Acoustic Ground Discrimination Research Articles

Acoustic Approach to Determining Seabed Substrates Distribution at Mandi Darah Island, Sabah

Marine ecosystems and natural habitat play the important role of the Earth’s life support system. They significantly contribute to economies and food safety and help preserve ecological processes. However, the devastation of the marine ecosystem in Malaysia due to the human factor and climate change is quite alarming. Therefore, spatial marine information, especially on the distribution of seabed substrates and habitat mapping, are of utmost importance for marine ecosystem management and conservation. Traditionally, seabed substrate and habitat mapping were classified based on direct observation techniques such as photography, video, sampling, coring and scuba diving. These techniques are often limited due to water clarity and weather conditions and only suitable for smaller scale surveys. In this study, we employed an acoustic approach using the RoxAnn Acoustic Ground Discrimination System (AGDS) with a high-frequency single-beam echo sounder to examine the distribution of seabed substrate at the Mandi Darah Island, Sabah. The acoustic signals recorded by AGDS are translated into hardness and roughness indices which are then used to identify the unique characteristics of the recorded seabed types. The analysis has shown that fifteen types of substrates, ranging from silt to rough/some seagrass, have been identified and classified. The findings demonstrated that the acoustic method was a better alternative for seabed substrate determination than the conventional direct observation techniques in terms of cost and time spent, especially in large scale surveys. The seabed substrate dataset from this study could be used as baseline information for the better management and conservation of the marine ecosystem.

Mapping of tropical marine benthic habitat: Hydroacoustic classification of coral reefs environment using single-beam (RoxAnn™) system

Field studies on the performance and discussion of RoxAnn™ Acoustic Ground Discrimination System (AGDS) in discriminating different coral growth forms are often fractional and limited in tropical waters. The purpose of this study is to bridge the information gaps by conducting field surveys at the surrounding benthic habitats of the largest bay in Borneo – Darvel Bay, Malaysia. Accuracy assessment was carried out to evaluate the integrity of the reef habitat map of a tropical island (Baik Island), using test plot and point sampling method. Despite the unsatisfactory test plot method, we found that point sampling method yielded a better internal accuracy of 78%. We demonstrated that factors such as orientation to shore and water depth do not impose significant influence to the AGDS performance using U-Mann-Whitney tests and regression analysis. Overall, the resulting island's habitat map exhibits decent repeatability with map accuracy higher than 80%. The penultimate section of this manuscript also evaluates and provides beneficial information on the potential and strength of AGDS system and its inherent limitation in classifying benthic substrate to a preferred level of details based on the user requirement.

Object-based classification of sub-bottom profiling data for benthic habitat mapping. Comparison with sidescan and RoxAnn in a Greek shallow-water habitat

Although Ground Discrimination Systems and Multi-beam Echo Sounders tend to be the preferred tools towards gaining knowledge about the seafloor, numerous surveys have been and are still being performed by simultaneously using conventional sidescan Sonars (SSSs) and Sub-Bottom Profilers (SBPs). This is in respect of gaining fast, wide-scale, three-dimensional imaging of the seafloor and its substrate, extracting maximum value from a single, time-limited survey. The combination of these two systems offers good knowledge of both the stratigraphy and the habitat extents on the seabed, aspects often linked to each other. However, a basic drawback is the inconsistency between their mapping scales, as SSS produces high-resolution backscatter maps and SBP produces substrata information of high vertical but very low horizontal density. In this work, 100-kHz SSS and 3.5-kHz SBP data, collected simultaneously during a geophysical survey at a 5.5 × 3.3 km, shallow (10–50 m depth) Marine Protected Area in Lourdas Gulf, Kefalonia Island, Greece, underwent post-processing and analysis, to extract numerous statistical features from both the seafloor and its substrate, towards automated seafloor classification. The SSS mosaic image was subjected to segmentation, coupling the mean-shift algorithm and unsupervised classification. The SBP images were processed using edge enhancement/detection techniques and numerous features were extracted regarding the acoustic appearance, transparency and density of the automatically detected seismic reflectors. Supervised classification of the SBP derivatives exhibited their high discrimination ability between different sea-bed types, making them suitable for ground discrimination. Linking of the SBP classes to the SSS segments (object-based analysis) led to full coverage, high detailed benthic habitat maps. Training of the classifiers was carried out using data points in proximity to underwater video inspected areas, while validation was performed using a manual habitat map made by expert interpreters. Three marine habitat classification schemes have been examined separately, namely NATURA, EUNIS and a custom one, successively including more Habitat types in a hierarchical manner, as to validate the method under different complexity levels. The SSS mosaic, as well as an independent data-set acquired using RoxAnn acoustic ground discrimination system, were also used, each individually, to classify Lourdas Gulf in yet again an object-based manner. The latter showed that SBP derivatives led to an equally significant and in most cases better classification of benthic habitats, compared to those produced by the widely tested and documented SSS and RoxAnn systems, implying that they deserve further exploitation.

Seafloor monitoring west of Helgoland (German Bight, North Sea) using the acoustic ground discrimination system RoxAnn

Marine habitats of shelf seas are in constant dynamic change and therefore need regular assessment particularly in areas of special interest. In this study, the single-beam acoustic ground discrimination system RoxAnn served to assess seafloor hardness and roughness, and combine these parameters into one variable expressed as RGB (red green blue) color code followed by k-means fuzzy cluster analysis (FCA). The data were collected at a monitoring site west of the island of Helgoland (German Bight, SE North Sea) in the course of four surveys between September 2011 and November 2014. The study area has complex characteristics varying from outcropping bedrock to sandy and muddy sectors with mostly gradual transitions. RoxAnn data enabled to discriminate all seafloor types that were suggested by ground-truth information (seafloor samples, video). The area appears to be quite stable overall; sediment import (including fluid mud) was detected only from the NW. Although hard substrates (boulders, bedrock) are clearly identified, the signal can be modified by inclination and biocover. Manually, six RoxAnn zones were identified; for the FCA, only three classes are suggested. The latter classification based on ‘hard’ boundaries would suffice for stakeholder issues, but the former classification based on ‘soft’ boundaries is preferred to meet state-of-the-art scientific objectives.

Single-beam acoustic variability associated with seabed habitats

This study is part of a wider predictive modeling work on benthic habitats along the Portuguese Continental Shelf. The aim here is to characterize the variability of the data collected by an acoustic ground discrimination system connected to a single-beam echo-sounder, considering different factors: the spatial survey step used to integrate the acoustic data (125, 250, 500, 750 or 1000m) and the sediment types. The surveyed area covered approximately 5400km2, with depth between 15 and 150m and superficial sediments ranging from fine gravel to mud. The acoustic survey followed 27 transects, perpendicular to the coast line, and 121 sediment samples were collected for ground-truth. The acoustic data integrated at 250m spatial step presented the lowest within sites variability and the highest among sites variance. Also, when the acoustic data was organized in groups according to the superficial sediment types, the 250m spatial step presented the highest variability among groups. Both results indicated that this spatial step should be used to integrate the acoustic data in future habitat modeling work.

High-Resolution Hydroacoustic Seafloor Classification of Sandy Environments in the German Wadden Sea

Mielck, F.; Hass, H.C., and Betzler, C., 2014. High-resolution hydroacoustic seafloor classification of sandy environments in the German Wadden Sea.The knowledge regarding the distribution of seabed environments in the North Sea is still fragmentary. Hydroacoustic devices can provide rapid and reliable information on the acoustic characteristics of the seafloor. Data of a typical, exclusively sandy and shallow investigation area located in the German Wadden Sea are here presented. The study area includes shallow sublittoral areas and deep channels. The acoustic ground discrimination system RoxAnn was used to measures backscatter intensities, which indicate roughness and hardness parameters of seafloor. Sidescan-sonar data provide acoustic images that are instrumental in seafloor characterization. For ground truthing, surface-sediment samples were collected. The results reveal surficial sediments that range from fine to coarse sand. Finer material is rather restricted to the shallow patches, while coarser sediments characterize the deeper tidal channels. The determined roughness and hardness parameters also strongly increase within these inlets following the change in grain size. The sidescan-sonar imagery shows flow-transverse subaqueous dunes of different sizes. Both ebb- and flood-dominated structures are present. RoxAnn reflects these zones of subaqueous dunes basically through the formation of distinct clusters of data points in hardness vs. roughness scatter plots. Further, the results show that areas characterized by similar grain-size spectra and sonar imagery occasionally reveal completely different roughness and hardness properties. This is caused by varying amounts of shell fragments on the seafloor. Both in situ and hydroacoustic methods are suitable for identifying seafloor properties, but the combination of methods yields more information at higher precision, especially about shell content and grain-size distribution.

Evaluation of WorldView-2 and acoustic remote sensing for mapping benthic habitats in temperate coastal Pacific waters

Maps of nearshore marine habitat are fundamental tools for the management and conservation of coastal ecosystems. While traditional field mapping techniques, such as towed video and diver surveys, are still commonly employed for generating highly detailed maps of seafloor habitat, acoustic and satellite remote sensing have proven to be efficient alternatives for generating habitat maps. To date remote sensing using satellite imagery has dominated mapping efforts over coral reefs and other tropical ecosystems with fewer studies applied in temperate marine regions where acoustic studies are more common. Few studies exist that have assessed the performance of high resolution satellite imagery in mapping seafloor habitat in temperate regions. This paper compares the efficacy of high resolution satellite imagery (WorldView-2) and a single-beam acoustic ground discrimination system (QTC View V) for mapping the distribution of submerged aquatic vegetation at a site within the Gwaii Haanas National Marine Conservation Area (GHNMCA) off the north coast of British Columbia, Canada. Ground-truth data for training and validation were collected using a towed underwater video camera. Prior to classification the WorldView-2 image (8 bands, 2m resolution) was processed following orthorectification, atmospheric correction, glint correction, land and optically deep water masking. An acoustic survey was conducted using a 200kHz echosounder and data were processed and interpolated using QTC IMPACT software. The WorldView-2 imagery performed best in mapping habitat in regions shallower than 3m, obtaining a total accuracy 75%, where it could identify the distribution of green algae (Ulva spp.), brown algae (Fucus spp.) and eelgrass (Zostera marina). The 200kHz data were unable to detect the distribution of brown and green algae but were able to map the distribution of eelgrass as well as a subtidal red algae (Chondrocanthus exasperatus) (total accuracy 80%). A final habitat map containing all habitat types present at the study site was produced using the output from both datasets. The study resulted in recommendations for remote mapping of submerged vegetation in temperate coastal area.

Predicting spatial kelp abundance in shallow coastal waters using the acoustic ground discrimination system RoxAnn

Kelp forests represent a major habitat type in coastal waters worldwide and their structure and distribution is predicted to change due to global warming. Despite their ecological and economical importance, there is still a lack of reliable spatial information on their abundance and distribution. In recent years, various hydroacoustic mapping techniques for sublittoral environments evolved. However, in turbid coastal waters, such as off the island of Helgoland (Germany, North Sea), the kelp vegetation is present in shallow water depths normally excluded from hydroacoustic surveys. In this study, single beam survey data consisting of the two seafloor parameters roughness and hardness were obtained with RoxAnn from water depth between 2 and 18 m. Our primary aim was to reliably detect the kelp forest habitat with different densities and distinguish it from other vegetated zones. Five habitat classes were identified using underwater-video and were applied for classification of acoustic signatures. Subsequently, spatial prediction maps were produced via two classification approaches: Linear discriminant analysis (LDA) and manual classification routine (MC). LDA was able to distinguish dense kelp forest from other habitats (i.e. mixed seaweed vegetation, sand, and barren bedrock), but no variances in kelp density. In contrast, MC also provided information on medium dense kelp distribution which is characterized by intermediate roughness and hardness values evoked by reduced kelp abundances. The prediction maps reach accordance levels of 62% (LDA) and 68% (MC). The presence of vegetation (kelp and mixed seaweed vegetation) was determined with higher prediction abilities of 75% (LDA) and 76% (MC). Since the different habitat classes reveal acoustic signatures that strongly overlap, the manual classification method was more appropriate for separating different kelp forest densities and low-lying vegetation. It became evident that the occurrence of kelp in this area is not simply linked to water depth. Moreover, this study shows that the two seafloor parameters collected with RoxAnn are suitable indicators for the discrimination of different densely vegetated seafloor habitats in shallow environments.

Measuring nearshore variability in benthic environments: An acoustic approach

Hardbottom and sedimentary habitats in the nearshore are continually affected by natural and anthropogenic disturbances. Currently used monitoring practices rely on visual techniques, aerial and underwater photography, which are conducted on an annual basis. These practices do not allow for rigorous assessment of the effects of human-induced or natural events on the distribution and composition of these habitats. We investigated the use of an acoustic ground discrimination system (RoxAnn) to determine changes in benthic habitats in the nearshore area of northern Indian River County, FL where hardbottom outcrops occur. It was determined that this method was precise and repeatable when deployed in a highly variable surf zone environment. Multiple surveys assessed temporal and spatial changes in bottom types. Spatial distributions of several bottom types were identified: bare rock surfaces, Sabellarid worm reef, sponge covered rock, and rock covered with two dominant algal species. Short-term (weekly) changes in the distribution of sand coverage were detected during a relatively calm hydrodynamic period. At one site (R-15), there was an increase of 11% in sediments, which attributed to approximately 2 750 m2 of coverage; 2.1 km south (R-22) there was a 6% decrease in sediments in six days. The latter equates to a change of almost 1 500 m2 of available habitat. These results suggest that the current practice of annual surveying is limited for assessing the magnitude and impact of episodic disturbances on benthic habitats. Based on these data, acoustic ground discrimination systems provide an unbiased, technology-based data collection method to distinguish benthic habitats in the surf zone. Employing this method will benefit researchers and coastal resource managers.

Detecting end-member structural and biological elements of a coral reef using a single-beam acoustic ground discrimination system

A thematic map of benthic habitat was produced for a coral reef in the Republic of Palau, utilizing hydroacoustic data acquired with a BioSonics DT-X echosounder and a single-beam 418 kHz digital transducer. This article describes and assesses a supervised classification scheme that used a series of three discriminant analyses (DAs) to refine training samples into end-member structural and biological elements utilizing E1′ (leading edge of first echo), E1 (trailing edge of first echo), E2 (complete second echo), fractal dimension (first echo shape) and depth as predictor variables. Hydroacoustic training samples were assigned to one of six predefined groups based on the plurality of benthic elements (sand, sparse submerged aquatic vegetation (SAV)) rubble, pavement, rugose hardbottom, branching coral) that were visually estimated from spatially co-located ground-truthing videos. Records that classified incorrectly or failed to exceed a minimum probability of group membership were removed from the training data set until only ‘pure’ end-member records remained. This refinement of ‘mixed’ training samples circumvented the dilemma typically imposed by the benthic heterogeneity of coral reefs, that is either train the acoustic ground discrimination system (AGDS) on homogeneous benthos and leave the heterogeneous benthos unclassified, or attempt to capture the many ‘mixed’ classes and overwhelm the discriminatory capability of the AGDS. It was made possible by a conjunction of narrow beam width (6.4°) and shallow depth (1.2 to 17.5 m), which produced a sonar footprint small enough to resolve the microscale features used to define benthic groups. Survey data classified from the third-pass training DA were found to: (i) conform to visually apparent contours of satellite imagery, (ii) agree with the structural and biological delineations of a benthic habitat map (BHM) created from visual interpretation of IKONOS imagery and (iii) yield values of benthic cover that agreed closely with independent, contemporaneous video transects. The methodology was proven on a coral reef environment for which high-quality satellite imagery existed, as an example of the potential for single-beam systems to thematically map coral reefs in deep or turbid settings where optical methods are not applicable.

Benthic habitat mapping: A review of progress towards improved understanding of the spatial ecology of the seafloor using acoustic techniques

This review examines the various strategies and methods used to produce benthic habitat maps using acoustic remote sensing techniques, coupled with in situ sampling. The applications of three acoustic survey techniques are examined in detail: single-beam acoustic ground discrimination systems, sidescan sonar systems, and multi-beam echo sounders. Over the past decade we have witnessed the nascence of the field of benthic habitat mapping and, on the evidence of the literature reviewed in this paper, have seen a rapid evolution in the level of sophistication in our ability to image and thus map seafloor habitats. As acoustic survey tools have become ever more complex, new methods have been tested to segment, classify and combine these data with biological ground truth sample data. Although the specific methods used to derive habitat maps vary considerably, the review indicates that studies can generally be categorized into one of three over-arching strategies; 1) Abiotic surrogate mapping; 2) Assemble first, predict later (unsupervised classification); 3) Predict first, assemble later (supervised classification). Whilst there is still no widely accepted agreement on the best way to produce benthic habitat maps, all three strategies provide valuable map resources to support management objectives. Whilst there is still considerable work to be done before we can answer many of the outstanding technological, methodological, ecological and theoretical questions that have been raised here, the review concludes that the advent of spatial ecological studies founded on high-resolution environmental data sets will undoubtedly help us to examine patterns in community and species distributions. This is a vital first step in unraveling ecological complexities and thus providing improved spatial information for management of marine systems.

Comparison of a Self-Processed EM3000 Multibeam Echosounder Dataset with a QTC View Habitat Mapping and a Sidescan Sonar Imagery, Tamaki Strait, New Zealand

A methodology for automatically processing the data files from an EM3000 multibeam echosounder (Kongsberg Maritime, 300 kHz) is presented. Written in MatLab, it includes data extraction, bathymetry processing, computation of seafloor local slope, and a simple correction of the backscatter along-track banding effect. The success of the latter is dependent on operational restrictions, which are also detailed. This processing is applied to a dataset acquired in 2007 in the Tamaki Strait, New Zealand. The resulting maps are compared with a habitat classification obtained with the acoustic ground-discrimination software QTC View linked to a 200-kHz single-beam echosounder and to the imagery from a 100-kHz sidescan sonar survey, both performed in 2002. The multibeam backscatter map was found to be very similar to the sidescan imagery, quite correlated to the QTC View map on one site but mainly uncorrelated on another site. Hypotheses to explain these results are formulated and discussed. The maps and the comparison to prior surveys are used to draw conclusions on the quality of the code for further research on multibeam benthic habitat mapping.

Mapping Coral Reef Habitats in Southeast Florida Using a Combined Technique Approach

To create maps of nearshore benthic habitats of Broward County, Florida, from 0 to 35 m depth, we combined laser bathymetry, acoustic ground discrimination, subbottom profiling, and aerial photography data in a geographic information system (GIS). A mosaic of interpolated, sun-shaded, laser bathymetry data served as the foundation upon which acoustic ground discrimination, limited subbottom profiling and aerial photography, and groundtruthing data aided in interpretation of habitats. Mapping criteria similar to NOAA biogeographic Caribbean mapping were used to allow for a comparable output. Expert-driven visual interpretation outlined geomorphological features at a scale of 1 : 6000 with a minimum mapping unit of 1 acre. Acoustic data were then used to differentiate areas of similar geomorphology by their acoustic diversity into areas of high and low scatter, which could be equated to rugosity created by either the substratum or benthic fauna. Of the approximately 112 km2 mapped, 56.62 km2 were coral reef and colonized hard bottom (50.42%), 54.78 km2 were unconsolidated sediments (46.80%), and 0.43 km2 were other categories (2.78%). Three linear reef complexes exist. The outermost linear reef has a mature windward reef morphology including a drowned spur and groove system, which was absent on the other two reef lines. The acoustic ground discrimination and groundtruthing showed different benthic habitats on the outer vs. middle and inner reefs. Higher acoustic scatter could be related to taller benthos and more rugose substratum. A considerable amount of colonized pavement (nearshore hard grounds) was found inshore. The map of Broward County yielded a high overall accuracy of 89.6%, only slightly less than the photo-interpreted NOAA Caribbean maps (overall accuracy of 91.1%). User and producer accuracies within each category were also similar. The combined technique approach was effective and accurate, and similar methodology can be used in other areas where photo interpretation is not feasible because of turbidity or depth limitations.

Single-beam acoustic ground discrimination of shallow water habitats: 50 kHz or 200 kHz frequency survey?

The single-beam acoustic ground discrimination system QTC View, Series V, was used in the Bay of Cadiz, Southwest Spain, for the identification and mapping of the bottom acoustic diversity. The acoustic data were obtained through two successive surveys, each conducted with one of the following echo sounder frequencies: 50 kHz and 200 kHz. The performance of each survey frequency for the identification of the sedimentary gradients was analyzed. The surveys were conducted during high tide given that the majority of the surveyed area is shallower than 5 m, although depth may occasionally reach 20 m in specific areas located in a navigation channel. The acoustic data obtained at the two different frequencies were, individually, submitted to manual clustering and a final solution consisting of three acoustic classes was reached for both datasets. However, only the geographical distribution of the acoustic classes obtained with 50 kHz echo sounder frequency was coincident with the spatial distribution of the superficial sediment groups (silty medium sand, very silty fine sand and mud), identified through multivariate analysis of the grain-size data of ground-truth sediment samples. The results obtained with the 200 kHz echo sounder frequency did not match the sedimentary gradients obtained for the area surveyed, not even the separation of muddy and sandy areas.

Small-scale variation within a Modiolus modiolus (Mollusca: Bivalvia) reef in the Irish Sea: I. Seabed mapping and reef morphology

Surveys by digital side-scan sonar, RoxAnnTM acoustic ground discrimination systems, multibeam echosounder and a sub-bottom profiling system showed that a Modiolus modiolus reef, in the Irish Sea off Pen Llŷn, north-west Wales, had a distinctive morphology and acoustic characteristics. The extent of the reef could therefore be determined and the benthic structure reliably mapped. The biogenic reef is in an area with moderately strong tidal currents and overlays lag gravel and cobbles with patchy sand veneers. The mussels form an undulating surface, orientated perpendicular to the current, with an average wavelength of 11.7 m and amplitude of 0.24 m that is significantly different from the surrounding seabed. Reef deposits reach a thickness of 1 m on top of the underlying lag gravels. The characteristic reef surface morphology helps distinguish the reef from the surrounding seabed on side-scan sonar and multibeam echosounder records and the undulations create the spatial complexity that influences the small-scale distribution of the associated epifauna, and infauna, reported in papers II and III of this series. The M. modiolus reef was recorded in the same location 40 y ago and has probably persisted there for over 150 y. Monitoring implications are discussed.

Thematic accuracy assessment of acoustic seabed data for shallow benthic habitat mapping

A single beam acoustic ground discrimination system (AGDS) was used to survey 1.25 km2 of shallow (< 20 m depth) seabed on the northeast coast of Tasmania, Australia. This paper investigates the uncertainties associated with the qualitative interpretation of real time and post‐processed acoustic signal, and the effect of track spacing on the mapping of rocky reef distribution. The survey was repeated with different track spacing (50 m, 100 m, 200 m and 50 × 200 m shore normal) to investigate the influence of data density and ‘knowledge based interpolation’ validated against direct measurements made with an underwater video camera. Habitat area calculations varied substantially only with the 50 × 200 m transect. These results have important implications for the qualitative assessment and application of AGDS technology in shallow water marine habitat mapping.

Sedimentary facies of the eastern Pacific's northernmost reef-like setting (Cabo Pulmo, Mexico)

Cabo Pulmo, Mexico, is often cited as the eastern Pacific's northernmost coral reef. We investigated sedimentary dynamics to verify whether the system functions sedimentologically as a reef, which would mean that it retained internal sediment in a framework and exported peri-reefal sediment creating clearly identifiable facies dominated by reefal fauna. We mapped distribution of sediments using grab samples, the acoustic ground-discrimination systems QTC View and Echo plus at 50 kHz signal frequency and an IKONOS satellite image. Morphologically, the bay of Cabo Pulmo can be divided into intrusive dikes and rocky hardgrounds in less than 10 m depth and a sandy interior with rocky outcrops sloping to over 40 m. Reefal fauna without lasting framework-building but with production of typically reef-associated carbonate sediments, as well as a patch-reef facies were found on the dikes and hardgrounds. Sediments in the bay were a mixture of carbonates and siliciclastics and were influenced by a seasonal river near the bay's center that imported siliciclastic material. To the north and south sediments had a higher carbonate content, although overall the bay was characterized by > 50% siliciclastics. The shallower subtidal sands were dominated by mollusk fragments, whereas in depths > 30 m planktonic foraminiferal sand was found. Acoustically and optically remote-sensed maps were used to differentiate between the rocky and sandy areas and to identify the presumed off-shelf transport pathway of reef-derived carbonates. Overall, the sedimentary system is characterized by carbonate production, but the coarser reef derived carbonates (coral and reef-dwelling mollusks) are transported off-shelf along a steep depth gradient and little typical peri-reef sediment remains. This transport is likely aided by storms. Since no sediment retaining frameworks are formed, the dikes cannot retain internal sediment, and the reefal fauna was not dominant in sediments throughout the bay, we conclude that Cabo Pulmo sedimentologically functions only in a limited way like a coral reef.

Sea-bottom classification across a shallow-water bar channel and near-shore shelf, using single-beam acoustics

An acoustic ground discrimination system (QTC VIEW, Series IV) was used to identify and map the bottom acoustic diversity in the bar channel of Ria de Aveiro, Western Portugal. The majority of the survey area presented shallow depth for this type of equipment, ranging mainly from 5 to 15 m. Depth occasionally reached 25 m in specific areas located across the entrance channel, dug by the strong tidal currents, reaching 3 m/s. The acoustic data were submitted to manual and auto-cluster and the results obtained from both procedures were coherent. Using aids to the acoustic classification and ground-truth sediment data, a final solution consisting of four acoustic classes was reached. Their geographical distribution was coincident with the spatial distribution of the major bottom types and sediment groups (hard bottom, coarse sand, medium sand and fine sand), identified through multivariate analysis of the grain-size data, and reflected the complex hydrodynamics of the entrance channel. The acoustic pattern was coincident at the intersections of the acoustic survey lines, assuring the repeatability of the acoustic procedure. Overall, the acoustic approach showed consistent results for the assessment and mapping of the benthic habitats in this shallow-water coastal area, providing a very valuable tool in an area where conventional sediment sampling is less favourable, namely due to strong tidal currents and frequent ship traffic, such as the entrance channel of Ria de Aveiro and the near-shore adjacent shelf.

Acoustic detection of a scallop bed from a single-beam echosounder in the St. Lawrence

Abstract Single-beam seabed echoes combined with epi-macrobenthos photographs were used to remotely detect a scallop bed and characterize the specific acoustic signal of Iceland scallop (Chlamys islandica). A dense scallop bed was surveyed in 2002, with a QTC VIEW Series IV acoustic ground-discrimination system (AGDS) connected to a 38 kHz, 7° split-beam SIMRAD EK60 scientific echosounder. In 2003, a 50 kHz, 42° single-beam SUZUKI ES-2025 echosounder was connected to a QTC VIEW Series V AGDS. The QTC VIEW data were analysed with QTC IMPACT following the standard procedures and classified into acoustic classes. Several approaches were tested: unsupervised and supervised survey strategies directed to specific benthic communities. The SIMRAD EK60 seabed volume-backscattering strength (Sv) was submitted to a principal component analysis (PCA), before and after removal of a depth trend, and the scores on the first 10 principal components were classed by a K-means cluster analysis. The same seabed Sv data were submitted to stepwise discriminant analysis whose training data sets were defined with the ground-truth photographs using different groupings: biotope types, community types, and finally scallop-density classes. All the QTC AGDS approaches failed to reveal the scallop bed, community structures, or biotopes. The QTC classifications mimicked the bathymetry with a strong correlation of the acoustic classes with depth. The seabed Sv PCA + K-means approach presented similar depth-dependence, but, the PCA + K-means on the Sv residuals revealed the scallop bed. The discriminant analysis was the best solution for the scallop density with a general classification success rate of 75% and up to 91% for the highest density class. The Sv signature of the scallop bed is presented, and the most discriminant part of the acoustic signal is identified.

Mapping seabed habitats in the Firth of Lorn off the west coast of Scotland: evaluation and comparison of habitat maps produced using the acoustic ground-discrimination system, RoxAnn, and sidescan sonar

Abstract In September 2003 a national workshop took place in the United Kingdom with the aim of assessing the accuracy of the acoustic ground-discrimination system (AGDS), RoxAnn™, as a tool for mapping seabed habitats in Special Areas of Conservation (SACs). A heterogeneous area of seabed, approximately 1 km2 in size, was selected for the study. The area was first surveyed using a sidescan-sonar system and a mosaic of the output was produced covering 100% of the survey area. Interpretation of the mosaic identified three acoustically distinct seabed types, the spatial distributions of which were mapped. Four RoxAnn data sets were then collected over the same area of seabed applying different survey parameters (e.g. different survey grids, track spacing, survey vessels, survey speeds, and RoxAnn systems). Extensive ground-truthing was carried out involving 26 drop-down video stations, and from these data six benthic classes (seabed habitats) were identified. Following interpolation of the RoxAnn track-point data to produce full spatial-coverage data, these six-seabed habitat categories were used to conduct supervised classification of the RoxAnn data to produce full-coverage habitat maps of the area for each of the four RoxAnn data sets. Comparisons were then made between the four RoxAnn maps and the sidescan-sonar interpreted map. The accuracy of each map was assessed and the application of this mapping approach for mapping seabed habitats in SACs is discussed.