copper plates in wine to create verdigris, a green pigment that comes after weathering the metal. A modern example is the green hue of the Statue of Liberty, New York which started off as a bright copper bronze when erected in 1886.

‘Green has a long history of being problematic’

Over time more green pigments were developed from natural materials, such as plants, but the colours would eventually fade. Some early Renaissance painters would paint faces with a green undercoat, then add pink. However, over the centuries the pink has faded, making some faces appear a sickly green.

Green over the centuries has not escaped being embroiled in religion and nationalism. These days Ireland’s embrace of green is associated with the Catholic Church and St Patrick of Ireland, although there is a more convoluted story of how Irish republicans also came to adopt green. In her very readable ‘Secret Lives of Colours’, Kassia St Clair notes that green was almost synonymous with paradise for Muslims in times past and was the favourite colour of the Prophet Mohammed. This is why so many predominantly Islamic countries such Iran, Iraq, Saudi Arabia, Pakistan, Libya and Bangladesh incorporate green in their national flags.

Today it is extraordinary how the environmental green has usurped other modern usages. In fact, a fun parlour game would be to come up with all the contexts in which green can be used, e.g., green for go, envy (Shakespeare’s ‘green-eyed monster’ in Othello), sickness, inexperience, hospital green, vegetable greens, green (in golf and other sports), green fingers (gardening), greenbacks (US money), etc. Who would have thought that in Britain, the leather seating in the House of Lords is red compared with the green in the House of Commons, the former regarded in the past as rich and regal and the latter seen as more common. In song,

The big but not entirely beneficial change in green dyes and pigments came with Scheele’s Green, named after a Swedish chemist Carl Wilhelm Scheele. In 1872, he introduced a deadly, bright green hue made with the toxic chemical arsenite. This was rapidly adopted and by the end of the 19th century had replaced previous mineral and vegetable dyes.

Scheele’s Green was used on paper, wall hangings, fabric, and even children’s toys. According to various sources, some 19th century journals reported children becoming ill in bright green rooms, and ladies in green dresses becoming sick from consuming the toxic vapours.

For a French company championing green, it is perhaps unfortunate that various historians attribute the death of Emperor Napoleon Bonaparte in exile on the island of St Helena to the poisonous green wallpaper in his bedroom. Later in the century a similar far more vibrant pigment called Paris Green replaced Scheele’s Green and was used by French Impressionists such as Claude Monet, Paul Cézanne, and Pierre-Auguste Renoir to create their lush green landscapes. But it was equally toxic. It may have been responsible for Monet’s blindness. Queen Victoria got wind of its dangers when a guest complained of feeling ill and had any green rooms redecorated.

Paris Green is no longer in use, but its story underlines the line sung by Muppet character Kermit the Frog – ‘It is not that easy being green’.

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

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UK has 100 days to save its North Sea oil and gas sector, say industry leaders

The next UK government will have 100 days to save 100,000 jobs in the North Sea energy sector as companies consider pulling investment from the UK.

The 39th Energy Transition Report from Aberdeen and Grampian Chamber of Commerce reveals a sharp decline in work across production, exploration and renewables as investors await the outcome of the UK general election on 4 July.

‘Industry confidence in UK activities has plunged to a record low, according to the long-running survey, with high taxes and a potential exploration ban threatening to bring our world-class domestic oil and gas industry to a premature end,’ says the report.

With a leading investment bank warning that up to half of the direct and indirect jobs supported by the North Sea could be lost inside just five years, the chamber estimates that the party which wins the election has 100 days to restore confidence or face losing investment worth £30 billion.

The chamber is also making fresh calls for an independent body, free from political influence, to oversee the energy transition. The body should be charged with developing recommendations which could command cross-party consensus.

The report also calls for a ‘relentless focus’ on renewable energy job creation in wind power manufacturing and operations, hydrogen, carbon capture and decommissioning.

The chamber’s Energy Transition Survey sponsored by KPMG and ETZ shows that companies expect only around half of their work (51%) to be in renewables by 2030 up from 34% currently. Profitability and the regulatory environment are listed as the biggest barriers to diversification into renewable energy.

The 39th edition also finds that despite the oil price remaining over $80 a barrel, confidence among companies working in the UKCS is now lower than the financial crash and the pandemic when oil prices had slumped to around $16 a barrel.

Tax, the political environment and market stability are the three biggest concerns facing companies based in the UK energy sector, the report says. UK-based companies are increasingly focusing their investment and resources in overseas projects and markets.

More than two-thirds of energy sector companies (77%) believe that none of the UK’s political parties are putting forward the correct strategy for energy transition.

All companies are facing acute recruitment challenges, with half losing more people than usual to retirement. A third are losing more staff than usual to overseas projects.

Russell Borthwick, chief executive at Aberdeen and Grampian Chamber of Commerce, said: ‘The Energy Transition Survey has charted the highs and lows of the UK’s energy sector for the past 20

years, but never before has its findings been so important; and the need for action so urgent. From our survey and listening to focus groups, we believe the next government has just 100 days to convince industry that there is a future in the UK Continental Shelf.

‘Failure to do so will result in the current apathy, which is evident throughout this report, turning to open revolt, where companies move their resources on to countries which offer a less hostile business environment and better returns. Privately, industry leaders are being very clear that this will be the outcome of an extended windfall tax with scaled back allowances. Should this transpire, our path to net zero could look more like a road to nowhere. A road that leaves the UK poorer, less energy secure, and beholden to foreign regimes for the energy we need to keep the lights on and our economy running.’

Paula Holland, office senior partner for KPMG in Aberdeen, said: ‘It is quite clear that political instability in an election year, ever changing tax policy, and uncertainty in the market understandably weigh very heavily on the minds of those who responded, outstripping concerns over the oil price for the first time. This difficult climate is seen as a significant barrier to the speed of UK diversification to renewables and other low carbon sources of energy, something which is closely tied to the investment decisions being made in this region every day.

PGS starts delayed 4D survey offshore Brazil

PGS has started a large 4D seismic acquisition contract over the Barracuda Caratinga fields offshore Brazil for client Petrobras.

The contract was awarded in December 2022 and was expected to start in the summer of 2023 but was delayed. The Brazilian Institute of Environment and Renewable Natural Resources (IBAMA) has now issued the final permit.

To optimise vessel resources during the 2023 summer season, the vessel Ramform Victory was reactivated and earmarked for this project. ‘While permitting was pending she undertook a series of attractive multi-client programs,’ said PGS.

Ramform Victory was expected to start data acquisition for Petrobras in early June. The contract has a duration of approximately eight months and will be completed in 2025. In September Ramform Victory will be joined by the PGS Apollo as a source vessel, to complete the more complex areas of the survey.

‘The operations and sales and services organisations of PGS have been working relentlessly to plan and optimise this project and as with previous large and complex 4D projects in Brazil the team is ready to deliver according to the plan in Barracuda Caratinga. We are happy to see Ramform Victory begin mobilisation,’ said Adrian Burke, PGS vice-president Brazil.

Meanwhile, PGS has won approval to reprocess and expand its Uruguay MC3D dataset.

Originally acquired in 2012 as survey 3DUR12 and covering an area of 15,600 km2, the data provides insights into the Pelotas Basin’s prospectivity.

The reprocessing sequence is designed to unlock further opportunities within these basins, said PGS.

The data rejuvenation will be conducted by a team of geophysicists in the PGS imaging centre in Houston. Innovative technology, including imaging with multiples (SWIM), Full Waveform Inversion (FWI) and Least Squares imaging will elevate the level of resolution and bring the fidelity required to further develop this frontier basin.

CGG becomes Viridien

CGG is changing its name to Viridien.

Shareholders approved the name change at the company’s Annual General Meeting on 15 May 2024.

Sophie Zurquiyah, CEO of Viridien, said: ‘Our new name, Viridien, connects our company’s history to our future,

confidently positioning us for accelerated growth as an advanced technology, digital and earth data company.’

The company launched the new Viridien brand on 10 June at the EAGE Annual Conference in Oslo, aiming to strengthen its focus across solutions including the

core businesses of geoscience, earth data and sensing and monitoring, as well as new offerings in both the low carbon markets of minerals and mining and CCS, and markets beyond energy in high-performance computing and infrastructure monitoring.

EMGS reports $1.3 million revenue from late sales

EMGS has signed several late sales licensing agreements related to its existing Norwegian EM multi-client library, with a total combined revenue of approx. $1.3 million.

Meanwhile, the company has reported first quarter revenues of $0.2 million in contract sales, down from $5 million in the first quarter of 2023 and down from $1.1 million in the fourth quarter of 2023. There was no multi-client revenue.

Adjusted EBITDA (including multi-client expenses and vessel and office expenses) was a loss of $3.8 million, down from $2.4 million in the first quarter of 2023.

Free cash decreased by $1.6 million during the quarter, to $8.7 million.

The vessel Atlantic Guardian completed the acquisition of a fully prefunded multi-client survey in the North Sea and started transiting towards Brazil.

Ramform Victory Vessel.

Atlantic Guardian vessel. Picture: Tom Gulbrandsen.

Polaris prepares large 2D survey in Namibia

Polaris has won a 200-line km 2D seismic project at PEL 93 licence in Namibia for a consortium of Monitor (37.5%), Legend (15%), Namcor (10%) and 88 Energy (37.5%).

The program, expected to commence mid-2024, will focus on confirming the structural closures of the 10 independent leads identified from airborne geophysical methods and partly verified using existing 2D seismic coverage.

The 2D seismic program is expected to be completed in Q3 2024, with processing of the data anticipated to be completed in Q4 2024.

Results from the 2D seismic program will then be incorporated into existing historical exploration data over the acreage and used to identify possible exploration locations for drilling next year in the unexplored 18,500 km2 Owambo basin.

‘Subsurface investigations completed to date on PEL 93 by Monitor have incorporated a range of geophysical and geochemical techniques to assess and validate the significant potential of the acreage,’ said 88 Energy. ‘This included

airborne geophysical methods, reprocessing of existing 2D seismic coverage, as well as measuring methane and ethane concentrations in soil samples over interpreted structural leads, validating the existence of an active petroleum system. Passive seismic anomalies from this work were noted to align closely to both interpreted structural leads and measured alkane molecules (c1-c5) concentrations in soil.’

A series of large, anticlinal structures have been identified in the onshore Owambo Basin, collectively expected to represent a resource of equivalent size to recent offshore Namibian discoveries.

Initial exploration in the Owambo Basin has focused on the shallow Karoo Play, but attention has turned to the substantial potential of the deeper and largely untested Damara Play.

The Damara Play resulted from a large fold and thrust episode, providing significant independent targets for exploration of an equivalent size to those discovered offshore Namibia, where there has been a ~88% success rate in exploration (with recent wells discovering oil).

The Damara Play is characterised by several coherent, mappable, large fold and thrust structures which generated 19 prospects and four leads. The target structures observed are up to 25 km in length and 3 to 5 km wide. Reservoirs are expected to be encountered in both the marine/fluvial sandstones of the Mulden formation as well as the fractured carbonates of the Otavi Supergroup. The Otavi group was penetrated in well 6-2 and contained oil shows on mud logs as well as oil saturations in side-wall cores.

The Pre-Rift Karoo play interval is interpreted to be late Neo-Proterozoic to Early Palaeozoic sedimentary rocks, which are analogous to the sediments of the Owambo Basin to the west.

Given their depth, modelled maturation, geological age and geochemical data from wells and seeps, the hydrocarbons are expected to be both light oil and gas.

Onshore Namibia, where a 200-line km acquisition is planned.

BRIEFS

Shell and Exxon Mobil are reported to be close to an agreement to sell their jointly owned gas fields in the Southern North Sea to independent British producer Viaro Energy.

Chevron is reported to be negotiating a sale of its remaining UK North Sea oil and gas assets, in a move that would mark its exit from the UK after more than 55 years. Chevron’s assets include a 19.4% stake in the BP-operated Clair oilfield in the West of Shetland region, the largest in the British North Sea with production of 120,000 barrels per day.

ConocoPhillips has agreed to acquire Marathon Oil in an all-stock transaction of $22.5 billion, inclusive of $5.4 billion of net debt. Marathon Oil shareholders will receive 0.2550 shares of ConocoPhillips common stock for each share of Marathon Oil common stock, representing a 14.7% premium to the closing share price of Marathon Oil on 28 May.

CNNOC has entered into petroleum exploration and production concession contracts (EPCCs) with the government of Mozambique for five offshore blocks: S6-A, S6-B, A6-D, A6-E and A6-G, located offshore Mozambique. The total area is approx. 29,000 km2, with water depths from 500 to 2500 m. The first stage of the exploration period of the blocks is four years.

Triindad and Tobago has received six bids on four blocks in its Shallow Water Competitive Bidding Round 2023/2024. Of the 13 offshore blocks on offer one bid was received for Block Lower Reserve L from EOG Resources; three bids for Block Modified U(c) from BG International, bp and EOG Resources; one bid for Block NCMA2 from bp and one bid for NCMA4(a) from EOG Resources. Data is available for the bid round via a Virtual Data Room. Production sharing contracts are expected to be awarded later in the year.

TGS launches wind measurement project offshore Germany

TGS has launched its latest multi-client wind measurement campaign using two LiDAR buoys offshore Germany in the North Sea.

The buoys are equipped with a multitude of sensors, allowing detailed measurements of wind, metocean and environmental data. The collected data is streamed to shore and used to enhance decision-making to support several current and future wind lease rounds in the German Bight.

The buoys offshore Germany, supplied by Green Rebel, will deliver a comprehensive wind data suite over a 24-month measurement campaign. In addition to wind speed measurements, the data package includes 12 months of critical metocean data, such as significant wave heights and ocean current profiles, acquired via Directional Waverider buoys and seabed-mounted Acoustic Doppler Current Profilers. Data will be continuously streamed, quality-controlled, and made available to customers daily through the Wind AXIOM platform, TGS’ site evaluation and wind data analytics tool.

Similar multi-client programs have already been deployed across the east coast of America, where five buoys are currently collecting data; and offshore Norway, where data availability has averaged 96% at 160 m measurement height throughout the deployment.

Kristian Johansen, CEO of TGS, said: ‘Early access to essential wind and metocean data significantly mitigates investment risks, empowers informed decisions, and ensures smooth project implementation across the wind development lifecycle. With this deployment, we can proudly boast eight offshore LiDAR campaigns concurrently collecting critical wind data worldwide, and more deployments are likely to follow as new areas open up globally

Meanwhile, TGS has won an Ocean Bottom Node (OBN) data acquisition contract in North America. The sixmonth-plus contract is expected to enhance a repeat client’s seismic data acquisition capabilities, facilitating more informed decision making.

UK offers 31 more oil and gas licences

The UK has offered a further 31 licences in phase 3 of the 33rd oil and gas licensing round. The licences comprise 88 blocks/ part blocks in the Central North Sea, East Irish Sea and Southern Northern Sea.

A total of 82 offers to 50 companies have now been made in the round which attracted 115 bids from 76 companies across 257 blocks and part-blocks.

The licences offered in the round would be expected to add an estimated 600 mmboe up to 2060, or 545 by 2050.

The 33rd UK Offshore Licensing Round opened on 7 October 2022 and closed for applications for licences on 12 January 2023.

On 30 October 2023 the NSTA offered 27 new licences and the merge of six blocks into five existing licences. On 31 January 2024 the UK North Sea Transition Authority offered 24 new licences made up of 74 blocks/part blocks in the West of Shetland, Northern North Sea and Central North Sea.

One of the company’s LiDAR buoys. Courtesy TGS LinkedIn.

PGS enhances data interpretation

PGS has enhanced its seismic data interpretation capability through a collaboration agreement with Onward, an Austin-based energy innovation platform.

The collaboration will facilitate on-demand data interpretation provided by Onward to enhance PGS’ existing multi-client and data on demand services and accelerate interpretation workflows.

‘The collaboration between Onward and PGS gives subscribers of PGS’ OnDemand services the option to access Onward’s interpretation services and unlock the subsurface potential of extensive seismic data sets,’ said PGS in a statement. ‘PGS subscribers that engage Onward’s additional services will have

access to Onward’s global community of geoscience and data science experts who leverage analysis tools in a cloud-based environment (subject to certain conditions).’

SeaBird wins six-month OBN contract

SeaBird Exploration has won a contract for OBN source work for the vessel Eagle Explorer in the western hemisphere with a repeat client.

The duration of the contract is six months and could be extended by six months. Mobilisation has started and the contract was expected in the second half of June.

‘The company’s backlog currently shows a combined 33 months of OBN

source work including options. Further, we see several opportunities in 2D and are exploring alternative ways of employing the company’s considerable track record and streamer capacity,’ said Ståle Rodahl, executive chairman SeaBird.

Meanwhile, SeaBird has reported first quarter revenues of $10.3 million and operational EBITDA of $4.6 million. Net interest-bearing debt is $13.1 million. Fleet utilisation is 80%.

Viridien reimages offshore Côte d’Ivoire data

Viridien has announced two multi-client 3D reimaging programs, CDI24 Phase I (3120 km2) and Phase II (6610 km2) offshore Côte d’Ivoire.

Phase I is starting immediately and Phase II is planned for H1 2025.

The multi-client programs, in association with Côte d’Ivoire’s state energy company Direction Générale des Hydrocarbures (DGH) and Petroci, will be merged with the CDI23 (6430 km2) reimaged data (originally marketed as 2023/2024 PDSM), to create a seamless and contiguous total volume of over

16,000 km2, overlapping the recently announced Calao discovery and adjacent to the Baleine field.

‘Tools, including time-lag FWI, Q-FWI, Q-Kirchhoff and advanced deghosting and demultiple techniques aim to produce clearer images of the deep structural plays not visible in the legacy data, as well as imaging the Calao Cenomanian and Baleine Carbonate fairways at unrivalled resolution, providing a step-change in understanding of the opportunities in the region,’ said Viridien (formerly CGG) in a statement.

ENERGY TRANSITION BRIEFS

UK-based Net Zero Technology Centre and German-based Cruh21 have launched a report on the potential of a green hydrogen collaboration between Scotland and Germany. The ‘Enabling Green Hydrogen Exports: Matching Scottish Production to German Demand’ report analyses hydrogen export and consumption, exploring multi-sector end use, technologies, infrastructure, and regulatory frameworks. The report indicates prospective Scottish hydrogen exports could potentially satisfy 22% to 100% of Germany’s hydrogen import volume by 2045.

Vår Energi has joined the UN-led initiative to improve accuracy and transparency of methane emissions reporting. The company has signed an agreement with OGMP (Oil and Gas Methane Partnership), an initiative of the United Nations Environment Programme (UNEP) in which nearly 140 oil and gas companies are working to improve accuracy and transparency of methane emissions reporting.

The UK North Sea Transition Authority (NSTA) has collected evidence between 4 December 2023 and 26 January 2024 on the potential principles, design and timing of a possible future levy on UK carbon storage licences. The NSTA said that it intends, unless there are major external developments, to introduce a carbon storage levy once the CCS industry is on a more self-sustaining footing.

Norway and Switzerland have signed a ‘declaration of intent’ to strengthen cooperation on carbon capture and storage (CCS) and carbon dioxide removals between the two countries.

Viridien (formerly CGG) has signed a memorandum of understanding with Baker Hughes to explore carbon capture and storage (CCS) solutions. The collaboration intends to support the rapid increase of CCS projects that is underway by providing high-quality and fully integrated end-to-end solutions to screen, select, characterise and monitor potential carbon storage sites worldwide, the companies said in a joint statement.

Image source: Anders Otnes, PGS.

Norway adds Barents Sea blocks to its APA 2024

Norway’s Awards in Predefined Areas in the Barents Sea has been expanded by 34 blocks, featuring ‘good coverage’ as a result of recently released seismic data.

Norway’s Ministry of Energy announced APA 2024 on 8 May, including blocks in the North Sea, the Norwegian Sea and the Barents Sea. The deadline for applications is 3 September.

The expanded area in the Barents Sea is located on the Finnmark Platform and covers an area of just under 9000 km2 The APA area in the Barents Sea has good coverage of 3D and 2D seismic data. Over the last five years alone, vast volumes of data have been released within the predefined areas.

The Norwegian Offshore Directorate has compiled a map of the seismic datasets released between 2019 and 2024.

The directorate pointed to a high density of released 3D and 2D seismic data in the areas west of the Loppa High and around the North Cape Basin. The released data include several modern datasets. They were processed or reprocessed in 2020/2021 and include available data in both time and depth as well as ‘angle/ offset stacks’.

The expanded area on the Finnmark Platform includes several exciting plays that can be mapped for further maturing of prospects. The seismic line from the released 3D dataset FIN13 shows fault structures at basement level with potential structural and stratigraphic traps in the Billefjorden Group, as well as overlying spiculites in the Røye Formation. The Billefjorden Group and the Røye Formation are part of proven plays.

TGS reports first quarter net loss of $16 million

TGS has made a first quarter net loss of $16 million on revenues of $227 million compared to a net loss of $8 million on revenues of $229 million in Q1 2023.

However, operating profit of $40 million was up on operating profit of $25 million in Q1 2023.

Early sales of $78 million were down on $97.5 million in Q1 20223, but late sales of $72 million were up on $45.5 million in Q1. Proprietary sales of $77 million were down on $86 million in Q1 2023.

Organic multi-client investments of $67 million were down from $133 million in Q1 2023. Free cash flow has dropped from $106 million to $10 million. Kristian Johansen, CEO of TGS, said: ‘We are pleased to see late sales of completed multi-client data in Q1 2024 showing good progress, both compared to the preceding quarter and the same quarter of last year. Further, we saw strong sales of surveys in the workin-progress phase, supporting the early sales rate of approximately 116% in the quarter. We continue to show good oper-

ating performance in our OBN business, although the activity level, as expected, remained seasonally low in Q1 2024. The strong start to the year, combined with a continued tight global oil market and increasing exploration ambitions among our customers makes me optimistic for the remainder of the year. With the PGS merger, which we expect to close on or around 1 July 2024, TGS will be perfectly positioned to support our customers’ exploration ambitions and capitalise on what we think will be a multi-year upcycle.’

PGS reprocesses data offshore Denmark for CCS projects

PGS has launched a 2D 50,000 km reprocessing project in Denmark that will provide a regional screening product for carbon storage sites in Norwegian-Danish Basin aquifers

A proven processing and imaging sequence has been developed based on PGS’ 2023 reprocessing of seven seismic sections from three 2D surveys over the Inez and Hanstholm fields.

The processing and imaging sequence, from pre-processing, through to tomography and final VTI Kirchhoff PSDM, illuminates potential CCS structures, said PGS.

The anticipated main CO2 storage targets are Triassic Bunter Sandstone in the Skagerrak-Gassum Formation and mid-Jurassic Haldager Formation. Aquifer potential in the Oligocene-Miocene is expected in a favourable 800–3000 m depth range. The area has marine shale seal lithology, with a regional chalk seal. Zechstein salt movement-induced structures are seen, with inter-salt mini basins and turtle structures.

CCS structures have been identified in the feasibility study, showing the uplift value in reprocessing legacy 2D seismic to evaluate regional CCS potential. This seismic line shows subsurface salt-induced structure and is close to the Inez-1 well, which was used in the velocity model building.

Noise Attenuation on SEAM002 Arid Model data

Mamadou S. Diallo1* and Nacim Brika2.

Abstract

The SEAM Arid model is a synthetic data set proposed by the SEG Advanced Model (SEAM) that simulates the complexity of seismic wave propagation in a desert environment characterised by surface and near-surface features such as karst, Waadi and sand dunes. The modelled data embodies typical challenges of processing land seismic acquired in the Middle East, characterised by complex near-surface conditions that include sand dunes, karst and rough topography. The complexity of the near-surface due to the strong and rapid velocity variation both vertically and horizontally produce a complex wavefield propagation that generates strong coherent and scattered noise arrivals in the acquired data. The presence of strong velocity contrast in subsurface geological horizons produces strong interbed multiples. Free-surface and interbed multiples constitute a type of coherent noise that needs to be attenuated for accurate imaging.

In this work, our main objective is to attenuate the coherent and scattered ground roll, the free surface and interbed multiples. We performed the noise attenuation on a decimated version of the original SEAM Arid model to emulate the orthogonal acquisition geometry of a conventional high-channel-count (HCC) survey often used in the Middle East. We demonstrate, through our processing workflow, how we progressively attenuate these coherent noises while minimising the damage to primary reflection arrivals. We performed isotropic Kirchhoff pre-stack time migration on the processed data using picked velocities for a qualitative assessment of imaging. Given the complexity of the near-surface conditions, an elaborate velocity model building, in depth domain, will be required for a more accurate imaging.

Introduction

The SEAM Arid model synthetics were built on a region of about 10 km x 10 Km x 3.75 km. The data simulation included 2D, 3D and VSP surveys. The 3D surface seismic Arid model was simulated with shot spacing of 50 m and receiver spacing of 12.5 m and had nearly 65000 shots. A very detailed description of the elastic properties of the Arid model and how the different features of the model were built and put together, can be found in Regone et al. (2017). The raw data used in the present study is a decimated version of the actual Arid model 3D surface seismic survey, to simulate an acquisition geometry similar to the high channel count (HCC) survey (Bakulin and Silvestrov, 2021 and 2023)

selecting dataset with shot lines/shot points spacings of 100 m and receiver line/receiver point spacings of 25 m. The seismic trace at each receiver point results from the summation of 3x3 array of receivers from the actual Arid Model data. Figure 1 shows an illustration of the decimated acquisition geometry and associated parameters. In addition to complexity brought into the data by the geologic features inserted in the near-surface (karst) and the strong vertical velocity variation caused by the superposition of unconsolidated sediment and hard rock, the data decimation provides stronger spatial aliasing into the frequency band of interest and brings in an additional challenge in data processing for noise attenuation.

1 Saudi Aramco | 2 SLB

* Corresponding author, E-mail: Mamadou.diallo@aramco.com DOI: 10.3997/1365-2397.fb2024053

Figure 1 High Channel Count (HCC) acquisition geometry design to generate decimated SEAM Arid Model data (Total number of traces 707,560,000 ~ 0.7 billion traces, size of the survey 13km x 13Km).

Source parameters

Source line spacing = 100 m

Source point spacing = 100m

Number of source line = 95

Number of shot point per line = 95

Total number of shot point = 9025

Receiver parameters

Receiver line spacing = 25m

Receivers point spacing = 25m

Number of receiver line per shot = 280

Number of receivers point per RL = 280

Number of traces per shot = 78400

Our processing workflow of the decimated data is organised in two parts. In the first part, we perform surface wave noise-attenuation using an approach that models the fundamental and higher modes including scattered noise from picked dispersion panels (Strobbia et al., 2014). We obtain the filtered data by adaptive subtraction between the raw data and the modelled – fundamental and scattered – surface waves. Then, we used curvelet-based decomposition and a local coherent noise modelling method based on Fourier-space (FX) least-square minimisation technique to attenuate residual coherent noise not associated with surface waves (Hildebrandt 1982; Seeman and Horowicz, 1993). Finally, we performed free surface and interbed multiple attenuation. In the second part, we used picked velocities to perform an isotropic Kirchhoff Pre-Stack Time Migration (KPSTM). For an assessment of the result, we stretched the time-image to depth to perform a qualitative comparison with a depth image of the processed data using the true (acoustic component) of the SEAM Arid model. Despite the known limitations of time-imaging on a dataset emulating survey environment with challenging near-conditions, we were encouraged by the ability to clearly delineate major fault features and horizons present in the SEAM Arid model.

Coherent noise attenuation

In this section we describe the methodology used to attenuate the dominant coherent noise arrivals in the SEM002 synthetic Arid Model data which consist mainly of surface waves, scattering noise and guided waves.

Surface wave attenuation

In land data, the strong surface-wave arrivals – i.e. Rayleigh waves – observed in raw seismic records and commonly known as Ground-roll result from the constructive interference of P and SV waves interacting with the free-surface. These waves propagate along the surface boundary and most of its energy is confined to the near-surface because of the exponential decay of its amplitude with depth as the distance from the surface boundary increases. In heterogenous media, these surface-waves are dispersive, meaning that different frequency components propagate with different phase velocities. The lower frequency/longer wavelengths components being generally faster and penetrating deeper into the subsurface. Analysis of surface waves yields dispersion curves that are combined with other geological and geophysical parameters to invert for a shear-velocity model than can be, for example, used in the study of the shallow earth crust as in earthquake seismology and soil characterisation in civil engineering.

In seismic exploration, surface waves are generally considered as coherent noise that need to be attenuated in order to

recover the reflection arrivals used for imaging of the deeper sub-surface. We should note however, that for near-surface velocity model building, surface waves can provide an additional source of independent information to constrain elastic full waveform inversion (FWI) (Adwani et al., 2021).

Modelling of surface waves in vertically heterogeneous media, requires first solving the elastic-wave equation supplemented with the stress-free boundary conditions at the free-surface, vanishing stress and displacement at infinity, and continuity of normal stress and displacement at vertical discontinuities of material properties (Aki and Richard 1980 Lai 1998). Imposing these boundary conditions results in a linear differential eigenvalue problem that can be solved to yield the characteristics of surface wave propagation modes defined by the multivalued eigenvalues kj(w) and eigenfunctions ri(z,kj,w) of Rayleigh wave dispersion equation. The eigenvalues kj(w) represent the frequency dependent wavenumbers and ri(z,kj,w) the associated depth-dependent displacement/stress eigenfunctions. Assuming the far field approximation conditions, the functional form of the vertical component of the Rayleigh wave displacement can be written as summation of M Rayleigh modes:

(1)

where [Az(z,x,w)]j is Rayleigh wave displacement amplitude. Detailed derivation of the above equation can be found in Lax

Table 1 Acquisition parameters.

Figure 2 Semblance plot showing a picked dispersion curve (black line).

and Lai (1998). For the purpose of forward modelling the vertical component of surface waves (both direct and scattered), recorded at the surface (i.e., z = 0), we follow the method presented in Strobbia et al., (2014) and references therein. Successful applications of the method can be found in Strobbia et al. (2011a, b) and Han et al., (2015). The surface wave modelling method detailed in Strobbia et al. (2014) bear some similarities with the method described in Lee et al., (2008). The input for forward modelling the surface-wave consists of the picked modal dispersion curves at the source or receiver stations. Figure 2 shows an example on of a semblance plot overlain with an automatically picked dispersion curve. The picked model dispersion curves are used to generate a dispersion curve volume for the entire survey. For each mode, conjugation of the integrated phase along the path from a source-to-receiver or from a source-to-scatter then finally to a receiver is used to flatten the surface wave component in the actual data, in the time-offset domain at a reference time t0, within a carefully selected time window ∇w, leaving the signal (reflected energy) non-flat with substantial differentiating curvature. Filtering is then used to separate the flat events (surface-waves) from the non-flat events (mostly signals). Removing the phase conjugation from the isolated surface wave component provides a first estimate of the noise model which is then adaptatively subtracted from the input data to yield the final output after surface wave noise attenuation(Strobbia et al., 2011).

Other linear noise attenuation

After attenuation of the dominant surface wave arrivals, we want to deal with linear arrival associated with potential guided waves and other low velocity coherent linear events in early arrival times. We used a frequency-space (f-x) domain, localised fan filters followed by mild pass of curvelet transform for high-frequency aliased events. A combination of these two steps helped us to attenuate residual coherent noise related either to higher order mode or second order scattered mode of surface wave arrivals that were not considered in the modelling step where we relied on the dispersion curve volume generated from picks of the fundamental mode and its associated first order scattering.

Figure 3 shows a raw shot gather with four cables (near and far) and illustrates the prominence of surface wave arrivals (direct and scattered). Reflection signal that can be identified by the

hyperbolic events in the individual cables clearly visible between 1.4 s and 2.4 s along the time axis. Beyond 3 s, the seismic arrivals are predominantly made of surface wave arrivals that overwhelm signal events from deeper reflectors (Figure 4a). The frequency-wavenumber plot shows evidence of spatial aliasing starting at around 10 Hz. The frequency spectrum shows a noticeable hump in the lower frequency (around 10 Hz) caused by the strong amplitude surface wave arrivals. Figure 4b depicts the result of the cumulative noise attenuation sequence applied on the raw data from Figure 3 where significant improvement in the coherency and continuity of the reflections arrivals across all offsets (both in near and far cables) and in late time arrivals are observed. We deliberately designed the attenuation to minimise the damage to the signal leading to some residual anomalous amplitude in the shot. We will attenuate these anomalous amplitude spikes in a later stage of the pre-processing sequence, after having sorted the data in the offset vector tiles (OVT) domain. Figure 5 shows a simple difference between the raw and filtered gathers of Figures 3 and 4 and demonstrates that there is no noticeable damage of reflection arrivals that constitute the signal we aim to recover. Figures 6a-c show raw and filtered stacks and the difference, demonstrating overall the successful attenuation of surface-waves with minimal impact to signal. Figures 7a-b show a 3D view a of a shot gather, and stacked data, before and after the noise attenuation, demonstrating a significant reduction in the amount of direct and scattered surface events, resulting in a significant improvement of the signalto-noise ratio (S/N) as evidenced by the strong and coherent stack responses of reflections in both the inline and crossline directions. We used the curvelet-based decomposition method to attenuate the residual high-frequency surface arrivals.

Multiple attenuation

The data after linear noise attenuation were used as input for the free-surface multiple attenuation (FSMA) flow. The data preconditioning prior to the application the free-surface multiple attenuation, included reduction of residuals amplitude spikes, incoherent and aliased low velocity events, present in the data after noise attenuation, and muting the direct/ refracted arrivals. Thus, we performed free-surface multiple modelling and adaptive subtraction in a manner that limits the damage to primary arrivals (Verschuur et al. 1992; Verschuur 2006; Dragoset et al., 2010).

Figure 3 Raw gathers. Subset of five cables from a raw shot gather of the decimated SEAM Arid Model.

Figure 8 shows an in inline stack before, after free-surface multiple attenuation and the associated noise model respectively. Considering the frequency content and character of the attenuated free-surface multiple energy in the right panel of Figure 8, we conclude that the strongest surface-related multiple energy arrivals are generated in the shallow part of the model. After free-surface multiple attenuation, we used the layer-based approach (Jakubowicz 1998, Berkout and Verschuur, 1999) to model and subtract interbed multiples. We ran several modelling iterations to identify the time window and number of layers susceptible to generating the strongest interbed multiple events. In the end, we settled on two generating layers, the top one between 800 ms

Figure 4 (a) Left hand side: Frequency-wavenumber spectrum of the shots before noise attenuation showing the aliased surface-wave noise. Right hand side panel: Frequency spectrum of a shot showing a hump in the low frequency. (b) Illustration of the of noise attenuation results. Example of noise attenuation applied to the raw data in Figure 3. Hyperbolic events (few shown with arrows) corresponding to reflection arrivals, can be clearly identified across all cables.

Figure 5 illustration of the performance of noise attenuation processing sequence showing the amount of noise removed for the raw data of Figure 3. Significant amount of linear noise mostly associated with surface-wave noise has been removed without noticeable damage to the reflection events (i.e., hyperbola in Figure 4).

and 1200 ms and the second one between 1200 ms and 1600 ms. The result is shown in Figure 9a with an inline stack before and after interbed multiple attenuation and the associated difference showing the model interbed multiple model. We evaluated the quality of both free-surface and interbed multiple attenuation by comparing the stack data after multiple attenuation with a timestretched version of the true reflectivity model (Figure 9b) and confirmed that the bulk amount of energy that was removed was indeed related to multiple arrivals. We confirmed the demultiple process by comparing the stacked section before and after attenuation of both free-surface and interbed-multiple with a version of the true reflectivity stretched to time.

Kirchhoff pre-stack time migration

In preparation for the time-migration, we sorted data into Offset Vector Tile (OVT) (Vermeer, 2012), performed additional data conditioning to reduce residual amplitude anomalies and picked velocities for Kirchhoff Pre-Stack Time Migration (KPSTM).

Figures 10a, b display results of KPSTM migration on selected in-lines cutting through the middle of the SEAM model. These cross-sections clearly outline two major fault lines present in the SEAM model as shown in Figure 7 of Regone et al., (2017). While these preliminary observations are encouraging, they only

Figure 6 Stacks’ comparisons before and after noise attenuation. a) Raw stack before noise attenuation. b) the stack after noise attenuation. Strong reflection events that were covered by noise in the raw stack are now clearly visible across the section. (c) illustration of the performance of noise attenuation processing sequence. Significant amount of linear noise mostly associated with surface-wave noise has been removed without noticeable damage to the reflection events.

Figure 7 (a) 3D view of a raw shot-gather and a stack with an inline, crossline and a time slice. On the shot gather, the strongest events correspond to surface waves. These events overwhelm the signals on the stack sections. The inset in the middle corresponds to an FK transform of a shot with evidence of a low velocity event associated with surface and evidence of an aliased surface wave arrival indicated by events dipping inward. (b) 3D view of the shot-gather and sections of Figure 7a after noise attenuation. Surface waves have been successfully attenuated, revealing coherent hyperbolic events corresponding to reflection arrivals. This results in much more coherent stacks on the inline/cross line section and a much more coherent time section. The inset in the middle with FK transform of the shot after noise attenuation does not contain the low velocity and aliasing associated with the surface waves that have been significantly attenuated.

highlight the performance of the noise attenuation processing and multiple attenuation. We still need to work on the velocity model building, with a full consideration of the complexity of the near-surface and anisotropy before we can make firm conclusions about the imaging quality.

Conclusions

We introduced a pre-processing flow to mitigate the challenging coherent noises present in a decimated SEAM Arid model. The decimation simulates an orthogonal data acquisition geometry

often used in the Middle East. We put particular emphasis on attenuating surface wave arrivals (both direct and scattered components) which constitute the dominant noise in the data and discussed some fundamentals of the model-based approach used for surface wave attenuation. The advantage of this approach over conventional methods that rely on multichannel filtering, is that it is less sensible to spatial sampling aliasing. We proceeded care-

Figure 8 Free-surface multiple attenuation: Stacked inline before (left panel), after application of surfacerelated multiple attenuation (middle panel). The left panel shows the amount of free-surface multiple that was removed.

Figure 9 (a) Interbed multiple attenuation: Stacked inline before (left panel), after application of interbed multiple attenuation (middle panel). The left panel shows the amount of interbed multiple that was removed. The two generating layers are situated between 800 ms and 1600 ms. (b) Free surface and interbed multiple attenuation: Stacked inline showing a good correlation in the time between 1300 ms and 1800 ms with synthetics (i.e., true reflectivity model stretched to time).

fully with the parametrisation of surface waves attenuation and leaned more towards leaving some residuals that we addressed in subsequent steps of the processing sequence. Considering our evaluation of processed data pre-stack/post-stack and the analysis of the spectral content, we conclude that the comprehensive sequence that included all major steps of land pre-processing sequence was effective in mitigating the bulk amount of coherent

Figure 10 (a) An inline after KPSTM migration. Two major faults are apparent in the middle of the section. Even though they may not be correctly postioned given the complexity of the near surface and lateral velocity variation, the quality of section suggests that with an elaborate velocity model building, much accurate imaging of the SEAM Arid model data can be achieved. (b) 3D visualisation of the KPSTM results with an inline, crossline and time-slice display depicting with more details the two majors vertical fault transecting the SEAM Arid model.

noise present in the recorded data. We performed Kirchhoff Prestack time migration on the processed data and were encouraged by the quality of preliminary time image. Our future work will focus on developing comprehensive velocity modelling building flow to address the imaging challenge caused by the complexity of near-surface features present in the SEAM Arid model.

Acknowledgements

We acknowledge benefiting from discussions with Saudi Aramco colleagues from the Geophysical Imaging Department (GID) and EXPEC Advanced Research Center whose effort resulted in the design of the HCC acquisition geometry of the data used in the work. We acknowledge the support of Saudi Aramco and SLB

colleagues who were involved in the early phase of testing the Omega seismic processing software.

References

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A novel simulator for probing water infiltration in rain-triggered landslides

Cassiano Antonio Bortolozo1,2, Tatiana Sussel Gonçalves Mendes1, Daniel Metodiev 2 , Maiconn Vinicius de Moraes1, Harideva Marturano Egas2, Marcio Roberto Magalhães de Andrade2, Tristan Pryer3 and Luana Albertani Pampuch1*.

Abstract

This study presents a specially designed dripping rainfall simulator, functional in both laboratory and field settings, developed to research water infiltration processes relevant to landslide studies. The simulator incorporates several advanced features, including adjustable rainfall parameters and precise monitoring and measurement capabilities for a range of experimental setups. The system’s calibration was achieved by measuring the volume of water over a set period, correlating it with the rainfall intensity. Experiments were conducted on a slope surface for up to five hours at a constant rainfall intensity. During this time, 3D electrical resistivity measurements were taken to assess the influence of rainfall on resistivity data, offering insights into the subsurface dynamics of water infiltration. The findings suggest that the combination of dripping rainfall simulation and 3D electrical resistivity analysis holds promise for advancing landslide risk reduction research. This paper provides an in-depth overview of the simulator’s design, functionality, and performance, emphasising its applicability for comprehensive landslide investigations.

Introduction

Slope failure due to heavy rainfall is prevalent in tropical and subtropical regions (Pinho and Filho, 2021; Mendonça et al., 2021; Cabral et al., 2021). In Brazil, landslides represent a significant problem associated with slope occupation (Coelho-Netto et al., 2009; Hirye et al., 2023). These disasters are particularly concentrated in mountainous areas with high rainfall, especially during the summer, resulting in numerous fatalities and material losses (Hirye et al., 2023; Bortolozo et al., 2019).

The rainwater infiltration process moistens the soil’s surface layer (Bordoni et al., 2015). This results in increased pore pressure, decreasing the soil’s shear resistance, making slopes more susceptible to landslides. Consequently, precipitation is typically recognised as the primary condition for predicting landslide occurrences (Sousa et al., 2023; Mendonça et al., 2021; Silva et al., 2021; Marino et al., 2020; Spolverino et al., 2019; Metodiev et al., 2018).

Detecting the hydrogeological conditions of unstable slopes can aid in monitoring rainfall-induced landslides (Lavalle et al., 2018a; Lavalle et al., 2018b; Hojat et al., 2017). Such detection necessitates a deep understanding of the soil’s hydromechanical behaviour, often represented by soil saturation trends (Bordoni et al., 2018).

Gathering data on soil dynamics during rainfall requires consistent precipitation that matches climatic averages. Rainfall simulators are pivotal in this experimental soil research (Rončević et al., 2022; Živanović et al., 2022). They are instrumental for

studying hydrologic processes involving rainwater-soil interactions (Lora et al., 2016; Živanović et al., 2022), as they best emulate local conditions (Spohr et al., 2015) and control the primary factors of natural rainfall (Naves et al., 2020; Rončević et al., 2023).

These simulators, which can replicate real rain conditions (Mendes et al., 2021), have been employed in a variety of studies beyond landslides, like soil infiltration and erosion assessments (Mhaske et al., 2019; Ricks et al., 2019), urban runoff monitoring (Mamoon et al., 2019), and runoff and infiltration characteristics evaluations in urban green spaces (Nielsen et al., 2019).

Rainfall simulators can be categorised as pressurised or gravitational (Živanović et al., 2022). Pressurised simulators use nozzles or sprinklers to emit rain-like droplets (Ricks et al., 2019) and operate under high pressure for a diverse drop size range (Lora et al., 2016). In contrast, gravitational simulators, known as dripping simulators, produce drops by free-fall from simulator tubes of varying diameters (Živanović et al., 2022).

Portable dripping simulators enable experiments on various surfaces (Iserloh et al., 2012), allowing researchers to generate precipitation at desired locations, times, and intensities. This capability is crucial for studying mass movements in landslide-prone areas with documented historical occurrences.

This study focuses on the benefits of a dripping simulator, emphasising its reliability, simplicity, adaptability, and uniform rain simulation. The system’s design enables the adjustment of rainfall intensities without the need for external power equipment. The goal

1 São Paulo State University (Unesp) | 2 CEMADEN | 3 University of Bath

* Corresponding author, E-mail: luana.pampuch@unesp.br DOI: 10.3997/1365-2397.fb2024054

of this paper is to present the features of the constructed dripping rainfall simulator: an affordable, sturdy tool suitable for both lab and field use, primarily for studying water infiltration and its link to rainfall-induced landslide susceptibility. Our specific objectives are to assess water infiltration using electrical resistivity and to evaluate the simulator’s utility in landslide risk mitigation studies.

Methodology

The proposed methodology consists of three main steps: designing the rainfall simulator, calibrating it, and conducting a field experiment using an electrical resistivity geophysics survey.

Rainfall simulator design

The rainfall simulator was designed based on the American Military standards (MILSTD-810G, 2014), as depicted in the schematic diagram shown in Figure 1.

The project demanded specific features, including:

• Proportional distribution of the drippers along the x and y axes, which need not be equal but should be adaptable for three-dimensional (3D) geophysical surveys.

• Stability on sloped surfaces.

• Easy transportability, ideally modular, with safe operation features like shielded needles to prevent damage or operator injury.

• Suitability for operation by a fieldwork team of 2 or 3 persons, as suggested by Loch et al. (2001). Construction from insulating material to avoid interference with electrical resistivity in geophysical surveys.

• The ability to continuously simulate rain of varying intensities.

• Assured accuracy of the rainfall intensity generated by the syringe-type simulator.

With the above features in mind, we designed the dripping rainfall simulator and constructed two simulation water tanks. The larger tank measures 1.10 m in length, while the smaller is 90 cm, with both being 70 cm wide. The larger tank houses 989 needles spaced 2.5 cm apart, while the smaller contains 759 needles, summing up to 1748 needles. The primary components such as structural support, water tank, drippers, and the rainfall intensity regulation mechanism are detailed below:

Structural Support: This support was constructed to facilitate the installation of the water tank with the drippers on top. As per Rončević et al. (2022), it needs to be rigid to prevent unwanted water tank oscillations during simulations. Additionally, this support should shield the needles from external impacts during operations or transport to avoid operator injuries. The design prioritised exceptional ground stability, enhanced by the weightier wooden feet (See Figure 2a).

Water Tank: Fabricated from an aluminum plate, the water tank boasts a box design with a top handle for transportation. Aluminum was chosen for its ease of drilling to accommodate the needles. Its flat base facilitates the proportional distribution of needles in both the x and y directions (Refer to Figure 2b). The tank’s open design ensures stability on uneven terrains without damaging the needles (See Figure 2c). It is versatile enough to be placed on the ground, on supports, or even to be suspended, as indicated by its reinforced handle and open side (Figure 2d).

Drippers (Needles): To comply with the MIL-STD-810 standard, which mandates drippers to have an internal diameter between 0.5 mm and 0.6 mm and a length of 25 mm, we explored suitable hypodermic needles. After comprehensive assessment, 21G needles were identified as having the required dimensions, as detailed in Table 1.

The holes for the needles were drilled at intervals of 2.5 cm, in accordance with the prescribed standards. The needles were secured purely by pressure, as they feature a tab on their upper edge. However, it’s essential to note that the drilled hole should be slightly smaller than the needle to ensure a snug fit and prevent water leakage. It’s also worth noting that the needles must be pressed until their entire edges make contact with the metal plates. Failure to do so can lead to uneven needle levels along the tub, resulting in misalignment, water leakage, and an increased risk of the needles falling.

With this system, there was no need for additional sealing for the needles; only the folds of the plates and the areas where the screws secure the lower supports of the devices required sealing.

Mechanism of rainfall intensity regulation: The rainfall intensity regulation is a critical task for accurately simulating various precipitation conditions. In our rain simulation system, the rainfall intensity is controlled by the height of the water column in the simulator body, which is regulated by the float valves installed at the water inlet in the water tank (Figure 3).

Figure 1 MIL-STD-810G standards for simulating rain on military equipment (image adapted from the MILSTD- 810G manual). Gaunge

Table 1 Suitable hypodermic needles characteristics classified as 21G.(Source: hamiltoncompany.com).

3 Illustration of the float valves installed in the tubs, highlighting their design and adjustment mechanism.

The selection of float valves for the water tank was selected by their compact dimensions and ability to function with small water columns, typically a few centimetres in height. Moreover, these float valves incorporate a dual height adjustment system, consisting of a lateral butterfly valve (Figure 3) and a finer adjustment mechanism that manipulates the length of the float arm. Notably, both adjustments can be made without disassembling the float valve from its position in the device. This system has been demonstrated to be an effective strategy for controlling the intensity of simulated rainfall.

Calibration

An essential aspect of calibration is assuring the accuracy of the rainfall intensity generated by the syringe-type rainfall simulation device. This is achieved by establishing a collection system beneath the device to capture the simulated rain. The collection system consisted of a round container, 34 cm in diameter and 15 cm high, placed in the centre of the device for 15 minutes. After a period of 15 minutes, the water content was measured in a volume measuring container and with this it was possible to calculate the volume of rain per hour.

During the calibration process, maintaining a constant water height is critical. To modify the desired rainfall intensity, the floats within the device can be adjusted, consequently changing the water column height. By meticulously adjusting the floats and measuring the resulting rainfall intensity using the methodology described previously, the device can be calibrated effectively.

To calculate the intensity of the simulated rainfall, I in mm/h (the standard scale), the following Equation (1) can be used.

(1)

Where:

Figure 2 Photos of the simulators showing how the needles were installed (a), the simulator supported on concrete blocks (b), the simulators supported from the sides (c), and how they can be used when lifted (d).

• I (mm/h) is the simulated rainfall,

• A (m2) is the area of the device used to capture the simulated rain,

• V (m3) is the volume of water captured in the device in t minutes.

This calibration procedure ensures the simulated rainfall accurately reflects the intended intensity for research purposes. It empowers researchers to recreate specific rainfall conditions in controlled environments, facilitating the collection of precise and dependable data for scientific investigations.

Field experiments

The simulators were tested continuously for up to five hours, maintaining a steady water level and, consequently, a constant rainfall intensity. Several key points emerged during the testing process. It was crucial to ensure the water tanks were level on both axes to prevent water pooling in specific areas (Figure 4a and Figure 4 b). Even in instances where the simulators were placed on slopes, it was mandatory to level the devices to ensure an equal amount of rainfall over the tubs’ area (Figure 4c). At the onset of each simulation, additional water was introduced to hasten the stabilisation of rainfall at the defined intensity, as reaching the desired column height in the water tank can take some time (Figure 4d).

For field experiments, the support structures were positioned on blocks to prevent needle contact with the ground, thereby improving the realism of the rain simulation. Moreover, it was also necessary to confirm that the water source had sufficient flow capacity to maintain the desired rainfall intensity, as higher intensities necessitated a significant volume of water. Regularly monitoring the water level with a ruler was recommended to ensure its stability throughout the experiment.

Figure

These considerations and precautions were vital for the accurate and reliable simulation of rainfall using the developed tubs. By adhering to these points, it was possible to ensure consistent and controlled rainfall conditions in the experiments, thereby enabling the collection of dependable data for scientific analysis.

Electrical resistivity surveys were carried out as initial experiments to verify changes in electrical resistivity in the soil during the simulation of rainfall, considering a controlled scenario. To this, the electrode mesh covered an area of 1.25 m x 2.7 m, with the rainfall simulator positioned in the middle (Figure 5a). For data acquisition, we used the Polo-Dipole Forward array with a 25 cm spacing between the electrodes.

The rain simulation was applied to the entire area of the resistivity block to ensure consistent coverage. Each 3D block of the survey comprised a total of 630 measurement points, acquired

3 Illustration of the float valves installed in the tubs, highlighting their design and adjustment mechanism.

Figure 4 Important points regarding the use of rainfall simulators. In a and b, the leveling of the tubs in both axes; in c, the use of the devices on a sloping surface; and in d, the addition of extra water at the beginning of the operation.

within less than 4 minutes. To capture the temporal variability, rain was simulated over a period of 180 minutes (3 hours).

Results and discussion

The experiments using a geophysical survey with the rain simulation enabled us to examine the impact of rainfall on resistivity data and better understand the subsurface dynamics associated with the study area (Bortolozo et al., 2023a; Bortolozo et al., 2023b). Figures 5b, 5c, and 5d display the resistivity after 1h, 2h, and 3h from the start of the rainfall.

This integrated approach offers valuable insights into the temporal changes in subsurface resistivity caused by rainfall, allowing for a more comprehensive analysis of hydrological processes, particularly in relation to infiltration and subsurface water movement.

Figure

The changes in the resistivity block are clear and demonstrate how water infiltration alters underground saturation (thereby decreasing the electrical resistivity). Such results are especially relevant in landslide studies that focus on determining changes in underground cohesion, which is directly related to changes in saturation.

Although the American military standards (MILSTD- 810G, 2014) served as a reference, the development faced more significant challenges for this research as the system was designed to operate beyond laboratory confines, specifically in the field.

In the context of an outdoor experiment, precise measurement of environmental variables and those within the experiment itself is critical for determining results. Factors such as slope, lithological conditions, terrain typology, and climatic conditions need to be accounted for.

Other experiments were conducted using rainfall simulators and to provide a comprehensive and detailed visualisation of the studied processes, some videos were created. These videos are divided into two groups, one in English and the other in Portuguese, showcasing the results of the experiments conducted. The links to the videos are provided below:

English Group:

1. Video 1: https://youtu.be/G_t2e1-Z4rw

2. Video 2: https://youtu.be/l23zeLiPLPM

Portuguese Group:

1. Vídeo 1: https://youtu.be/g1G7oNojwBs

2. Vídeo 2: https://youtu.be/17HBDmmOk2c

These videos offer a visual representation of the experimental setups, rainfall simulation, and the corresponding outcomes. The videos serve as a valuable resource for researchers, practitioners, and interested stakeholders in gaining deeper insights into the dynamics of the studied processes. They enhance the clarity and accessibility of the research outcomes, allowing for a more comprehensive comprehension and interpretation of the experimental data.

Although the American Military standards (MILSTD- 810G, 2014) served as a reference, the development faced more significant challenges for this research as the system was designed to operate beyond laboratory confines, specifically in the field.

In the context of an outdoor experiment, precise measurement of environmental variables and those within the experiment itself is critical for determining results. Factors such as slope, lithological conditions, terrain typology, and climatic conditions need to be accounted for.

Conclusion

This paper presents the development and calibration process of a dripping rain simulator. Additionally, initial experiments were conducted to verify the equipment’s capability to simulate real rainfall and to understand water-soil interactions through geophysical surveys.

The developed simulator meets the initial requirements set for experimental research. Based on experimental results, the rainfall simulator proves its potential for both laboratory and field studies, especially in landslide research.

The device detailed in this article boasts several features that establish it as an invaluable tool in this domain. This rainfall simulator offers a cost-effective solution that’s easily

Figure 5 Field experiments. a) the field setup. b) the resistivity block after 1 hour of rainfall. c) after 2 hours. d) after 3 hours.

transportable and adaptable to various environments. Its solid construction ensures durability and stability, permitting consistent and trustworthy simulations over extended durations.

During calibration, the device’s proficiency in accurately controlling rainfall intensity was affirmed, thereby enabling realistic rain simulations. This is crucial for landslide studies and for evaluating possible risk mitigation strategies.

Pairing the rainfall simulator with a 3D geophysical survey allows for an in-depth understanding of the subsurface’s response to rainfall and its subsequent hydrological implications. The seamless integration of the rainfall simulator and the geophysical survey system presents a potent method for exploring the dynamic interplay between rainfall and subsurface properties, furthering our grasp on landslides and associated events.

Acknowledgements

Cassiano Bortolozo would like to thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the Postdoctoral Scholarship (grant 152269/2022-3), for the Research Fellowship Program (grant 301201/2022-6) and also for the Research Financial Support (Universal Project grant 433481/2018-8). All authors would like to thank FINEP (Financiadora de Estudos e Projetos) for financing the REDEGEO project (Carta Convite MCTI/FINEP/FNDCT 01/2016), responsible for the PCD Geo network installation.

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Spolverino, G., Capparelli, G. and Versace, P. [2019]. An Instrumented Flume for Infiltration Process Modeling, Landslide Triggering and Propagation. Geosciences, 9, 108. https://doi.org/10.3390/geosciences9030108.

Živanović, N., Rončević, V., Spasić, M., Ćorluka, S. and Polovina, S. [2022]. Construction and calibration of a portable rain simulator designed for the in situ researchof soil resistance to erosion. Soil and Water Research, 17(3), 158-169. https://doi.org/10.17221/148/2021SWR.

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Modelling and interpretation techniques are vital to identifying the hydrocarbons that the world still needs but also for modelling reservoirs for carbon capture, storage and utilisation (CCUS) and other renewable energy sites.

Artificial intelligence and machine learning are leading to advances. The latest high-performance computing and algorithmic innovations aimed at enhancing the quantity and quality of modelling and interpretation are presented here.

Brian Russell explains why an EAGE president’s address over 60 years ago is as relevant as ever.

Christoph Georg Eichkitz et al present survey planning of geoelectric measurements and sub-bottom profiling in the entire area between Spielfeld and Bad Radkersburg in Austria.

Neil Hodgson et al present seismic processing and imaging techniques for building a geologically driven velocity model to successfully explore the basin.

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Revisiting Baranov’s thoughts on mathematics and geophysical interpretation

Brian Russell1,2* explains why an EAGE president’s address more than 60 years ago is as relevant as ever.

Introduction

I have always felt that there is a prominent role for mathematics in geophysical interpretation and this is a sentiment shared by many of my colleagues. Recently, when doing a literature search into the origins of seismic inversion, I came across an article by Vladimir Baranov which made this argument extremely well.

Baranov (1897-1985) was a researcher at CGG in the 1950s and 60s having come from Russia where he fought with the White Army before being evacuated in 1920 from Gallipoli to Bulgaria arriving in France in 1925. He studied geophysics at the École Normale in Paris, and worked in the Middle East before joining CGG.

This article was a presidential address in 1959 when Baranov was president of the European Association of Exploration Geophysicists, as our society used to be called, in those days. Even though this address is 65 years old, I think it is still as pertinent today as it was then.

Interestingly, Baranov’s address was published in French. In those days, the EAEG was a truly European society and accepted publications in English, French or German. In the late 1970s, while I was with Chevron, I remember laboriously translating an article called Analyse Sequentielle by Patrick Bois (another CGG researcher), from French to English because I wanted to understand his

application of the Hadamard Transform to seismic data. Luckily, thanks to Google Translate, translation is much easier and I was able to translate Baranov’s address with a lot less effort. It only needed light editing.

What follows is the translation of the 1960 EAGE Presidential Address, originally entitled Role des mathematiques dans l’art de l’interpretation by V. Baranov. I hope you find it as interesting as I did. One final point I should make is that the Compagnie Générale de Géophysique, or CGG (now Viridien), received the 2006 SEG Distinguished Achievement Award, and in the list of key researchers, V. Baranov’s name was listed second.

The role of mathematics in the art of interpretation

By V. Baranov

August Comte said that a set of facts relating to a specific subject cannot constitute a real scientific discipline without being ordered by mathematics, the fundamental science.

But can we really talk about science when discussing applied geophysics? Personally, I have always thought that applied geophysics was more of an art. Malicious spirits will no doubt claim that it is simply a profession. But we prospectors know that imagination is one of the essential attributes of our profession. When the imagination submits to a discipline based on reasoned knowledge, the profession rises to the rank of an art.

Also, even if it means appearing presumptuous, I dare to declare that applied geophysics is an art guided by several sciences: physics, geology and mathematics. Geology, however, holds a special place in this trio. It is permissible to wonder, indeed, if it is geology that serves geophysics, or the reverse. Philosophia ancilla theologiae (Philosophy is the handmaiden

1 Geosoftware | 2 University of Calgary Alberta

* Corresponding author, E-mail: brian.russell@geosoftware.com

DOI: 10.3997/1365-2397.fb2024055

of theology), says an old maxim; likewise, applied geophysics is admitted to the honour of serving geology. But, in return, geology provides support for the interpretation of geophysical data.

The geophysicist takes measurements in the field and then acts as a physicist. Once the measurements have been taken, he or she undertakes to interpret them: the results are attributed to complex causes which the geophysicist tries to characterise and evaluate exactly.

We know that this problem is almost always indeterminate, very diverse causes being able to have the same practical effect. This is where geology comes to the aid of the geophysicist, in two ways. On the one hand, even incomplete, the geological data constitute a starting point, which the interpreter extrapolates beyond the limits that the geologist cannot cross: this work is controlled by calculation and by mathematical reasoning. On the other hand, certain considerations of geological likelihood, based on a general knowledge of the region, make it possible to choose

between several constructions that are also compatible with the measurements.

All these combined approaches lead any interpreter worthy of the name to a valid result: not rigorous, nor even guaranteed to be correct, but the most plausible and satisfactory result for both the physicist and the geologist. This approximate solution, sometimes fragile, is at the mercy of additional measurements, or geological information plus the human factor which is not absolutely excluded. It is an honest result, however, at which the geophysicist sees fit to stop.

If I expose such commonplace approaches at length, it is to insist on the fact that geophysicists find themselves behaving like physicists when they make measurements. Then, like geologists they interpret the results, examine their deductions, and finally, like mathematicians, they verify their hypotheses and justify their deductions.

How do we assess the relative importance of these functions? How far should measurement count and how much interpretation effort makes sense?

When an engineer studies a bridge project, he or she must think of the deck and the piles that support it. If you increase the number of piles, the construction of the lighter deck will be that much easier and less expensive, but the piles will cost too much. Another extreme solution would be to build an extremely solid deck, without any piles: the work would not be balanced, and the total expenditure would still be very high. It is easily demonstrated that the cost price will be minimal if one spends about as much money for the piles as for the deck. And the work will be more harmonious.

In prospecting, things probably happen in a fairly similar way. It would be unwise to have recourse to varied methods, making excessively numerous, detailed, and precise measurements while limiting oneself to a hasty interpretation. But, of course, a solid experimental basis is necessary to establish a reasonable interpretation.

I do not want to push the analogy too far and compare geophysical measurements to the piles of a bridge, and the interpretation to the deck which crowns it. The measuring devices are more and more expensive, the auxiliary means that must be implemented absorb a very large part of the expenses. I would simply like to express the wish that a little more time be granted to the interpreters, and that the best chance should be given to them for arriving at summaries that can shed more light on the facts.

All the detours

Geophysicists need to think, so they shouldn’t be too rushed. Too often, they are forced to rush an interpretation without examining all the detours. They are not always given the time to check their summaries, and to prove by mathematical calculation that the hypotheses developed are compatible with the results of measurements.

Admittedly, mathematical calculation cannot replace geological thought, but only mathematics can demonstrate the validity of hypotheses. This is why in order to serve geology well and loyally, interpreters have the advantage of resorting to the precise and effective tool which is applied mathematics.

Geophysical measurements have now reached a very high degree of perfection: they are increasingly rapid and precise. However, interpreters have, it seems, neither the time nor the means to take full advantage of them and use all the information contained in the gravity and aeromagnetic maps, in the seismic sections, etc.

In fact, methods of mathematical interpretation are under-studied and far behind measurement techniques. How could it be otherwise? Geophysicists are hardly encouraged in this respect. Geologists distrust formulas, and mathematical theories annoy them. For these reasons, no doubt, quantitative interpretation and above all direct interpretation, i.e., the direct passage from experimental data to tectonic structure, has made very little progress up to now.

Seismic methods should be considered separately. It is in seismic reflection that the method of interpretation comes closest to this ideal which, in my opinion, constitutes direct interpretation. Undoubtedly, this is why the seismic reflection technique has become the undisputed queen of seismic prospecting methods, because magnetic recording tapes contain so much information that seismologists do not yet know how to use in full.

Much of what is called ‘noise’ is nothing but real information, but too dense, too complex, for its elementary constituents to be easily separated. The synthetic films with multiples that we now know how to construct confirm this point of view, since they are very analogous to real films. Seismologists prefer to get rid of excess information, to keep only the salient features. Unfortunately, conventional filtering devices are only a palliative, as the signal-to-noise ratio is barely improved. I should mention in this connection the recent work of Merlini on an interesting process of composition of traces greatly facilitating the correlation between sections. The progress achieved seems to me worthy of praise. However, I am inclined to think that there is still a long way to go in this direction. We must probably find a way to transform each isolated seismic trace to make the correlation between the traces as easy as possible.

Gravimetry

To clarify my thoughts, I will venture a comparison with gravimetry. A Bouguer anomaly map is a mixture of localised anomalies superimposed on a regional anomaly. The problem here is to remove the latter and interpret the average anomalies while striving not to exaggerate the importance of the minor anomalies. This is achieved by well-known and very varied procedures. To shed light on the reasons for this comparison, I will allow myself a slight exaggeration. Assuming that the seismic filter admits a very narrow passband, so that it passes practically only one frequency, the trace will then look a bit like a sinusoid with slowly varying amplitude. However, arrivals of reflections will necessarily bring small disturbances to the normal steady state of the filter, and for short moments other frequencies may pass. Finally, we will have an almost pure sinusoid affected by very small deviations that probably escape direct observation. Everything happens here as in gravimetry. The sinusoid is analogous to a constant or slowly varying regional anomaly, obviously in time, not in space. The weak disturbances are analogous to the local anomalies: to

highlight them, it is necessary to remove the regional anomaly, i.e. the sinusoid.

I propose this problem for the attention of seismologists and have every reason to believe that it is not insoluble. There are many other issues worth considering. I said that the art of the geophysicist consists in judiciously extrapolating certain acquired data: this is a key question. Seismologists, for example, will try to correctly interpolate the information provided from drilling results. So it is vital to focus whenever the geophysicist enters into direct competition with the driller. Without minimising the importance of improvements in equipment and work in the field, I think that we must accentuate our efforts in the interpretation office. Applied mathematics can play a great role here, and provide a reliable guide to the geologist-geophysicist.

These remarks apply just as well to other methods, to gravimetry, for example. Apparently, gravimetry currently forms a well-developed body of doctrine. We have effective ways to separate the anomalies and eliminate the regional anomaly. Many formulas have been developed for the calculation of vertical derivatives. The effectiveness and validity of these methods are disputed by some geophysicists. I do not intend to repeat their arguments here: each of the opponents is right, for different reasons. Just as the musician chooses the scale that suits him to build his harmony, so the geophysicist in the exercise of his art has the right to choose a formula to his liking, even a graphic method. And if we remain in the domain of qualitative interpretation, the values of the coefficients involved in the formulas are only of secondary importance.

Things take on a completely different aspect if we want to verify by calculation the tectonic hypothesis point at which the interpreter has stopped, and then prove that the result of its synthesis is compatible with the numerical experimental data. It is preferable for the gravity map to represent one of the derivatives as exactly as possible. We must reject simplified formulas and not shrink from the pain of a slightly longer calculation. Between an insufficiently exact derivative and a residual plotted by the graphical method, there is no need to hesitate: it is the second solution that must be adopted. However, I do not want to deny the interest of approximate vertical derivatives. Transformed maps can guide the geophysicist more easily.

Bott and Smith

Direct interpretation is more difficult in gravimetry than in seismic and has not made much progress. The only serious test seems to have been made by Bott and Smith. They established a series of formulas for estimating the limiting depth of heavy masses. Unfortunately, these formulas are abstract, and they often lead to values far exceeding the real depths. For quick checks these depth indices can be useful but for a real interpretation it is necessary to start from geological data and, above all, to be able to think like a geologist and not like an abstract mathematician. (Editor’s note: I knew both Bott and Smith when I did my MSc at the University of Durham. They both lived in the nearby town of Shincliffe and often walked home together. I sometimes followed them since I lived in High Shincliffe, just up the hill from them. But I rarely interrupted their walk, as perhaps they were developing some new geophysical ideas!)

However, I am sure that in this area, too, mathematics has not said its last word. Indeed, imagine a conscientious geophysicist (by the way, can it be otherwise?), who has just developed a tectonic hypothesis consistent with the facts and geologically acceptable. It is desirable, for greater certainty, to calculate the theoretical gravimetric influence to compare it with the experimental map. He has at his disposal a whole arsenal of processes because the calculation of integrals is a very simple mathematical operation. If there is a good match, the success of the interpretation is confirmed. But most often the interpreter is not completely satisfied and thinks that by retouching his construction he will succeed better. That’s when he can ask for the help of mathematics. How must he or she modify the theoretical structure to obtain a good match for sure?

Thus formulated, the problem probably admits of no reasonable solution. Indeed, the gravimetric problem is essentially indeterminate, and the mathematician – geologically blind, if left to himself – could arrive at a numerically exact solution, but deviating considerably from the starting point, therefore taken at random from an infinite set. To avoid this, it will suffice for the geophysicist to add an additional restrictive condition by requiring the mathematician to deviate as little as possible from the initial hypothesis.

I think that there is here a subject of study likely to interest geophysicists. If such a study led to the development of a process of successive approximations, the gravimetric, magnetic, and aeromagnetic methods could greatly benefit from it.

If I spoke to you about these two problems of interpretation – a seismic and a gravimetric – do not believe that I consider them the most important: it is simply because they interest me on a personal level. Many other issues are also important. I am firmly convinced that the future of applied geophysics depends largely on the efficiency and accuracy of interpretation. It would serve no purpose to improve the precision of the measurements, nor to increase their number, if at the same time the means were not found to use all the information contained in the considerable mass of digital data. The fineness of the interpretation must go hand in hand with that of the measurements. You must be convinced that the interpretation will become more and more difficult as more complex problems are tackled. Geophysicists must be ready to solve them.

Will the task become inextricable? I do not believe that. Part of the work will be done by analogue computers, another part by digital calculating machines. Modern electronic calculators are a very powerful means at the service of interpreters. They deserve to be placed on the same level as measuring devices. We cannot do applied geophysics without geophones, gravimeters, etc. Similarly, measurement results cannot be properly interpreted without appropriate instruments. And, among these indispensable auxiliaries, modern calculating machines occupy a place of choice. We must not forget either the help they bring to research, to the study of new problems.

Precise formulas

Let us again emphasise the fact that fast and powerful calculating machines change the way of thinking of the mathematician and, consequently, that of the geophysicist. The solution of

Geophysical description of a groundwater aquifer using the combination of geoelectric measurements and sub-bottom profiling

Christoph Georg Eichkitz1*, Marcellus Gregor Schreilechner1 and Erwin Heine2 present survey planning of geoelectric measurements and sub-bottom profiling in the entire area between Spielfeld and Bad Radkersburg in Austria.

Introduction

In the upper part of the Mur river several hydropower plants are located where sediments are accumulated above the dam. As a consequence, less amounts of sediments reach the lower parts of the Mur river in Austria. This reduced supply of sediments led to partial erosion of the aquitard of the Mur river in the lower parts. In the course of a research project for the local government the application of geophysical methods for description of the aquifer, the aquitard, and possible eroded sections should be tested. On both sides of the Mur geoelectric measurements were carried out along and perpendicular to the river to describe the top and bottom of the aquifer. Within the Mur river sub-bottom profiling was applied to describe the internal structures of the sediments. The test site was located between Mureck and Bad Radkersburg, where the Mur river is also the state boundary between Austria and Slovenia (Figure 1). In this area several groundwater wells are located on both sides of the Mur river to calibrate the measurements. A lowering of the groundwater level is observed in the wells, which could be related to the erosion of the aquifer and the aquitard. Based on these test measurements a general statement about the principal applicability of the used methods and specifically about the possibility to detect erosion of the aquifer and current aquitard. Furthermore, these test measurements are used for survey planning of geoelectric measurements and sub-bottom profiling in the entire area between Spielfeld and Bad Radkersburg (around 30 km).

Geoelectrical measurements

Geoelectric tomography is the combination of geoelectrical exploratory mapping using many electrodes arranged along a profile (multi-electrode geoelectrics) and two-dimensional evaluation in the form of a computer-aided inversion calculation. The field measurements were carried out on several days between 2-13 October, 2023 in the municipalities of Halbenrain (Austria) and Apače (Slovenia). Electrical data from six profiles with lengths of 166 m (profile 6), 415 m (profile 3), 498 m (profile 4 & 5) and 664 m (profile 1 and 2) were acquired. In this test project all geoelectric profiles were measured with a ‘dipole-di-

1 Geo5 GmbH | 2 University of Natural Resources and Life Sciences

* Corresponding author, E-mail: christoph.eichkitz@geo-5.at

DOI: 10.3997/1365-2397.fb2024056

Figure 1 Overview of the project area showing the measured geoelectric profiles, SBP profiles, and available wells at the border of Austria and Slovenia.

pole’ electrode configuration, profiles 2 and 5 were additionally recorded with a ‘Wenner’ electrode configuration (Figure 2). Dipole-dipole configurations have a large point density and therefore good resolution, but are sensitive to interference such as natural and artificial electrical potential. Wenner configurations are less sensitive to interference and to poor electrical coupling conditions (high contact resistance), but small-scale vertical resistance changes are suppressed and are also unsuitable for inclined layers. Furthermore, due to the use of an eight-channel measurement unit the measurement time with a Wenner configuration is on average three times longer and therefore the project costs are higher. Therefore, a Wenner configuration should only be selected if there are legitimate reasons.

In Figure 3 the results after inversion are shown for profile 2 (parallel to the river, Austrian side), profile 5 (parallel to the river, Slovenian side), profile 6 (perpendicular to the river, Slovenia). For profiles 2 and 5 the results for both electrode configurations are shown. In the geoelectric profiles, fine-grained sediments (silts, clays, ...) can be distinguished as low-resistance areas from coarse-grained sediments (sands, gravels, ...). The good conductivity of the fine-grained sediments (high clay content) is due to its interface properties (cation exchange capacity) in addition to the electrolytic conductivity of the pore water. The Wenner measurements show laterally a lower resolution and tend to smear horizontally. On the other hand, the horizontal boundary between the Quaternary aquifer and the Miocene aquitard is more pronounced in the Wenner measurements than in the dipole-dipole measurements.

Sub-bottom profiling

Sub-bottom profiling (SBP) systems are high resolution acoustic measurement systems that are usually single channel sources mounted to a vessel. Their field of application reaches from riverbed structure description (Wang et al., 2018; Alaamer, 2015;

Eichkitz et al., 2023), interpretation of lake sediments (Daxer et al., 2018; Fabbri et al., 2021), illumination of subsea mass movements (Vanneste et al., 2011; Freire et al., 2014) to object detection (Gregory and Manders, 2015; Geraga et al., 2020). For the acquisition of SBP data several systems with different frequencies and thus penetration depths are available. In the given project a “parametric system” was used which is based on the concept of non-linear generation of acoustic waves. This system transmits two high frequency signals simultaneously (both around 100 kHz), which leads to the generation of a new frequency that is equal to the difference between the two primary frequencies. The secondary signal has a lower frequency (between 4 kHz and 12 kHz) which allows for a better sub bottom penetration. By this

Figure 2 Acquisition of geoelectric profiles next to the Mur river.

Figure 3 Results of geoelectric measurements for profiles 2, 5, and 6. For profiles 2 and 5 Wenner and Dipole-Dipole configurations are shown. The black circles indicate positions of groundwater wells. The black line represents the interpreted top aquitard horizon.

approach a resolution of approximately 5 cm with an accuracy of ±2 cm + 0.02% of the water depth can be achieved (Heine et al., 2017). Similar to a common seismic reflection survey, these sources send sound pulses into the shallow sub-sea floor sediments. Sound pulses are then reflected at sediment layer boundaries where differences in the acoustic impedance of the upper and lower layer are present. The reflected sound pulses are then recorded by the sub bottom profiling measurement system.

On October 19th, 2023, sub-bottom profiling data with a total length of approximately 14.3 km was acquired on the Mur in the area between Mureck and Bad Radkersburg (Figure 4).

The positioning of the measurement data was carried out by a Leica GS25 GNSS receiver using real-time kinematic (RTK) positioning) via the APOS NTRIP server (virtual reference station). Unfortunately, measurements were done during a time of relative low water depths, which led to several problems during the acquisition. First, the originally planned boat ramp in the vicinity of the test site couldn’t be used due to too low water levels, hence an alternative boat ramp 3.5 km upstream needed to be taken. As a consequence, and positive side effect, measurements also started further upwards and additional data could be acquired. The low water level also posed problems for this trip on the Mur, as in some places the water depth was too shallow for the outboard engine, which was stuck several times on the riverbed. In such areas the engine was throttled and the dinghy only drifted downstream with the current. Next to the outboard engine, the low water level also led to problems with the SBP acquisition itself. The Innomar SES2000 was mounted approx. 10 cm below the water surface for the measurements. In order not to damage the measuring device, it was lifted out of the water in the shallow water areas and the measurements were interrupted in these parts.

The actual test measurements were carried out in deeper sections of the Mur river in the vicinity of the Slovenian groundwater well field near Apače, where geoelectric measurements were carried out as well. In this test site several secondary frequencies (4 kHz, 5 kHz, 6 kHz, 8 kHz, 10 kHz, 12 kHz and 15 kHz) were

tested (Figure 5), but due to partial low water level the measurements for the lower frequencies were sometimes problematic as depths of more than 80 cm are recommended for those.

All these measurements were carried out with idle speed downstream or across the river. As a final measurement test it was planned to acquire SBP data while driving back to the boat ramp. These upstream measurements would be with strong engine power and thus vibration of the engine and boat, which might cause interference signals. Due to the low water level the engine was stuck several times and in order to not damage the propeller it was decided on short notice to continue downstream and search for an alternative boat ramp, which resulted in approximately 6 km of additional SBP data.

of resolution and penetration depths for different frequencies used during acquisition.

Figure 4 Acquisition of sub-bottom profiling data in the Mur river.

Figure 5 Comparison

After acquisition, the SBP data was converted to segy-format and coordinates were assigned to it. For further interpretation structural oriented filtering (Al-Dossary and Marfurt, 2007) was applied. In this process for each sample point the volumetric vector dip is estimated using a discrete scan approach (Marfurt et al., 1998), followed by median filtering along the volumetric vector dip is applied to filter the sub bottom profiling data. This allows the data to be smoothed

Figure 6 Interpretation examples for profiles measured in flow direction. The red circles in the small satellite images on the right indicate position of profiles. The partially eroded aquitard is visible.

Figure 7 Interpretation examples for profiles measured perpendicular to flow direction. The red circles in the small satellite images on the right indicate position of profiles.

without ‘smearing’ reflector discontinuities. Due to the different frequencies, different penetration depths and resolutions can be achieved. In principle, the lower frequencies are very suitable for visualising the internal structures of the aquitard. However, in the current test measurements the profiles with 10 kHz and 12 kHz are to be preferred due to the problems that occurred in the measurements because of low water level. Profiles with 15 kHz have the highest vertical resolution, but the attenuation

is also the strongest and therefore the penetration depth into the river sediments is very small. For the interpretation of the SBP sections a sequence stratigraphic approach is used (Galloway, 1989; Posamentier and Allen, 1999; Embry, 2002; Catuneanu, 2002, 2006). In this interpretation approach at first reflection terminations are marked (onlaps, downlaps, and toplaps) and these are then used to map geometrically connected sedimentary facies (Christie-Blick and Driscoll, 1995). Furthermore, the seismic facies (wavelength, continuity of reflectors, relative amplitudes, reflection pattern) is analysed and also integrated in the mapping process (e.g. Figure 6).

Based on this interpretation approach two main events were interpreted over the whole test site. First, the river bed, and second the surface of the aquitard. The surface of top aquitard was also correlated with interpolated results from geoelectric measurements and well information (stippled orange line in figures 6 and 7). In some areas a clear erosion of the Quaternary river sediments could be observed, with erosion most probably also affecting the Miocene aquitard which is inclined to east. Furthermore, some of profiles indicate internal structures of the aquitard, which haven’t been further investigated yet, but will be part of future projects in this area. This angular unconformity can be correlated with seismic sections of the East Styrian Basin. Contact of the Quaternary groundwater with formation waters of the Miocene sediment can therefore not be ruled out.

Conclusion

In the course of this pilot project, six geoelectric profiles were recorded in the immediate vicinity of the Mur between Halbenrain (Austria) and Apače (Slovenia) with dipole-dipole electrode configuration and two profiles with Wenner electrode configuration. Four profiles are located on the northwest side of the Mur (Austria; orographically left-sided) and four profiles on the southeastern side (Slovenia; orographically right-sided). A profile on each side was recorded perpendicular to the Mur (profile numbers 3 and 6). Different electrode configurations and the different electrode spacings were used to compare the resolution and penetration depths under the given (hydro-)geological conditions. This means that a total of eight geoelectric profiles with a total length of around 4 km are available as a geophysical basis for hydrogeological and sedimentological interpretation. On the Slovenian side, information from 14 wells and on the Austrian side information from 3 wells were successfully included in the geoelectric profiles. The resistance contrasts of the high-resistance Quaternary aquifer compared to the (Miocene)

low-resistance aquitard correlate with the geological information from the well information. The measurements of the sub-botton profiling data included 31 profiles with different frequencies, with a total length of around 14 km and ranged downstream from Mureck to Bad Radkersburg. Different acquisition frequencies (4, 5, 6, 8, 10, 12, and 15 kHz) were tested for applicability and resolution. In the area between Halbenrain and Apače, the number of river-parallel profiles was increased and additional cross profiles were included. In the SBP data, the upper edge of the (Miocene) aquitard, partially the Quaternary river sediments and the geometry of the river bed could be identified. With this test project it was proven that the Quaternary river sediments in the Mur river bed have been eroded in sections and that the Mur is in direct contact with the aquitard or is already eroding it. This could be linked to the falling groundwater level. Based on these results it is necessary for the future to acquire additional SBP data in a dense pattern to fully map these areas. This information can then be used for a better understanding of the current erosion processes and furthermore, for a better control of sedimentation processes in this river system.

References

Al-Dossary, S. and Marfurt, K.J. [2007]. Lineament-preserving filtering. Geophysics, 72(1), 1-8.

Alaamer, H. [2015]. Characteristics of sub-bottom profile acquired in Shatt Al-Arab River, Basrah-Iraq. International Journal of Marine Science

Catuneanu, O. [2002]. Sequence stratigraphy of clastic systems: concepts, merits and pitfalls. Journal of African Earth Sciences, 34, 1-43.

Catuneanu, O. [2006]. Principles of sequence stratigraphy. Elsevier Netherlands, Amsterdam.

Christie-Blick, N. and Driscoll, N.W. [1995]. Sequence stratigraphy. Ann. Rev. Earth Planet. Sci., 23, 451-478.

Daxer, C., Moernaut, J., Taylor, T., Haas, J.N. and Strasser, M. [2018]. Late Glacial and Holocene sedimentary infill of Lake Mondsee (Eastern Alps, Austria) and historical rockfall activity revealed by reflection seismics and sediment core analysis. Austrian Journal of Earth Sciences, 111(1), 111-134.

Eichkitz, C.G., Schreilechner, M.G., Heine, E., Golja, M., Hauer, C. [2023]. Application of High-Resolution Sub Bottom Profiling Data for the Interpretation of Shallow Lake Sediments. First Break, 41(7), 53-57.

Embry, A.F. [2002]. Transgressive-regressive (T-R) sequence stratigraphy. 22nd Annual Gulf Coast SEPM Foundation Bob F. Perkins Research Conference, 151-172.

Figure 8 Combined 3D view of geoelectric profiles and sub bottom profiles.

Fabbri, S.C., Affentranger, C., Krastel, S., Lindhorst, K., Wessels, M., Madritsch, H., Allenbach R., Herwegh, M., Heuberger, S., Wielandt-Schuster, U., Pomella, H., Schwestermann T. and Anselmetti, F.S. [2021]. Active Faulting in Lake Constance (Austria, Germany, Switzerland) Unraveled by Multi-Vintage reflection Seismic Data. Frontiers of Earth Sciences, 9 Freire, F., Gyllencreutz, R., Jafri, R.U. and Jakobson, M. [2014]. Acoustic evidence of a submarine slide in the deepest part of the Arctic, the Molloy Hole. Geo-Marine Letters, 34

Galloway, K.A. [1989]. Genetic stratigraphic sequences in basin analysis I: architecture and genesis of flooding-surface bounded depositional units. AAPG Bulletin, 73, 125-142.

Geraga, M., Christodoulou, D., Eleftherakis, D., Papatheodorou, G., Fakiris, E., Dimas, X., Georgiou, N., Kordella, S., Prevenios, M., Iatrou, M., Zoura, D., Kekebanou, S., Sotiropoulos, M. and Ferentinos, G. [2020]. Atlas of Shipwrecks in Inner Ionian Sea (Greece): A Remote Sensing Approach. Heritage, 3, 1210-1236.

Gregory, D. and Manders, M. [2015]. SASMAP Guideline Manual 2:

Best practices for locating, surveying, assessing, monitoring and preserving underwater archaeological sites

Heine, E. [2017]. Multi-Transducer Sediment Echo Sounder for 3D Documentation of Submerged Archaeological Sites - a Case Study at a Prehistoric Pile Dwelling at Lake Mondsee (Austria). FIG Working Week 2017, Helsinki, Finland.

Marfurt, K.J., Kirlin, R.L., Farmer, S.L. and Bahorich, M.S. [1998]. 3-D seismic attributes using a semblance-based coherency algorithm. Geophysics, 63(4), 1150-1165.

Posamentier, H.W. and Allen, G.P. [1999]. Siliciclastic sequence stratigraphy: concepts and applications. SEPM Concepts in Sedimentology and Paleontology 7.

Vanneste, M., Forsberg, C.F., Glimsdal, S., Harbitz, C.B., Issler, D., Kvalstad, T.J., Løvholt, F. and Nadim, F. [2011]. Submarine Landslides and Their Consequences: What Do We Know, What Can We Do? Proceedings of the Second World Landslide Forum, Rome.

Wang, R., Li, C. and Yan, X. [2018]. Application of Sub-Bottom Profiler to Study Riverbed Structure and Sediment Density. IOP Conference Series: Earth and Environmental Science, 128

14-16 OCTOBER 2024 NAPLES, ITALY

Delivering sands to Venus and all the traps between: Orange Basin, Namibia

Neil Hodgson1*, Lauren Found2 and Karyna Rodriguez3 present seismic processing and imaging techniques for building a geologically driven velocity model to successfully explore the basin.

Introduction

When Ernest Hemingway wrote The Old Man and the Sea, he created one of the most gripping thrillers ever told, of human fortitude and cunning pitted against nature – the majestic and overwhelming adversaries and capricious luck. The metaphor for perseverance in exploration is inspirational, but also instructional.

In the Orange Basin of Namibia, the Cretaceous Shelf, Slope and Basin Floor are characterised by very different geologies, connected through time of a rifting continent, subsidence, sediment supply, chaos and instability. Decades of exploration have finally achieved extraordinary success, yet this journey has only been achieved by utilisation of the industry’s technological superpower – 3D imaging. Illumination and imaging of the now proven playfairways in this basin have presented the brave explorers of the past with complex problems. Although the Orange Basin has no mobile salt, it is characterised by hiding its prospectivity below complex fold and thrust belt macro-structuring and mass-transport complex micro-structuring. These geologies bring challenges not only in velocity models (and for the processing geoscientists – the unravelling of complex velocities to allow data to be migrated correctly) but also in the anisotropy of the rock properties in the sequences above the targets. These issues require state-of-the-art seismic processing and imaging techniques to build a geologically driven velocity model to successfully explore this basin.

Geological background

The structural domains of the Orange Basin can be viewed as four very different geological settings, connected by time, plumbing and gravity. That physicists continue to argue about the existence of two of these is of little concern – because the most important factor in exploration is the plumbing; both the systems that move sands from source to sink, and also the systems that move oil from source to reservoir.

Firstly, imaging subtle Turonian and Cenomanian prospects where one can apply advanced attribute analysis workflows and absolute seismic inversion methods, requires good low-frequency signal content. Therefore, it is essential to build a robust and detailed velocity model for pre-stack depth migration which in Namibia is derived by full waveform inversion (FWI) and tomographic methods. These capture both the vertical and lateral variability (Anisotropy) of the overburden to ensure precise positioning of the seismic events. All of these multi-client 3D datasets are being processed by Shearwater Geoservices, and the results to date are just stunning. Depth imaging is key and building a reliable, geologically meaningful velocity model that allowed accurate imaging of the geological structures at multiple detachment levels and the underlying Aptian Source was one of the main challenges to overcome.

There were some key complexities identified and resolved during velocity model-building such as the erosional small-scale

1 Searcher

* Corresponding author, E-mail: n.hodgson@searcherseismic.com DOI: 10.3997/1365-2397.fb2024057

Figure 1 Sketch of the Structure of the Orange Basin.

migrating pockmarks with circular/oval shapes (Hodgson et al. 2023), which were nicely captured in the FWI model, and the geometries in the gravity-driven fold and thrust belt complex.

Geologically, near shore is an uplifted and eroded continental crust shelf, with late Karoo age syn-rift hydrocarbon systems. Figure 1. Outboard of the shelf, the basement breaks into an Early Cretaceous ‘Inner Basin’ syn-rift separated from the early drift ‘Outer Basin’ slope by the Outer High. The Outer High is quite variable on the margin, sometimes onlapped by Aptian (from both the east and the west), and sometimes absent. It seems to represent a local high, often comprised of syn-rift and early drift volcanics, perhaps themselves deposited on the last fragment of true continental crust. Beyond the Outer High, the basin floor is underlain by both Seaward Dipping Reflectors and Oceanic crust.

Whilst the Cretaceous section is thick and stable on the shelf (despite the uplift) the late cretaceous slope was amazingly unstable, repeatedly collapsing (there are at least three quite discrete episodes of instability in the Late Cretaceous visible on Searcher’s modern 2D and 3D data) and depositing up to 2 km of anisotropic mass-transport-system goo on the basin floor. This provides the top seal for the Early Cretaceous Venus target, and no doubt provided episodic pulses of loading and thermal blanketing onto the source rock, particularly useful during the late Tertiary thermal pulse from mantle-derived volcanic intrusions. Up-dip and thrusting into this astonishing accumulation of MTS is one of the wonders of the geological world – the Orange Basin gravity-driven fold and thrust belt which has its extensional roots near Kudu but extends over both the entire Inner Basin and the slope.

Why is Venus there?

The ultra deep water portion of the Orange basin comprises Early Cretaceous oceanic crust overlain by Aptian source rocks and reservoirs that comprise the target for the Venus and Mangetti discoveries. The transition from rift to volcanic drift is rarely seen on Earth today, with the exception of the Afar Triangle in Africa, and can be hard to unravel. At some point though volcanic lava flows deposited with a mix of sub-areal clastics into syn-rift half grabens changes to subareal flood basalts deposited onto the last extended fragments of continental crust. Subsidence of the root of

the volcanism makes the outer parts of these lava flows dip down away from land, and they become ‘Seaward Dipping Reflectors’. In detail, subsidence is more chaotic and reflectors dip in all sorts of directions, but useless as it is, the name ‘SDR’ has stuck. As magma erupted, the dyke swarm associated with the vent loads and subsides into the depleting magma chamber, giving these SDRs a characteristic ‘arcuate’ or ‘bent’ morphology. We can observe at some point a change in character of these volcanics as they suddenly stop being SDRs and start being nobbly MORB style oceanic crust. This change is not caused by a tectonic event but just the first marine incursion into the basin. Oceanic crust is formed by the sub-marine eruption of basalt that is immediately quenched in a shallow restricted and mostly anoxic basin stretching from South Africa to Brazil’s Pelotas basin.

It is in this shallow anoxic basin that the Aptian source rock is deposited, or at least the organic component of the sediments is preserved. That these are basin floor sands is uncontested. However, the actual water depth of this Aptian Basin is open to some debate. That the Venus sands are deposited so soon after the first marine incursion and pinching out on Oceanic Crust and Slope SDRs with so little basin relief, suggests a shallow water origin. The basin margin was irregular and so in some places the Aptian source rock on-laps against the Outer High, in others the basin extends inboard through gaps in the Outer High over the inner basin almost to the foot of the shelf. Into this geology, Aptian sands, swept down complex slope systems into the shallow restricted marine basin are then swept north by contourite currents draping the lateral fan edges elegantly against the western ramp of the Outer High.

A thick source rock, high geothermal gradient and a couple of km of ‘gloop’ above the reservoir as a top seal and this trap set the stage for Venus.

Why is Mopane there?

Two years ago the industry focus was on Venus type basin floor fan traps, or lower slope channels and traps such as Graff and La Rona. However, although related, the new intrigue on the margin is the discovery of oil in PEL 83, operated by Galp with the Mopane-1 and 2 wells. A discovery in the Inner basin, inboard of the Outer High suggests the Aptian Source Rock here is mature.

Figure 2 Strike line across Late Cretaceous channels of the Mopane Discovery. Fast-track preliminary processing Link and Bridge multi-client surveys. In 2022 and 2023 Searcher and Shearwater acquired four multi-client 3D surveys in Namibia’s Orange Basin.

As this is ubiquitously deposited across the whole inner basin and is penetrated even by Kudu well on the east side of the inner basin – this is a startling discovery. New plays are observed not only on PEL 85, charged from this source but the play can be extended south into South Africa, inboard of the outer high and northward into adjacent blocks, and indeed even up through Luderitz to the Southern Walvis basin. The Mopane discovery in PEL 85 comprises stacked sand-filled channels and levees (Figures 2 and 3).

These sedimentary systems were taking sands that came off the shoreface on the shelf and delivering them towards Venus on the basin floor. However, some channels are deflected by the geometries on the outer high and other sands are ponding against the high as the systems develop to find places to wriggle past the Outer High and down to Venus. Exploring in this basin, it is not only important to find the sands at the base of slope – where the largest apron fan prospects are usually found, but it is also key to follow the channel pluming and find the sands trapped against topology along the way. Some of these may be the biggest prizes of all.

It has been extremely difficult to use 2D data to chase these plays that are best explored by looking for AVO effects, especially far-offsets on modern PSDM 3D data (see Figure 3). Not only do the offset stacks prove useful in identifying hydrocarbons ahead of the drill bit, but the high-quality imaging identifies the

Figure 3 Left-hand side; map of base Aptian source rock across the Bridge 3D PEL 85. Arrows indicating the direction of sediment input and channel systems onlapping onto this surface. Right-hand side; the base Aptian source rock surface in 3D with AVO-supported leads hi-lit in depth. Seismic data is the 2022 Bridge multi-client 3D.

sedimentological complexities on the constrained channel systems that are bringing sand from the shelf to basin floor, leaving huge low risk plays and traps along the way.

The exploration message is clear and echos the lessons from The Old Man and the Sea; If you are fishing for the biggest plays and prospects in the world, arm yourself with modern 3D seismic, tuned with an optimum lure (far-offset responses) and get in early to a basin where you can hunt alone, the hydrocarbon system is strong and unexplored. But, as Hemingway’s story ruefully reminds us, if you are skilled at fishing and you hunt the big fish – just be prepared to actually catch one.

Acknowledgements

We would like to thank our partners Shearwater Geoservices for its amazing processing and hydrocarbon prospectivity understanding skills, particularly Olga Litvyakova, Javier Martin, Samuel Winters, Kenan Erdogan and William Johnson. Shearwater GeoServices and Searcher Seismic are delighted to be working on these amazing projects with the staff of Namcor in Namibia.

References

Hodgson, N., Rodriguez, K. and Found, L. [2023]. Namibia’s Orange Basin and the holey slope mystery. GEOExPro (https://geoexpro. com/namibias-orange-basin-and-the-holey-slope-mystery/).

Integrating regional 2D seismic mapping and 3D seismic spectral decomposition to understand the fairway evolution of offshore Benin

Pauline Rovira1* discusses how 2D and 3D mapping combined helps us to understand the differences in sediment input directions and faulting impact on the fairways of the Cretaceous, Paleogene and Miocene.

Abstract

Offshore Benin, and the wider Keta Basin, remains an underexplored area of the West Africa Transform Margin. The evolution of the different sediment fairways and their depocentres can be identified on structure maps from the mapping of a regional 2D seismic dataset. The 3D seismic offshore Benin supports the 2D interpretation but in addition, allows for a more complete evaluation through detailed seismic attribute analysis. The use of 3D spectral decomposition highlights the changes in fairway directions with clear imaging of the channel systems and their orientations, correlating with the thicknesses observed from regional 2D seismic mapping.

The transform faults strongly control the overall structuration of offshore Benin and the depositional style during the Cretaceous period. The onshore Togo and Benin river systems supply sediment directly to the basin in a north to south direction which is limited and directed by the transform movement and ridges outboard. Towards the end of the syn-transform deposition, the main fairway input changes from a directly northern source to a north eastern source in the Dahomey Embayment. Finally, in the Cenozoic, the Niger River system drainage increased leading to the Benin Ultra Deep area to form part of the Niger prodelta, with a predominant easterly sourced sediment input. The transform faults are no longer active and no

1 TGS

* Corresponding author, E-mail: Pauline.Rovira@tgs.com

DOI: 10.3997/1365-2397.fb2024058

Figure 1 Map of the TGS seismic data library.

longer control sediment distribution, leading to an unconfined channelised system.

Introduction

Offshore Benin is located within the Gulf of Guinea, forming part of the West Africa Transform Margin, and borders Togo to the west and the prolific Nigerian Delta province to the east. Since exploration began in the late 1960s, 18 exploration wells have been drilled offshore but success has been limited with the discovery of only the Seme North and Seme South fields to date. The majority of the wells have been drilled within the shelf-slope setting and the deep basin remains unexplored. The dataset used for this study comprises regional 2D (reprocessed in 2019) spanning the length of the basin extending from Ghana to Nigeria, as well as two high-quality 3D surveys located offshore Benin (Figure 1 and Table 1).

The 2D data has been used to map and understand the key mega-sequences from a regional, large-scale perspective, tying into the wells where available. Following on from the regional interpretation, structure and isopach maps were produced to gain initial insight of the main depocentres and how that extends in the outboard and basinal setting of offshore Benin, away from well control. The 3D data was then utilised to garner more specific detail on the depositional systems of each mega-sequence. The 3D results show strong support for the 2D evidence but also allow for infilling of knowledge gaps in terms of fairway style and possible reservoir sweet-spots. Spectral decomposition images from the Block 5&6 3D are shown but other attribute workflows are being developed to further characterise the fairways.

Geological setting

The Benin Basin marks the eastern continuation of Ghana’s Keta Basin, bound to the north by the distal end of the Romanche Fracture Zone and to the south by the Chain Fracture Zone and the Charcot Fracture Zone. These faults are major dextral-slip transform faults that have accommodated movement during the opening of the Central Atlantic throughout the Cretaceous. Rifting is initiated in the Lower Cretaceous, with rapid infilling of these newly formed pull-apart basins by continental and lacustrine clastics (Brownfield & Charpentier, 2006). Further outboard, a more restricted marine setting is developed. The Mid-Albian unconformity is clearly visible regionally, and marks the end of the main rifting event. However, transform movement on the faults continues into the Upper Cretaceous. The basin fill becomes fully marine in the Late Cretaceous. The onset of the passive margin occurs in the Palaeocene once transform faulting movement ends, driven by thermal subsidence (Kjemperud et al, 1992). The key seismic sequences were interpreted across the regional 2D in the first instance, tying into the wells where possible. A lack of well penetrations in the deeper parts of the basin make the exact age correlation of horizons difficult from shelf to basin. However, the

combination of published reports and good quality seismic data extending and connecting Ghana, Togo, Benin and Nigeria allows for a stratigraphic framework of some confidence.

Clastic reservoir intervals are found throughout the stratigraphy, from Mid-Albian fluvio-deltaic rift sediments to Miocene deepwater turbiditic channels and fans. The main source rocks, that have been proven are the Neocomian and Aptian-Albian lacustrine and marginal marine source rocks. There is a risk of over-maturity for these where the greatest sedimentary cover occurs. However, for much of the basin, based on simple basin modelling, these source rocks are located within a present-day oil-gas window, assuming a base case geothermal gradient of 35 °C/km. The Cenomanian and Palaeocene source rocks, proven by the Capitaine-1 and Barracuda-1 wells, and Palaeocene source rocks, would be within the early to main oil window at present-day. Structural traps are mainly present within the rift and transform sequences, and stratigraphic traps being the primary closure mechanism for the passive margin plays.

Seismic stratigraphy

Figure 2 shows a number of faulted rift basins within the continental crust domain in the present day shelf-slope setting. The Upper Cretaceous shows limited extent beyond the Chain Transform Fault Zone (CTFZ) and is primarily confined to the basin inboard. The Upper Cretaceous seismic package is composed of high reflectivity, bright amplitudes and a channelised facies. Across the regional 2D dataset, this sequence shows a mixed turbidite and contourite system with stacked and laterally migrating channels and associated basin floor fans. The ‘Paleocene Wedge’ sequence denotes a localised package to this basin that pinches out westwards in the Ghanaian portion of the Keta Basin and northwards against the Upper Cretaceous slope. This turbiditic wedge marks the first substantial post-rift deposition beyond the CTFZ.

A seismically unreflective package of Eocene-Paleocene age, the Imo Shale Formation (Brownfield, 2016), is mappable regionally and appears to correlate with Nigeria’s Akata Formation. Another local sequence to the Benin Basin develops in the Oligo-Miocene, but conversely to the Paleocene Wedge, the Dahomey Wedge pinches out southwards, onlapping onto the Oligocene unconformity, and is restricted to the inboard part of the basin. The final mega-sequence of note is the Miocene channelised package, which has a similar response seismically to the Upper Cretaceous. The sequence is dominated by bright reflectivity and local erosive surfaces and forms the deepwater equivalent to the Agbada Formation of the Niger Delta.

Fairway evolution

Upper Cretaceous

A general deepening trend is noted from the inboard to the outboard of the Benin Basin. From the structure, and in particular

Table 1 2D and 3D survey specs used in this study.

the thickness map (Figure 3a), it is clear that faulting is having a large impact on deposition of the Upper Cretaceous, with progressive thinning of the package to the south and minimal Upper Cretaceous is deposited beyond the CTFZ. The main rifting event is over by the time of Upper Cretaceous sedimentation. However, transform movement along the E-W faults continues and strongly controls the thickness variations observed with generally a thicker depocentre in the hangingwall area of the faults where ponding is occurring. From this 2D overview, it can already be determined that input to the basin is coming from immediately to the north of the Benin Basin. The 3D data on the other hand shows a similar story, but the additional detail highlights that more minor splay faults to the transform faults are also impacting Upper Cretaceous deposition (Figure 3b) and localised thinning is noted over horst structures.

Spectral decomposition carried out on slices within the megasequence shows dominant north to south fairways, particularly in the east of the Block 5&6 3D that highlihts a long-lived channel belt (Figure 4). Sediment wave fields can be seen inbetween the main channel belts, which may be associated levee type deposits or contourite/sand wave systems.

Paleogene

The Palaeocene sequence, like the Cretaceous, continues to show influence from faulting on deposition. The Palaeocene Wedge thickness (Figure 5) reveals a greater input from the Niger Delta area with a depocentre thick being developed in the east of Benin and the CTFZ causing funnelling of the deposition parallel to the fault. From the 3D attribute analysis, a chaotic facies is noted near the pinch-out edge, likely to be the result of local slope instability,

Figure 3 a) Isopach map for the Upper Cretaceous megasequence (Top Cretaceous to Mid-Albian Unconformity) from 2D interpretation. b) Isochron map for the Upper Cretaceous megasequence from 3D interpretation.

Figure 2 Regional 2D transect (N-S) offshore Benin highlighting the type geology and key mega-sequences.

but away from the edge in the relative depocentre (the 3D survey is located up-dip of the main package thick for this sequence), a NE-SW fairway trend is observed suggesting additional increased input from the Dahomey Embayment in the NW (Figure 6a). The Eo-Oligocene package shows little seismic reflectivity and distinct

Figure 4 Spectral decomposition images (RGB blended) of two slices within the Upper Cretaceous, syn-transform package. a) slice through approx. Cenomanian level. b) slice through approx. Santonian level.

Figure 5 Isopach map for the Palaeocene Wedge sequence from 2D interpretation.

channel features are not discernible from the spectral decomposition slices. Mass transport complexes (MTCs) are prevalent within this likely non-reservoir interval due to uplift in the Oligocene resulting in slope instability. However, a general NW-SE depositional trend remains (Figure 6b) and this also correlates with the

interpretation from the regional 2D dataset. Away from the slope, the very bland seismic response suggests a very shale prone section and regionally this shale is well developed. This section forms the Benin Basin equivalent to the Niger Delta’s Akata Formation, and is expected also to be a good-quality seal.

Miocene

The regional 2D thickness reveals two dominant depositional trends. In the nearshore, there is still a strong influence from a northerly input on the western side of Benin, but basinward, an easterly influence from the Niger delta area becomes much

more prominent, with a much thicker package in the deepwater domain (Figure 7a). The transform faults are no longer expressed in the depositional fabric, with the area being in a passive margin setting by the Miocene time. The 3D is located north of the main eastern and basinal depocentre. However, the isochron (Figure 7b) supplements the 2D overview and shows the general thickening outboard as well as further highlighting a potential western fairway.

The seismic data shows that the Miocene mega-sequence is composed of bright and reflective turbidite-type facies encased in a low-reflectivity shale prone facies. The spectral decomposition images showcase these deepwater systems adding detail to the

Figure 6 Spectral decomposition images (RGB blends) of two slices within the Paleogene mega-sequence. a) slice through the Palaeocene Wedge. b) slice through the Eo-Oligocene MTC.

Figure 7 a) Isopach map for the Miocene megasequence (Top Miocene to Top Dahomey Wedge) from 2D interpretation. b) Isochron map for the Miocene megasequence from 3D interpretation.

thickness interpretations. Interestingly, the slices taken through the older parts of the sequence reveal a dominant NE-SW channel belt, continuing the trend observed from the Paleogene of a primary clastic input from the Dahomey Embayment (Figure 8a). Meandering channel belts are observed, with individual tortuous channels within these belts discernable. Clear associated levee systems are present and potentially some reworked sand waves in between the turbidite belts.

The younger Upper Miocene slices on the other hand show this combined E-W fairway, with input from the Niger Delta, and the N-S fairway, with input from onshore Benin (Figure 8b) as noted from the 2D and 3D thicknesses. These Miocene systems are unconfined, and eroding into the older NE-SW system. Not depicted in Figure 8, but additional attribute work shows that throughout the Upper Miocene the N-S fairway eventually dwindles whereas the Niger Delta input continues to increase leading to an even stronger E-W fairway system.

Imaging these channel belts allows for identification of reservoir sweet-spots and although untested in offshore Benin, these Cenozoic clastics have the potential to be viable, good quality reservoirs, with some of the Nigeria fields having up to 30% porosity, e.g. Bosi North and South Fields, Hatch Field. Additionally, the N-S Benin-sourced system results in potential reservoirs with a much shorter transport distance than the Nigeria fed turbidites.

Conclusions

From the Cretaceous through to the Miocene, sediment input progressively moves from a dominant northern source, to a northeastern source, to an eastern source. Transform faulting strongly influences fairway orientation throughout the majority of the Cretaceous. The northern input along with E-W oriented faults

Figure 8 Spectral decomposition images (RGB blends) of two slices within the Miocene mega-sequence. a) slice through the Tortonian. b) slice through the Messinian. Termination of the E-W system is likely to be owing to faulting and/or potential down-cutting from an overlying MTC.

leads to a confined and mixed system with depocentres confined by the transform faults. The passive margin stage that follows results in an unconfined channelised system that is first fed from the Dahomey Embayment area to the north east and then more recently from the east as part of the Niger prodelta. Interestingly, a Benin-sourced input is still recognised in the Miocene, even if less dominant that the input coming from the Niger Delta. This leads to a north to south channel fairway with a shorter transport distance than the easterly sourced turbidites. The combination of regional 2D mapping along with more detailed observations and interpretations of the fairway type and distribution from 3D attributes allows for an improved understanding of Benin’s depositional systems.

Acknowledgements

SNH Benin (Société Nationale des Hydrocarbures du Bénin) own the 2D and 3D seismic data, and TGS would like to thank SNH for their partnership and their permission for TGS to share these findings.

References

Brownfield, M.E. [2016]. Assessment of undiscovered oil and gas resources of the Niger Delta Province, Nigeria and Cameroon, Africa. Geologic assessment of undiscovered hydrocarbon resources of Sub-Saharan Africa: US Geological Survey Digital Data Series Brownfield, M.E. and Charpentier, R.R. [2006]. Geology and total petroleum systems of the Gulf of Guinea Province of West Africa, U.S. Geological Survey Bulletin 2207-C.

Kjemperud, A., Agbesinyale, W., Agdestein, T., Gustafsson, C. and Yükler, A. [1992]. Tectono-stratigraphic history of the Keta basin, Ghana with emphasis on late erosional episodes. In: Géologie Africaine: Coll. Géol. Libreville, recueil des Communications 6-8 May 1991, 55–69.

Deep learning-based Low Frequency Extrapolation: Its implication in 2D Full Waveform Imaging for marine seismic data in the Sadewa Field, Indonesia

Sonny

Winardhi1*, Asido S. Sigalingging3, Wahyu Triyoso1, Sigit Sukmono1, Ekkal Dinanto1, Andri Hendriyana1, Pongga D. Wardaya2, Erlangga Septama2 and Rusalida Raguwanti2 demonstrate how with Low Frequency Extrapolation the proposed algorithm can reliably extrapolate the low-frequency content of the field data with minimal errors and exhibits good agreement with the deghosting results.

Abstract

Seismic data with a low-frequency content is crucial for full waveform imaging (FWI) as it can improve the resolution of subsurface features and offer insight into the underlying geological characteristics. A lack of low-frequency content may cause cycle-skipping, which can distort the results of the inversion process. Low frequency content in seismic data is usually estimated using seismic processing-based deghosting techniques. In this study, an attempt is made to reconstruct the low-frequency content using an artificial intelligence approach through a deep learning algorithm. A convolutional neural network (CNN) approach was used to automatically extrapolate the low-frequency content of the band-limited common-shot-gather data, without the need for preprocessing steps. The model was first tested and validated with synthetic data. The optimised model was applied to the Sadewa field, Indonesia, and the obtained low-frequency extrapolated data were used as input for the FWI process. The results show that the proposed algorithm can reliably extrapolate the low-frequency content of the field data with minimal errors and exhibits good agreement with the deghosting results. The FWI results also demonstrate that our proposed method can be a reliable and efficient substitute for determining the low-frequency component of seismic reflection data.

Introduction

Full Waveform Inversion (FWI) is a seismic tomography-based subsurface modelling technique that provides high-resolution images and physical properties (Pratt, 1990; Tarantola, 1984).

FWI uses all the seismic waveforms to update the subsurface model. The optimisation process, however, often struggles to reach a global minimum, getting trapped at a local minimum instead due to various factors. These factors include source wavelets, lack of low-frequency (LF) content in seismic data, initial models that do not represent complex geological structures, and seismic noise. One common challenge faced in FWI, and seismic data processing, is the deficiency of LF content in the data, which

inherently offers information about geological trends (Bunks et al., 1995). To prevent the velocity model from converging to a local minimum, it is imperative to include LF content during the FWI process (Hu et al., 2017).

One method used to address this deficiency is deghosting, which aims to remove the effects of the water layer on seismic data. In marine seismic surveys, ghost effects occur on both the source and receiver sides when the upward and downward components of

Institut Teknologi Bandung

* Corresponding author, E-mail: swinardhi@itb.ac.id

DOI: 10.3997/1365-2397.fb2024059

Figure 1 Location of the Sadewa field, Kutei basin, Indonesia (Thompson et al., 2009).

Figure 2 The CNN regression model architecture consists of six (6) convolutional blocks with regularisation and a fully-connected layer (Sun and Demanet, 2018).

the seismic wavefield interact at the water’s surface (Ghosh, 2000). Ghosts, the downward-moving wavefield described by Beasley et al. (2013), are seismic events with reversed polarity or the same polarity as the primary signals, which can interfere with those primary signals and cause a loss of LF information.

Multiple approaches exist for eliminating ghost signals in seismic data, including (i) wave-equation deghosting, which involves using mathematical representations to generate a model of the sea surface’s interaction with seismic waves. By predicting ghost signal patterns, these can be subtracted from the seismic recordings (Beasley et al., 2013); (ii) receiver- and/or sourcebased deghosting, which employs specialised algorithms that pinpoint and filter out the ghost signals by recognising their unique attributes in the seismic data (Jayaram et al., 2015); and (iii) implementing convolutional neural networks (CNNs) to leverage machine learning to detect and eradicate ghost signals (de Jonge et al., 2022).

Following the previous study by Sigalingging et al. (2024), this study diverged from the conventional use of CNN for 2D image classification. Instead, it employs CNN as a regression for 1D signal processing (Sun and Demanet, 2018, 2020). The decision to utilise deep learning was driven by its inherent ability to directly predict LF content without preprocessing raw seismic data. This task is often challenging to achieve using conventional methods. Consequently, through a comprehensive end-to-end

Figure 3 The 12 submodels were extracted from the Marmousi model to compile the training dataset. In these submodels, 30 shots per model and 256 receivers per shot were utilised. Approximately 70% of the total traces generated were utilised for training data, while the remainder was used for testing.

process, the model is used to extrapolate LF content in 2D seismic data, which includes both synthetic and field data, thus allowing the FWI process to use the LF extrapolated results.

Study area

In 2020, Pertamina and Institut Teknologi Bandung conducted a collaborative study on anisotropy velocity model building and subsurface imaging to improve oil and gas exploration success using core, log, and seismic data in Sadewa field as a case example. Situated in the Makassar Strait, between the islands of Sulawesi and Kalimantan, the Sadewa field (Figure 1) is an integral part of the Kutei Basin (Thompson et al., 2009). It lies at a depth of 1500-2500 feet in the sea, approximately 5 km from the shelf edge. With oil reserves of 2.47 MMBO and a gas reserve of 28.1 TCF, the Kutei basin stands as Indonesia’s second-largest hydrocarbon basin (Rose and Hartono, 1978). The prospective Sadewa sand reservoirs are believed to have been deposited in a fluvio-deltaic environment during the Upper Miocene period.

Several works have been conducted to identify the best seismic attributes to map prospective fluvio-deltaic reservoirs. Alamsyah et al. (2008) and Sari et al. (2018) show that amplitude and its derivative attributes can map the prospective fluvio-deltaic Talang Akar Formation (TAF) deposits in South Sumatra Basin (SSB). Sukmono et al. (2017) found that for the

areas with no well data, the amplitude attributes effectively map young fluvial-deltaic channel deposits in the Gulf of Mexico. Alamsyah et al. (2023) evaluated several post-stack attributes to map the thickness of Lower TAF deposits in SSB. They concluded that the Sum of Negative Amplitudes and Acoustic Impedance are the most effective ones. Ambarsari et al. (2020) delved into analysing reservoir quality using core samples and illustrated how critical porosity and clay content can be utilised for rock typing.

Particularly for the Sadewa field, Ambarsari et al. (2021) highlighted that the lithology and facies type influence anisotropy parameters and upscaling factors. Oktariena et al. (2021) explored the use of core anisotropy parameters and log anisotropy in seismic data processing. Sigalingging et al. (2021) examined the extrapolation of seismic data’s LF content using

Figure 4 Training is carried out with the same dataset, but the training batch size parameter is allowed to vary by 16, 32, 64, and 128, respectively; the corresponding error results are shown.

deep learning methodology based on synthetic data. Oktariena et al. (2023) investigated the effect of saturated fluid variation on seismic anisotropy. Additionally, Oktariena et al. (2023) conducted research on the reconstruction of velocity logs in deviated wells driven by anisotropy. Lastly, Winardhi et al. (2023) presented the use of curved pseudo-elastic impedance to predict reservoir petrophysical properties by rotating elastic attributes.

Convolutional neural network methodology

Artificial neural networks, also known as ANNs, are machine learning techniques that mimic the learning mechanisms of biological systems. In these networks, perceptrons represent neurons, which are the cells found in the human nervous system. Every perceptron receives information, processes it, and then produces an output by a linear calculation. An activation function is used to transform outputs to accomplish non-linear processing. Mathematically, the perceptron is expressed as y = f(x,w), where x and y are the input and output, respectively, and w is a parameter that sets the function. CNNs, or Convolutional Neural Networks, are a specific kind of ANN typically utilised for analysing data with a grid structure, like images. The CNN scanned and analysed image data using convolutional layers, employing filters and pooling layers to reduce dimensionality (Aggarwal, 2023). This process allowed CNN to extract features from images effectively. The feature extraction process is carried out by convolving the input data with a filter kernel, resulting in increased insight and pattern sensitivity in the input data.

Figure 5 The seismic low-frequency (LF) content was predicted using several models that varied in batch size. The results showed that the model with a batch size of 32 produced a smaller error value than other models. Thus, this model was selected as the optimal model for predicting LF component of real data.

Figure 6 The results of a low-frequency (LF) prediction from a BP-Tooth velocity model; the predicted results (left panel), the true actual data (centre panel), and the difference between them, predicted and actual (right panel). Comparison between the two results indicates that the deep learning is robust, effective, and accurate in making LF predictions, considering the data being predicted differs in both velocity as well as acquisition geometry.

In this work, the Convolutional Neural Network (CNN) technique is utilised to extrapolate the low-frequency content of seismic data. The CNN receives band-limited seismic data as input and produces seismic data’s low-frequency content as output. The CNN’s structure comprises six blocks, including convolutional, dropout, and fully connected layers, as depicted in Figure 2. The initial five convolutional layers contain 32 filter kernels, each measuring 80 in size, whereas the last layer consists of just one filter kernel, thereby decreasing the total model parameters. Regularisation is accomplished by incorporating a dropout layer of 20%, consisting of two dropout layers, to mitigate the risk of overfitting. The fully connected layer contains several units, doubling the number of samples in the input layer. PreLU is the activation function in both the convolutional and fully connected layers. Meanwhile, the method of optimisation

Figure 7 Results of testing our SIFWI algorithm on synthetic Marmousi data. It shows that an overall high resolution velocity model can be achieved, and closely resembled the true Marmousi model after only 10 iterations.

Figure 8 The FWI result using band-limited synthetic CSG (upper panel) and using CNN LF-extrapolated CSG as input (lower panel) which gives much sharper image and better velocity details.

used is adaptive moment estimation (Kingma and Ba, 2014), with a learning rate of 5x10-4

The study’s dataset consists of velocity models derived from the Marmoussi models (shown in Figure 3) and seismic synthetics generated using the finite difference method simulating the propagation of acoustic waves. To conduct the simulation, 30 shots, and 256 receivers were used for each model, with a 15 Hz Ricker wavelet as the source and a 6 s time length, with a 10 m grid space in both the horizontal and vertical axes. The simulations were conducted using Devito (Luporini et al., 2020), an open-source Python library. After a Butterworth filter is applied to partition the synthetic data into low- and high-frequency contents, the input and output were sampled at 8 ms intervals. This sampling technique was crucial in minimising the parameters and normalising the data.

Training could commence only after the dataset had undergone all preprocessing steps.

The CNN model depicted in Figure 2 was implemented utilising the widely used deep learning frameworks in Python, namely TensorFlow and Keras. With the aid of a Quadro M2000 GPU, the training process was conducted for over 48 hours. It involved 300 epochs and utilised a batch size of 64. A swifter convergence of MSE error was evident with smaller batch sizes, as shown in Figure 4. A smaller batch size accelerates the convergence of the training

process as the model is exposed to fewer data patterns. However, this outcome also leads to a decline in the model’s capacity to forecast the data accurately, emphasising the necessity of carefully selecting the appropriate batch size. The optimal batch size of 32 was determined based on the results obtained from the parameter tuning analysis. Nevertheless, upon conducting tests on different combinations of models using various batch sizes, it was concluded that a batch size of 32 yielded more consistent and reduced average prediction results and prediction error, as demonstrated in Figure 5.

Figure 9 The results of a low-frequency prediction performed on real data from the Sadewa Field. The results demonstrate the efficacy of the deep learning model on real data, as the predicted low-frequency trend agrees with the actual data. However, it is noted that some shallow-time seismic events may not be accurately reconstructed due to the presence of strong seismic noise.

Figure 10 Results of comparison between application of FWI to 2D Sadewa field data using CSG without LF-extrapolation (upper panel) and CSG with CNN LF-extrapolated as input (lower panel). Note the resulting high resolution velocity model, which better correlates to the P-wave sonic data at the well location, ensuring that the resulting FWI model is geologically plausible.

The chosen deep learning model underwent training and testing on synthetic data of the BP-Tooth velocity model, utilising a batch size of 32 and 300 epochs. The BP-Tooth velocity model varies from the submodels employed in the deep learning training, while the wave propagation simulation was conducted using distinct geometries. The presentation of the test results can be observed in Figure 6. The deep learning model produced a low-frequency forecast that exhibited a high level of accuracy, demonstrated by an RMS error value of under 1%, and closely resembled the trend observed in the actual data. This outcome shows that the deep learning model developed is proficient at predicting LF content with varying velocity models and geometries.

Full Waveform Inversion methodology

Full waveform inversion (FWI) is an imaging technology that attempts to produce high-resolution subsurface velocity

models using all information of the seismic wavefield (Virieux and Operto, 2009). The main issues with FWI are that it is highly non-linear and intensive in the use of computational resources, its dependency on initial models and its accuracy in source wavelet estimation. However, in the case of real seismic data, accurate estimation of the source wavelet cannot be guaranteed. To deal with this issue, a source-independent acoustic full-waveform inversion (SIFWI) using convolutional-based objective function (Zhang, et al., 2016) is adopted.

We utilised L1-norm objective function and implemented a multi-scale inversion strategy to improve ability in handling noisy data and to reduce the sensitivity to initial models, respectively. In this work, forward modelling simulations were conducted using the open-source package Devito (Luporini et al., 2020). Meanwhile, our SIFWI algorithm was implemented in the JUDI,

Figure 11 Application of the FWI velocity model to pre-stack depth migrate the 2D-Sadewa data demonstrates the reliability of the method. Note the flatness of the reflection events shown on the depth migrated (upper-right panel) and on the residual velocity plot that clusters around the centre zero value (upper-mid panel). The lower panel shows the FWI velocity model used as the background of the resulting pre-stack depth migrated data.

a framework for large-scale seismic modelling and inversion in the Julia language (Witte et al., 2019).

We have tested and validated our SIFWI algorithm on synthetic Marmousi data using a very smooth initial velocity model, as shown in Figure 7. The results show that an overall high-resolution velocity model can be achieved, with a final model that closely resembles the true model, which can be obtained after only ten iterations. The experiment was also conducted for full waveform inversion using two different inputs, namely (i) band-limited input and (ii) the band-limited input that has gone through CNN LF extrapolation. The results of the experiment are shown in Figure 8. The FWI result of the CNN LF-extrapolated synthetic data shows a much sharper image and better velocity details, which is very similar to the actual model.

Results of the Sadewa field data

An examination has been undertaken to evaluate the effectiveness of our proposed CNN deep-learning model and SIFWI algorithm utilising synthetic data. The CNN deep-learning model has proven its capability to predict LF seismic content in the Sadewa field. Before being used as input for the deep learning model, the seismic data underwent preprocessing, which involved amplitude normalisation and high-pass filter application with a cut-off frequency of around 10 Hz. The accuracy of the deep learning model’s predictions was evaluated by comparing results from the CNN LF-extrapolated with the conventional de-ghosting methods, as shown in Figure 9. Based on the comparison results, it can be concluded that the deep learning model’s predictions exhibit effectiveness when applied to real data. This is evident from the alignment of the predicted LF trend with the actual data.

Subsequently, the CNN LF-extrapolated common shot gather (CSG) field data were used as input for the FWI, and the results are shown in Figure 10. Results show that FWI model obtained using CSG field data with CNN LF-extrapolated as input achieves high-resolution velocity details, which better correlates to the P-wave sonic data at the Sadewa-4 well location. This ensures that the resulting FWI model is geologically plausible. Application of the FWI velocity model to pre-stack depth migrate the 2D-Sadewa data further demonstrates the reliability of the proposed methods, as shown in Figure 11. Flatness of the reflection events shown on the depth-migrated CSG and the residual velocity plot that clusters around the centre zero value indicates the accuracy of the resulting velocity.

Conclusion

Evaluation of the CNN model on synthetic data revealed its proficiency in predicting low frequency, regardless of disparities in the velocity and acquisition geometry models utilised for training. The most optimal model was used for analysing actual seismic data in the Sadewa Field. According to the outcomes, deep learning successfully extrapolates LF content in the seismic data, demonstrating an error below 2%. However, removing any existing noise from the data is crucial before utilising the model. Applying the CNN LF-extrapolated data as input for the FWI significantly improves the resulting velocity model. It achieves better high-resolution depth migrated images, ensuring better

interpretation. Nevertheless, given its accuracy and effectiveness, the successful validation of deep learning models on synthetic and real data indicates this approach’s potential for predicting low-frequency content in seismic data across different fields.

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Revealing the hidden paleomagnetic information from the airborne total magnetic intensity (TMI) data

Michael S. Zhdanov1,2*, Michael Jorgensen1,2 and John Keating3 demonstrate that the remanent magnetisation of rocks can be determined remotely from the airborne magnetic data, opening up the possibility of paleomagnetic study on a large scale without extracting specific rock samples from the ground.

Introduction

Earth’s magnetic field is a vector field characterised by both amplitude and direction, and it is a vector function of the coordinates of the observation point (horizontal and vertical coordinates). Conventional airborne, ground, and marine magnetic surveys, however, collect the amplitude of the magnetic field only, or Total Magnetic Intensity (TMI), which is a scalar field.

Until recently, the interpretation of the TMI data was based on different types of transformation of this field and the qualitative analysis of the resulting maps. The first step of TMI data interpretation usually involves the calculation of the anomalous magnetic intensity (AMI) field by subtracting the scalar intensity of the earth’s magnetic field, usually represented by the International Geomagnetic Reference Field (IGRF). The most widely used transformations of the AMI data include different derivatives of the magnetic field intensity, reduction to the pole (RTP) and the equator (RTE), residual field calculations, and upward and downward analytical continuations (Blakely, 1995). The maps of the transformed magnetic field provide useful qualitative information about the main geological features of the survey area, like locations of magnetic anomalies, geological strike directions, major geological faults, and edges of the different geological formations, etc.

The methods of magnetic field inversion developed over the last decades resulted in a paradigm change in the interpretation of the TMI data (e.g., Li and Oldenburg, 1996; Portniaguine and Zhdanov, 2002). These methods invert for magnetic susceptibility, linking induced magnetisation to the Earth’s magnetic field. 3D images of the subsurface magnetic susceptibility distribution help us to identify the locations of the rock formations with anomalous magnetisation, thus providing information about zones of potential mineralisation. These methods have become widely used in the quantitative interpretation of the magnetic survey data.

However, conventional inversion methods for Total Magnetic Intensity (TMI) data assume that rocks do not possess remanent magnetisation, attributing the observed magnetic field solely to induced magnetisation. In fact, rocks exhibit both induced and remanent magnetisations, preserving the historical Earth’s magnetic field at their time of formation.

Two primary natural mechanisms contribute to remanent magnetisation. First, in igneous rocks, as magnetic minerals cool through the Curie point, their magnetic domains align with the Earth’s magnetic field during formation, creating a lasting record of orientation. The second mechanism occurs during sedimentation, where tiny grains in sedimentary deposits align with the Earth’s magnetic field during deposition before the rock consolidates.

The remanent magnetisation is characterised by magnetic vectors recorded in rock. It plays a critical role in paleomagnetic applications, such as magnetostratigraphy, paleointensity studies, and apparent polar wander. Remanent magnetisation is also used to investigate sedimentary, igneous, and metamorphic rocks (Tauxe, 2003).

The vector of remanent magnetisation can be described by its inclination, declination, and intensity. Until recently, these parameters were determined from paleomagnetic samples in the labs only using specific laboratory procedures, including measurements and demagnetisation, which are very laborious and time-consuming.

In this paper, we demonstrate that the remanent magnetisation of the rocks can be determined remotely from the airborne magnetic data. This opens the possibility of paleomagnetic study on a large scale without extracting specific rock samples from the ground.

Figure 1 Representation of magnetisation vector as a superposition of remanent and induced magnetisation (after Jorgensen et al., 2023).

1 TechnoImaging | 2 The University of Utah | 3 PJX Resources Inc

* Corresponding author, E-mail: mzhdanov@technoimaging.com

DOI: 10.3997/1365-2397.fb2024060

Induced and remanent magnetisations

In a general case, the total magnetisation vector, can be represented as a superposition of induced, , and remanent, magnetisations (Figure 1):

Thus, the parametric functional for the regularised solution of the inverse problem takes the following form: (5)

The induced magnetisation is parallel and linearly proportional to the inducing magnetic field, , , where is the magnetic susceptibility.

We should note that paramagnetic and ferromagnetic materials tend to align in the direction of the inducing field, while diamagnetic materials tend to align in the opposite direction (Figure 1).

The remanent magnetisation can manifest itself as a vector pointing away from the inducing field (Figure 1):

where are the scalar components of the remanent magnetisation.

In recent decades, there has been considerable focus in research and applications on extracting magnetisation vectors from observed Total Magnetic Intensity (TMI) data, as evidenced by studies such as those by Ellis et al. (2012), Zhu et al. (2015), Li (2017), Jorgensen and Zhdanov (2021), and Jorgensen et al. (2023). Numerous developed methods have concentrated on inverting TMI data to reveal the distribution of the underground magnetisation vector (Magnetisation Vector Inversion – MVI).

However, as Figure 1 shows, the magnetisation vector is a superposition of the induced and remanent magnetisation, which makes it challenging to extract the remanent magnetisation using MVI. We propose a rigorous approach by simultaneously inverting TMI data to reveal both the induced and remanent components of the magnetisation vector. To address the non-uniqueness inherent in the inverse problem, we incorporate Gramian regularisation (Zhdanov, 2015, 2023).

The studies conducted by Zhu et al. (2015) and Jorgensen and Zhdanov (2021) illustrated the viability of obtaining a dependable solution to the inverse problem related to the magnetisation vector by strengthening correlations among its various components. We suggest applying this additional constraint specifically to the components of the remanent magnetisation. This can be achieved by minimising the following Gramian stabilisers,

where is a data misfit term, is a regularisation parameter, and coefficients balance the stabilisers. The parametric functional is minimised through a reweighted regularised conjugate gradient scheme, as outlined by Zhdanov (2015). In cases where no prior susceptibility information is available in the region, is established by conducting an inversion solely for susceptibility first, which is subsequently employed as a soft constraint.

Analysis of TMI data collected over a Sullivanstyle massive sulphide target near the historical Estella Mine in British Columbia, Canada

PJX Resources Inc., a Toronto-based Canadian exploration company, recently discovered sediment hosted semi-massive to massive sulphide boulders near the historical Estella Mine in British Columbia, Canada, exhibiting Sullivan deposit-style and grade zinc, lead, silver, cadmium, and indium magnetisation. This is the first Sullivan-style and grade discovery of this kind outside the Sullivan deposit area in more than a century. The sulphide boulders with zinc (sphalerite mineral), lead (galena), and iron (pyrite and pyrrhotite) are magnetic.

PJX’s Dewdney Trail property in this study was surveyed in May 2021 using a helicopter-borne MobileMT, VLF-EM, and magnetic system by Expert Geophysics Limited of Aurora, Ontario, Canada. The survey was flown with a Eurocopter AS 350 B3 at an average survey speed of 12 m/sec, average terrain clearance of 195 m, average magnetometer clearance of 116 m, and an average EM sensor clearance of 98 m.

where is the vector of discrete values of the component of remanent magnetisation, and is the vector of discrete values of magnetic susceptibility formed by their values in every cell. Symbol (... , ...) denotes the L2 inner product operation (Zhdanov, 2015). To mitigate uncertainty regarding the contributions of induced and remanent magnetisation to the total field, we can integrate prior information about magnetic susceptibility, , into the inversion process. This can be achieved by imposing the following constraint:

In total, 895 line-km of TMI data, flown at 100 m spacing, were inverted using the developed method incorporating a GPU-accelerated inversion algorithm with a moving sensitivity domain (Cuma and Zhdanov, 2014; Jorgensen and Zhdanov, 2021). The concept of the moving sensitivity domain approach can be described as follows (Cox and Zhdanov, 2008; Zhdanov, 2018). For a given receiver (magnetic sensor), we compute and store the sensitivities for those inversion cells within a predetermined horizontal distance from this receiver, i.e., the sensitivity domain. The radius of the sensitivity domain is based on the rate of sensitivity attenuation. Typically, the size of the sensitivity domain is less than the size of an airborne survey. The size of the sensitivity domain for the magnetic field is proportional to 1⁄ 3, where is the distance from a given receiver. The sensitivity matrix for the entire 3D earth model could be constructed as the superposition of the sensitivity domains from all receivers in the survey area. This approach helps to reduce the required computer memory and speed up the computations dramatically.

Figure 2 shows the map of the observed TMI data over the airborne survey area.

The TMI data were inverted to magnetic, susceptibility and remanent magnetisation vector models on a detailed fine grid

Figure 3 The horizontal section of the inverse magnetic susceptibility model at a depth of 150 m. The bold black line shows the profile crossing the Sullivan-style target area near the historical Estella Mine.

Figure 2 The map of the observed TMI data over the airborne survey area.

Figure 4 The horizontal section of the amplitude of the induced magnetisation model at a depth of 150 m. The bold black line shows the profile crossing the Sullivan-style target area near the historical Estella Mine.

Figure 5 The horizontal section of the amplitude of the remanent magnetisation model at a depth of 150 m. The bold black line shows the profile crossing the Sullivan-style target area near the historical Estella Mine.

discretisation of 10 x 20 m laterally with a logarithmic vertical discretisation to the depth of 2.6 km.

Figure 3 presents the horizontal section at a depth of 150 m of the inverse magnetic susceptibility model. Figures 4 and 5 show the horizontal sections at a depth of 150 m of the amplitudes of the induced and remanent magnetisation models, respectively. The bold black line in these figures indicates the profile crossing the Sullivan style target area near the historical Estella Mine, which will be analysed in detail below.

In Figure 6, we show a geologic cross-section of the target area interpreted from surface mapping and limited underground Estella mine data. Figures 7 and 8 represent the induced and remanent mag-

Figure 6 Geological cross-section of the Sullivan-style target area near the historical Estella Mine along the profile shown by the black line in Figures 3, 4, and 5 (looking north).

Figure 7 The vertical section of the induced magnetisation model superimposed over the geological cross-section of the Sullivan-style target area near the Estella Mine along the profile shown by the black line in Figures 3, 4, and 5.

Figure 8 The vertical section of the remanent magnetisation model superimposed over the geological cross-section of the Sullivan-style target area near the Estella Mine along the profile shown by the black line in Figures 3, 4, and 5.

netisation models superimposed over the geologic cross-section. The sulphide boulders with pyrrhotite likely to have belonged to the horizon imaged in red in Figure 7. However, the two anomalies in red in Figure 8 identify the potential for separate magnetic horizons with mineralisation. The possibility of multiple mineralised horizons is similar to what occurs at the Sullivan deposit. The low magnetic signature between the two red anomalies coincides with non-magnetic mineralisation (predominantly sphalerite with minor pyrrhotite) in the outcrop that is located about 100 m north of the section. The non-magnetic mineralisation is only visible because erosion has created a window through a thin alkalic porphyry dyke that is masking a potential deposit beneath the dyke.

Figures 9 and 10 show three-dimensional columns of the induced and remanent magnetisation models, respectively.

The red isobody in Figure 9 clearly delineates a possible source of the magnetic boulders observed downslope with high pyrrhotite content, clearly generating the induced magnetic response. However, Figure 10 provides a more complete picture of the geology of the area —delineating a broad feature relating to potentially multiple stacked mineralised sedimentary horizons. This would be consistent with the multiple styles of magnetic and non-magnetic mineralisation discovered in boulders and outcrop. Multiple stacked mineralised horizons are also similar to what occurs at the Sullivan deposit. The model also suggests that the mineralisation may plunge to the south along stratigraphy.

Conclusions

Conventional total magnetic intensity (TMI) data inversion algorithms generate a distribution of the magnetic susceptibility in the subsurface. This paper explores a unique method for extracting the remanent magnetisation from observed TMI field data. This approach involves representing the magnetisation vector as a combination of induced magnetisation, aligned parallel to the inducing magnetic field, and remanent magnetisation, positioned arbitrarily in relation to the inducing field. The inversion process targets four unknown scalar parameters — magnetic susceptibility and three scalar components of the remanent magnetisation vector. All four parameters are simultaneously inverted, revealing the comprehensive structure of the rock’s magnetic properties. This is significant because remanent magnetisation reflects the historical Earth’s magnetic field, which was present during the formation of igneous or sedimentary rocks and was preserved over time. The orientation and intensity of remanent magnetisation indicates the displacement of rock formations by tectonic forces, providing a

more accurate representation of complex geology compared to magnetic susceptibility alone.

We validated this novel approach using the TMI data collected over a Sullivan-style massive sulphide target near the historical Estella Mine in British Columbia, Canada. The direct reconstruction of induced and remanent magnetisation, coupled with the simultaneous recovery of magnetic susceptibility and remanent magnetisation, provides crucial geological and mineralisation insights within the surveyed area.

Acknowledgements

The authors acknowledge the support of the Consortium for Electromagnetic Modelling and Inversion (CEMI) at The University of Utah and TechnoImaging for this research project. We also thank PJX Resources Inc. for permission to publish and Expert Geophysics for gathering high-quality data.

References

Blakely, R.J. [1995], Potential theory in gravity and magnetic applications: Cambridge University Press.

Cox, L.H. and Zhdanov, M.S. [2008]. Advanced computational methods of rapid and rigorous 3-D inversion of airborne electromagnetic data: Communications in Computational Physics, 3(1), 160-179.

Cuma, M. and Zhdanov, M.S. [2014]. Massively parallel regularized 3D inversion of potential fields on CPUs and GPUs: Computers and Geosciences, 62, 80-87.

Ellis, R.G., De Wet, B. and Macleod, I.N. [2012]. Inversion of magnetic data from remanent and induced sources: 22nd ASEG Geophysical Conference and Exhibition, Brisbane, Australia, Expanded Abstracts

Jorgensen, M. and Zhdanov, M.S. [2021]. Recovering magnetization of rock formations by jointly inverting airborne gravity gradiometry and total magnetic intensity data: Minerals, 11, 366.

Figure 10 3D column of the remanent magnetisation model near the Estella Mine. The broad red isobody may more accurately reflect the potential for multiple mineralized horizons underlying the thin alkalic intrusive dyke.

Figure 9 3D column of the induced magnetisation model near the Estella Mine. The red isobody may correspond to the bedrock source of the high pyrrhotite boulders.

Application of simultaneous inversion of velocity and angle-dependent reflectivity in frontier exploration

Nizar Chemingui1*, Sriram Arasanipalai1, Cyrille Reiser1, Sean Crawley1, Mariana Gherasim1, Jaime Ramos-Martinez1 and Guanghui Huang1 describe how a simultaneous velocity and angle-dependent reflectivity inversion workflow can provide accurate and high-resolution attributes that could help derisk frontier exploration.

Introduction

Seismic attributes can play a crucial role in hydrocarbon exploration by identifying potential prospects. Therefore, seismic inversion has proven to be an effective approach for generating earth models, which are then used for attribute calculations to aid interpretation. We previously introduced a novel seismic inversion technique for the joint estimation of velocity and reflectivity (Yang et al., 2022). This solution employs a vector reflectivity parameterisation of the wave equation (Whitmore et al., 2021) and uses efficient scale separation as the basis for the simultaneous inversion. The scale separation approach, based on inverse scattering theory, has been effectively employed in Reverse Time Migration (RTM) (Whitmore and Crawley, 2012) and Full Waveform Inversion (FWI) (Ramos-Martinez et al., 2016). The new inversion approach enables iterative estimation of both velocity and reflectivity within a single framework, allowing us to derive additional attributes such as relative impedance and density to improve prospectivity assessment.

Additionally, seismic amplitude variations with angle can provide valuable insights into fluid content, porosity, and lithology of subsurface formations for a deeper understanding of subsurface geology. However, conventional solutions for FWI do not straightforwardly compute pre-stack reflectivity, which is crucial for Amplitude Versus Angle (AVA) analysis.

In this work, we expand on our simultaneous inversion workflow which updates velocity and stacked 3D reflectivity, by incorporating angle and azimuth-dependent pre-stack reflectivity (Chemingui et. al, 2023). Our method extracts geometric information from the vector reflectivity and the pressure wavefield as it propagates in the earth’s subsurface. This enables us to compute the angle between the incident wavefield and the vector reflectivity, thereby facilitating the construction of angle gathers. During the simultaneous inversion process, the velocity model and angle gathers are continuously updated, leading to improved model resolution and compensating for incomplete acquisition and variations in illumination.

We demonstrate the application of our simultaneous inversion workflow using 3D seismic data from the frontier Salar and

1 PGS

* Corresponding author, E-mail: nizar.chemingui@pgs.com

DOI: 10.3997/1365-2397.fb2024061

Orphan Basins, offshore Newfoundland and Labrador, Canada. This approach yields more reliable attributes and better insight into prospectivity, which can help derisk exploration efforts in these frontier areas.

Methodology – Simultaneous inversion of velocity and angle-dependent reflectivity

In general, seismic inversion relies on a modelling relation defined by the wave equation, which connects the recorded seismic data to subsurface models. The process involves iteratively adjusting model parameters to minimise discrepancies between observed and predicted seismic responses. In our formulation, we use the acoustic wave equation, parameterised in terms of velocity and vector reflectivity (Whitmore et al., 2021):

(1)

In this equation, P represents the pressure wavefield, V denotes the velocity, and R(x) is the vector reflectivity defined as,

(2)

where Z is the acoustic impedance, and the source term is represented by S(x, t).

In the above formulation, velocity and reflectivity are explicitly set as model parameters eliminating the need to construct a density model. The sensitivity kernels for velocity and impedance obtained through inverse scattering theory (Whitmore and Crawley, 2012; Ramos-Martinez et al., 2016), are combined with the special representation of the wave equation, forming the basis for our simultaneous inversion of velocity and reflectivity (Yang et al., 2022).

To construct angle gathers, it is essential to compute the incidence and reflection angles (or reflector dip direction) at each image point. The vector reflectivity wave equation includes a fundamental characteristic that facilitates the calculation of these angles. Specifically, the gradient of the forward propagation

wavefield provides the incident wavefield’s direction, whereas the vector reflectivity contains information about the reflector. Consequently, the reflection angle required for constructing the pre-stack angle gathers can be derived as follows:

data identified several fan systems along the margin, interpreted as Oligocene in age. The main prospectivity is believed to reside in these fans, originating from the shelf and shelf-edge deltas. Within the reservoir interval, Class II AVA anomalies are observed, along with Class IV responses in the deeper section, analogous to a modelled source rock in the region. The average water column depth exceeds 3 km in this area.

In such frontier exploration areas, the lack of well information for calibration of the seismic response puts more demand on optimising the seismic images and their associated attributes in a timely fashion to identify and derisk prospects as efficiently as possible. Therefore, the primary goal of this study was to develop a detailed, high-resolution velocity model to better define the target fan system. The aim was to refine the velocity over the lead and provide reliable pre-stack angle-dependent reflectivity for enhanced interpretative analysis. Successfully achieving these objectives would significantly reduce exploration risks in this basin.

Figure 1 illustrates the geometric definition of these elements and their relationship with the reflection angle and its azimuth. The generation of angle gathers follows a process similar to the one used in RTM where angle and azimuth maps are computed for each individual shot and used for the binning of the pre-stack images.

During each iteration, the modelling engine incorporates reflectivity extracted from the current angle gathers, which are continuously updated in subsequent cycles. This process can also produce azimuth gathers by integrating azimuth maps, enabling further characterisation of subsurface structure variations with azimuth.

Application in the Salar Basin – South Bank 3D

The first study area showcasing the benefits of our inversion workflow is in the Salar Basin, located in southeast Newfoundland and Labrador, Canada (Figure 2). The 3D seismic data were acquired in 2020 using multisensor streamer technology. The survey comprised 16 cables with 100 m streamer separation and an 8 km streamer length. Previous analyses of existing seismic

The simultaneous inversion workflow started with a smoothed tomographic velocity model and minimally processed field (shot-ordered) data. A maximum frequency of 40 Hz was selected to delineate the stacked sands in the reservoir interval.

Figure 3 shows examples of the starting and inverted models, demonstrating significantly improved resolution of the velocity field resulting from the simultaneous inversion. In addition to the inverted velocity, angle-dependent reflectivity volumes were simultaneously produced. A comparison with similar volumes from a conventional Kirchhoff migration reveals consistent amplitude behaviour and an improved signal-to-noise ratio in the inverted angle gathers (see Figure 4).

The availability of prestack reflectivity volumes from simultaneous inversion enabled the estimation of elastic properties such as relative acoustic impedance and relative Vp/Vs. The relative attributes calculated from the simultaneous inversion are similar and exhibit improved resolution over the main leads in the Tertiary section compared to the attributes from the Kirchhoff migration (Figure 5). The similarity in the computed attributes is reassuring as Kirchhoff migration has long been accepted as an AVA-friendly migration solution.

Figure 1 Geometric relation between the vector reflectivity (R), the gradient of the pressure field , and the reflection angle (θ) and azimuth (φ)

Figure 2 Map showing the study areas in the Salar and Orphan basins, offshore Newfoundland and Labrador.

Application in the Orphan Basin – Cape Anguille 3D survey

The second study area demonstrating the advantages of our simultaneous inversion is in the Orphan Basin, a highly prospective and underexplored area offshore Newfoundland and Labrador, Canada. Previous exploration endeavours in this basin focused on structural highs in the Cretaceous and Jurassic intervals with limited success. However, the availability of modern, high-quality, 3D regional seismic data has significantly enhanced our understanding of this frontier basin (McCallum et al., 2017). Exploration targets now encompass Late Jurassic and

Figure 3 Salar Basin: Example of starting (A) and 40 Hz inverted (B) velocity models. The resolution of the velocity field has significantly improved after the simultaneous inversion, clearly defining the details along the stacked sands.

Figure 4 Salar Basin: Comparison of angle gathers from Kirchhoff migration and simultaneous inversion (top row) and the product stack (multiplication of the estimated Shuey 2 term intercept and gradient) (bottom row). Note the similarity between the Kirchhoff migration and inversion results, with a clear improvement in the signal-to-noise ratio in the latter.

Figure 5 Salar Basin: Example of attribute extraction at the reservoir interval, showing relative acoustic impedance (Ip) (top row) and Vp/Vs (bottom row). The elastic attributes from the simultaneous inversion of shot data align with the results from the conventional Kirchhoff imaging workflow. However, direct inversion from data to seismic attributes significantly reduces the cycle time for building ground models.

Early Cretaceous reservoirs potentially sourced by Late Jurassic marine shales. Notably, Class II AVA anomalies are observed in the reservoir interval.

The contributing Cape Anguille 3D survey for this study was acquired in 2021 using multisensor streamer technology. This narrow azimuth data was acquired using 16 cables, 100 m streamer separation and 8-km streamer length with water depths around 2 km over the area of interest. The objective of this study was to improve the structural image of the Cretaceous and Jurassic sections at approximate depths of 5-7 km, while appropriately resolving the velocities of the stacked sands in

the Cretaceous basin. One of the challenges identified in the conventional imaging workflow was that remnant multiples compromised the estimation of reliable velocities in the reservoir interval, negatively affecting the structural image. Therefore, a simultaneous inversion workflow was selected to better manage

Figure 6 Orphan Basin: Comparison of starting (A) and inverted (B) velocity models. The inverted velocity model shows significant detail over the stacked sands in the reservoir interval (4 km-5 km in depth).

Figure 7 Orphan Basin: A) Full-Stack Reflectivity. B) Near-Angle Stack Reflectivity. C) Far-Angle Stack Reflectivity. The insets in B and C show the amplitudes along a horizon following the top of the lead (blue arrow), highlighting the Class II AVA behaviour.

the multiple energy in the recorded data, leading to a more accurate understanding of the potential petroleum system in this basin.

A 40 Hz simultaneous inversion was performed over the study area, outputting inverted velocity and angle-dependent reflectivity volumes, similar to the approach used in the Salar basin described

above. The initial lower-frequency inversions utilised raw hydrophone data, and the starting velocity model was a smoothed tomographic model. The vector reflectivity parameterisation of the wave equation in the simultaneous inversion workflow enables reliable modelling of both primaries and multiples, aiding in the estimation of accurate background velocities in areas affected by multiple energy contamination. Essentially, using data closer to what was acquired in the field helped to build a reliable background velocity model. This approach significantly impacts project turnaround times and exploration decisions. Figure 6 presents a comparison between the 40 Hz inverted velocity model and the starting model. The simultaneous inversion workflow yielded a high-resolution velocity model, revealing intricate details and delineating various sections within the reservoir interval.

The full-stack reflectivity volume output from the simultaneous inversion reveals a detailed section with a robust structural image below the unconformity in the reservoir section at around 5 km depth (Figure 7a). This full reflectivity stack represents the sum of all angles at each image point. The image quality can be further enhanced by excluding non-specular angles, especially in the deeper sections. A comparison of amplitudes over a horizon in the target interval from near and far angle reflectivity volumes illustrates the expected Class II AVA behaviour (Figures 7B and 7C).

Discussion

The application of our enhanced simultaneous inversion workflow is demonstrated with field data examples from offshore Newfoundland and Labrador, Canada. These study areas, located in deep-water settings, feature diverse underlying geology and target intervals. Given the acquisition configuration, it is expected that the model updates are primarily driven by reflection energy in the recorded data. Our proposed multi-parameter inversion utilises the full acoustic wavefield through an innovative formulation of the wave equation in terms of velocity and vector reflectivity. This approach allows for the joint updating of velocity and reflectivity while refining the angular information at each image location. The resulting pre-stack reflectivity volumes provide robust structural images with improved focusing and better fault imaging. Additionally, the results demonstrate the expected amplitude fidelity and an improved signal-to-noise ratio compared to conventional Kirchhoff migration.

Our multi-parameter inversion leverages the similarities between Full Waveform Inversion (FWI) and Least-Squares Reverse Time Migration (LS-RTM). By using scale separation based on the inverse scattering imaging condition, velocity and reflectivity are updated with minimal crosstalk between the models, ensuring that reflectivity changes due to density variations are not falsely mapped as velocity updates. With accurately inverted velocity and reflectivity models, we can easily estimate additional properties, such as relative acoustic impedance and relative Vp/Vs, which are crucial for prospectivity analysis and reservoir studies.

Starting with minimally conditioned field data allows for a faster turnaround of reliable products from the inversion. Consequently, the inverted models and derived properties can provide a direct understanding of the petroleum system and facilitate reliable lead identification, ultimately contributing to reducing exploration risk.

Conclusions

We explored the implementation of our simultaneous inversion solution using two field datasets from offshore Newfoundland and Labrador. Our workflow has been enhanced to produce angle gathers at each subsurface image location, defining angle-dependent earth reflectivity. The resulting images exhibit the expected amplitude fidelity and superior signal-to-noise ratio compared to conventional migration-based workflows. Furthermore, the inverted models were utilised to derive additional subsurface attributes such as relative impedance and relative Vp/Vs, thereby enhancing prospectivity analysis.

By employing minimally processed field data and a simple starting velocity model for the inversion, we can expedite the turnaround time from data acquisition to critical exploration drilling decisions. To this point, the technology aids in derisking potential prospects in areas with imaging challenges, yielding notably different yet reliable results compared to conventional processing techniques.

These simultaneous inversion applications were conducted in frontier exploration basins with limited or no well information, constraining calculations to relative attributes from the inverted results. Directly deriving absolute density from inversion results and well data would significantly advance our understanding of an area and potentially drive up exploration activity. This progression would be the next phase in refining our workflow.

Acknowledgements

The authors would like to thank PGS for permission to publish this paper and for providing the datasets used in this study. We would also like thank the Oil and Gas Corporation of Newfoundland and Labrador (OilCo) for its support and fruitful discussions.

References

Chemingui, N., Yang, Y., Ramos-Martinez, J., Huang, G., Whitmore, D., Crawley, S., Klochikhina, E. and Arasanipalai, S. [2023]. Simultaneous Inversion of velocity and angle-dependent reflectivity. Third International Meeting for Applied Geoscience & Energy, Expanded Abstracts.

McCallum, D.S., Carter, J.E., Cameron, D.E. and Mitchell, V. [2017]. A revised distribution of Mesozoic sediments and its implications on play type elements and interpreted leads within the Orphan basin, offshore Newfoundland and Labrador, Canada. AAPG/SEG International Conference & Exhibition 2017.

Ramos-Martinez, J., Crawley S., Zou, Z., Valenciano, A.A., Qui, L. and Chemingui, N. [2016]. A Robust Gradient for Long Wavelength FWI Updates. 78th Conference and Exhibition, EAGE, Extended Abstracts, SRS2.

Whitmore, N.D. and Crawley, S. [2012]. Applications of RTM inverse scattering imaging conditions. SEG Technical Program, Expanded Abstracts: 1-6.

Whitmore, N.D., Ramos-Martinez J., Yang, Y. and Valenciano A.A. [2021]. Full wavefield modeling with vector reflectivity. 83rd Annual International Conference and Exhibition, EAGE, Extended Abstracts

Yang, Y., Ramos-Martinez, J., Whitmore, N.D., Huang, G. and Chemingui, N. [2022]. Simultaneous inversion of velocity and reflectivity. First International Meeting for Applied Geoscience & Energy, Expanded Abstracts.

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Why numbers matter

Doug Crice (Geostuff) takes exception to loose use of number conversions.

When you started your college education, if you came up through the engineering track instead of earth sciences, you probably had a lecture about numbers.

It went something like this: ‘The precision of a number is implied by the number of digits used’. For example, if a measurement (not a count) is given by ‘1’, that implies that the item being measured is more than ½ and less than 1½. If you want to be more precise, add some digits. 1.1 means more than 1.05 and less than 1.15. Notice that I am already breaking a rule here by using more precision than is implied.

To make an example another way, assume that you see a 1in long caterpillar. Now you know that caterpillars come in different sizes, and you know that this is an approximate number. But suppose I am writing for a scientific journal where some readers will think in metric, I convert the number and in parentheses say the caterpillar is 2.54 cm long. Obviously we don’t know the typical length of the caterpillar to 0.1 of a millimetre, so it would make more sense to say 2.5 cm, still implying more precision than our little worm justifies, or even better - 2½ cm.

Why am I dwelling on these numbers? Because geophysicists routinely make number conversions that are non-sensical. If a geophysicist is describing a resistivity survey using an array that will measure to a depth of 100 m, as a service to people who think in English units, it will be put in parentheses that the setup will work to depths of 328 ft (even worse, 328.1 ft). This is an actual example from a student’s poster paper at an SEG meeting. Obviously, that conversion is wrong: the depth penetration of a resistivity survey is always an approximate number and not precise to three digits. The correct conversion of that 100 m depth should be 300 ft.

So, the first rule of numbers is to use only enough digits to describe the precision of the measurement, adding some zeros when necessary to show the scale/magnitude.

Lest we think that our student was early in his/her career, let’s use an example from a published paper on soil classification for a seismic site by some experienced scientists:

‘The Vs30 values calculated for the forward and reverse shots are 285.62 m/s and 285.36 m/s, respectively, while the mid-point shot shows a Vs30 velocity of 253.59 m/s’.1

This is egregiously common now that we have replaced our slide rule by computers. Probably many of my readers have never used a slide rule, but it had the advantage that you only got the precision a calculation deserved.

Here’s another example of bad math; in this case from the Nuclear Regulatory Commission: ‘Comparison of WUS U-HS at surface of rock site (solid line) and UHS at free surface with a shear-wave velocity of 914m/sec (3,000 ft/sec) using transfer function corresponding to surface acceleration of 0.483g (Figure 6-170). Modified spectrum represents modification of surface soft rock motions to base-of-soil motions’.2

In this case, the writer started out in English Units with a perfectly reasonable number, 3000 ft/sec, and converted it to metric units. But of course, the shear-wave velocity isn’t known to three-digit precision. The correct conversion is 900 m/sec, not 914. And the legitimacy of calculating the acceleration to 0.001 g is certainly not reasonable.

There are exceptions. An American football field is 300 ft in length, or 91.44 m. In this case, 300 ft is an exact number and the metric conversion is correct.

There are actually precise rules which determine how numbers should be rounded: ‘The number of significant digits retained must be such that accuracy is neither sacrificed or exaggerated’.3

The author hopes that earth science instructors will pass on this simple rule to their students so that future geophysicists won’t be embarrassed by inappropriate use of significant digits in this rather inexact science.

1 Savas¸ Karabulut, Soil classification for seismic site effect using MASW and ReMi methods: A case study from western Anatolia (Dikili -izmir), Journal of Applied Geophysics Volume 150, March 2018, Pages 254-266. https://www.sciencedirect.com/science/article/abs/pii/S0926985117305943

2 Technical Basis for Revision of Regulatory Guidance on Design Ground Motions: Hazard- and Risk-consistent Ground Motion Spectra Guidelines, Risk Engineering, Inc, U.S. Nuclear Regulatory Commission. https://www.nrc.gov/docs/ML0131/ML013100012.pdf

3 The International System of Units (SI) Conversion Factors for General Use. NIST special publication 1038. https://www.govinfo.gov/content/pkg/GOVPUB-C13-f10c2ff9e7af2091314396a2d53213e4/pdf/GOVPUB-C13-f10c2ff9e7af2091314396a2d53213e4.pdf

Views expressed in this article are solely those of the author, who can be contacted at dcrice@geostuff.com

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