Side-scan Sonar Mapping Research Articles

Development of Active Numerating Side-scan for a High-Density Overwintering Location for Endemic Shortnose Sturgeon (Acipenser brevirostrum) in the Saint John River, New Brunswick

In 1979, the Shortnose Sturgeon (Acipenser brevirostrum) population of the Saint John River, New Brunswick, was estimated at 18,000 ± 5400 individuals. More recently, an estimate of 4836 ± 69 individuals in 2005, and between 3852 and 5222 individuals in 2009 and 2011, was made based on a single Shortnose Sturgeon winter aggregation in the Kennebecasis Bay of the Saint John River, a location thought to contain a large proportion of the population. These data, in combination with the Saint John River serving as the sole spawning location for Shortnose Sturgeon in Canada prompted a species designation of “Special Concern” in 2015 under Canada’s Species at Risk Act (SARA). A three-decade span of scientific observations amplified by the traditional knowledge and concerns of local indigenous groups have pointed to a declining population. However, the endemic Shortnose Sturgeon population of the Saint John River has not been comprehensively assessed in recent years. To help update the population estimate, we tested a rapid, low-cost side-scan sonar mapping method coupled with supervised image classification to enumerate individual Sturgeon in a previously undescribed critical winter location in the Saint John River. We then conducted an underwater video camera survey of the area, in which we did not identify any fish species other than Shortnose Sturgeon. These data were then synchronized with four years of continuous acoustic tracking of 18 Shortnose Sturgeon to produce a population estimate in each of the five identified winter habitats and the Saint John River as a whole. Using a side-scan sonar, we identified > 12,000 Shortnose Sturgeon in a single key winter location and estimated the full river population as > 20,000 individuals > ~40 cm fork length. We conclude that the combined sonar/image processing method presented herein provides an effective and rapid assessment of large fish such as Sturgeon when occurring in winter aggregation. Our results also indicate that the Shortnose Sturgeon population of the Saint John River could be similar to the last survey estimate conducted in the late 1970s, but more comprehensive and regular surveys are needed to more accurately assess the state of the population.

Natural and Anthropogenic Sediment Mixing Processes in the South-Western Baltic Sea

Natural and anthropogenic sediment mixing can significantly impact the fidelity of sedimentary records of climate and environmental variability and human impact. This can lead to incorrect interpretations of the previous state(s) of a given ecosystem, its forcing mechanisms, and its future development. Here, natural and anthropogenic sediment mixing processes (i.e. bioturbation, hydroturbation and direct anthropogenic impact) are investigated in the south-western Baltic Sea by sedimentological, ichnological, geochemical, and radionuclide analyses to assess their impact on time-marker profiles and sediment deposition. Depth profiles of mercury and caesium-137 display a varyingly strong disturbance down to 5-25 cm. The deviations from undisturbed profiles can be used to estimate the relative degree of sediment mixing. Sedimentary fabric analysis of high-resolution X-radiographs provides further insight into bioturbation. Ichnofossils identified in short sediment cores suggest that the primary sedimentary structure is partly overprinted by burrowing organisms living in the upper 5-7 cm of sediment. Meteorological and hydrological data from automated measuring stations combined with results from resuspension experiments show that hydrodynamic events, such as storms and saline water inflows from the North Sea, have the potential to resuspend and laterally transport sediment particles in the south-western Baltic Sea. Partially graded layers in the scale of mm to cm in the investigated cores are likely associated with such hydrodynamic events. Multiple generations of linear traces on the seafloor are made visible through sidescan-sonar mapping and document a strong anthroturbation in the muddy sediments of the Mecklenburg Bight and the Arkona Basin, e.g. by bottom trawling. Depending on the core location and assumed mixing depth, determined mean net linear sedimentation rates range between 0 and 3.5 mm/a and reconstructed net mass accumulation rates range between 0 and 1.86 kg/m2/a in the mud basins. The calculable inventory of anthropogenic contaminants in the sediments illustrates the important, at least temporary storage function of the mud basins and of the adjacent sandy areas over industrial times. The findings of this study help identify the influence of natural and anthropogenic mixing on sediment profiles, and demonstrates the use of mixed sediment profiles for inferring information about past environmental conditions and anthropogenic impact.

Benthic Habitat Morphodynamics-Using Remote Sensing to Quantify Storm-Induced Changes in Nearshore Bathymetry and Surface Sediment Texture at Assateague National Seashore

This study utilizes repeated geoacoustic mapping to quantify the morphodynamic response of the nearshore to storm-induced changes. The aim of this study was to quantitatively map the nearshore zone of Assateague Island National Seashore (ASIS) to determine what changes in bottom geomorphology and benthic habitats are attributable to storm events including hurricane Sandy and the passage of hurricane Joaquin. Specifically, (1) the entire domain of the National Parks Service offshore area was mapped with side-scan sonar and multibeam bathymetry at a resolution comparable to that of the existing pre-storm survey, (2) a subset of the benthic stations were resampled that represented all sediment strata previously identified, and (3) newly obtained data were compared to that from the pre-storm survey to determined changes that could be attributed to specific storms such as Sandy and Joaquin. Capturing event specific dynamics requires rapid response surveys in close temporal association of the before and after period. The time-lapse between the pre-storm surveys for Sandy and our study meant that only a time and storm integrated signature for that storm could be obtained whereas with hurricane Joaquin we could identify impacts to the habitat type and geomorphology more directly related to that particular storm. This storm impacts study provides for the National Park Service direct documentation of storm-related changes in sediments and marine habitats on multiple scales: From large scale, side-scan sonar maps and interpretation of acoustic bottom types, to characterize as fully as possible habitats from 1 to 10 m up to many kilometer scales, as well as from point benthic samples within each sediment stratum and these results can help guide management of the island resources.

Applying the Coastal and Marine Ecological Classification Standard (CMECS) to Nearshore Habitats in the Northeastern Gulf of Mexico

Many countries have classification standards for their environmental resources including criteria for classifying coastal and marine ecosystems. Until 2012, the United States just had a nationwide protocol for classifying terrestrial and aquatic habitats with no national standard for marine and most coastal habitats. In 2012 the Coastal and Marine Ecological Classification Standard (CMECS) was implemented to address this need. In the past, coastal and marine classifications were developed at the regional or local level. Since its inception, the CMECS has not been applied in many geographic areas. My study was one of the first to apply the CMECS to the benthic habitats in the nearshore Northeastern Gulf of Mexico. Sidescan sonar mapping and dive surveys were completed at 33 sites at depths 10–23 m. Hardbottom and sand habitats characterized the study area, and the underwater surveys revealed hard corals, sponges, and macroalgae as the dominant taxa on the hardbottom. The CMECS was applied to the overall study area rather than each individual site or groups of similar sites because habitat and environmental characteristics, primarily outside the context of the CMECS, appeared to influence the distribution of taxa across sites more than the CMECS geoform, substrate, and water column components. The CMECS worked well for classifying the entire study area, but was not adequate for classifying the complex fine-scale habitats and temporal variations observed; modifications to the CMECS could help resolve these issues.

Inversion of Side Scan Sonar Motion and Posture in Seabed Geomorphology

Abstract Side scan sonar measurement platform, affected by underwater environment and its own motion precision, inevitably has posture and motion disturbance, which greatly affects accuracy of geomorphic image formation. It is difficult to sensitively and accurately capture these underwater disturbances by relying on auxiliary navigation devices. In this paper, we propose a method to invert motion and posture information of the measurement platform by using the matching relation between the strip images. The inversion algorithm is the key link in the image mosaic frame of side scan sonar, and the acquired motion posture information can effectively improve seabed topography and plotting accuracy and stability. In this paper, we first analyze influence of platform motion and posture on side scan sonar mapping, and establish the correlation model between motion, posture information and strip image matching information. Then, based on the model, a reverse neural network is established. Based on input, output of neural network, design of and test data set, a motion posture inversion mechanism based on strip image matching information is established. Accuracy and validity of the algorithm are verified by the experimental results.

Innovative method for optimizing Side-Scan Sonar mapping: The blind band unveiled

Over the past few years, the mapping of Mediterranean marine habitats has become a priority for scientists, environment managers and stakeholders, in particular in order to comply with European directives (Water Framework Directive and Marine Strategy Framework Directive) and to implement legislation to ensure their conservation. Side-scan sonar (SSS) is recognised as one of the most effective tool for underwater mapping. However, interpretation of acoustic data (sonograms) requires extensive field calibration and the ground-truthing process remains essential. Several techniques are commonly used, with sampling methods involving grabs, scuba diving observations or Remotely Operated Vehicle (ROV) underwater video recordings. All these techniques are time consuming, expensive and only provide sporadic informations. In the present study, the possibility of coupling a camera with a SSS and acquiring underwater videos in a continuous way has been tested. During the ‘PosidCorse’ oceanographic survey carried out along the eastern coast of Corsica, optical and acoustic data were respectively obtained using a GoPro™ camera and a Klein 3000™ SSS. Thereby, five profiles were performed between 10 and 50 m depth, corresponding to more than 20 km of data acquisition. The vertical images recorded with the camera fixed under the SSS and positioned facing downwards provided photo mosaics of very good quality corresponding to the entire sonograms's blind band. From the photo mosaics, 94% of the different bottom types and main habitats have been identified; specific structures linked to hydrodynamics conditions, anthropic and biological activities have also been observed as well as the substrate on which the Posidonia oceanica meadow grows. The association between acoustic data and underwater videos has proved to be a non-destructive and cost-effective method for ground-truthing in marine habitats mapping. Nevertheless, in order to optimize the results over the next surveys, certain limitations will need to be remedied.

Quantification of Walleye Spawning Substrate in a Northern Minnesota River using Side‐Scan Sonar

AbstractEvaluating aquatic habitats is an important component of many ecological studies and natural resource assessments, but traditional habitat evaluations are time and labor intensive and do not provide continuous data. Side‐scan sonar (SSS) can provide a low‐cost method that collects continuous aquatic habitat data. We used SSS mapping to quantify suitable spawning substrate available to Walleye Sander vitreus during the 2015 spring spawning migration in a 10.8‐km reach of the Tamarac River, Minnesota. The SSS map had 78.0% agreement with reference points classified in the field, and the proportion of reference points predicted as suitable using the SSS map was not significantly different than the proportion of reference points observed to be suitable. Suitable substrate for Walleye spawning comprised 8.4% (26,392 m2) of the total area mapped. The estimated number of females that suitable substrate could support was lower than the number that likely migrate up the Tamarac River and suggests that access to spawning substrate may sometimes limit reproductive success. This study demonstrates that a relatively inexpensive SSS unit can be used to efficiently map aquatic habitat while acquiring quantitative and qualitative data.Received July 6, 2016; accepted November 5, 2016 Published online March 10, 2017

Reprint of “Evaluating change in seagrass meadows: A time-framed comparison of Side Scan Sonar maps”

Side Scan Sonar (SSS) is a common method for mapping seagrass meadows that allows to define meadow extent and typology. Should sonogram interpretation be efficient, repeated mapping would allow evaluation of spatial meadow dynamics through time. Here, maps of Posidonia oceanica (L.) Delile meadows from western Liguria (NW Mediterranean Sea) produced in 1990, 1991 and 2006 through SSS surveys, were processed with GIS (Geographical Information System) and compared by means of diachronic analysis to establish concordances and discordances. While any difference between maps of 1990 and 2006 might provide information about change in seagrass cover with time, the availability of maps taken at only one year of distance (1991 vs 1990) offered the unique opportunity to evaluate the efficiency of SSS mapping, as the slow dynamics of P. oceanica makes significant change over such a short time scale unrealistic in absence of any large-scale environmental catastrophe. Results showed obvious differences in P. oceanica meadow distribution among the three periods in most of the areas. The largest differences were found between the maps of 1990 and 1991. This discrepancy was probably due to erroneous interpretation of small, fragmented shallow-water meadows on sonograms. Errors in sonogram interpretation combine with inefficient positioning in old cartographies to blur the evaluation of change over large time scales (i.e. between 1990 and 2006). Extensive sea-truthing based on a rigorous design is mandatory for efficient acoustic mapping of seagrass meadows.

서태평양 해저산 고코발트 망간각 자원평가를 위한 광역 탐사 방안

서태평양 해저산 사면에 부존하는 고코발트 망간각은 코발트, 니켈, 백금, 희유금속 등을 다량 함유하고 있으며, 최근 국제해저기구에서 공해상 탐사규칙이 제정됨에 따라 개발 대상으로 더욱 주목받고 있다. 해저산에 분포하는 망간각의 개발을 위해서는 경사가 낮고 지형기복이 완만하여 채광에 유리한 지형조건을 갖추면서 망간각이 두껍게 분포하는 유망지역을 선별하여야 한다. 따라서 광역단계의 망간각 탐사는 음향 수심탐사를 통한 지형 및 경사도의 확인, 음향산란 자료의 획득을 통한 기저면 표층 매질 분포 파악, 그리고 해저면 영상 관찰과 시료채취를 통한 망간각의 분포 두께 파악이 필요하다. 또한 음향산란 자료를 이용하여 망간각 분포 지역을 확인하기 위해서는 영상관찰 및 시료 채취를 통한 망간각 음향매질 특성분석이 필요하다. 기존의 탐사를 통해 수행된 망간각 기초탐사 자료를 분석한 결과 해저산 지형 해석과 망간각 광역분포와 같은 망간각 유망지역 선별을 위한 일부 자료를 확인할 수 있었다. 하지만, 음향산란 자료를 확보하지 못하여 넓은 탐사지역을 대상으로 망간각 부존 유망지역을 선별하는 데 필요한 망간각 분포 변화는 파악하지 못하였다. 따라서 향후 탐사는 망간각 탐사후보 지역을 대상으로 음향산란 자료의 확보가 선행되어야 하며, 해저면 관찰 및 시료채취를 병행하여 해저면 음향매질 특성과 망간각 분포의 상관성을 파악하기 위한 탐사가 수행되어야 한다. Co-rich ferromanganese crusts (Fe-Mn crusts) distributed on the seamounts in the western Pacific are potential economic resources for cobalt, nickel, platinum, and other rare metals in the future. Regulations for prospecting and exploration of Fe-Mn crusts in the Area, which enables the process to obtain an exclusive exploration right for blocks of the fixed size, were enacted recently by the International Seabed Authority, which led to public attention on its potential for commercial development. Evaluation and selection of a mining site can be established based on abundance and grade of Fe-Mn crusts in the site as well as topography that should be smooth enough for mining efficiency. Therefore, acquisition of shipboard echo-sounding and acoustic backscatter data are prerequisite to select potential mine sites in addition to visual and sampling operations. Acoustic backscatter data can be used to locate crust-covered areas in a regional scale with the understanding of acoustic properties of crust through its correlation with visual and sampling data. KIOST had collected the topographic and geologic data to assess the resources potential for Fe-Mn crusts in the west Pacific region from 1994 to 2001. However, they could not obtain acoustic backscatter data that is crucial for the selection of prospective mining sites. Therefore, additional exploration surveys are required to carry out side scan sonar mapping combined with seafloor observation and sampling to decide the blocks for application of an exclusive exploration right.

Evaluating change in seagrass meadows: A time-framed comparison of Side Scan Sonar maps

Side Scan Sonar (SSS) is a common method for mapping seagrass meadows that allows to define meadow extent and typology. Should sonogram interpretation be efficient, repeated mapping would allow evaluation of spatial meadow dynamics through time. Here, maps of Posidonia oceanica (L.) Delile meadows from western Liguria (NW Mediterranean Sea) produced in 1990, 1991 and 2006 through SSS surveys, were processed with GIS (Geographical Information System) and compared by means of diachronic analysis to establish concordances and discordances. While any difference between maps of 1990 and 2006 might provide information about change in seagrass cover with time, the availability of maps taken at only one year of distance (1991 vs 1990) offered the unique opportunity to evaluate the efficiency of SSS mapping, as the slow dynamics of P. oceanica makes significant change over such a short time scale unrealistic in absence of any large-scale environmental catastrophe. Results showed obvious differences in P. oceanica meadow distribution among the three periods in most of the areas. The largest differences were found between the maps of 1990 and 1991. This discrepancy was probably due to erroneous interpretation of small, fragmented shallow-water meadows on sonograms. Errors in sonogram interpretation combine with inefficient positioning in old cartographies to blur the evaluation of change over large time scales (i.e. between 1990 and 2006). Extensive sea-truthing based on a rigorous design is mandatory for efficient acoustic mapping of seagrass meadows.

Very-high resolution side-scan sonar mapping of biogenic reefs of the tube-worm Lanice conchilega

Reefs of the tube-building polychaete Lanice conchilega are known to represent hotspots of biodiversity within inter- and subtidal soft sediments of the North Sea. However, because of their patchy distribution, point sampling does not appropriately map their subtidal spatial distribution. This study evaluated the feasibility to detect L. conchilega reefs by very-high resolution side-scan sonar imagery. A subtidal very-high resolution (410 kHz) side-scan survey, combined with grab sampling, revealed high densities of L. conchilega (up to 1979 ind. m − 2 ) to coincide with a higher reflectivity, patchy and grainy acoustic facies. From the side-scan sonar imagery, individual reefs were estimated to reach a maximum size of 15 m 2. To ground truth the acoustic facies, the distribution of intertidal L. conchilega reefs was mapped at low tide and side-scan sonar imagery was recorded during the following high tide. Intertidal L. conchilega reefs had a patch size of 0.8 m 2 up to 11.6 m 2, elevated 7.5 to 11.5 cm above the surrounding seafloor and covered approximately 10% of the selected area. The very-high (445 kHz) resolution side-scan sonar imagery revealed a similar acoustic facies as in the subtidal. Lower-resolution (132 kHz) side-scan sonar imagery was less efficient to detect physically less developed L. conchilega reefs. We conclude that (1) there are no major technical restrictions to map L. conchilega reefs using side-scan sonar, (2) the developmental stage of L. conchilega reefs impacts the detectability of the reefs, and (3) very-high resolution side-scan sonar imagery is considered a necessity when mapping small-scale structures, such as L. conchilega reefs.

Evolution of the Glaciated Shelf and Coastline of the Northern Gulf of Maine, USA

The Gulf of Maine (northeast US coast) records shelf evolution since deglaciation ca. 15 ka. Glacial erosion of bedrock left a complex coastline of bays, peninsulas and islands. Till and outwash provided coarse sediment for reworking in littoral systems throughout the Holocene transgression. Glaciomarine mud, the Presumpscot Formation, was an abundant source of fine sediments that were reworked in estuaries, embayments and back-barrier systems. Relative sea-level change was driven by both isostatic and eustatic components. Initial submergence to 70–130 m above present sea level was contemporaneous with marine-based ice-sheet retreat at 15-13 ka. Rapid emergence followed to 60 m below present, during continuing isostatic rebound at 13-11 ka. Finally, submergence and transgression occurred 10.8 ka to present as isostatic rebound was overtaken by eustatic sea-level rise. Reworking during emergence and lowstand brought sand and gravel to the present coast and inner shelf, building paleodeltas at the mouths of the Merrimack River and Kennebec River. Other rivers, such as the Penobscot, drained landscapes with fewer coarse-grained sources, and have primarily mud-filled estuaries, such as Penobscot Bay.Detailed seismic reflection profiling and sidescan sonar mapping provide data for a model of inner-shelf evolution based on principles of sequence stratigraphy. The lower sequence boundary is the unconformity created on top of the Presumpscot Fm. and other glacial sediments during emergence. Lowstand systems tracts are best recognized in paleodeltas. Transgressive systems tracts are thin, but interrupted by parasequences of prograding deltaic and estuarine facies in some estuaries. Significant examples such as the newly discovered Penobscot Paleodelta (8–9 ka, 30 m below present sea-level) may relate to a slowing of relative sea-level rise. Highstand systems tracts formed in the late Holocene as the rate of sea-level rise slowed and the rate of sediment supply allowed stabilization and progradation of barriers and tidal deltas. Preservation potential of these features is controlled by the open coast ravinement unconformity, and by the tidal ravinement unconformity and bluff ravinement unconformity in embayments. Variability in preservation potential results both from paleotopography and differing energy of modern processes.

Volcanic facies associations in a modern volcaniclastic apron (Lipari and Vulcano offshore, Aeolian Island Arc)

Multibeam bathymetric and reflectivity data, high-resolution sidescan sonar mapping and ROV images have allowed reconstruction of the present-day facies distribution within the volcaniclastic apron offshore from Lipari and Vulcano. Primary volcanic, resedimented volcaniclastic and volcanogenic sedimentary facies have been distinguished. Primary volcanic facies are volumetrically subordinate but play a major role in controlling the distribution of the other facies and the development of the main submarine canyons, and as the source of volcanic detritus in resedimented volcaniclastic facies. Erosion and transport prevail in the canyons. Conversely, deposition of volcanogenic sediments, in response to flow spreading and gradient reduction, occurs at the canyon mouths. The influence of the adjacent islands on the offshore processes is particularly evident in the area facing the active sector of Vulcano Island. Here, large volumes of volcanogenic sediment from the emergent volcanic edifice are transferred downslope by subaqueous gravity flows allowing the construction of a volcaniclastic fan.

UK Atlantic Margin Environmental Survey: Introduction and overview of bathyal benthic ecology

The recent expansion of the Oil and Gas Industry in to the deep waters of the UK Atlantic Frontier prompted the industry and its regulator to reappraise the needs and means of environmental monitoring. In concert, deep-sea academics, specialist contractors, the regulator and the Industry, through the Atlantic Frontier Environmental Network (AFEN), devised and implemented a large-scale environmental survey of the deep waters to the north and west of Scotland. The AFEN-funded survey was carried out during the summers of 1996 and 1998, and involved two steps; an initial sidescan sonar mapping of the survey areas, followed up with direct seabed investigations by coring and photography. This contribution deals with the latter step. Seabed samples were collected to assess sediment type, organic content, heavy metals, hydrocarbons and macrobenthos. Photographic and video observations were employed to provide both ‘routine’ seabed assessments and to investigate particular sidescan features of note. Although essentially intended as a ‘baseline’ environmental survey, anthropogenic impacts are already evident throughout the areas surveyed. Indications of the effects of deep-sea trawling were frequently encountered (seabed trawl marks and areas of disturbed sediments), being present in almost all of the areas studied and extending to water depths in excess of 1000 m. Evidence of localised contamination of the seabed by drilling muds was also detected, though background hydrocarbon contamination is predominantly of terrestrial origin or derived from shipping. The benthic ecology of the UK Atlantic Margin is dominated by the marked differences in the hydrography of the Faroe–Shetland Channel (FSC) and the Rockall Trough (RT). Comparatively warm North Atlantic Water is common to both areas; however, in the FSC, cold (subzero) waters occupy the deeper parts of the channel (>600 m). The extreme thermal gradient present on the West Shetland Slope has a substantial influence on the distribution and diversity of the macrobenthos. While there is continuous variation in the fauna with depth, warm and cold water faunas are nonetheless quite distinct. The boundary region, centred on 400 m water depth, may be best characterised as an ecotone, having a mixed warm and cold water fauna with a distinctly enhanced diversity. The Wyville–Thomson Ridge largely prevents the cold waters of the deep FSC from entering the RT (they certainly do not influence the areas of the Malin/Hebrides Slope assessed during the survey). Consequently, the deep-water faunas north and south of the ridge are highly distinct. There is also a very marked difference in the diversity of the two faunas: diversity declines with depth in the FSC but increases with depth in the RT. The distribution of macrobenthos in the RT is largely continuous with depth, with little indication of local variations but some evidence of enhanced rates of change at around 1200 m, possibly associated with the presence of Labrador Sea Water. Other observations made during the course of the survey include: (a) the occurrence of sponge dominated communities (‘ ostebund’) at mid-slope depths (ca. 500 m) north and west of Shetland; (b) the discovery of a population of sediment surface dwelling enteropneusts associated with a sandy contourite deposit at the base of the West Shetland Slope (ca. 900 m); (c) the widespread and abundant occurrence of phytodetritus in the RT but not the FSC; and (d) the discovery of the ‘ Darwin Mounds’ at ca. 1000 m in the northern RT, a field of numerous, small seabed mounds that support significant growths of the coral Lophelia pertusa. These mounds also have ‘acoustically visible tails’ with dense populations of xenophyophores ( Syringammina fragilissima), a species found to be common elsewhere in the RT.

The Norwegian-Barents-Svalbard (NBS) continental margin: Introducing a natural laboratory of mass wasting, hydrates, and ascent of sediment, pore water, and methane

Side-scan sonar mapping and ground-truthing of the Norwegian–Barents–Svalbard continental margin shed new light on shelf glaciation, mass wasting, hydrates, and features like the Hakon Mosby mud volcano (HMMV), reflecting upward mobility of gas, pore fluids, and sediments. Detailed HMMV examination revealed thermal gradients to 10°/m, bottom-water CH4 and temperature anomalies, H2S- and CH4-based chemosynthetic ecosystems, and subbottom methane hydrate (to 25%). Seismic and chemical data suggest HMMV origins at 2–3 km depth within the 6-km-thick depocenter. The HMMV and mound fields bordering the Bjornoyrenna slide valley and pockmarks bordering the Storegga slide may all have formed in response to sediment failure.

Side-scan sonar mapping of bedforms in the middle Tay Estuary, Scotland

Side-scan sonar, augmented with echo-sounding and sediment sampling, has been employed to examine and map the distribution of sediments, bedform types and bedform geometries in the middle reaches of the macrotidal Tay Estuary of eastern Scotland. Much of this dynamic sector is characterised by mobile, medium to slightly gravelly sands. The most abundant bedforms are small to medium dunes (wavelength 2-10m, height up to 0.5m) of varying sinuosity and superposition the asymmetries of which are indicative of the dominance of flood currents and the marine derivation of sands. However, previously unrecognised abrupt, non-gradational changes in bedform morphology and sediment type occur both with and without apparent bathymetric control. A hitherto unreported acoustic signature, referred to as 'patchy discontinuous', was recorded in many areas of channels. This is attributed to the patchy colonisation of mussels preferentially on the gentle stoss slopes of dunes, with resultant sediment stabilisation, whilst uncolonised sediments are exposed on the steeper lee slopes.

Crepidula fornicata L. (mollusque, gastéropode) en baie de Marennes-Oléron : cartographie des fonds par sonar à balayage latéral et estimation du stock

Extensive and detailed subtidal ground mapping of the Marennes-Oléron Bay and Fouras-Aix areas was performed with a side-scan sonar. Side-scan sonographs gave a comprehensive view of sedimentological bottom environments and also allowed us to outline locations of grounds colonised by the gastropod mollusc Crepidula fornicata L. with a precise estimate of their surface. A sedimentological map of the centre of the Marennes-Oléron Bay is given, describing spatial organisation of the following four types of sediment: pure mud, fine sand, coarse sand and rocks. Crepidula beds were also recognised on sonographs and were estimated to cover 181 hectares in the Fouras-Aix area and 615 hectares within the Marennes-Oléron Bay. Standard sampling methods were then combined with the side-scan sonar mapping results, allowing an estimate to be made of the stocks of Crepidula fornicata (live specimens and dead shells). A stratified sampling procedure was performed in April–May 1995, including 80 stations sampled with a Smith McIntyre grab (two grabs per station). The live stock of Crepidula was estimated to be 2 494 ± 3 344 tons at Fouras-Aix and 2 644 ± 1 137 tons within the bay of Marennes-Oléron. This latter estimate is similar to the previous one performed in this bay in spring 1984, i.e. 1 800 ± 900 tons, due to the overlap of 95 % confidence intervals. These comparable estimates within the Marennes-Oléron Bay give no evidence to suggest that dredging operations (1 000–1 500 tons-yr −1) performed in the bay for 15 years are not efficient. However, annual landing operations of slipper limpet cannot prevent further spread of the species, as pointed out by the large development of a new Crepidula ground in the north-western centre of the bay (north Lamouroux sand bank). Both natural processes (current action and bed load transport) and bottom-trawl activities can facilitate spread of the species in the bay, as revealed by side-scan sonar image analysis.

Cenozoic Tectonic and Sedimentary Development of the North American-Caribbean Transform Boundary: ABSTRACT

The North American-Caribbean plate boundary consists of strike-slip deformation extending 3,200 km from the Middle American volcanic arc in western Guatemala to the northern Lesser Antilles. The authors have reconstructed the complex sedimentary and tectonic evolution of this zone over the past 65 m.y. by integration of onshore geologic data and offshore geophysical data. Onshore geologic data consists mainly of detailed structural maps and biostratigraphic studies from uplifted sedimentary basins in Hispaniola and Jamaica. Offshore data consists of single and multichannel seismic profiles and side-scan sonar maps. Both onshore and offshore data have been synthesized into a set of computer-based, paleogeographic maps for the following Cenozoic periods: late Paleocene, middle Eocene, late Oligocene, early Miocene, late Miocene, Pliocene, and Holocene. These data allow them to generalize the tectonic evolution of the boundary into three stages. Stage 1: Paleocene to early Eocene rifting and bimodal volcanism associated with formation of the Cayman Trough pullapart basin. Nonmarine to shallow marine sedimentation was controlled by northwest-northeast-striking normal faults formed at a high angle to the direction of plate motion. Stage 2: middle Eocene to early Miocene strike-slip faulting across a broad, California Borderlands-type margin. Shallow marine to marine sedimentation was controlled by strike-slipmore » faults parallel or at a low angle to the direction of plate motion. Stage 3: middle Miocene to present strike-slip faulting along a single major fault. Deep-marine sedimentation was controlled by a major strike-slip fault scarp formed approximately parallel to the direction of plate motion over most of the length of the boundary. Eustatic sea level effects are recognizable in all three stages.« less

Side-Scan Sonar Mapping of Lake Trout Spawning Habitat in Northern Lake Michigan

Native stocks of lake trout Salvelinus namaycush were virtually or completely extirpated from the lower four Great Lakes by the early 1960s. The failure of early attempts to reestablish self-sustaining populations of lake trout was attributed partly to the practice of stocking hatcheryreared juveniles at locations and over substrates that had not been used in the past for spawning by native fish. Subsequent attempts to improve the selection of stocking locations were impeded by the lack of reliable information on the distribution of substrates on historical spawning grounds. Here we demonstrate the potential of side-scan sonar to substantially expand the data base needed to pinpoint the location of substrates where lake trout eggs, fry, or juveniles could be stocked to maximize survival and help ensure that survivors returning to spawn would encounter suitable substrates. We also describe the substrates and bathymetry of large areas on historical lake trout spawning grounds in the Fox Island Lake Trout Sanctuary in northern Lake Michigan. These areas could be used to support a contemporary self-sustaining lake trout population in the sanctuary and perhaps also in adjacent waters.

Spatial variability of bottom types in the lower Chesapeake Bay and adjoining estuaries and inner shelf

The spatial distributions of the bed textural and morphologic properties that influence boundary-layer roughness characteristics in the lower Chesapeake Bay, the lower portions of the York, James and Elizabeth Rivers, and the adjacent inner continental shelf were systematically mapped. A high resolution, fully-corrected side-scan sonar mapping system (100 kHz) was used for remote acoustic detection of bottom roughness, supported by ‘ground-truthing’ by direct in situ observations by divers. These complementary methods proved to be especially effective in detecting a wide range of roughness-controlling bed surface properties at various scales. Fine-scale variations in sediment size and associated bottom texture are considered to be the main source of heterogeneity in Nikuradse (skin friction) roughness. A wide variety of small- and intermediate-scale morphologic elements provide meso-scale and small-scale distributed (form drag) roughness. Depending upon location, the distributed roughness may be either biogenic or hydrodynamically induced (by currents and waves), although anthropogenic roughness prevails in certain instances (e.g. port areas). In terms of particular combinations of roughness scales and types, combined sonar and diver observation data allow the beds to be systematically but qualitatively classified into 10 bottom types, each of which is associated with a particular type of subenvironment.