Autonomous Underwater Vehicle Surveys Research Articles

Magmatism at an ultra-slow spreading rift: high-resolution geomorphological studies of a Red Sea Rift segment in Hadarba Deep

The mid-ocean rift in the Red Sea is one of the youngest rifting systems on Earth. Only recently, state-of-the-art methods and modern deep-sea instruments have been used to explore this young and unique volcanic system. During the first autonomous underwater vehicle surveys of the Red Sea Rift in Spring 2022, we collected multibeam bathymetry, backscatter, sub-bottom profiler data, and water column data over a 9 km long ridge segment in the Hadarba Deep between 22.49°N and 22.56°N to investigate the volcano-tectonic processes of this ultra-slow spreading segment (12 mm/year spreading rate). The high-resolution hydroacoustic data was used to (1) delineate and quantify the geometry of tectonic structures and individual lava flows, (2) define lava flow morphology and eruption style, (3) estimate relative ages of flows and features, and (4) retrace the evolution of the volcanic activity. In addition, the geochemistry of several young lava flows provides information on the relation between the different magma that supply these eruptions. About 90 eruptive units with variable sedimentary cover have been identified within the 43 km2mapped region. The oldest lava flows are buried under 3 to 4.2 m of sediment, indicating ages of up to ~30 ka based on average sedimentation rate estimates (~14 cm/ka), while the youngest eruptions are covered by<10 cm of sediment, and are thus younger than 700 years. Three volcanic phases have been identified based on changes in flow morphology and distribution, and tectonic pattern. All three axial phases have an average eruptive frequency of ~100-250 years. The segment displays an overall low tectonic extension (<10% of the total extension) and low vertical offset. Our geomorphological maps, analyses, and statistics reveal a moderately faulted, ultra-slow spreading MOR segment in the Red Sea with a surprisingly large amount of magmatic extension, implying that the segment has been underlined by a large magma supply for at least 15 ka. All these observations provide valuable implications for the formation history of the Red Sea Rift and the formation of ultra-slow spreading crust.

Bedform evolution along a submarine canyon in the South China Sea: New insights from an autonomous underwater vehicle survey

ABSTRACTTraditional mapping of bedforms in submarine canyons relied on vessel‐mounted and towed sensors, but their fine‐scale geomorphology and shallow structure requires higher resolution datasets. This study utilizes a high‐resolution dataset obtained from an autonomous underwater vehicle, combined with seismic reflection profiles and sediment cores, to analyse bedform sets within a 25.6 km long submarine canyon (canyon C14) in the northern South China Sea. A train of crescent‐shaped axial steps, indicative of cyclic steps formed by supercritical turbidity currents, is imaged along the canyon. Axial steps in the upper course show erosional truncations and sub‐horizontal reflectors on the lee and stoss sides, respectively, pointing to erosional–depositional cyclic steps formed by confined flows with high erosional capacity. This is facilitated by canyon narrowness and steeper axial gradient. After a transition segment, the lower course widens, with a gentler axial gradient, resulting in increased asymmetry and wavelength of axial steps. Backset bed deposits on the stoss sides of these steps indicate depositional cyclic steps with higher aggradation. Sediment filling, almost padding each cyclic step associated scour suggests the reworking of previously formed bedforms by gravity flows fed by destabilization processes on the canyon sidewalls and upstream lee faces and, possibly, by shelf‐edge and uppermost slope spillover into the canyon. At the lowermost course, cyclic steps transition to a furrow field, likely associated with flow velocity reduction facilitated by canyon floor widening and a further decrease in slope gradient. Flow braiding and re‐convergence, related to the erosion of fine‐grained deposits within the canyon floor, should have played a role to produce furrows under supercritical conditions. This work enhances our understanding of the detailed morphology and shallow relief configuration of bedforms in deep‐water submarine canyons, providing insights into their causative processes and evolution.

Schrödinger's fish: Examining the robotic observer effect on pelagic animals

AbstractRobust sampling of animals is necessary for understanding ocean ecology, but evaluating the effectiveness of our samplers is a challenge. Scientific echosounders were added to two robotic platforms carrying video imaging systems: a remotely operated vehicle (ROV) and an autonomous underwater vehicle (AUV). The vehicles were used to quantitatively sample midwater life in Monterey Bay along horizontal transects at incremental depths ranging from 25 to 1000 m. The echosounders allowed the bulk behavioral responses of animals to be observed up to 200 m forward of each platform. These responses observed included no response, continual avoidance, avoidance to a fixed range resulting in a patch, and attraction. There were strong and interacting effects of depth and platform type on behavioral responses. Measurements of acoustic backscatter showed that animals responded more strongly to the AUV than the ROV. During AUV surveys, there were effects of day/night and the use of artificial illumination on animal responses. Behavioral responses to our sampling were both species‐ and context‐dependent. These data inspired the expansion of an existing mathematical framework that formalized the processes affecting the sampling of motile ocean organisms. Originally developed for net sampling, we generalized the equations to be platform‐ and sensor‐agnostic and incorporated animal decision‐making processes to allow for behaviors consistent with the full range of responses we observed. These results and the framework can help move toward more effective sampling of motile animals in the ocean.

Online Stochastic Variational Gaussian Process Mapping for Large-Scale Bathymetric SLAM in Real Time

Rao-Blackwellized particle filter (RBPF) SLAM solutions with Gaussian Process (GP) maps can both maintain multiple hypotheses of a vehicle pose estimate and perform implicit data association for loop closure detection in continuous terrain representations. Both qualities are of particular interest for SLAM with autonomous underwater vehicles (AUVs) in the open sea, where distinguishable features are scarce. However, the applicability of GP regression to parallel, real-time mapping in an RBPF framework remains limited by the size of the area to survey and the computational cost of the GP training. To overcome these constraints, in this letter we propose the adaption of Stochastic Variational GP (SVGP) regression to online mapping in combination with a novel, efficient particle trajectory storing in the RBPF. We show how the resulting RBPF-SVGP framework can achieve real-time performance in an embedded platform on two AUV surveys containing millions of points. We further test the framework on a live mission on an AUV and we make the implementation publicly available.

Horizontal distribution of benthic and demersal fish assemblages on three seamounts in the Papahānaumokuākea Marine National Monument

Deep-sea fishes on seamounts have recently been shown to have strong changes in assemblage patterns with depth. However, horizontal variability patterns within a single seamount and among seamounts remain drastically understudied. The Papahānaumokuākea Marine National Monument (PMNM) is part of an extensive seamount chain that is relatively unimpacted by human activity as well as highly variable in physiographic and oceanographic characteristics. Three PMNM seamounts, Necker Island, French Frigate Shoals (FFS), and Pioneer Bank, were explored using Autonomous Underwater Vehicle surveys to document the deep-sea fish assemblages and their horizontal patterns of distribution at three different depths. Quantitative comparisons were made among the sides of the seamount and among the three seamounts while controlling for depth. The Gadiformes were the most abundant order with the highest number of individuals at 300 and 450 m on Pioneer and FFS. At 600 m the Myctophiformes showed the highest number of individuals overall and were the most abundant on FFS. Significant differences in abundance by seamount, side, and their interaction were observed at 300 and 600 m. Significant differences in some diversity metrics occurred in one, both, or the interaction of the factors of seamount and side at each depth. At all depths, the structure of the fish assemblage showed significant variation among seamounts and sides, which was explained mostly by the interaction term. After accounting for the variability explained by the confounding factor of time of day, the most common environmental factors related to assemblage structure for at least two depths were the east-west component of currents (u), latitude, chlorophyll a (chl a), and particulate organic carbon (POC). At 300 m, oxygen and salinity were also important, at 450 m mean direction of the substrate was an additional factor that was correlated with assemblage structure. At 600 m the factor that explained the most variation in community structure was percent rugosity. The results obtained in this study show significant horizontal variability in seamount fish assemblages within a given depth range; this variability should be taken into consideration in the management and conservation of the Monument and other seamounts.

Feature Space Exploration For Planning Initial Benthic AUV Surveys

Special-purpose Autonomous Underwater Vehicles (AUVs) are utilized for benthic (seafloor) surveys, where the vehicle collects optical imagery of the seafloor. Due to the small-sensor footprint of the cameras and the vast areas to be surveyed, these AUVs can not feasibly collect full coverage imagery of areas larger than a few tens of thousands of square meters. Therefore it is necessary for AUV paths to sample the surveys areas sparsely, yet effectively. Broad-scale acoustic bathymetric data are readily available over large areas and are often a useful prior of seafloor cover. As such, prior bathymetry can be used to guide AUV data collection. This research proposes methods for planning initial AUV surveys that efficiently explore a feature space representation of the bathymetry, in order to sample from a diverse set of bathymetric terrain. This will enable the AUV to visit areas that likely contain unique habitats and are representative of the entire survey site. We propose several information gathering planners that utilize a feature space exploration reward, to plan freeform paths or to optimize the placement of a survey template. The suitability of these methods to plan AUV surveys is evaluated based on the coverage of the feature space and also the ability to visit all classes of benthic habitat on the initial dive. Informative planners based on Rapidly expanding Random Trees (RRT) and Monte Carlo Tree Search (MCTS) were found to be the most effective. This is a valuable tool for AUV surveys as it increases the utility of initial dives. It also delivers a comprehensive training set to learn the relationship between acoustic bathymetry and visually derived seafloor classifications.

First Autonomous Underwater Vehicle Observations of a Potential Petrale Sole (Eopsetta jordani) Spawning Aggregation Off the US West Coast

Understanding the timing and reproductive behavior of commercial fish species is a key part of well-informed stock assessments and fishery management, but this information is often limited, particularly for species that spawn in deep water. Petrale sole (Eopsetta jordani) is one of the most commercially important flatfish species in the US and is known to spawn off the West Coast during winter months. A number of spawning grounds have been identified using catch data and tagging studies, but to our knowledge there have been no direct visual observations of aggregating petrale sole. In 2018, we observed unusually high densities of petrale sole in autonomous underwater vehicle (AUV) imagery collected at a feature inshore of Santa Lucia Bank off the California coast. In addition to aggregations of fish, we observed fish in positions and configurations that we believe to be related to spawning behavior including physical contact between individuals and evidence of consistent size differentiation within clusters. We present images of the aggregating petrale sole and information on the physical and environmental conditions at this site. We also compare these observations to the results of AUV surveys carried out in 2005 at the same site. Analysis of commercial catch data from this area supports the hypothesis that this is a seasonal petrale sole aggregation as fishing at this location occurs mainly during winter months with catches dominated by petrale sole. In addition to the potential identification of a spawning aggregation, these observations show how advanced technologies can provide insight into the reproductive behavior of a commercially important species in-situ.

Extent and Volume of Lava Flows Erupted at 9°50′N, East Pacific Rise in 2005–2006 From Autonomous Underwater Vehicle Surveys

AbstractSeafloor volcanic eruptions are difficult to directly observe due to lengthy eruption cycles and the remote location of mid‐ocean ridges. Volcanic eruptions in 2005–2006 at 9°50′N on the East Pacific Rise have been well documented, but the lava volume and flow extent remain uncertain because of the limited near‐bottom bathymetric data. We present near‐bottom data collected during 19 autonomous underwater vehicle (AUV) Sentry dives at 9°50′N in 2018, 2019, and 2021. The resulting 1 m‐resolution bathymetric grid and 20 cm‐resolution sidescan sonar images cover 115 km2, and span the entire area of the 2005–2006 eruptions, including an 8 km2 pre‐eruption survey collected with AUV ABE in 2001. Pre‐ and post‐eruption surveys, combined with sidescan sonar images and seismo‐acoustic impulsive events recorded during the eruptions, are used to quantify the lava flow extent and to estimate changes in seafloor depth caused by lava emplacement. During the 2005–2006 eruptions, lava flowed up to ∼3 km away from the axial summit trough, covering an area of ∼20.8 km2; ∼50% larger than previously thought. Where pre‐ and post‐eruption surveys overlap, individual flow lobes can be resolved, confirming that lava thickness varies from ∼1 to 10 m, and increases with distance from eruptive fissures. The resulting lava volume estimate indicates that ∼57% of the melt extracted from the axial melt lens probably remained in the subsurface as dikes. These observations provide insights into recharge cycles in the subsurface magma system, and are a baseline for studying future eruptions at the 9°50′N area.

Geoacoustic inversion for a 14-km autonomous underwater vehicle survey on the Malta Plateau

We consider signal processing and inversion of 1487 source instances recorded on a towed array along a 14-km seabed survey on the Malta Plateau. The data were acquired by autonomous underwater vehicle (AUV) and processed as reflection coefficients versus grazing angle and frequency. Data acquisition caused several artifacts that are studied by various data representations. These representations expose periodic fluctuations of reflection coefficients with angle and frequency that do not depend on seabed location. Averaging over source instances, frequencies, and angles reduces the artifacts. The Bayesian inversion assumes a one-dimensional seabed structure for each data set and is parametrized by an unknown number of homogeneous layers, sound velocities, densities, and attenuations. Results from individual inversions and sequential Monte Carlo sampling are compared. We demonstrate that removing data artifacts reduces over-fitting problems from previous considerations of the same data. Comparisons to piston and gravity core estimates, and separate wide-angle data show good agreement with the AUV results for two locations along the track. However, results at greater depths exhibit high uncertainty and strong influence of chosen prior boundaries. When considering results for all 1487 data sets, dipping and terminating layers are found along the track with high resolution (∼10 cm).

Recent fishing footprint of the high-seas bottom trawl fisheries on the Northwestern Hawaiian Ridge and Emperor Seamount Chain: A finer-scale approach to a large-scale issue

A standing data gap for management of high-seas seamounts of the Northwestern Hawaiian Ridge and Emperor Seamounts (ES-NHR) by the North Pacific Fisheries Commission is the footprint of fisheries activities on these seamounts. Using satellite AIS data and the algorithms of the publicly available Global Fishing Watch database, a spatial map of trawling in a 0.01-degree latitude by 0.01-degree longitude square grid was created to review the data available to map this footprint. From 2012 to 2018 much of the trawling effort for all countries focused on Koko, Yuryaku, Kammu, and Colahan Seamounts at depths between 400 m (summits) and the depth limit currently set by the North Pacific Fisheries Commission of 1500 m. Additional seamounts with fishing activity included Annei (North Koko), Kinmei, Jingu, and Suiko. The remaining ES-NHR seamount locations show no trawling in those years. Bottom contact fishing was predominately carried out by ships with flag states of Japan and Korea. To date there appears to be compliance with the recent small-scale closures on C–H seamount and Koko.An additional source of data comes from scientific Autonomous Underwater Vehicle (AUV) surveys in 2014 and 2015 from three of these seamounts, in which scars from bottom contact gear are readily visible. These cover a smaller spatial area and depth range than the satellite data, but indicate the full footprint is not encompassed by the satellite data, suggesting either the fishing footprint is not fully captured by the AIS approach or that the footprint has shifted through time. AUV surveys also provide data on areas where abundant megafauna occur, which can provide candidate starting points for VME management efforts and further closures, similar to ones already in effect in the ES-NHR. The combination of satellite and AUV data provide a finer-scale fisheries footprint for this region that can aid in management of these sites.

Cross‐Shore Structure of Tidally Driven Alongshore Flow Over Rough Bathymetry

AbstractA tidally driven alongshore flow on the western coast of O'ahu Hawai'i is examined using velocity measurements from an autonomous underwater vehicle (AUV) along with time series observations of the alongshore pressure gradient. Depth‐averaged velocities over the forereef shelf are reconstructed from AUV‐based velocity observations as a function of cross‐shore distance assuming a sinusoidal tidal periodicity. Ensemble phase averages of the alongshore pressure gradient and velocities from multiple AUV surveys reveal characteristics akin to an oscillatory boundary layer, with the nearshore flow leading the offshore flow in phase and with a corresponding velocity attenuation at shallower depths. Analysis of the depth‐averaged alongshore momentum equation indicates that the cross‐shore structure and evolution of the tidal boundary layer is well described by a balance between the local acceleration, the barotropic pressure gradient, and bottom drag. This primary balance allows estimation of the drag coefficient as a function of cross‐shore distance over depths spanning from 24 to 6 m. Results indicate that drag coefficients range from 0.004 to 0.010 [±0.002] over a 600 m cross‐shore forereef section. These estimates are in good agreement with previous results obtained at the 12 m isobath using fixed observations and compare favorably with roughness estimates from LIDAR and AUV‐based mapping. Roughness data suggest that larger scales, with wavelengths of O(10 m), play a more significant role than smaller meter‐scale roughness in determining the drag on the tidal flow.

Tracking the Movements of Juvenile Chinook Salmon using an Autonomous Underwater Vehicle under Payload Control

An autonomous underwater vehicle (AUV) under payload control (PC) was used to map the movements of juvenile Chinook salmon (Oncorhynchus tshawytscha) tagged with acoustic transmitters. After detecting a tag, the AUV deviated from its pre-programmed route and performed a maneuver designed to enhance the location estimate of the fish and to move closer to collect proximal environmental data. Nineteen fish were released into marine waters of southeastern Alaska. Seven missions with concurrent AUV and vessel-based surveys were conducted with two to nine fish present in the area per mission. The AUV was able to repeatedly detect and estimate the location of the fish, even when multiple individuals were present. Although less effective at detecting the fish, location estimates from the vessel-based surveys helped verify the veracity of the AUV data. All of the fish left the area within 48 h of release. Most fish exhibited localized movements (milling behavior) before leaving the area. Dispersal rates calculated for the fish suggest that error associated with the location estimates was minimal. The average movement rate was 0.62 body length per second and was comparable to marine movement rates reported for other Chinook salmon stocks. These results suggest that AUV-based payload control can provide an effective method for mapping the movements of marine fish.

Postcaldera volcanism and hydrothermal activity revealed by autonomous underwater vehicle surveys in Myojin Knoll caldera, Izu‐Ogasawara arc

AbstractMyojin Knoll caldera, one of the submarine silicic calderas lying on the volcanic front of the northern Izu‐Ogasawara arc, has attracted increasing attention since the discovery of a large hydrothermal field called the Sunrise deposit. Although numerous submersible surveys have been conducted in Myojin Knoll caldera, they have not sufficiently explored areas to produce a complete picture of the caldera and understand the origin of the Sunrise deposit. We conducted comprehensive deep‐sea surveys using an autonomous underwater vehicle and obtained high‐resolution bathymetric and magnetic data and sonar images from ~70% of the caldera. The detailed bathymetric map revealed that faulting and magma eruptions, possibly associated with an inflation‐deflation cycle of the magma reservoir during postcaldera volcanism, had generally occurred in the caldera wall. The main dome of the central cone was covered with lava flows and exhibits exogenous growth, which is unusual for rhyolitic domes. The magnetization distribution in the central cone indicates preferential magma intrusion along a NW‐SE direction. It is presumed that magma migrated along this direction and formed a rhyolite dome at the foot of the southeastern caldera wall, where the Sunrise deposit occurs. The Sunrise deposit is composed mainly of three ridges extending in slope directions and covers ~400 × ~400 m. Magnetization reduction in the deposit area is small, indicating that the alteration zone beneath the Sunrise deposit is slanting rather than vertical. It is presumed that several slanting and near‐vertical volcanic vents serve as pathways of hydrothermal fluid in Myojin Knoll caldera.

The Palos Verdes Fault offshore Southern California: Late Pleistocene to present tectonic geomorphology, seascape evolution, and slip rate estimate based on AUV and ROV surveys

AbstractThe Palos Verdes Fault (PVF) is one of few active faults in Southern California that crosses the shoreline and can be studied using both terrestrial and subaqueous methodologies. To characterize the near‐seafloor fault morphology, tectonic influences on continental slope sedimentary processes and late Pleistocene to present slip rate, a grid of high‐resolution multibeam bathymetric data, and chirp subbottom profiles were acquired with an autonomous underwater vehicle (AUV) along the main trace of PVF in water depths between 250 and 600 m. Radiocarbon dates were obtained from vibracores collected using a remotely operated vehicle (ROV) and ship‐based gravity cores. The PVF is expressed as a well‐defined seafloor lineation marked by subtle along‐strike bends. Right‐stepping transtensional bends exert first‐order control on sediment flow dynamics and the spatial distribution of Holocene depocenters; deformed strata within a small pull‐apart basin record punctuated growth faulting associated with at least three Holocene surface ruptures. An upper (shallower) landslide scarp, a buried sedimentary mound, and a deeper scarp have been right‐laterally offset across the PVF by 55 ± 5, 52 ± 4 , and 39 ± 8 m, respectively. The ages of the upper scarp and buried mound are approximately 31 ka; the age of the deeper scarp is bracketed to 17–24 ka. These three piercing points bracket the late Pleistocene to present slip rate to 1.3–2.8 mm/yr and provide a best estimate of 1.6–1.9 mm/yr. The deformation observed along the PVF is characteristic of strike‐slip faulting and accounts for 20–30% of the total right‐lateral slip budget accommodated offshore Southern California.

AUV technology for seabed characterization and geohazards assessment

Autonomous underwater vehicles (AUV) are untethered submarine robots that can be used to carry out deepwater mapping and seabed-characterization surveys (seafloor to 150-m depth). AUV surveys are used in the marine petroleum industry for various exploration, environmental, geohazard, and engineering applications. Typically, AUVs are launched from a mother ship. They execute a preprogrammed survey pattern and are recovered, and the survey data are downloaded for analysis. Data on water depth, geomorphology, stratigraphy, and structure ranging downward to 150 m below the seafloor routinely are collected using a variety of sensing technology. Principal survey tools include a multibeam echo sounder to provide water column, bathymetric, and seafloor-reflectivity data; a side-scan sonar to provide high-resolution seafloor imagery; and a subbottom profiler (1- to 24-kHz seismic-reflection tool) to show subseafloor stratigraphy and structural features. Other AUV survey tools can include still cameras, lidar scanners, magnetometers, geochemical sensors (CO2, CH4, PAH, dissolved oxygen), and temperature and salinity sensors. AUV surveys help deepwater exploration and development to proceed efficiently and safely and thus have become an indispensable tool for deepwater operations.

Integrating field survey data with satellite image data to improve shallow water seagrass maps: the role of AUV and snorkeller surveys?

Repeatable and accurate seagrass mapping is required for understanding seagrass ecology and supporting management decisions. For shallow (<5 m) seagrass habitats, these maps can be created by integrating high spatial resolution imagery with field survey data. Field survey data for seagrass are often collected via snorkelling or diving. However, these methods are limited by environmental and safety considerations. Autonomous underwater vehicles (AUVs) are used increasingly to collect field data for habitat mapping, albeit mostly in deeper waters (>20 m). Here, we demonstrate and evaluate the use and potential advantages of AUV field data collection for calibration and validation of seagrass habitat mapping of shallow waters (<5 m), from multispectral satellite imagery. The study was conducted in the seagrass habitats of the Eastern Banks (142 km2), Moreton Bay, Australia. In the field, georeferenced photographs of the seagrass were collected along transects via snorkelling or an AUV. Photographs from both collection methods were analysed manually for seagrass species composition and then used as calibration and validation data to map seagrass using an established semi-automated object-based mapping routine. A comparison of the relative advantages and disadvantages of AUV and snorkeller-collected field data-sets and their influence on the mapping routine was conducted. AUV data collection was more consistent, repeatable and safer in comparison with snorkeller transects. Inclusion of deeper water AUV data resulted in mapping of a larger extent of seagrass (~7 km2, 5% of study area) in the deeper waters of the site. Although overall map accuracies did not differ considerably, inclusion of the AUV data from deeper water transects corrected errors in seagrass mapped at depths to 5 m, but where the bottom is visible on satellite imagery. Our results demonstrate that further development of AUV technology is justified for the monitoring of seagrass habitats in ongoing management programmes.

Fast Target Detection in Synthetic Aperture Sonar Imagery: A New Algorithm and Large-Scale Performance Analysis

In this paper, a new unsupervised algorithm for the detection of underwater targets in synthetic aperture sonar (SAS) imagery is proposed. The method capitalizes on the high-quality SAS imagery whose high resolution permits many pixels on target. One particularly novel component of the method also detects sand ripples and estimates their orientation. The overall algorithm is made fast by employing a cascaded architecture and by exploiting integral-image representations. As a result, the approach makes near-real-time detection of proud targets in sonar data onboard an autonomous underwater vehicle (AUV) feasible. No training data are required because the proposed method is adaptively tailored to the environmental characteristics of the sensed data that are collected in situ. To validate and assess the performance of the proposed detection algorithm, a large-scale study of SAS images containing various mine-like targets is undertaken. The data were collected with the MUSCLE AUV during six large sea experiments, conducted between 2008 and 2012, in different geographical locations with diverse environmental conditions. The analysis examines detection performance as a function of target type, aspect, range, image quality, seabed environment, and geographical site. To our knowledge, this study-based on nearly 30 000 SAS images collectively covering approximately 160 km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of seabed, and involving over 1100 target detection opportunities-represents the most extensive such systematic, quantitative assessment of target detection performance with SAS data to date. The analysis reveals the variables that have the largest impact on target detection performance, namely, image quality and environmental conditions on the seafloor. Ways to exploit the results for adaptive AUV surveys using through-the-sensor data are also suggested.

A Camera-Based Autonomous Underwater Vehicle Sampling Approach to Quantify Scallop Abundance

The use of an autonomous underwater vehicle (AUV) survey method for population density estimates of Iceland scallops Chlamys islándica (O.F. Müller) was evaluated. The main focus was on statistical evaluations and survey design. The underlying objective was to illustrate how a repeated AUV survey approach can be used to generate variance estimates of population parameters to verify statistical precision of population density estimates. As a case study, 5 small-scale surveys were conducted at a site off West Iceland. The scallops were enumerated visually from a random selection of photos of the seafloor and the count data were used to estimate mean scallop population abundance with confidence limits. A nested random effects model yielded no significant difference among the 5 surveys, showing that such small-scale surveys are repeatable. The application was small scale, but the methodologies established can be applied to larger areas of a similar nature.

Seabed corrugations beneath an Antarctic ice shelf revealed by autonomous underwater vehicle survey: Origin and implications for the history of Pine Island Glacier

Ice shelves are critical features in the debate about West Antarctic ice sheet change and sea level rise, both because they limit ice discharge and because they are sensitive to change in the surrounding ocean. The Pine Island Glacier ice shelf has been thinning rapidly since at least the early 1990s, which has caused its trunk to accelerate and retreat. Although the ice shelf front has remained stable for the past six decades, past periods of ice shelf collapse have been inferred from relict seabed “corrugations” (corrugated ridges), preserved 340 km from the glacier in Pine Island Trough. Here we present high‐resolution bathymetry gathered by an autonomous underwater vehicle operating beneath an Antarctic ice shelf, which provides evidence of long‐term change in Pine Island Glacier. Corrugations and ploughmarks on a sub‐ice shelf ridge that was a former grounding line closely resemble those observed offshore, interpreted previously as the result of iceberg grounding. The same interpretation here would indicate a significantly reduced ice shelf extent within the last 11 kyr, implying Holocene glacier retreat beyond present limits, or a past tidewater glacier regime different from today. The alternative, that corrugations were not formed in open water, would question ice shelf collapse events interpreted from the geological record, revealing detail of another bed‐shaping process occurring at glacier margins. We assess hypotheses for corrugation formation and suggest periodic grounding of ice shelf keels during glacier unpinning as a viable origin. This interpretation requires neither loss of the ice shelf nor glacier retreat and is consistent with a “stable” grounding‐line configuration throughout the Holocene.

Deep‐sea channel evolution and stratigraphic architecture from inception to abandonment from high‐resolution Autonomous Underwater Vehicle surveys offshore central California

AbstractThe Lucia Chica channel system is an avulsion belt with four adjacent channels that progressively avulsed to the north‐east from a single, upslope feeder channel. Avulsion occurred from underfilled channels, leaving open channels that were reactivated by flows stripped from younger, adjacent channels. Differences in relief (height from channel thalweg to levée crest), sinuosity and levée stratigraphy between adjacent channels correspond to relative channel age, and indicate a change in channel morphology and architecture with time. Potential triggers for the change over time include differences in gradient, flow behaviour and characteristics, and channel evolution. Gradient does not appear to be a major control on channel formation and avulsion because adjacent channels formed on the same gradient. Based on available ultra‐high‐resolution remote imaging obtained with an Autonomous Underwater Vehicle, differences in adjacent channel morphology are interpreted to be primarily a result of differences in channel maturity. The interpreted sequence of channel maturity involves erosional channel inception through scouring and incipient channels (defined by linear trains of scours) prior to development of continuous thalwegs. Channel narrowing, formation and growth of levées, increasing channel relief and development of sinuosity occurred as channels evolved. The evolutionary sequence interpreted from the high‐resolution Lucia Chica dataset provides a unique perspective on intrinsic controls of architecture for single channel elements. In addition to helping bridge the gap between outcrop and industry‐standard reflection‐seismic data resolutions and scopes, interpretations in this study also expose potential problems with hierarchical classifications in three‐dimensional imaging of distributary systems, and provide potentially important analogues for evolutionary morphologies not resolved in other deep‐water channel systems.