Vol. 35 32 No. No. 232 2018 2014 Vol.
UNDERWATER TECHNOLOGY
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A Personal View... SUT-US: volunteering to advance education in underwater technology
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Zenon Medina-Cetina
Book Review Sensing and Control for Autonomous Vehicles: Applications to Land, Water and Air Vehicles
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Detecting human-knapped flint with marine high-resolution reflection seismics: A preliminary study of new possibilities for subsea mapping of submerged Stone Age sites
Ole Grøn, Lars Ole Boldreel, Jean-Pierre Hermand, Hugo Rasmussen, Antonio Dell’Anno5, Deborah Cvikel, Ehud Galili7, Bo Madsen and Egon Nørmark
Book Review NOAA Diving Manual – Diving for Science and Technology, Sixth Edition ISSN 1756 0543
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Technical Briefing Dive computer decompression models and algorithms: philosophical and practical views
Sergio Angelini
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UNDERWATER TECHNOLOGY Editor Dr MDJ Sayer Scottish Association for Marine Science Assistant Editor E Azzopardi SUT Editorial Board Chairman Dr MDJ Sayer Scottish Association for Marine Science Gavin Anthony, GAVINS Ltd Dr MA Atamanand, National Institute of Ocean Technology, India LJ Ayling, Maris International Ltd Commander Nicholas Rodgers FRMetS RN (Rtd) Prof Ying Chen, Zhejiang University Jonathan Colby, Verdant Power Neil Douglas, Viper Innovations Ltd, Prof Fathi H. Ghorbel, Rice University G Griffi ths MBE, Autonomous Analytics Prof C Kuo FRSE, Emeritus Strathclyde University Dr WD Loth, WD Loth & Co Ltd Craig McLean, National Ocean and Atmospheric Administration Dr S Merry, Focus Offshore Ltd Prof Zenon Medina-Cetina, Texas A&M University Prof António M. Pascoal, Institute for Systems and Robotics, Lisbon Dr Alexander Phillips, National Oceanography Centre, Southampton Prof WG Price FRS FEng, Emeritus Southampton University Dr R Rayner, Sonardyne International Ltd Roland Rogers CSCi, CMarS, FIMarEST, FSUT Dr Ron Lewis, Memorial University of Newfoundland Prof R Sutton, Emeritus Plymouth University Dr R Venkatesan, National Institute of Ocean Technology, India Prof Zoran Vukić, University of Zagreb Prof P Wadhams, University of Cambridge Cover Image (top): zoonar.com/syrist Cover Image (bottom): Steve Crowther Cover design: Quarto Design/ kate@quartodesign.com
Society for Underwater Technology Underwater Technology is the peer-reviewed international journal of the Society for Underwater Technology (SUT). SUT is a multidisciplinary learned society that brings together individuals and organisations with a common interest in underwater technology, ocean science and offshore engineering. It was founded in 1966 and has members in more than 40 countries worldwide, incIuding engineers, scientists, other professionals and students working in these areas. The Society has branches in Aberdeen, London and South of England, and Newcastle in the UK, Perth and Melbourne in Australia, Rio de Janeiro in Brazil, Beijing in China, Kuala Lumpur in Malaysia, Bergen in Norway and Houston in the USA. SUT provides its members with a forum for communication through technical publications, events, branches and specialist interest groups. It also provides registration of specialist subsea engineers, student sponsorship through an Educational Support Fund and careers information. For further information please visit www.sut.org or contact: Society for Underwater Technology 1 Fetter Lane EC4A 1BR London UK e info@sut.org t +44 (0)20 3440 5535 f +44 (0)20 3440 5980
Scope and submissions The objectives of Underwater Technology are to inform and acquaint members of the Society for Underwater Technology with current views and new developments in the broad areas of underwater technology, ocean science and offshore engineering. SUT’s interests and the scope of Underwater Technology are interdisciplinary, covering technological aspects and applications of topics including: diving technology and physiology, environmental forces, geology/geotechnics, marine pollution, marine renewable energies, marine resources, oceanography, salvage and decommissioning, subsea systems, underwater robotics, underwater science and underwater vehicle technologies. Underwater Technology carries personal views, technical papers, technical briefings and book reviews. We invite papers and articles covering all aspects of underwater technology. Original papers on new technology, its development and applications, or covering new applications for existing technology, are particularly welcome. All papers submitted for publication are peer reviewed through the Editorial Advisory Board. Submissions should adhere to the journal’s style and layout – please see the Guidelines for Authors available at www.sut.org.uk/journal/default.htm or email elaine.azzopardi@sut.org for further information. While the journal is not ISI rated, SUT will not be charging authors for submissions.
in more than 40 countries worldwide, including over 190 Corporate Members of the Society.
Disclaimer and copyright The Society does not accept responsibility for the technical accuracy of any items published in Underwater Technology or for the opinions expressed in such items. The copyright of any paper published in the journal is retained by the author(s) unless otherwise stated. All authors are supplied with a PDF version of their papers once published. Authors are encouraged to make the PDF version of their papers free to download from their own websites.
Open Access Underwater Technology is available as Open Access. PDF versions of all published papers from Underwater Technology may be accessed via ingentaconnect at www. ingentaconnect.com/content/sut/unwt. All issues from Volume 20 (1995) onwards are available as Open Access. The Society for Underwater Technology also encourages Underwater Technology authors to make their papers available online on their personal and/or institutional websites for Open Access. Through this arrangement, the Society supports the Open Access policy not only in the UK (the Research Councils UK (RCUK) policy) but also the drive towards Open Access in other countries.
Abstracting and indexing Underwater Technology is included in Emerging Sources Citation Index. Additional abstracting and indexing services include American Academy of Underwater Sciences (AAUS) E-Slate; Aquatic Sciences and Fisheries Abstracts (Biological Sciences and Living Resources; Ocean Technology, Policy and Non-Living Resources; and Aquatic Pollution and Environmental Policy); Compendex; EBSCO Discovery Service; Fluidex; Geobase; Marine Technology Abstracts; Oceanic Abstracts; Scopus; and WorldCat Discovery Services.
Subscription Subscription to the print version of Underwater Technology is available to non-members of the Society at the following rates per volume (single issue rates in brackets). Prices are given in GBP. Accepted methods of payment are cheque or credit card (MasterCard and Visa). Foreign cheques must be in GBP and drawn on a British bank otherwise a currency conversion surcharge is incurred. UK subscription Overseas subscription
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Underwater Technology is also available in electronic format via ingentaconnect as Open Access. To subscribe to the print version of the journal or for more information please email Elaine Azzopardi at elaine.azzopardi@sut.org
Publication and circulation Underwater Technology is published in March, July and November, in four issues per volume. The journal has a circulation of 2,400 copies to SUT members and subscribers
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Advertising To book an advert or for more information please contact Elaine Azzopardi at elaine.azzopardi@sut.org
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A Personal View...
doi:10.3723/ut.35.033 Underwater Technology, Vol. 35, No. 2, pp. 33–34, 2018
SUT-US: Volunteering to advance education in underwater technology “We need not one, but three reforms!” The room went quiet and some eyebrows were raised after my opening remarks. We were passing through the hardest of the times in our industry’s history, and I was presenting my case to be elected Chair and President of the SUT in the US almost three years ago. Since then, and thanks to the support of extraordinary volunteers, our branch has voted and passed three reforms and is about to complete their implementation.
Executive reform This reform requires us to reorganise and re-define how we operate and managed our branch, from staffing to its day-to-day operations. Currently, our branch has no employees and no office. Still, it has flourished from having no social media presence to becoming SUT’s leading media marketeers, with the highest class on every detail we present at our events and leading to the launch of a distance education programme after innovating on the use of combined webinars and networking for years. These changes could only happen thanks to our staff members Araceli and Patsy, who wear the colours of SUT with so much pride, making all our volunteers feel at home. Such is their enthusiasm that we forget that they own their own companies while giving it all to SUT! Also, thanks goes to the strong steering of our Honorary Treasurer Don Schlater, who was able to pull our finances out of a severe loss and are now a profitable branch!
Constitutional reform This reform has redefined how we exercise our governance and how we organise our committees. Our tireless executive committee, led by our Honorary Secretary Jan van Smirren, revisited our bylaws starting with the clarification of who we want to be and what expectations we wanted to set for our members. Democratically elected Chairs of all committees (technical and non-technical) are now, by default, members of the executive committee – as long as they prove that they lead a committee with a constitution and a leadership organisation with governance defined by its own members. Since these requirements apply to our student chapters, our new bylaws opened seats of our board to students for the first time in the history of SUT. What has made our branch feel so proud is that our constitution now acknowledges that the way to provide true value to our Society is to keep a diverse and inclusive branch with zero-tolerance of industrial espionage or any type of harassment.
Technological reform This reform went beyond expectations. From my trench in academia, we conduct research that will define the future of energy, in my case from risk-based informed decisions. Therefore, it was clear to me that our branch needed to expand its technological base from our traditional main focus on hydrocarbon to new emerging technological sectors.
Zenon Medina-Cetina is Chair and President of the SUT-US and Associate Professor, Departments of Civil, Petroleum and Ocean Engineering, Department of Geography Texas A&M University. At TAMU, he leads the Stochastic Geomechanics Laboratory (SGL) and is the Associate Director of the Centre for Geospatial Sciences, Applications and Technology (GEOSAT).
For many years, we had two very active technical committees: Subsea Operations and Engineering (SEO) and Offshore Site Investigation and Geotechnics (OSIG). Within the past year, a new team of volunteers have joined SUT-US to help launch three new technical committees: • Group of Environmental Forces (GEFUS); • Underwater Robotics and Automation (URA); and • Offshore Renewables (OR). These are adding much broader opportunities for the use of underwater technology, from improving our understanding on the effects of climate variability in the environment, society and economy; to new technology integration using big data and artificial intelligence
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Zenon Medina-Cetina. SUT-US: Volunteering to advance education in underwater technology
(and sometimes even Bayesian inference). The proposed reforms had a single objective: to focus our volunteering efforts in the future. My vision for the future of SUT started while I was Chair of OSIG, which let me build a bridge between university and industry through the launching of the SUT-Texas A&M University (TAMU) student chapter. This spring, SUT-TAMU just inaugurated its fifth student president, and is now joined by SUT-US student chapters at the University of Houston, Texas A&M University Galveston, Rice University and most recently the Instituto Tecnologico del Petroleo y la Energia in Merida Yucatan, Mexico (inaugurated just last month). Our branch sponsors all of them. This effort has also taken our youngest professional volunteers to witness the re-emergence of our branch’s Young Professional
(YP) committee. And you guess it right, our latest committee to be formed is now led by former members of our student chapters! We are about to close a virtuous circle that will reshape our branch and our Society very soon. And the best part of it is that since the YP leadership was democratically elected and has a constitution with a governance defined by the committee leadership, that means that the YP Chair has a seat by default in our branch’s executive committee. As if these SUT-US efforts were not enough to bet on the future of our Society, also during my first tenure as Chair of OSIG, we launched one of SUT’s most successful outreach programmes called the School Touring Program (STP). It aims to introduce students from K-12 into all possible disciplines ‘touching’ underwater technology (we passed beyond STEM a long time ago). Since its inception, the STP has
served tens of thousands of students – at school fairs, during regular classes, at museums. Its success made me take the programme out of OSIG and extend it branch-wide. STP this year will start a mentoring programme engaging our volunteers from our student chapters, and our YPs to professionals represented across all our committees. My view of SUT-US’ success lies in the enormous time and effort given by our volunteers. It is no secret that at SUT-US, we favour the need of committed volunteers over membership. Membership comes as a corollary of committed volunteering. This is why I don’t see us doing great yet. I think that we will, when we manage to support every single candidate that applies to our scholarship programme. But for that we need committed volunteers, willing to advance education in underwater technology.
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doi:10.3723/ut.35.035 Underwater Technology, Vol. 35, No. 2, pp. 35–49, 2018
Technical Paper
www.sut.org
Detecting human-knapped flint with marine high-resolution reflection seismics: A preliminary study of new possibilities for subsea mapping of submerged Stone Age sites Ole Grøn*1,2, Lars Ole Boldreel1, Jean-Pierre Hermand3, Hugo Rasmussen4, Antonio Dell’Anno5, Deborah Cvikel6, Ehud Galili7, Bo Madsen8 and Egon Nørmark9 1 Department of Geosciences and Natural Resource Management, University of Copenhagen, Østervoldgade 10, DK 1250 Copenhagen K, Denmark 2 Culture & Preservation, Markstien 23, 9690 Fjerritslev, Denmark 3 Acoustics & Environmental HydroAcoustics Lab, Université libre de Bruxelles (ULB), Avenue F.D. Roosevelt 50 – CP 165/57, B-1050 Brussels, Belgium 4 Danish Nitro-Electric, Allegade 3, 7600 Struer, Denmark 5 Department of Life and Environmental Sciences (DiSVA), Università Politecnica delle Marche (UNIVPM), Via Brecce Bianche, 60131 Ancona, Italy 6 Leon Recanati Institute for Maritime Studies, University of Haifa, Haifa 3498838, Israel 7 The Zinman Institute of Archaeology, University of Haifa, 199 Aba-Hushi Avenue, Haifa 3498838, Israel 8 East Jutland Museum, Stemannsgade 2, 8900 Randers C, Denmark 9 Department of Geoscience, Aarhus University, Høeg-Guldbergs Gade 2, 8000 Aarhus C, Denmark
Abstract Seismic high-resolution Chirp profiles from the welldocumented submerged Stone Age settlement Atlit-Yam, located off Israel’s Carmel coast, display systematic disturbances within the water column not related to sea-floor cavitation, vegetation, fish shoals, gas or salinity/temperature differences, where flint debitage from the Stone Age site had been verified archaeologically. A preliminary series of controlled experiments, using identical acquisition parameters, strongly indicate that human-knapped flint debitage lying on the sea floor, or embedded within its sediments, produces similar significant responses in the water column. Flint pieces cracked naturally by thermal or geological processes appear not to do so. Laboratory experiments, finite element modelling and controlled experiments conducted in open water on the response to broad-spectrum acoustic signals point to an excited resonance response within humanknapped flint even for sediment embedded debitage, with acoustic signals within the 2–20 kHz interval. The disturbances observed in the water column on the seismic profiles recorded at Atlit-Yam are, therefore, based on these results, interpreted as resonance from human-knapped flint debitage covered by up to 1.5 m of sand. Such a principle, * Corresponding author. Email address: og@ign.ku.dk
if substantiated by further research, should facilitate efficient and precise mapping of submerged Stone Age sites. Keywords: maritime archaeology, Stone Age, survey methods, acoustic mapping
1. Introduction Acoustic methods are increasingly being used to map archaeological sites under water. The emphasis so far has been on side-scanners and multibeam systems, which are well-suited to detecting features visible above and on the sea floor (Bates et al., 2011). These methods cannot be applied when the archaeological sites are covered by sediments. For this purpose, high-resolution sub-bottom profilers (highresolution reflection seismic methods) have begun to play a role in the detection of archaeological sites (such as shipwrecks and pole structures) embedded in sea-floor sediments (Plets et al., 2009; Grøn et al., 2015; Grøn et al., 2018; Boldreel et al., 2018). Acoustic discrimination between cultural layers of submerged Stone Age sites and natural layers of
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Fig 1: Left: Atlit-Yam, located ~15 km SSE of Haifa (based on satellite images from Google Earth). Right: the coastline and the sailing lines. The circle marks the approximate outline of the Atlit-Yam Stone age settlement.
similar appearance has, however, proved difficult (Grøn and Boldreel, 2014). Surveys focusing on submerged Stone Age sites are today, therefore, mainly based on topographical/bathymetrical prediction (e.g. Fischer, 2004; Benjamin, 2010). The application of this approach to maritime archaeology in Denmark has been shown to detect less than 1 % of the Stone Age sites that would be present in similarly surveyed areas on land: a figure that seems unreasonably low (Grøn, 2012, 2018; Gross et al., 2018). A more reliable way of identifying these settlements is therefore needed. A geophysical approach, such as the one discussed in this paper, could be used to outline target areas with much greater efficiency for inspection by divers or other verification methods. In 2014, a collaboration was established between marine archaeologists from Norway and Israel and marine geophysicists from Denmark to test whether high-resolution seismic profiling could be applied successfully to the sandy, shallow sediments off the Israeli coast on a general basis as in Denmark (Boldreel et al., 2010), and specifically for the identification of sediment-embedded archaeological artefacts, as successfully as in Danish waters (Grøn et al., 2007; Grøn and Boldreel, 2014; Grøn et al., 2015). In 2014–15, field campaigns were carried out in the near-coastal parts of northern Israel to test
whether high-resolution seismic profiling (Chirp III) could be used to identify shipwrecks, harbour constructions, settlements, including those from the Stone Age, and poles located in sandy sediments below the sea floor (Grøn et al., 2015; Cvikel et al., 2017). One of the sites investigated was the archaeologically well-documented Stone Age settlement of Atlit-Yam, located off Israel’s Carmel coast (Fig 1), at a water depth of approximately 10–12 m. In addition to large amounts of flint debitage† (Figs 2a–c), the cultural remains recorded at AtlitYam include wall foundations, stone-built wells, burials and megaliths (Galili and Rosen, 2011) (see Fig 5b in section 5.1). The site is dated to 7500–4600 cal BC, and it thereby represents the earliest known evidence for a ‘agro-pastoral-marine subsistence’ (Galili and Rosen, 2011). The large extent of the site is due, not least, to the fact that it was located on the coast and withdrew progressively in the face of rising sea levels (Galili and Rosen, 2011). Over the years, systematic recording and investigation have been undertaken for most of the 40 000 m2 area of the site. These show that the remains are generally covered by 1 m–1.5 m of sand, although a † The expression ‘debitage’ is used here to include all the pieces of flint removed from a core by knapping: blades, flakes and waste. It also includes tools produced on the basis of these, but excludes cores and core tools. This is because of clear evidence that pieces in the first category respond acoustically to certain frequency ranges, while this has not been observed for the latter.
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Fig 2: (a) Flint-knapping utilising the Hertzian cone phenomenon to remove long, sharp pieces of debitage (blades) from a prepared flint core. (b) A blade with the characteristic waves on its ventral side (inner surface in relation to the core). (c) Flint core with 5 cm–8 cm long, thin pieces of flint debitage (blades) removed.
small but variable part of the cultural deposits is generally exposed due to sediment dynamics (Galili and Rosen, 2011). Seismic profiling had not been employed previously in the investigation of the Atlit-Yam settlement, and it was decided to test whether high-resolution seismic methods could provide new information about the site. The recorded data were of good quality and were analysed on a workstation using
the computer software Petrel. Interpretation of the seismic profiles revealed that several of them displayed characteristic ‘haystack-like’ disturbances in the water column (Fig 3). Most of these were located in the parts of the investigated area where settlement remains from the Stone Age had been identified and were therefore related to the site’s cultural layer, which contained significant amounts of human-knapped debitage. They were different from the employed equipment’s signatures for sea-floor cavitation, vegetation, gas or salinity/temperature differences. They could resemble the signature of fish shoals but did not move over time. It was therefore suggested that the ‘haystacks’ observed at AtlitYam could be related to the human-knapped flint. This called for the first study of its kind, combining controlled field experiments, laboratory measurements and finite element modelling. The aim of this paper is to underpin, on a broad methodological basis, the possibility of using acoustics to identify ancient Stone Age sites, based on their content of human-knapped flint debitage. This was achieved through the analysis of the spatial distribution of acoustic phenomena (haystack features) in relation to the recorded settlement features at Atlit-Yam, and the use of corroborating data from the Danish site of Møllegabet as well as data from two controlled seismic offshore experiments undertaken in Denmark. In the latter experiments, human-knapped flint debitage and flint cracked by geological processes was placed on the sea floor in bags and embedded in sediment in plastic buckets placed on the sea floor. It was then exposed to similar acoustic signals to those employed at Atlit-Yam
Fig 3: Four characteristic examples of haystack features in a part of profile 21 from Atlit-Yam (see Fig 5a). The grey bars at the bottom mark the horizontal extent of the haystack features. The black arrow to the right marks the sea floor, shown as a dark ‘wavy’ line below the haystacks. Horizontal noise bands can be seen in the water phase.
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to investigate whether an excited response could be detected. The results obtained from this research in Israel and Denmark correspond well to those obtained in sound laboratory experiments and from theoretical finite element modelling.
2. Flint-knapping and the submerged Stone Age Flint is a hard, sedimentary cryptocrystalline form of the mineral quartz (Rykart, 1995). Owing to its widespread occurrence and good knapping properties, it played a central role as a raw material for the human production of sharp-edged artefacts during the ‘Stone Age’, dating back as far as Clark’s ‘Mode I Industry’, ~ 2.5 million years ago (Clark, 1977; Ambrose, 2001). Flint-knapping technology (Fig 2) survived from the Stone Age to the 20th century in local technological niches, for example in the production of flint-spiked threshing sledges (Whittaker, 2014). Surviving flint-knapping has also been recorded in ethnographic contexts (e.g. Shackley, 2003). Consequently, it was still possible for prehistoric archaeologists to study a live flint-knapping tradition as a basis for modern experimental flint-knapping in the 20th century (Clarke, 1935). Knapping flint involves the intentional removal, from a prepared core, of sharp pieces of debitage (Figs 2a–c). Subsequently, some pieces are shaped further (typically ‘retouched’) to make tools (Inizan et al., 1999). Our understanding of the flint-knapping process is that debitage (i.e. sharp pieces of flint) is removed from the edge of a prepared core as sections of ‘Hertzian cones’ (Fig 2). The shock waves that bring this about are created in a controlled way by the flint-knapper, for instance with a hammer stone (Fig 2a) (Inizan et al., 1999; Knapp, 2010). Stone Age settlements located in now submerged areas of northern Europe have been shown to date from a period beginning almost 1 million years ago and extending up until the end of the Stone Age, about 4000 years ago. The high biomass production of marine coastal zones suggests that they would have been very attractive to humans, compared with other types of environment (Odum and Barrett, 2005; Grøn, 2015). Since extensive parts of the coastal zone, on the continental shelf extending down to 140 m below the present sea level, had potential for human settlement as related to the fluctuating sea level, they can be expected to have hosted relatively dense populations at various times in the past (Dix and Westley, 2006; Parfit et al., 2010; Bailey, 2011; Flemming et al., 2011; Tizzard et al., 2015). The recovery from the sea floor of the southern North Sea of a large and diverse assemblage of
remains of a so-called mammoth fauna dating from the Weichselian glaciation, and covering at least the time interval 45–30 ky BP, documents the potential value for humans of such open coastal areas (Glimmerveeen et al., 2004; Mol et al., 2006; Grøn, 2015; Moree and Sier, 2015). Many of the submerged Stone Age sites located in deeper waters today can be expected to have been damaged or totally reworked and redeposited due to marine currents and fluctuations in sea level (e.g. Dix and Westley, 2006). However, experience from the shallower and therefore more dynamic waters with strong currents occurring in sounds and straits, which cause disturbance and destruction, show that pockets with good conditions for preservation of such sites do exist, in some cases even with extremely good preservation of organic material (Skaarup and Grøn, 2004; Bendixen et al., 2017). Where systematic surveys in Denmark have focused on mapping submerged Stone Age settlements, they have recorded a considerable number of potential sites. A good example of this is the pioneering approach adopted in 1972 by Langelands Museum. By 2004, these efforts had resulted in the recording of 126 such sites (Skaarup and Grøn, 2004). Even relatively vulnerable structures, such as graves containing skeletons, fish weirs and the remains of dwellings with stakes and, in one case, a bark-covered sleeping platform – have been found preserved under water using this strategy (Skaarup and Grøn, 2004; Uldum, 2011). Systematic surveys with divers have their limits since they can mainly be used to record relatively shallow sites exposed on the sea floor and not the better protected ones embedded in sea-floor sediments. To limit the survey area, surveys were, to some degree, directed by the ‘fishing-site model’ which has recently been shown to be rather ineffective for mapping Stone Age sites (Grøn, 2018; Gross et al., 2018). The records are, in accordance with these limitations, strongly dominated by sites from the Late Mesolithic, whereas older and deeper sites are only rarely featured (Fischer, 2011). Therefore, the number of sites recorded today can only represent a fraction of the total, and locally their density will vary according to the level of effort invested in surveys for such sites.
3. Acoustic detection of flint flake- and blade-based tools and other debitage During experimental flint-knapping, it was observed that the production of pieces of debitage produced a high, loud and characteristic tone as they were
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Fig 4: The frequency peaks measured for 16 pieces of flint debitage at the Bang & Olufsen sound lab. The two upper samples each consist of two pieces of debitage (Rasmussen, 1982).
detached from the core. To test whether this phenomenon could be used for acoustic mapping of Stone Age sites, 16 pieces of human-knapped flint debitage (blades and flakes) (Fig 4), from nine geographically diverse sites in Denmark representing various Stone Age periods, were tested for resonance in the Bang & Olufsen sound laboratory. All the pieces responded when excited with acoustic resonance in the interval 3–23 kHz, with the main area being 7–12 kHz (Fig 4). The pieces of debitage had to be damped during the tests so that the magnitude of their acoustic response did not damage the receiver. It was therefore concluded that it should be possible to detect flint debitage of this kind acoustically even when damped by being embedded in sediment. To investigate whether it would be possible to detect the response from flint debitage embedded in sediments, finite element modelling of the response to various acoustic signals from a digital seafloor model was undertaken (Hermand et al., 2011; Hermand and Tayong, 2013). A high-resolution 3D digital model was constructed using minimal elements (finite elements) with physical characteristics ascribed to them that were identical to the materials they represented (Caiti et al., 2006). The 3D model consisted of thin layers of sand and mud overlying a till substrate. The model allowed humanknapped debitage pieces to be inserted and removed digitally without disturbing its sediment sequence (Hermand and Tayong, 2013). The results corroborated the findings from the Bang & Olufsen Sound Laboratory and showed that human-knapped flint debitage inserted into the model produced a different response to that without flint debitage, when exposed to identical emitted signals. This experiment confirmed that human-knapped flint debitage embedded in
sediment responds acoustically to some types of acoustic signal with frequencies around 10 kHz, as observed in the Bang & Olufsen sound laboratory. This reinforces the assumption that humanknapped flint can be detected acoustically when embedded in sediments (Hermand et al., 2011; Hermand and Tayong, 2013). To test further the potential of acoustic resonance of flint, Chirp recordings were undertaken off Stevns Klint, Denmark, with a similar setup to that used at Atlit-Yam. Stevns Klint is a steep coastal chalk formation which, through erosion, has deposited large amounts of natural flint, including a significant fraction of flint cracked by geological processes, in the 10 m–15 m deep waters adjacent to it. From the boat used for the seismic recording, large patches of flint could be seen on the sea floor but no ‘haystacks’ were identified in the recorded seismic profiles. This strongly suggests that flint cracked by geological processes does not respond in the same way as human-knapped flint debitage. This therefore explains why the method appears to facilitate a focus on the latter category.
4. Methodology 4.1. The Atlit-Yam recordings A small boat with an outboard motor was used for the recordings at the Atlit-Yam site. A single-beam Teledyne Chirp III sweeping the frequency interval 2 kHz – 20 kHz was the acoustic source and system for recording. It consists of an emitter and a receiver mounted within the same instrument, directly beside each other on the 61 cm long recording platform (the fish). This facilitates precise recording of the positions of the dense recording points. The navigational offset is negligible, as no hydrophone cable is used with this method. The Chirp 39
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III instrument is generally well-suited for high-resolution recording of sediment-embedded archaeological features that may accompany areas containing human-knapped flint (Grøn and Boldreel, 2014; Cvikel et al., 2017). The differential global positioning system (DGPS) was mounted in the middle of the Chip III fish, between the emitter and the receiver. C-Nav calibration of the navigation was used to obtain a precision of ±10 cm. The fish was mounted on the side of the boat to ensure that the recordings avoided disturbance from bubbles produced by the propeller, a serious potential problem for recording good data (Grøn and Boldreel, 2014). Apart from these elements, the acquisition setup was the same as described in Grøn and Boldreel (2014). Acquisition was undertaken at a speed of ~1 kn relative to the sea floor. Calm weather conditions are important for obtaining good results with highresolution recordings of archaeological features because this allows the instrument to be kept close to a specific reference level. The recorded data were automatically processed in the Chirp’s processor and saved in SEG-Y format, with the navigation being stored as UTM coordinates. Analysis of the recorded data was carried out on a workstation with Petrel software (other types of interpretation software can also be used). Despite some contamination by almost horizontal noise bands on the displayed seismic sections, high-resolution information was obtained from the water column, the sea floor and the sub-sea floor. During the interpretation and mapping of features evident in the seismic data from the Atlit-Yam site, horizontally restricted acoustic disturbances (haystacks) in the seismic sections were present in the water column (see Fig 3). These cannot be ascribed to the almost horizontal noise bands. As a second step in the analysis, maps containing archaeological information from the research area were imported into the workstation and integrated into the interpretation process. This enabled the haystack features at the Atlit-Yam site to be correlated with the known archaeological features in the area.
4.2. Two controlled high-resolution seismic experiments, Denmark Two controlled high-resolution seismic experiments were carried out in Denmark using the same Chirp III setup as at the Atlit-Yam site. The purpose of these experiments was to investigate further the acoustic characteristics of human-knapped flint debitage and flint pieces cracked naturally by thermal or geological processes. Seismic profiles were acquired before the experiments were undertaken to ensure that the sites
were free of disturbance in the water column, such as for instance ‘haystacks’. In experiment 1, two 14 kg samples of flint were used. One sample consisted of various types of human-knapped flint debitage originating from the Stone Age and varied in size from less than 10 mm to more than 200 mm, with a continuous distribution and with the main emphasis on the smaller pieces. The second sample consisted of a representative collection of various sizes of flint pieces cracked by thermal and geological processes (e.g. frost cracks, pressure cracks), which had been neither produced nor shaped by human knapping. The natural pieces were chosen for their resemblance to flint pieces knapped by humans. A systematic comparison of the pieces in the two samples was not possible, because there is a much more elaborate morphological terminology for human-knapped flint debitage than for flint pieces cracked by thermal and geological processes. ‘Blades’ and ‘flakes’ were used as central categories for human-knapped flint debitage and the naturally cracked pieces included more than 300 ‘blade-like’ and ‘flake-like’ pieces. These were evenly distributed within the size range 20 mm –140 mm, with the main part being in the interval 20–50 mm, as well as several larger irregular pieces matching the ‘core pieces’ in the human-knapped sample. The two flint samples were placed separately in two cotton bags soaked in water to avoid the inclusion of air that could disturb the seismic signal. They were slowly lowered on to the sea floor at a water depth of 3.5 m from a small inflatable rubber dinghy that was also used for the recording, so as not to produce turbulence in the water column. The bags were placed 43 m apart on the sea floor to ensure that potential responses from them could be distinguished individually. They were left for ~1 hr to stabilise before recording commenced. In the test area, seismic profiles were acquired before commencing the experiment to ensure that there was no acoustic disturbance found in the water column in the seismic recordings. The recordings were made at a speed of 1 kn, crossing the positions of the bags and their surroundings numerous times. In experiment 2, the 14 kg of human-knapped flint used in experiment 1 was buried in water-saturated sand in a plastic bucket and left for one month to ensure that no air bubbles were present. Before the bucket was lowered into the water, seismic profiles were recorded to ensure that there was no disturbance in the water column in the seismic profiles. The bucket was slowly lowered into the water and placed at a depth of 2.5 m on the sea floor and left for some time to allow the water column to stabilise. After this, numerous high-resolution profiles were acquired that crossed over the bucket and its surrounding areas.
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5. Results 5.1. Recordings from Atlit-Yam, Israel Fifty-nine high-resolution seismic profiles were recorded at Atlit-Yam (Fig 1) at a water depth of 10 m –12 m, with the sandy sea floor sloping slightly downwards out from the coast and to the north into the investigated area. Conditions were calm with only minor swells during the experimental survey and with clear visibility in the sea. For the recording, the surveyed area was subdivided into three parts: a northern (I), a central (II) and a southern (III) area. Area II is shown in Fig 5a, areas I, II, III in Fig 5b. In the central area (II), remains from the settlement have been previously recorded through marine archaeological fieldwork, whereas only a few similar remains have been recorded in the northern area (I), and none in the southern area (III) (Galili and Rosen, 2011) (Fig 5b). Of the 13 profiles orientated east to west through the northern area (I), four show haystack features (0.31 per recorded profile). Of the 30 profiles acquired in the central area (II), haystack features are evident in 28 profiles as 57 separate observable haystack features (1.90 per recorded profile). Of the 21 profiles cutting through the southern area (III), only one profile shows a single haystack feature. The haystack features are widest at their base, nearest the sea floor, and become narrower, culminating in a peak or a rounded plateau well before
they reach the sea surface. Some variation can be observed in their height and width (Fig 3). The phenomenon is particularly evident in the central part of area II, where the archaeological features are concentrated, in adjacent and crossing profiles (Fig 5) acquired at different times of the day. In area II, a concentration of haystack features is noted just around the position where a flint workshop was excavated, which contained a higher concentration of human-knapped flint debitage than was observed in the other parts of the cultural layer (Fig 5b) (Galili and Rosen, 2011). This dense zone of significant haystack features extends from the area around the flint workshop and 50 m to the north-northeast. Fig 5a shows that the locations of the haystacks are grouped within several smaller zones, as well as the large zone associated with the flint workshop. Within the part of the site covered by sailing lines, the haystack features coincide consistently with the extent of the site’s archaeological remains as recorded through several years of archaeological fieldwork, even though the main part of the cultural layer to which they are related is covered by up to 1.5 m of sand. The haystack features appear consistently as horizontally restricted phenomena in permanent positions in the straight parts of the sailing lines. They cannot therefore be ascribed to the turning of the vessel at the end of the crossings or to the presence
Fig 5: (a) The sailing lines recorded in area II Atlit-Yam. Profile 21 (see Fig 3) is shown as a thick red line. The blue line fragments mark locations where haystacks were observed in the profiles. (b) The surveyed part of the settlement area is marked with a broken black line. Coordinates are in Universal Transverse Mercator (UTM) zone 36N.
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of schools of fish. The slow speed of 1 kn ensured that they were not caused by turbulence/bubbles from the outboard motor. The fact that the tops of the haystack features are well below the sea surface, and that no air bubbles were observed coming up from the sea floor, shows that the haystack phenomena are not caused by gas leaking from the subsea floor into the water column. Furthermore, no signs of shallow gas accumulation are evident on the seismic profiles. The haystacks do not mimic the sea-floor bathymetry (see Fig 3 in section 1), which would have been the case if these phenomena had been related to the morphology of the seabed. There was no seafloor vegetation in the study area, so this cannot be the reason for the observed anomalies. No indications were observed, or could be hypothesised, which could explain the observed anomalies as reflecting salinity/temperature differences. A clear spatial correlation can therefore be observed between the haystack features and the settlement’s cultural layer with physical structures and containing significant amounts of human-knapped flint debitage (detailed in the discussion below). It has been demonstrated experimentally that this type of feature (i.e. haystack) is related to humanknapped flint debitage, even when embedded in sediments, but not to naturally cracked flint. Furthermore, it has been demonstrated in the Bang & Olufsen sound laboratory that human-knapped flint debitage displays a strong resonance feature (see Fig 4 in section 3) around 10 kHz. In addition, finite element modelling underpins the Bang & Olufsen lab results (Hermand et al., 2011; Hermand and Tayong, 2013). It can therefore be concluded that the haystacks observed at Atlit-Yam are most likely related to the human-knapped flint debitage occurring in the site’s cultural layer, and should most probably be interpreted as an acoustic response from this.
5.2. Results of two controlled experiments, Denmark The two Danish experiments outlined in section 4.2 were carried out to test whether the occurrence of human-knapped flint debitage located on the sea floor or embedded within its sediments can produce haystack features like those observed in the recorded seismic profiles at Atlit-Yam. The results of the seismic recordings are shown in Fig 6. A1–A3 in Fig 6 show three seismic profiles crossing the bag containing naturally cracked pieces, and B1–B3 show three seismic profiles crossing the bag containing human-knapped pieces. In all six profiles, the bags are visible on the sea floor, thereby demonstrating that these seismic recordings transect
the target bags. In the water column directly above the bag with human-knapped flint debitage, all three profiles display disturbances like the haystack features observed at Atlit-Yam (Fig 6b), whereas the bag containing naturally cracked flint pieces does not produce a similar disturbance in the water column (Fig 6). C1–C3 shows the results for experiment 2, where the three profiles transect the sample of humanknapped flint debitage used in experiment 1, now buried in a plastic bucket filled with water-saturated sand. Disturbances similar to the haystack features evident at Atlit-Yam can be observed directly above the bucket, which is also visible on the sea floor in the recordings.
5.3. Results from Møllegabet, Denmark A submerged Mesolithic settlement, Møllegabet, in the southern part of Denmark, was excavated in the period 1976–93. The settlement dates from the Early and Middle Ertebølle culture and was related to different palaeo-coastlines, representing the rising sea level from around 5250–4400 cal. BC (Skaarup and Grøn, 2004). The deeper part of the settlement yielded a boat grave containing the skeletal remains of a male and remains of a dwelling with its still partly bark-covered platform preserved (Skaarup and Grøn, 2004). In the eastern part of the settlement, at a water depth around 2.5 m below present sea level, there was an ~75 cm thick kitchen midden, which accumulated as a result of Late Mesolithic hunter-gatherers’ habitation adjacent to the 4400 cal BC coastline. Human-knapped flint was abundant at the site but was removed during the excavations from the limited areas excavated in the kitchen midden and to the offshore side of it, as well as in and around the boat grave and the dwelling, which were both excavated in full (Skaarup, 1995; Skaarup and Grøn, 2004). A few high-resolution seismic profiles across the site were acquired in 1996, using a Datasonics Chirp II instrument very similar to the Chirp III instrument (Skaarup and Grøn, 2004). In the light of the results obtained at Atlit-Yam, and in the controlled seismic experiments, the profiles from Møllegabet were re-examined for this study to determine whether similar acoustic disturbances were present within the water column. The recording instrument was, however, an older version than that used at Atlit-Yam and in the Danish experiments, and lacked some of the later improvements. On the seismic profile cutting through the shell midden, two clear acoustic responses in the water phase can be observed from the central part of the kitchen midden’s northern half, which was not excavated prior to the recording (Fig 7a). It is
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Fig 6: (A–C) Seismic profiles recorded during the experiments. Grey arrows indicate the level of the sea floor. The bags/bucket with flints are marked with white arrows and can be discerned on the sea floor. (A1–A3) Experiment 1: naturally cracked flint in a bag. No flint response is evident in the water column. (B1–B3) Experiment 1: human knapped flint in a bag. A clear acoustic flint response can be observed in the water column. (C1–C3) Experiment 2: human knapped flint in sediment filled bucket. A clear acoustic flint response is evident in the water column. (D) The sailing lines for experiment 1. The blue dot represents the bag containing human-knapped flint debitage and the red dot represents naturally cracked flint. (E) Experiment 1 in the southern part of the lagoon of Amager Strandpark. Experiment 2 in the little harbour of Sundby Sailing Union north of the lagoon.
known from repeated diving activity in the area that these responses do not represent physical structures, air-filled seaweed or gas leaking from the sea floor.
6. Discussion The high-resolution seismic profiles obtained from Atlit-Yam, the profiles from the Møllegabet site and the controlled seismic experiments in Denmark revealed systematically occurring acoustic disturbances, so-called haystack features, in the water column. These features are found directly above areas
containing human-knapped flint debitage and do not reflect sea-floor cavitation, vegetation, fish shoals, gas or temperature/salinity differences. In the many seismic profiles acquired from the central part of the Atlit-Yam site (zone II), the haystacks occurred most frequently and with the highest density at locations characterised by the presence of prehistoric structures and a cultural layer containing human-knapped flint debitage. The intersecting profiles recorded here display a number of zones consistently producing haystack features, regardless of the varying time of recording through the day. In the northern part of the recording area
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Fig 7: Submerged Stone Age settlement Møllegabet, Denmark. (a) the profile cutting north to south through the kitchen midden at Møllegabet with two haystack features, 1 and 2. (b) Bathymetric plan over the excavated archaeological features and the coastlines they date (Skaarup and Grøn, 2004).
(zone I), similar features were evident in four profiles, apparently signifying one coherent haystack zone. Only one haystack feature was observed in the south-eastern zone III (see Fig 5). The height, width and general magnitude of the haystacks show some variation but can be generalised as being widest near the sea floor and their top below (i.e. not touching) the surface of the water phase. Their horizontal stability at Atlit-Yam means they cannot be explained as reflections of biological phenomena. The fact that they do not reach the surface of the water column strongly indicates that the haystacks cannot be explained as gas/bubble phenomena. It is also clear that they are not related to the sea floor morphology (see Fig 3). Their distribution coincides closely with the location of the preserved settlement features and the cultural layer containing human-knapped flint debitage (Fig 5b). It is normal at Stone Age settlements that human-knapped flint debitage appears in several smaller concentrations. These can be interpreted as smaller activity/living areas, which form part of the settlement’s overall spatio-chronological activity configuration (e.g. Grøn, 2003; Grøn and Kuznetsov, 2004). The four responses obtained from the northern area (zone I), which apparently
reflect only one haystack zone, and the single small haystack feature recorded from the southern area (zone III) at Atlit-Yam, are both interpreted as small occurrences of human-knapped flint debitage that have not, as yet, been recorded by archaeologists. At the Danish settlement of Møllegabet, earlier Chirp II data were re-interpreted and revealed haystack features over the kitchen midden containing flint debitage similar to those observed at Atlit-Yam (Skaarup and Grøn, 2004). The experimental testing in Denmark was carried out with identical instrumentation and acquisition parameters to those employed at Atlit-Yam. This demonstrated the repeatability of the phenomenon and the relationship between the occurrence of human-knapped flint debitage and haystack features, both with flint debitage exposed on the surface of the sea floor and when covered by 1 m–1.5 m of sea-floor sediments, as at Atlit-Yam. The control experiment involving a 14 kg sample of naturally cracked flint produced no acoustic response. Based on these experiments, it is concluded that the haystack features observed at Atlit-Yam represent acoustic responses from the site’s large quantities of human-knapped flint debitage, most of which are covered by up to 1.5 m of sea-floor sediments. It is suggested that the haystack features occur because the debitage is excited by the highfrequency seismic signals emitted by the acoustic system. This conclusion was further corroborated by seismic recordings and excavation findings from Møllegabet, Denmark; the results of 3D finite element modelling of human-knapped flint debitage embedded in sea-floor sediments (Hermand et al., 2011; Hermand and Tayong, 2013); and measurements of human-knapped flint debitage in the Bang & Olufsen sound laboratory. It seems that strong, measurable resonance is excited in the flint debitage by the high- frequency signal emitted by the Chirp III. The fact that the haystacks are evident in the water phase, and therefore appear to be recorded before the first signal reflection from the sea floor, could well reflect a signal delay. The shapes of the haystacks on the Atlit-Yam seismic profiles show variation in their extent, form and height (Fig 3). It is presently uncertain what causes this variation, but it is speculated that it may reflect differences in the amount of flint debitage excited, the size of the debitage pieces and their orientation in relation to the acoustic signal. These aspects are, however, beyond the scope of this preliminary empirically based discussion. The ongoing method development and investigation of the physical principle behind it will hopefully provide a good theoretical understanding of the phenomenon and facilitate optimal application of it.
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The apparent consequence of this previously unreported acoustic phenomenon is that it appears possible to generate resonance in human-knapped flint debitage when excited with the appropriate frequencies. It has been demonstrated that it is possible to identify and map areas characterised by the occurrence of human-knapped flint debitage on the sea floor or embedded at least 1.5 m into the sea-floor sediments. Not only that, but it can be done with an affordable basic technical setup such as a Teledyne Chirp III sweeping the critical frequency interval of 2–20 kHz, and a precise navigation unit, combined with expertise and experience. Flint cracked by thermal or geological processes produced no similar acoustic response when subjected to the same signals and frequencies. Haystack features were also not observed in recordings made offshore from Stevns Klint, Denmark, where there is an abundance of flint cracked by thermal and geological processes on the sea floor. An important implication of these findings is that haystack features, such as those discussed here, appear to serve as indicators of human activity. The present state of the technology represents a significant advance in relation to the hitherto employed methods for mapping submerged Stone Age sites embedded in sediments, even though the sensitivity of the method discussed here is still undergoing improvement. Deep towing of the Chirp III fish also allows efficient mapping and/or verification of sites that are not accessible by divers. The method can therefore already be used in its present state to improve the detection rate for submerged Stone Age settlements. With current use of topographical/ bathymetric predictive modelling, the detection rate appears to be around, or even less than, 1 % of the total number of the Stone Age sites present. Given sufficient signal development on application of the acoustic method, it should theoretically be possible to raise detection to an estimated 50 % of the submerged sites containing human-knapped flint debitage in the areas subject to direct survey, and perhaps even higher (Fischer, 2004; Benjamin, 2012; Grøn, 2012, 2018). A further corollary of the method presented here will be facilitation of a considerable improvement in the mapping and dating of the fluctuations evident in submerged prehistoric coastlines, because in the Stone Age, coasts represented the most attractive environments for exploitation and settlement by humans (Odum and Barrett, 2005). There has been increasing recognition of this by archaeologists in recent years, even though these coastal sites often lie submerged today and are therefore difficult to locate and study (Schackleton and van Andel, 1986; Parfitt et al., 2005; Richards et al., 2005;
Jaksland, 2008, 2009; Bjerck, 2009; Wikell et al., 2009; Parfitt et al., 2010; Cortés-Sánchez et al., 2011; Ramos et al., 2011; Cuenca-Solana et al., 2013; Fischer et al., 2013). Submerged prehistoric coastal settlements with good preservation of organic material can facilitate a more precise dating of the coastline than is possible using purely geological features. This is because, in some cases, it will be possible to obtain samples for dating (radiocarbon, etc.) from human structures positioned only a few metres from the contemporaneous coastline. It will also be possible to distinguish overlapping cultural deposits, representing short time intervals, that can be used to obtain ‘sharpened’ radiocarbon dating through wiggle-matching (Van Geel and Mook, 1989). This presents the opportunity of obtaining much more detailed dating of the sediments, thereby providing high-quality input for the mapping of sea-level variation and an understanding of geological evolution. Stone Age settlements should not only be understood as habitation areas for prehistoric humans, but also as dumps for the remains of the organisms (animals and plants) they extracted from their immediate environment for food, clothes, etc. (e.g. Wilmsen, 1973; Sahlins, 1974; Service, 1979; Binford, 1980). Good examples of this are the enormous shell middens, the accumulated shells of consumed molluscs, together with other food remains, which formed in some places (e.g. Muller et al., 2002; Jerardino, 2010; Balbo et al., 2011). Common features of such sites are so-called waste/refuse layers, directly associated with the living areas and containing discarded organic materials such as mammal, bird and fish bones, as well as the remains of acorns, hazelnuts and other nuts, fruits and berries. The waste was often deposited in water, probably to reduce the stink of decomposing organic material (e.g. Rust, 1937; Balbo et al., 2011; Conneller et al., 2012; Andersen, 2013). Such accumulations would generally lead to the formation at the settlements of significant concentrations of animal and plant DNA derived from the immediate environment, relative to the general level of ancient DNA in the landscape. It is possible to retrieve ancient animal and plant DNA directly from sediments with good DNA preservation, for example permafrost (Willerslev et al., 2004). Leaching out of the ancient DNA appears not to be a problem in permafrost sediments and sediments that have recently thawed (Willerslev et al., 2014). Ancient DNA from non-frozen sediments also appears to be a rich potential source of environmental information, given that it is recovered from an uncompromised stratigraphic sequence. As the
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DNA fragments themselves are negatively electrically charged, they bind to the positively charged sediment particles and generally only leach out when the latter become displaced (Romanowski et al., 1991; Lorenz and Wackernagel, 1992; Haile et al., 2007; Lalonde et al., 2012; Rawlence et al., 2014). The risk of such disturbance should be less in buried and water-saturated sea-floor sediments than in terrestrial sediments, which have a dynamic exchange between water/ground water and air. Consequently, the possibility of obtaining undisturbed ancient DNA samples from cultural deposits at submerged Stone Age sites should generally be better than on land. As saline environments, such as saltwater-saturated sediments, appear to preserve ancient DNA surprisingly well (Corialdesi et al., 2011; Coolen et al., 2013; Lejzerowicz et al., 2013; Smith et al., 2015), this means that the potential for studying prehistoric environmental processes through ancient DNA accumulated and preserved in settlement-related sediments under water is significantly better than for non-frozen sediments on land. Acoustic mapping of Stone Age sites, is of further interest to archaeology because the positions of these settlements can reflect human dispersal routes and man-nature interaction in prehistory. They therefore provide access to a huge body of hitherto unrealised data for an understanding of sea-level change and for reconstruction of the related environmental processes. In relation to the construction of offshore installations and the extraction of raw materials from the sea floor, the importance of protection and management of submerged Stone Age sites, in both national and international waters, is promoted by the 2001 UNESCO Convention on the Protection of the Underwater Cultural Heritage (UNESCO, 2001). For proper practical management of such submerged Stone Age sites, it is important to develop more efficient mapping methods than the bathymetrically/ topographically-based predictive modelling methods currently practised today. Scanning of the potential impact areas with a direct detection-centred method prior to any detailed planning would be a methodologically and economically sensible strategy.
7. Conclusions In high-resolution (Chirp III) reflection seismic profiles recorded at the submerged Stone Age site of Atlit-Yam, Israel, acoustic disturbances in the water column resembling haystacks were observed. These show a pronounced correlation with the location of the large amounts of human-knapped flint debitage present in the cultural layer of this submerged Stone Age settlement.
Earlier recordings from the submerged Møllegabet site in Denmark, reanalysed during this study, show similar disturbances in the water column. In Møllegabet, archaeological investigations undertaken prior to the recordings show a correspondence between haystack features and the occurrence of flint debitage. Controlled Chirp III seismic experiments, employing similar acquisition parameters to those used for the recordings at Atlit-Yam, show that a test sample of human-knapped flint debitage located on the sea floor, or embedded within its sediments, produces a similar response to those observed at Atlit-Yam when acoustically excited. Flint cracked by thermal and geological processes was tested in the same experimental setup, with the same acquisition parameters, and displayed no similar response in the water column. Laboratory-based measurements of the resonance of human-knapped flint debitage shows an acoustic response in the frequency area around 10 kHz, even though the flint pieces are damped. 3D finite element modelling confirms that acoustic responses can be recorded from such sediment-embedded pieces of flint. It is therefore concluded that human-knapped flint debitage on the sea floor, as well as that embedded within its sediments, can be brought to resonance by certain acoustic signal types emitted by a high-resolution seismic system, and that this consequent acoustic response can be recorded. The experimental application of high-frequency seismic equipment instruments constitutes a new way of detecting and mapping human-knapped flint and thereby of mapping the presence of submerged Stone Age settlements.
Acknowledgments The survey at Atlit-Yam (IAA permit S-688/2016) was supported by the Israel Science Foundation (grant no. 1899/12) and conducted with the assistance of Amir Yurman and Moshe Bachar from the maritime workshop of the Leon Recanati Institute for Maritime Studies, Haifa; and Peer T. Jørgensen, Department of Geosciences and Natural Resource Management, Copenhagen, who served as a technician. Thanks to Schlumberger for the Petrel university grant issued to the Department of Geosciences and Natural Resource Management, Geology Group, University of Copenhagen.
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Technical Briefing
doi:10.3723/ut.35.051 Underwater Technology, Vol. 35, No. 2, pp. 51–61, 2018
Dive computer decompression models and algorithms: philosophical and practical views S Angelini* MARES S.p.A., Salita Bonsen, 4, 16035 Rapallo (Ge), Italy
Abstract The functioning of diving decompression computers is based on predictive models that are made operational through algorithms. Relatively simple models can be constructed to manage diving decompression obligations with a high degree of confidence, as long as the dive profiles fall within the model’s ‘range of applicability’. The same degree of confidence cannot be assumed where dive profiles are outside of that range – for instance by diving deeper, or for longer or more frequently than what had been considered in the development of the model, or because of individual physiological particularities. A common method to deal with this is to increase the level of conservatism of the model by reducing inert gas load. Depending on the dive computer, this is achieved by allowing the diver to set predefined ‘personal levels’ or through ‘gradient factors’, which is a more transparent method of obtaining a reduced inert gas load at the end of a dive. This paper outlines models and algorithms in general, and then discusses gradient factors in further detail. Keywords: dive computers, decompression models, decompression algorithms, range of applicability, M-values, gradient factors
1. Introduction 1.1. Distinction between model and algorithm In its simplest form, a model is a mathematical representation of a physical event, while an algorithm is the coding of the model in a form that can be solved by a microprocessor. Models are developed in order to predict future outcomes, and algorithms are the tools to calculate this outcome based on given initial or boundary conditions. Developing a model requires strong understanding of, and insight into, the phenomenon that is being * Email address: s.angelini@mares.com
reproduced. Moreover, the key to developing a good model is the ability to capture the essential aspects of the phenomenon and to identify those aspects that are, if not negligible, at least less relevant to the final result. For example, one could start with basic laws of physics such as conservation of mass, momentum and energy, applying them to the process at hand and deciding that, for the process being considered, heat transfer by radiation could be neglected in favour of conduction and convection because of the low temperatures involved. Radiation is very complex to model, computationally intensive for a microprocessor and only significant when temperatures are very high. Thus, when modelling the heat exchange of a first-stage regulator in water, the impact of radiation could be neglected, resulting in a simplification of the model without loss of accuracy in the result. Writing an algorithm, on the other hand, requires a strong mathematical background and advanced programming skills. So modelling is really the world of physics and physicists, while writing algorithms is the world of programmers. Mathematics is a fundamental bridge between the two, because a physicist who cannot put their model into a mathematical formulation will not be able to communicate their ideas. Similarly, a programmer who cannot apply, for example, Taylor expansions will not be able to turn the formulas into step-by-step commands.
1.2. Empirical models A model can be heavily based on theory, but some models are purely empirical, i.e. based primarily on observations of physical phenomena and interpretation thereof. An empirical model does not necessarily have to be correct to yield the correct results – that is, an empirical model can give the right results for the wrong reasons. 51
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Angelini. Dive computer decompression models and algorithms: philosophical and practical views
not just a detail within a much bigger picture. Therefore, a fundamental concept in modelling, especially in empirical modelling, is the definition of a ‘range of applicability’. This is the range within which there is a high degree of confidence that the model will yield useful results. In an empirical model, the range of applicability is the most important concept to consider. Generally, interpolating is safer than extrapolating. When interpolating, two data points are inside the range of applicability and a new one is fit in between the two existing data points, thus staying within the range of applicability. Conversely, when extrapolating, two or more data points are inside the range of applicability and the position of a point outside of that range is guessed. If data on dives to 30 m and 40 m are available, they can be used to make an educated guess for what happens at 35 m, but the same cannot be said for dives to 80 m. Fig 1: Ptolemy’s prediction of the movement of Mars around the Earth
An example of this is Ptolemy and his predictions of the position of Mars with respect to the green planet in his Earth-centred model: here Mars revolves around the stationary Earth in a flower-shaped pattern, as depicted in Fig 1 (inspired by Singh, 2004). We know this to be completely wrong, but Ptolemy was able to predict with good accuracy where the planet would be in a week or three months. By having enough data points obtained from observing a certain phenomenon, it is possible to build a model that will yield exactly those data points. The constant repeatability of the motion of the planets lends itself beautifully to this approach because, once observed, a data point will reoccur at defined intervals and, once the model has been fitted to account for that data point, it will be perpetually correct. This lends credibility to the model in spite of it being erroneous. This approach is called ‘data fitting’ and is based on empirical observations only. The model can be incorrect, as in this case, but it is difficult to dispute it since it continues to give accurate predictions. Galileo tried to dispute it, but when it became apparent that he was to follow Giordano Bruno’s fate – who was burned at the stake for heresy – he recanted (Aquilecchia, 2017). But he left us the exquisite e pur si muove (“and yet it moves”) expression.
1.3. Range of applicability Data fitting can lead to mistaken interpretations, which in turn can lead to disastrous consequences. The field of observation must be wide enough to give some confidence that what is being observed is
1.4. Decompression models Physical events governed by laws of physics can be complex to model, but in most cases experiments can be set up to yield reproducible data with which to determine the validity of the model. A decompression model, however, adds physiology into the mix, and this carries a lot of complications with it. One first has to develop a model of the human body, and then model decompression and decompression illness on top of that. A mathematical representation of the human body is probably possible, but incredibly difficult if everything is to be taken into account. Aside from physical phenomena such as blood flow, gas diffusion, bubble formation and growth, there are a plethora of chemical processes taking place as well. On top of this baseline complexity, physiology varies not only from individual to individual, but also for the same individual from one day to the next. Sleep, hydration and nutrition are just a few aspects that influence how a person will react to external stimuli. Wanting to put all this into a set of mathematical formulae is quite a daunting task. At present, there are essentially two types of decompression models†: dissolved gas models and bubble models. For simplicity’s sake, this paper restricts itself to binary mixes as breathing gas (oxygen and an inert gas, such as nitrogen or helium). Conceptually, it applies to trimix as well, although there are some other factors that may be †
In addition, there are probabilistic decompression models, in which parameters of known statistical models are fitted to a set of empirical data concerning decompression illness incidences in subjects exposed to various decompression profiles. These are not commonly found in dive computers and therefore not covered in this paper.
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important with regard to trimix such as isobaric counter diffusion. A dissolved gas model describes the human body as a number of tissues or compartments, each of which is defined by two parameters. One parameter defines how quickly the tissue absorbs and off-gases the inert gas in the breathing mix (tissue half-time), and the other defines how much overpressure of this gas the tissue can tolerate before a controlling criterion is broken (maximum tolerated supersaturation, also known as M-value). In a bubble model, one or more bubbles are tracked as they grow or shrink during the dive as a result of gas migrating in or out of it, caused by changes in ambient pressure and breathing gas. In such models, the controlling criterion is the size of the bubble(s). The dissolved gas model essentially dates back to 1908, when John Scott Haldane and his team published a paper on experiments carried out on goats (Boycott et al., 1908). This established the foundation of what is still very much in use today, and is generally referred to as the Haldanian model. The Haldanian model is brilliant for its simplicity and its flexibility in adapting to additional conservatism. Over the years, numerous studies have been carried out by the US Navy, Dr Bühlmann in Zürich and others, all aimed at better correlating the empirical data. They have mostly focused on redefining the number of tissues with which to represent the body; the tissues’ respective half-times and M-values; and the difference in speed between absorbing and releasing the inert gas, e.g. the Exponential-Linear model in the V-VAL 18 (Thalmann, 1983). Since development of the Haldanian model, two world wars took place, astronauts landed on the moon and the internet was invented. Yet the most widely accepted decompression model still stands as it did 110 years ago. This model has its limitations and certainly cannot be used to extrapolate results outside of its range of applicability. Still, when comparing the staggering difference between the simplicity of the Haldanian model and the complexity of the human body, it is impressive that it still provides useful results as the historical records on diving safety demonstrate. Implementing this model in a dive computer means that, for each tissue, every few seconds a simple equation is calculated and the results are compared with the maximum overpressure tolerated by the tissue itself. This is easily done by just about any microprocessor. Bubble models started with the research of David Yount at the University of Hawaii (Yount et al., 2000). As technology advanced and Doppler recorders or ultrasound imaging became available, it became obvious that in many, if not all, dives
(even those with no symptoms of decompression illness), a portion of the inert gas absorbed during the dive is released in the form of bubbles in the tissues or blood stream. The idea behind the model was to track a hypothetical bubble in its evolution during a dive as a function of the exposure to changing ambient pressure and partial pressures of inert gas. The research of Dr Yount eventually led to the variable permeability model (VPM). The reduced gradient bubble model (RGBM), by Dr Bruce Wienke of the Los Alamos National Lab (Weinke, 2001), shares its beginnings with the VPM but then diverges. Both are significantly more complex than a straight Haldanian model and require very powerful processors to solve the nonlinear differential equations pertaining to bubble dynamics. And both have been adjusted, at least to some extent, against dive profiles with known outcome, i.e. data fitted, because they lack a comprehensive physiologically correct model of the human body.
1.5. Extending the range of applicability Over the years, researchers have limited their work to a certain depth range, a certain dive time, and to one repetitive dive (at most), in order to keep testing manageable. For example, 3000 dives would be necessary for the following test parameters: • Five depth values – say 20 m, 30 m, 40 m, 50 m and 60 m; • Five values of bottom time – say 10 mins, 15 mins, 20 mins, 25 mins, 30 mins, • Four profile shapes – square, multilevel forward, multilevel reverse, triangular; and • Ten testers to perform the dive three times to ensure some kind of statistical significance. Apply the same parameters for one repetitive dive, with surface intervals of, say 30 mins, 60 mins, 90 mins, 120 mins and 180 mins, and 45 million dives are required. It is generally accepted that the model works when calibrated against data collated by many researchers throughout the years (notably Workman, 1965; Bühlmann, 1995). Contributions of organisations like Divers Alert Network (DAN) and even training agencies contributing their databases help to extrapolate to a range outside of the tested range. After all, several million dives are performed every year and it all adds up to some pretty impressive statistics. We convince ourselves that we can trust our dive computer when we go for the fifth dive of the day on the fourth day of a live-aboard trip. Rarely, decompression illness appears, though the dive computer had given a green light, and we call it an ‘undeserved hit.’ Lately, a growing number of divers have been performing long, deep dives, such as exploring cave
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systems in 24 hr submersions. These dives are outside of the range of any testing (except for research regarding saturation dives, but that’s the opposite end of the spectrum), and these pioneering divers have found that the dissolved gas model does not work anymore, at least not in the standard implementation. Making it more conservative is done simply by lowering the tolerated supersaturation, which implies longer and deeper decompression stops for a given exposure. These divers did not go from recreational dives to deep 24 hr dives overnight, but gradually increased the exposure. Incrementally, they found what worked and what did not, and they started providing data points that had been missing until then, conceptually extending the range of the database and allowing modellers to account for these exposures by tweaking their parameters, both in dissolved gas models and in bubble models. On the dissolved gas side, a prominent contribution is that of Baker (1998). With the introduction of the concept of gradient factors, Baker provided the ultimate transparency in adapting a model. On the bubble model side, RGBM provides some predefined levels of conservatism, while in VPM, the parameters that can be tweaked are available, yet are all but intuitive. But what is interesting is that whether simple or complex, these models owe their functioning not so much to an underlying theory, but to data fitting. Other aspects concerning the range of applicability are multiday repetitive dives, as well as physiology of the individual. Research by Ljubkovic et al. (2012) points to two main characteristics defining each diver: the propensity to produce bubbles, and the propensity to pass these bubbles from the venous side to the arterial side, whether by patent foramen ovale (PFO‡) or pulmonary shunt. Both aspects are unrelated to overall fitness: for example, a fit US Navy diver does not, by default, have less propensity to decompression illness than a deskbound employee. People fortunate enough not to produce bubbles or not to pass them from the venous to the arterial side, can tolerate dive profiles which would have dire consequences for people that produce bubbles and also pass them easily. Extending the range of applicability is thus a matter of compensating for the simplifications in the original models, which did not capture all of the essential physics and physiology. Mars does not revolve around the Earth after all. In the absence of a physiologically complete and correct decompression model, ‡
A hole in the wall of tissue between the left and right upper chambers of the heart, which allows venous blood to leak into arterial blood before the latter is circulated through the body.
such compensation is possible and can be obtained by increasing the conservatism of the model itself.
2. M-values and pressure gradients Fig 2 shows the maximum tolerated supersaturation values of nitrogen (M-values) for all 16 tissues in the unmodified ZH-L16C model (Bühlmann, 1990) in comparison with the nitrogen partial pressure in the saturated tissues before a dive§. The tissues are lined up along the horizontal axis, with half-times increasing (from left to right) from 4 mins (tissue 1) to 635 mins (tissue 16). The vertical axis represents nitrogen partial pressure expressed in bar. The dots represent the partial pressure of nitrogen in each tissue (also called tissue tension) before the dive. As the dive progresses and the diver breathes gas at higher than atmospheric partial pressure, nitrogen will diffuse into the tissues thereby increasing their partial pressure, causing the dots to travel upwards. The triangles represent the values for each tissue which are not to be exceeded upon returning to the surface (M-values). A safe dive is defined as one in which either the dots are kept below the triangles or staged decompression stops** are introduced at the end of the dive to bring the dots below the triangles prior to reaching the surface. In essence, for each tissue a limit is imposed on the amount of nitrogen that can be accumulated during the dive and brought back to the surface. And this is at the heart of the Haldane and Bühlmann approach: controlling the amount of nitrogen in each tissue. This graph represents a significant portion of Bühlmann’s work. Haldane had defined the maximum allowed supersaturation as being double the tension tolerated on the surface, and this applied equally to all tissues (his model utilised five tissues, from 5 to 75 mins). Bühlmann’s work, and that of others before him, showed that fast tissues can tolerate much more, while slow ones actually tolerate less. Fast tissues, by definition, will take on nitrogen quickly but because of this they will also start releasing it early on during the ascent. Fig 3 depicts a hypothetical distribution of tissue tensions halfway through a dive profile in which the maximum depth §
The value used here is 0.79 bar, i.e. 1 bar atmospheric pressure multiplied by 79 % of nitrogen fraction in the air. In reality one should deduct 5 mbar of water vapor pressure, but this detail is neglected as it does not change the essence of this discussion. ** The term ‘staged decompression stop’ is used to differentiate a pause during the ascent to offgas excess inert gas as opposed to the decompression a diver undergoes as he or she offgasses nitrogen during the final part of any dive, also a dive within the no decompression limits. In the remainder, when a decompression stop is mentioned, the word ‘staged’ is implied.
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Fig 2: M-values in ZH-L16C
Fig 3: Hypothetical tissue tension distribution
is reached at the beginning and is followed by a slow and gradual ascent. In this particular example, we see that tissues 3 to 7 have tensions above the maximum tolerated value, so these tissues will require one or more decompression stops prior to reaching the surface. In the Haldanian approach, decompression stops are defined in 3 m (10 ft) increments††, and at each there are corresponding maximum tolerated ††
In the so-called Hills approach (Hills, 1978), there are no predefined decompression stop depths, but rather a continuously evolving ceiling that represents the minimum reachable depth for the current nitrogen load. As tissues offgas this ceiling keeps decreasing. Ideally this method allows for more effective offgassing since one always has the maximum pressure gradient available. In practical terms, there are two disadvantages: one has to constantly track the ceiling and move accordingly in order to take advantage of this effectiveness; and one loses the ability to plan gas consumption based on predefined depths for a certain amount of time.
tissue tensions, relating to the surface value augmented by the increase in ambient pressure. Fig 4 uses the same hypothetical dive as Fig 3 except the M-value triangles are replaced with a line and the M-value lines at 3, 6 and 9 m have been added. When a dot is above one of the lines, it means that it has to be brought below that line by stopping at the next deepest stop (e.g. a tissue tension higher than the maximum tolerated supersaturation at 3 m results in a 6 m stop). Fig 4 shows that tissues 3 and 7 only require a 3 m stop, but tissues 4, 5 and 6 also require a 6 m stop. Once these tissues have tensions below the green line, the diver can move up to 3 m and stay there until all dots have fallen below the black line. The diver moves up to the next permissible depth as soon as the next deepest obligation is absolved, in order to maximise the
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Fig 4: Hypothetical tissue tension and M-values for several depths
Fig 5: Impact of stop depth on offgassing gradients
pressure gradient available for offgassing. This is given by the difference in the partial pressure in the tissue and the partial pressure in the inhaled gas, the latter of course diminishing as the depth decreases. Fig 5 shows the same hypothetical profile as Fig 4 but with the addition of a line corresponding to the partial pressure of air at 3 m and 6 m. It shows that at 6 m the pressure gradient available for offgassing (‘A’) is smaller than that at 3 m (‘B’). So if the computer says 3 mins at 3 m but the diver stays at 6 m, then the offgassing will take longer. The computer simply states what the decompression time would be if a diver were at 3 m, but staying deeper means that there is less pressure difference and hence the release of nitrogen is slowed down. This in turn results in a longer time to reach the desired reduction.
Fig 6 compares a dive to 40 m using air and nitrox EAN32‡‡. For the sake of illustrating a concept, the hypothetical load is always the same. While at 40 m, on air the diver is constantly submitted to a higher partial pressure (‘A’), which leads to a quicker rise of tissue tensions during the dive and slower offgassing during decompression (‘C’). Conversely with EAN32, slower rise of tissue tensions occur during the dive (‘B’) and quicker offgassing during decompression (‘D’). Thus, for the same dive profile, using Nitrox implies slower ongassing and more efficient offgassing of nitrogen. It may not seem much on the graph, but the difference is substantial and can ‡‡
EAN stands for Enriched Air Nitrox, 32 represents the concentration of oxygen in the mix, the balance being nitrogen. Similarly EAN80 is 80 % oxygen and 20 % nitrogen.
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Fig 6: Comparison between air and EAN32 at depth and during decompression
Fig 7: Comparison for a deep dive on air with and without EAN80 for decompression
be easily demonstrated by carrying two computers, one set to air and the other to EAN32. Fig 7 shows the effect of adding a dedicated decompression gas for a dive to 50 m on air. Air allows a diver to reach this depth and the slow offgassing is compensated by switching to a gas with very low inert gas content in order to accelerate the offgassing. The pressure gradient driving nitrogen into the tissues is high in both cases (‘A’), but decompression on EAN80, which in Fig 7 is used at 9 m, allows for much quicker offgassing (‘C’ instead of ‘B’). There is another important factor to consider in favour of a high oxygen concentration decompression gas, which is exemplified in Fig 8. Fig 8 shows the hypothetical nitrogen load of Fig 7 after the required decompression time has
elapsed so that all dots are now below the M-values. Segment B represents the pressure gradient available for further offgassing when using air while staying at 3 m; segment C represents the pressure gradient available for further offgassing when breathing EAN80 while staying at 9 m; and segment D represents the pressure gradient available for offgassing upon ascending to the surface and breathing air. Staying underwater, even at 9 m, on EAN80 is much more efficient than starting the surface interval. Staying on air on the other hand results in losses in efficiency because of the small pressure gradient, which (slowly) becomes even smaller as any offgassing reduces the gradient further. In light of this, is a safety stop more significant at the end of a no decompression dive or at the end of
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Fig 8: Pressure gradients at end of dive
a decompression dive? A common belief is that it would be more significant in the former scenario as in the latter a stop is already required. Without wishing to minimise the importance of doing a safety stop between 3 and 5 m after a no decompression dive, Fig 8 illustrates how doing a safety stop after a decompression dive is even more significant. After a no decompression dive, by definition none of the dots are on or above the black line, so there is an inherent margin between the tension in each tissue and the criterion for a safe ascent. Clearing decompression, however, means at least one of the tissues barely clears the criterion for a safe ascent (the same is true of dives to the ‘no decompression’ limit, which have an appreciable risk of decompression illness). Performing a safety stop will move the dot below the line, which within the Bühlmann model is beneficial. The extent of the movement away from the black line is a function of time and available pressure gradient. Therefore, it is recommended to perform a safety stop after a decompression dive and after a dive to the ‘no decompression’ limit. Divers on EAN80 should extend this as much as possible. On air, as the dot descends and gets closer to the blue line (partial pressure of nitrogen in air at 3 m), the pressure gradient will become small enough that the diver should ascend to take advantage of the (slightly) higher pressure gradient available there.
3. Increasing conservatism by reducing inert gas load The previous section outlined the basics of managing decompression using algorithms, which are model information coded for use in dive computers. An
algorithm has a baseline conservatism, but in most dive computers it is possible to choose an alternate, more conservative setting (Smart et al., 2015). There can be many reasons why one may want to increase the conservatism, be it out of caution, or out of consideration for ‘internal’ (predisposition to bubble formation, fitness level, temporary lack of sleep or hydration, etc.) or ‘external’ factors (current, water temperature, etc.). The conservatism settings are often referred to as P0, P1, P2 or similar (Sayer et al., 2016). In a Haldanian framework, increasing conservatism is easily achieved by lowering the M-values as shown in Fig 9. This is the same as Fig 4, but with a second set of M-value lines that represent a 15 % reduction of the original ones. The immediate effect is less nitrogen in the tissues at the end of the dive. But looking at Fig 9, we see also that: • Tissue 2 does not require a decompression stop with the original M-values, but it does require one with the 15 % reduction. • Tissue 3 requires a 3 m decompression stop and, after the 15 % reduction, requires a 6 m stop. • Tissues 4 and 5 require a 6 m decompression stop and, after the reduction, require (barely) a 9 m stop. • The overall effect is to lengthen the decompression, since all red dots have to reach a lower final value. This, by and large, is what lies behind P0, P1, P2 or other monikers in dive computers. The M-value reduction may not be evenly distributed over all tissues. In addition, some introduce mathematical tricks to speed up ongassing and slow down offgassing. The concept, however, remains the same: each
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Fig 9: M-value reduction as a means to increase conservatism
tissue is viewed as a bucket that fills with inert gas during the dive and which must be emptied to a certain safe level before returning to the surface. Increasing the conservatism means lowering the safe level in the bucket, and this is accomplished by reducing ‘no decompression’ limits (less time available for inert gas to get in), or extending decompression times (more time available for inert gas to get out). Unfortunately, these monikers do not help the diver in appreciating the impact of the new conservatism level on the dive.
4. Gradient factors The Bühlmann ZH-L16C algorithm has been used extensively by technical divers pushing the frontiers of diving. The original pushes divers as close to the surface as possible in order to maximise the pressure gradient available for offgassing. However, there is also evidence that in some particular dive profiles, a slower ascent with deeper stops may present benefits. Fig 9 shows that this can be achieved by reducing the M-values until that same hypothetical load would generate a 21 m or 24 m stop. This, however, would imply a long stop at 3 m. Baker (1998) came up with a simple but ingenious idea. He defined two values: one that represented the percentage of the original Bühlmann values accepted at the surface (GF high); and one that represented the accepted reduction of Bühlmann’s values to define the depth of the first stop during the ascent (GF low). So revisiting Fig 9, where a 15 % reduction was applied to everything (and thus would be defined a GF85/85), Baker’s approach would allow a diver to apply, say, a 15 % reduction at the surface (GF high of 85) and a 60 % reduction to define the first stop
(GF low of 40), and then interpolate between those two values to define all the stops and their duration in between. This would be termed ‘GF40/85’ and the comparison between this and GF85/85 is shown in Fig 10, with the gradient factor of 40 applied at 9 m, and which, by interpolation, results in a gradient factor of 55 (45 % reduction) at 6 m and 70 (30 % reduction) at 3 m. As a consequence of this mathematical manipulation, tissues 3, 4 and 5 now require a 12 m decompression stop, since they are placed above the M-values at 9 m, while decompression duration at 3 m is not increased. It could actually be slightly decreased if a decompression gas high in oxygen is used, since control may be passed from a faster tissue to a slower one during decompression, and the slower tissue may have benefited from the deeper stop already. There are three aspects that make this approach very appealing: 1) The increase of conservatism introduced by the GF low does not kick in until there is sufficient inert gas uptake to require decompression. Then, it applies to the 6 m stop only (since the 3 m decompression stop is what allows the diver to reach the surface, and thus defined by GF high). As the inert gas uptake increases, the GF low value is gradually applied to 9 m, then to 12 m and so on, with corresponding interpolation of GF values for the stops in between. It’s a dynamic application of M-value reduction that ‘penalises’ the diver more as the severity of the dive increases. This can be seen in Fig 11, calculated using the dive planner function on one of many commercially available dive computers. Fig 11 illustrates the total ascent time (i.e. the sum of all decompression stops and time required to travel the
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Fig 10: M-values at GF85/85 and GF40/85
Fig 11: Effect of extending the bottom time on a 50 m dive
vertical distance to the surface at 9 m/min) for a dive to 50 m on air as a function of bottom time for GF85/85 (no dynamic adjustment) and GF40/85 (dynamic adjustment to determine deepest stop). 2) The definition of the two values, GF high and GF low, is simple and easy to understand, yet allows one to reproduce any ascent schedule that a bubble model may predict. Divers wishing for slightly longer or shorter overall decompression, can decrease or increase both values. If they want to start staged decompression deeper and spend less time at 3 m, they can decrease GF low and increase GF high. If they do not have a decompression gas that allows them to offgas at 20 m, then they can raise GF low and maybe reduce GF
high. And for recreational divers wanting to add a bit of conservatism, they can take the standard values and reduce them in 5 % steps until they feel comfortable with the result. 3) This approach does not shift decompression time from shallow stops to deep stops. Current research carried out by the US Navy (Doolette et al., 2011) suggests that deep decompression stops may not be appropriate for all dive profiles, when such deep stops are introduced to partially replace a shallower stop. With the gradient factor approach, a deep decompression stop can be viewed as part of a multilevel dive with the GF high as the determining parameter defining the return to the surface.
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5. Practical applications Technical divers are comfortable with the use of gradient factors and know what works for them. A recreational diver might be more comfortable with the P0, P1 and P2 approach, though it would be helpful if dive computers combined these settings with a description of the increased conservatism. For example, this could be P0-85/85, P1-70/80, P2-60/70.§§ Personal customisation to address an overall assessment of one’s fitness to dive, or dayrelated deviations caused by internal or external factors could be defined on a scale from 1 to 3 which, when selected, would cause a deduction of 5, 10 or 15 percentage points from the starting values of the gradient factors. Similarly, a GF reduction can be used to account for repetitive dives, for instance by subtracting 15 percentage points from the set GF values upon surfacing, and adding back 1 percentage point every 12 mins. Thus, for surface intervals shorter than 3 hrs there is an additional conservatism that decreases as the surface interval increases. A similar logic can be applied to multiday dives.
References Aquilecchia G. (2017). Giordano Bruno. Encyclopaedia Brittanica. Available at: www.britannica.com/biography/ Giordano-Bruno, <last accessed 7 February 2018>. Baker EC. (1998). Understanding M-values. Immersed: International Technical Diving Magazine 3: 23–27. Boycott AE, Damant GCC and Haldane JS. (1908). The prevention of compressed-air illness. Journal of Hygiene 8: 342–43. Bühlmann AA. (1990). Tauchmedizin, Second edition. Berlin: Springer Verlag. Bühlmann AA. (1995). Tauchmedizin: Barotrauma Gasembolie Dekompression Dekompressionskrankheit. Berlin: SpringerVerlag.
Doolette DJ, Gerth GA and Gault KA. (2011). Redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives. US Navy experimental Diving Unit technical Report 2011-06. Panama City, FL: Navy Experimental Diving Unit, 60 pp. Available at: http:// archive.rubicon-foundation.org/xmlui/handle/123456789/ 10269 <last viewed 1 May 2018>. Hills BA. (1978). A fundamental approach to the prevention of decompression sickness. Journal of the South Pacific Underwater Medicine Society 8: 20–47. Ljubkovic M, Zanchi J, Breskovic T, Marinovic J, Lojpur M and Dujic Z. (2012). Determinants of arterial gas embolism after scuba diving. Journal of Applied Physiology 112: 91–95. Sayer MJ, Azzopardi E and Sieber A. (2016). User settings on dive computers: reliability in aiding conservative diving. Diving and Hyperbaric Medicine 46: 98–110. Singh S. (2004). Big Bang: the most important scientific discovery of all time and why you need to know about it. New York: Harper Perennial, 562 pp. Smart D, Mitchell S, Wilmhurst P, Turner M and Banham N. (2015). Joint position statement on persistent foramen oval (PFO) and diving: South Pacific Underwater Society (SPUMS) and the United Kingdom Sports Diving Medical Committee (UKSDMC). Diving and Hyperbaric Medicine 45: 129–131. Thalmann ED. (1983). Computer algorithms used in computing the MK15/16 constant 0.7 ATA oxygen partial pressure decompression tables. US Navy Experimental Diving Unit Technical Report 1-83. Panama City, FL: Navy Experimental Diving Unit. Wienke BR. (2001). Technical diving in depth. Flagstaff, AZ: Best Publishing Company, 460 pp. Workman RD. (1965). Calculation of decompression schedules for nitrogen-oxygen and helium-oxygen dives. US Navy Experimental Diving Unit Research Report 6-65. Washington, DC: Navy Experimental Diving Unit, 33 pp. Yount DE, Maiken EB and Baker EC. (2000). Implications of the varying permeability model for reverse dive profiles. In: Lang MA and Lehner CE. (eds). Proceedings of the Reverse Dive Profiles Workshop. Washington, DC: Smithsonian Institution, 29–61.
§§
No recreational dive computer manufacturer uses 100 % of the original Bühlmann values but rather the baseline conservatism that corresponds to approximately 85/85.
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SUT Publications The SUT publishes a peer-reviewed technical journal Underwater Technology; a quarterly magazine UT2 and e–magazine UT3; a series of conference proceedings Advances in Underwater Technology, Ocean Science and Offshore Engineering and The Operation of Autonomous Underwater Vehicles; and in–house conference proceedings and collected papers from seminars. All SUT books and conference proceedings are available to purchase from the SUT website www.sut.org/publications/books-and-conference-proceedings/ This is a selection of the larger collection of the Society’s books and conference proceedings available to purchase online.
Can a Lobster be an Archaeologist? Quirky Questions and Fascinating Facts about the Underwater World From exploring lost treasure to sea monsters, ocean rubbish and how to build your own ROV, the book is packed with factual and fun illustrated stories.
Offshore Site Investigation and Geotechnics: Integrated Geotechnologies – Present and Future Proceedings of the international conference held in September 2012
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The Operation of Autonomous Underwater Vehicles, Volume One: Recommended Code of Practice for the Operation of Autonomous Marine Vehicles, Second Edition
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The Collaborative Autosub Science in Extreme Environments: Workshop on AUV Science in Extreme Environments Proceedings for the international science workshop held at the Scott Polar Research Institute, University of Cambridge, 11-13 April 2007
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Sensing and Control for Autonomous Vehicles: Applications to Land, Water and Air Vehicles Edited by Thor I Fossen, Kristin Y Pettersen and Henk Nijmeijer Published by Springer
E-book edition, 2017 ISBN 978-3-319-55372-6 518 pages
Sensing and Control for Autonomous Vehicles: Applications to Land, Water and Air Vehicles is an instalment in Springer’s Lecture Notes in Control and Information Sciences series. Springer promotes the series as aimed at a high-level audience in the field of control and it explores new developments quickly and informally. This particular volume is a compilation of 23 chapters spanning several research themes arising from an invited workshop held in Ålesund, Norway, in June 2017. The book is divided into six parts comprised of different research areas: i. ii. iii. iv. v.
Vehicle navigation systems; Localisation and mapping; Path planning; Sensing and tracking systems; Identification and motion control of robotic vehicles; and
vi. Coordinated and cooperative control of multi-vehicle systems. Each part comprises two to six chapters or articles and, as the title indicates, involves sensing and vehicle technology for unmanned aerial vehicles (UAVs), autonomous underwater vehicles (AUVs), remotely operate vehicles (ROVs), autonomous surface vessel (ASV) and autonomous ground vehicles (AGV). This is a quality reference book with many articles at a level suitable for a controls or autonomous systems specialist. The text is well written and cleanly presented. The mathematics are presented clearly, as are most of the figures and images. However, some graphs rely on colour scales that suffer when printed in black and white. Each chapter is well referenced and the index is comprehensive; both are essential characteristics for a good text book. There is a great deal of reference content written by subject area experts in navigation, control and path planning, which will be helpful for systems engineers or researchers seeking to implement newer control and sensor methodologies for autonomous vehicle applications. It is also nice to see sections of some chapters that extend beyond the lab and into the field, where experimental data are used for validation and, in some cases, actual field experiments are reported. In terms of Society for Underwater Technology readership, about a third of the chapters involve underwater technology; either AUVs, ROVs or underwater sensors. However, some of the theory and techniques discussed can likely be transferrable in some
www.sut.org
Book Review
doi:10.3723/ut.35.063 Underwater Technology, Vol. 35, No. 2, pp. 63–64, 2018
way to the domain of underwater vehicles, whether autonomous or remote. There are interesting articles on new localisation applications using smartphones, selfdriving vehicles and UAV target tracking of floating objects, but my attention was mostly drawn to the AUV related chapters. I highlight only a couple of chapters to provide some sense of the scope of the text with respect to autonomous marine technology. ‘Motion Control of ROVs for Mapping of Steep Underwater Walls’ by Nornes et al. is an interesting article describing the use of a non-standard doppler velocity log (DVL) orientation on a light work-class ROV to collect 2D still photographic images suitable for a 3D photogrammetric model of a rock wall. Nornes et al. develop and validate a controller able to maintain a constant vehicle standoff distance to the wall. Results from a field experiment in April 2016 that involved mapping a 70° face are described, including some shaded relief and 3D textured images of the steep wall. The authors conclude the article with some suggested improvements to their system and approach. Sharing lessons learned from field exercises is in itself a valuable contribution to research and development. The Sayre-McCord et al. article ‘Advances in Platforms and Algorithms for High Resolution Mapping in the Marine Environment’ is a gateway article into some of the extensive work undertaken using the SeaBED class AUVs. It describes work in underwater imaging, image compression, data transmission, image recognition and online processing for adaptive mission planning and mapping
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dynamic environments, such as under ice. The article concludes with discussion and some results using accessible hobby component technologies to support ASV and UAV marine mapping applications. Although not quite underwater technology, there are two interesting articles from different groups at the Norwegian University of Science and Technology (NTNU) that use Maritime Robotics’ 8.45 m Telemetron ASV. Wilthil et al. describe a complete radar tracking system on the ASV for the purposes of collision avoidance. Their tracking system was shown to work using data collected in Trondheimsfjord. In another article, entitled ‘Modeling, Identification and Control of High-Speed ASVs: Theory and Experiments’ by Eriksen and Breivik, the Telemetron was used for at sea experiments using several controller combinations (proportional-integral feedback (FB), feed forward (FF), FF-FB
combined and FB linearised) to facilitate speed and heading control. The experiments entailed top speeds of ~16 m/s (~31 kts) and turning rates of ±20° in relatively calm sea states. What an absolutely fun day on the water! Given the speeds involved in the Eriksen and Breivik work, I would think that any mariners in Trondheimsfjord should take some solace that there is also ongoing collision avoidance work being undertaken by Wilthil et al. These two research themes complement each other quite nicely. Overall, this is a well-produced, comprehensive compilation of research articles on control theory and sensors for autonomous vehicle systems. Much of the content focuses on theory and/ or literature survey, with some examples of recent experimental results. Approximately 75 authors contributed to this volume, including notable groups
from France, Italy, Japan, Netherlands, Portugal, Spain, Sweden and particularly strong representation from the United States, and the workshop’s host, NTNU. The material is not for the faint of heart, but rather a control or autonomous system specialist. If readers are inclined to work on some of the material, then they better be prepared to roll up their sleeves and delve into some fairly heavy mathematics. But fear not, there are some novel ideas in Sensing and Control for Autonomous Vehicles that will capture the imagination of any underwater technology enthusiast. Controlling underwater snake robots and experiments involving ASVs zipping around at 30 kts makes for some pretty fascinating work! (Reviewed by Dr Ron Lewis, Autonomous Ocean Systems Lab and the Ocean Frontier Institute, Memorial University, Newfoundland)
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NOAA Diving Manual – Diving for Science and Technology, Sixth Edition Edited by Greg McFall, John N Heine and Jeffrey E Bozanic Published by Best Publishing Company
Paperback edition, 2017 ISBN-10: 1930536887 ISBN-13: 978-1930536883 800 pages
The US National Oceanic and Atmospheric Administration (NOAA) Diving Manual was first published in 1977 and has since become the predominant reference source for divers working in the underwater science and technology industry sectors around the world. From the second edition onwards, new editions appeared at a rate of every 10–12 years, but there has been only four years between the publication of the fifth and sixth editions. No reason is given in the foreword or the preface of the new edition for this accelerated revision, although it may have been driven, in part, by the changes in NOAA’s use of nitrox (see below). In addition, there may have been a feeling that the fifth edition had not made the usual significant level of improvement expected between editions of this Manual. My own review of the fifth edition for
Underwater Technology (Sayer, 2013) concluded that the opportunity to create a worthwhile update had been missed. The first notable change in the sixth edition is a new layout format for the Manual. Rather than the usual straightforward list of chapters, the Manual is now divided into five themes: Diving Basics, Advanced Modes of Diving, Diving Planning and Procedures, Environmental Considerations, and Emergency Procedures. The number of chapters in each section ranges from 3 to 6, and the overall total of 22 is 2 more than were in the fifth edition. The two new chapters are: ‘Advanced Platform Support’ and ‘Underwater Photography and Videography’. The ‘Advanced Platform Support’ chapter discusses remotely operated vehicles (ROVs), autonomous underwater vehicles (AUVs), submersible support, and atmospheric diving systems (ADS). Along with explaining the basics of all four platforms, there is also guidance related to either joint operations with divers and/or diver support. The discussion of the uses of AUVs, submersible support and ADS is new, or has been expanded upon, while the sections on ROVs reproduce mostly what was included in Chapter 2 (Dive Equipment) in the previous edition. The new ‘Underwater Photography and Videography’ chapter is credited in the preface to Brett Seymour. This dedicated chapter for these techniques and technologies, which are used widely in scientific diving, replaces the more diverse approach in the previous edition where these applications were spread mainly over two chapters (2: Dive Equipment;
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Book Review
doi:10.3723/ut.35.065 Underwater Technology, Vol. 35, No. 2, pp. 65–66, 2018
12: Procedures for Scientific Dives). As such, the content of the new chapter is expanded significantly from the previous edition, and is much more up-to-date being based nearly entirely on digital formats and detailing most of the new camera systems. There is also discussion of some of the more recent applications for photography and videography, such as fluorescence photography and 3D modelling, plus a much larger and more useful section on camera maintenance. The editors state in their preface that there has been a lot of revision and updating to Chapter 2 (Dive Equipment), Chapter 8 (Rebreathers), and Chapter 13 (Procedures for Scientific Dives). In addition, the new NOAA Nitrox No-Decompression Dive Tables are reproduced in Appendix A. The main revision to Chapter 2 (Dive Equipment) is in the sections on: a new generation of electronics and technologies being introduced into areas such as diver tracking, diver communications and dive logging; and a discussion of the numerous applications that are being developed for the underwater use of smartphones and tablets. There is also a completely new section on cylinder configuration, including the use of side-mount and no-mount cylinders. The only changes I could find in Chapter 8 (Rebreathers) were three new photographs and some minor alterations in the text, and so I would disagree that this chapter has received any significant revision. Chapter 13 contains some revisions and an update on a few new technologies and techniques, plus a number of new photographs. Many of the sections in this chapter, however, did
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McFall et al. NOAA Diving Manual – Diving for Science and Technology, Sixth Edition
not receive true updates and there are still too many badly outof-date sections. For example, it continues to describe belt transects, the use of which has not been approved by reviewers for many years because of the lack of independence between units. There was also very little reference to the use of 3D photogrammetry techniques, which are now widespread in the scientific diving community. In addition, there are apparently very few updates to the references cited, too many of which are 30-plus years old. This chapter deserves a more fundamental update in a future edition. The editors don’t explain what the changes are to the NOAA Nitrox Tables from the fifth edition, but it would appear to be in formatting and design only. The tables, which are numbered by the percentage oxygen content of the Nitrox mixture being breathed but now without the ‘R’ after each table number, are now reproduced in landscape layout and with some subtle changes in colouration. The introduction of highlighting for the parts of the tables where a PO2 limit of 1.40 bar would have to be exceeded recognises that the NOAA Diving Programme reduced its accepted PO2 limits down from 1.60 to 1.40 between the publication of the fifth and sixth editions of its Manual.
Inevitably for a subject matter where some areas change less rapidly than others, there are chapters where there has been little revision. There can be no criticism here and in subjects such as Diving Physics, Diving Physiology, Air Diving and Decompression, replication is unavoidable. In dip-testing, some of the chapters where I would expect little change, alterations are limited, but even in chapters where there has been revision (e.g. Chapters 2, 8, 13), there is a considerable volume of their content that is copied straight from the previous edition. However, efforts have been taken to refresh some of the images and figures used, even where the main text has not been altered. As in my review of the fifth edition of the manual, the References Appendix in this new edition is disappointing. There has been a very limited number of references added and very few deleted. As a result, this section appears woefully outdated and is another part of the Manual that needs fundamental improvement. A focus on a smaller number of relevant references would be preferable to the current situation, where there are many references but a high proportion of which have no contemporary significance.
In conclusion, I recognise that the fifth edition marked a significant change in the appearance and content of the Manual. The sixth edition has built on that change in format and content but only in a select few areas. There is, of course, an acceptance that the different editions will retain a large degree of similarity and that only a small number of the chapters will require revision, removal or addition. It will always remain a Manual that should be purchased and used by all diving units operating in the science and technology sectors, and this edition is no different. The Manual appears now to have settled on a good clear and well organised layout. Future updates will obviously always be necessary and it may be that new editions, such as this one, that are published more frequently but contain fewer changes, will serve the community better than waiting longer periods for more comprehensive revisions. (Reviewed by Dr Martin Sayer, Tritonia Scientific Ltd)
References Sayer MDJ. (2013). Book review: NOAA Diving Manual, Diving for Science and Technology. Underwater Technology 31: 217–218.
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CALL FOR PAPERS Underwater Technology: InternaƟonal Journal of the Society for Underwater Technology The Society for Underwater Technology is calling for papers for its internaƟonal journal, Underwater Technology. The journal publishes peer-reviewed technical papers on all aspects and applicaƟons of underwater technology, including: • • • • • • • • • • • • •
diving technology and physiology environmental forces geology/geotechnics marine polluƟon marine renewable energies marine resources oceanography subsea systems underwater acousƟcs underwater roboƟcs underwater science underwater vehicle technologies salvage and decommissioning
Original papers on new technology, its development and applicaƟons, and papers covering new applicaƟons for exisƟng technology, are parƟcularly welcome. Submissions should adhere to the journal’s guidelines available at www.sut.org/publicaƟons/underwater-technology/guidelines-for-authors/ For more informaƟon or to make a submission, please contact the Assistant Editor, Elaine Azzopardi, at Elaine.Azzopardi@sut.org
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UT2 and UT3 The magazines of the Society for Underwater Technology
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UT2 covers a focused range of underwater subjects including offshore, marine renewables, subsea engineering, ocean resources, diving and manned submersibles, underwater science and robotics.
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UT3 is the online magazine of the Society for Underwater Technology, and covers the subsea industry.
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It consists of the content of the print magazine UT2, greatly expanded with other information.
UT22 and UT33 are available online at http://issuu.com/ut-2_publication http://issuu.com/ut 2_publication www.sut.org 05-SUT67425-35(2).indd 68
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Society for Underwater Technology International multidisciplinary learned society This non-aligned membership-based organisation seeks to further the dissemination of knowledge and lessons learned in the underwater environment through networking, events and publications
Its membership covers the following activity areas:
diving and manned submersibles environmental forces marine policy marine renewable energies ocean resources offshore site investigation and geotechnics salvage and decommissioning subsea engineering and operations
For further information For membership, publications or general enquires, contact SUT Head Office Unit LG7, 1 Quality Court, London WC2A 1HR t +44 (0)20 3440 5535 e info@sut.org For events, contact SUT Aberdeen Office Enerprise Centre Exploration Drive Bridge of Don Aberdeen AB23 8GX UK t +44 (0)1224 823 637 e events@sut.org
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