Seabed Classification System Research Articles

Characteristic properties of seabed fluvial-marine sediments in front of Damietta promontory, Nile Delta, Egypt

Acoustic bottom classification utilities have been developed in favor of routine investigation of the seabed texture and grain size, as well as type of sediments. Single beam echosounder, seabed classification systems and QTC VIEW series V were used to map out and identify sediment gradients offshore Nile Delta in front of Damietta promontory. The survey area has an asymmetric shape and of very gentle slope parallel to the shore line at Damietta promontory. Ground truth observation analyses were carried out for 24 grab samples using the classification technique. The acoustic survey data were analyzed with the QTC IMPACT software and classified into five acoustic classes namely (silty clay, clayey silt, silt, sandy silt and very fine sand). The fine sediments (clay and silt) cover the front area of Damietta outlet, forming a stream like shape cutting the center of the area of study, which composed mainly of fluvial sediments. While the sandy sediments, appear as small patches to the east and west. The dominant of the small mean size (fine fractions) is corresponding to the fluvial depositions that form the Nile Delta fan of mainly mud sediments. Actually, since the building of Aswan High Dam in 1964, sediment discharge at the Nile promontories has diminished to almost zero, the recent situation of seabed characteristics is related mainly to the effect of the oceanographic circulation regime dominating the study area.

The performance, application and integration of various seabed classification systems suitable for mapping Posidonia oceanica (L.) Delile meadows

In the context of current global environmental changes, mapping and monitoring seagrass meadows have become highly important for management and preservation of coastal zone ecosystems. The purpose of this research was to determine the numerical precision of various cost-effective benthic habitat mapping techniques and their suitability for mapping and monitoring of Posidonia oceanica meadows in the Croatian Adriatic. We selected ultra-high resolution aerial imagery, single-beam echo sounder (SBES) seabed classification system from Quester Tangent Co. (QTC), and surface based underwater videography as affordable, non-destructive and simple to use systems for data acquisition. The ultra-high resolution digital imagery was capable of detecting P. oceanica meadows up to 4m depth with 94% accuracy, from 4m to 12.5m depth the accuracy dropped to app. 76%, and from 12.5 to 20m the system was only capable of distinguishing seabed biota from substrata, though with 97% accuracy. The results of the QTC system showed over 90% detection accuracy for Cymodocea nodosa covered seabed, excellent separation capabilities (>92%) of different sediment types (slightly gravelly sand, gravelly muddy sand and slightly gravelly muddy sand) and reasonable accuracy for mapping underwater vegetation regardless of the bathymetric span. The system proved incapable of separating P. oceanica from dense macroalgae on the same type of substratum. Surface-based underwater videography demonstrated great potential for estimating P. oceanica cover in a sampled region using either a single human rater or a computer estimate. The consistency between two human scorers in evaluating P. oceanica bottom coverage was near perfect (>98%) and high between digital and human scorers (80%). The results indicate that although the selected systems are suitable for mapping seagrasses, they all display limitations in either detection accuracy or spatial coverage, which leads to a conclusion that suitable system integration is essential for producing high quality seagrass spatial distribution maps.

Geomorphic Characterization of Reef Fish Aggregation Sites in the Upper Florida Keys, USA, Using Single-Beam Acoustics*

The objective of this study was to determine if multiple reef fish aggregation sites in the upper Florida Keys share characteristic geomorphic features. A commercial single-beam acoustic seabed classification system was used to map the seabed in the vicinity of reported or observed black grouper (Mycteroperca bonaci), mutton snapper (Lutjanus analis), and yellowtail snapper (L. chrysurus) aggregation sites. Results showed that drowned, margin-parallel, rocky ridges, known locally as outlier reefs, were features found in proximity to all of the mapped aggregation sites. In particular, three geomorphic characteristics were observed at most sites: a steep slope of the landward boundary of the upper slope terrace, an exposed outlier reef forming the seaward boundary of the upper slope terrace, and at least one other exposed outlier reef on the upper slope terrace. The results suggest that the acoustic-mapping technologies used herein could be valuable when considering the placement of marine reserves.

Assessment of Recent Habitat Conditions of Eastern Oyster Crassostrea virginica Bars in Mesohaline Chesapeake Bay

AbstractEastern oysters Crassostrea virginica in Chesapeake Bay, USA, have been subjected to intense harvest pressure since about 1850 and to disease epizootics since the 1950s. Combined, these processes have degraded the once extensive eastern oyster bottom; the remaining oyster shells, which are required as substrate for successful larval oyster settlement, are thus rendered increasingly susceptible to siltation. Between 1999 and 2001, we used an acoustic seabed classification system and underwater videography to assess oyster habitat conditions throughout Maryland's portion of Chesapeake Bay relative to eastern oyster recruitment and habitat restoration activities. We performed 16 surveys that covered a total of 39 km2 of bottom that were classified in 1911 as supporting productive oyster populations. Over 90% of this area has degraded from productive oyster bottom to mud, sand, or heavily sedimented oyster shell. Seventy percent of the locations we identified as containing unsedimented shell came from within the approximately 90 km2 of oyster bottom that had been subject to the management activity of spreading large volumes of shell at some time between 1960 and 1999. However, video observations of 49 of these locations that had been replenished with shell at various times indicated that any improvements in oyster habitat were short lived, as interstitial spaces in spread shell became highly sedimented after an average period of 5.5 years (n = 49; SE = 1.03). We conclude that the majority of oyster bottom in Maryland is extremely degraded and that no reasonable increase in the scale of present management practices will reverse this habitat decline.

Acoustic seabed classification in a coastal environment (outer Weser Estuary, German Bight)—a new approach to monitor dredging and dredge spoil disposal

Acoustic seabed classification provides a sophisticated technique to discriminate seabed characteristics and to map their distribution at high spatial and temporal resolution. In the present study, the seabed classification system QTC View/Impact was applied to investigate a 9- km 2 area at water depths between 6 and 20 m in the outer Weser Estuary (German Bight, southeastern North Sea). The survey area comprises parts of a routinely dredged shipping channel as well as of a dredge spoil disposal site. The acoustic data, collected by means of a singlebeam 200-kHz echosounder combined with a QTC system, were classified into three acoustic classes. These classes are identified with (1) fine to medium sand, with a low content of shell fragments; (2) medium sand, with a moderate content of shell fragments; and (3) medium to coarse sand, with a high content of shell fragments. By comparing this point-related acoustic classification with sidescan sonographs, it can be convincingly demonstrated that the acoustic data obtained by the QTC system reflect more than simply sediment type distribution, but that rather the occurrence of bedforms also appears to be important. Furthermore, in a time series of three QTC surveys in a period of 14 months, it is apparent that dredging as well as the disposal of dredge spoil constitute a strong impact on the sediment distribution and seabed morphology of the study site.

RoxAnn bottom classification system, sidescan sonar and video-sledge: spatial resolution and their use in assessing trawling impacts

Abstract Three complementary seabed characterization tools with different spatial resolution were used to locate a research site and to assess physical effects of experimental otter trawling in the Barents Sea: an acoustic seabed classification system (RoxAnn), sidescan sonar and a video-sledge. The marine protected area (MPA) around Bear Island was chosen as it offered unfished reference sites. The area was topographically complex which resulted in certain challenges for choice of the experimental site due to the requirements of representativity and homogeneity and suitable sampling substrate. Systematic waylines with RoxAnn gave broad-scale patterns of bottom conditions, the more informative sidescan revealed topographic reliefs, whilst detailed information on sediment composition and small-scale seabed features was provided by the video-sledge. Accurate positioning of towed gears (trawl, sidescan and video-sledge) ensured unbiased data acquisition. Trawl doors and rockhopper gear created furrows that were visible by sidescan sonar and video. Intensive trawling also caused changes in the acoustic properties by increasing roughness and decreasing hardness. Results are consistent with a possible resuspension of the sediment and a homogenizing effect from the trawl doors and ground gear ploughing the area. The suitability and advantages of using spatially overlapping tools in trawl impact studies are discussed.

Benthic biotopes remote sensing using acoustics

The present work applies novel methodologies to the study of sublittoral benthic biotopes, by combining the information given by underwater acoustic and biological survey techniques. The acoustic seabed classification system QTC VIEW™ was used to map the acoustic diversity between 5 and 40 m water depth on the shelf off “Ria de Aveiro”, Western coast of Portugal. Ground-truth was undertaken using an analysis of superficial sediments grain-size, and compared to the species composition and distribution of macrofaunal communities. Sedimentary and biological data were submitted to ordination analysis, and the acoustic data to both ordination and cluster analysis. The acoustic classes identified were mapped using a geographical information system. The acoustic results showed a very clear geographic pattern, with the acoustic classification being coincident where survey lines crossed, confirming the stability of the classification procedure. At the optimal splitting level, three acoustic classes were obtained. These classes were correlated to differences in coarse, fine and very fine sands. Additional real bottom differences in the grain-size of the coarser sand classes were not detected by the acoustic method, possibly due to the fact that they showed a similar degree of compactness. The benthic ecological data suggests only two main communities, which correspond to the outcomes of a two-class acoustic split. Therefore, a mismatch was noticed between the optimal acoustic split and the number of major biological communities present. However, by dropping the acoustic solution down to two classes, an optimal relationship to benthic communities is achieved. Overall, results suggest that the acoustic system provides very valuable and important data for mapping soft sediment biotopes, even in areas of relative bottom monotony such as the one analysed, but careful ground-truth is required to ensure that the acoustic class splits are biologically relevant.

Acoustic seabed classification of marine habitats: studies in the western coastal-shelf area of Portugal

Abstract Two single-beam, seabed-classification systems, QTC VIEW Series IV and QTC VIEW Series V, were used to identify and map biosedimentary gradients in a mid-shelf area off Western Portugal. The survey area has a moderate slope, a depth ranging from 30 to 90 m along a 3.5-km axis perpendicular to the shoreline, and is characterized by smooth sedimentary and biological gradients. Ground truth for sediment grain size and macrofaunal communities was based on grab sampling at 20 sites. The sedimentary and biological data were analysed using classification and ordination techniques. The acoustic data were analysed with qtc impact software and classified into acoustic classes. The affinity groups obtained in each data set were mapped using a Geographics Information System. All showed good agreement and identified prevailing gradients along a northwest–southeast direction. Three acoustic classes were identified, corresponding to the predominant sediment types, namely fine sand with low silt and clay content, silty, very fine sand, and mud. A close relationship with benthic communities was also verified, although less marked because benthic communities continuously change along the northwest–southeast gradient. Overall, the acoustic system coupled with ground-truthing data was able to discriminate and characterize the various benthic biotopes in the survey area.

Acoustic classification of seabed habitats using the QTC VIEW™ system

Ellingsen, K. E., Gray, J. S., and Bjornbom, E. 2002. Acoustic classification of seabedhabitats using the QTC VIEW system. – ICES Journal of Marine Science, 59:825–835.The QTC VIEW acoustic seabed classification system was used in Fraenfjorden,western Norway (62 N, 7 E), to map the spatial scales and variability of benthichabitats, and to evaluate its performance. Position and water depth were loggedcontinuously along with the acoustic data. Water depth in the survey area ranged from5 to 72 m over a spatial scale of ca. 1.5 8 km and there was considerable variation insediment characteristics at 20 ground-truth sites. Six acoustically-distinct seabed typeswere identified and the QTC system defined their spatial scales and variability.Generally, the acoustic classifications were consistent at track crossings. The classifi-cation was generally in accordance with the sediment grain size and was most likelyinfluenced by the softness of the substrate. However, these sediment characteristicscould not alone explain the diversity of acoustic classes and the classes were probablyalso related to other physical or biotic seabed characteristics. The QTC systemprovides new insights but the usefulness of the acoustic classification depends upon theamount and quality of ground-truth data, which is the key aspect when relatingacoustic class to seabed type. The acoustic system collects continuous data from largeareas relatively quickly, and defines seabeds that would be missed by traditional coreor grab point samples and will therefore be highly relevant to both geological- andbenthic-diversity studies.

The effect of slope and vessel speed on the performance of a single beam acoustic seabed classification system

Although the emergence of an acoustic single beam seabed classification system appears promising as a cost-effective tool to acquire information about bottom types, some limitations of this technology with respect to vessel speed and rough terrain have been identified. To further refine this technology, we examined the QTC View system to determine if it would work in an area such as the Gulf of Alaska, which is characterized by deep water, steep slopes, heterogeneous substrate. Studies were undertaken to evaluate the operational limits with respect to vessel speed and bottom slope. Results indicate that speeds between 3 and 12 kn have no significant effect on classification performance, but bottom slopes exceeding approximately 5–8° appear to cause a complete breakdown in classification accuracy. The potential of trawl-mounted sonar systems, currently under evaluation, as a potential solution to expanded range of operation over bottom slopes is discussed.

Acoustic classification of marine habitats in coastal Newfoundland

A digital acoustic seabed classification system, QTC View (Series IV) was used in the coastal waters of Newfoundland to characterize and classify marine benthic habitats. The QTC View system was calibrated in Placentia Bay at sites identified independently during a submersible research program. Four different habitats were used for calibration of the QTC View system: mud, gravel, rock, and macroalgae on rock. These different habitats were used as a ‘‘training’’ catalogue for real-time classification of marine habitats carried out in Bonavista Bay. The classification data were based on over 2000 km of survey tracks ranging in depth from approximately 10-m to 220-m depth. Post classification analyses were carried out using data visualization techniques, simultaneously comparing the classification data in mathematical and geographic settings. Following post classification, eight different marine habitats were identified using the acoustic system: mud, loose gravel, gravel, rock, sparse algae/cobble, macroalgae, high relief/deep cobble, and wood chips. Throughout the surveyed area, rock habitat dominated, followed by sparse algae/cobble and high relief/cobble habitat types. The wood chip habitat type was identified within a small area that historically had been associated with logging in coastal Newfoundland.

The effect of slope and vessel speed on the performance of a single beam acoustic seabed classification system

Although the emergence of an acoustic single beam seabed classification system appears promising as a cost-effective tool to acquire information about bottom types, some limitations of this technology with respect to vessel speed and rough terrain have been identified. To further refine this technology, we examined the QTC View system to determine if it would work in an area such as the Gulf of Alaska, which is characterized by deep water, steep slopes, heterogeneous substrate. Studies were undertaken to evaluate the operational limits with respect to vessel speed and bottom slope. Results indicate that speeds between 3 and 12 kn have no significant effect on classification performance, but bottom slopes exceeding approximately 5–8° appear to cause a complete breakdown in classification accuracy. The potential of trawl-mounted sonar systems, currently under evaluation, as a potential solution to expanded range of operation over bottom slopes is discussed.

Relationships between herring school distribution and seabed substrate derived from RoxAnn

A series of three fine-spatial-scale acoustic studies was carried out to study the relationships between herring-school distribution and the seabed substrate. The study was carried out on three separate bank areas east of Shetland in July 1993. The study areas were characterised respectively as: mainly mud with some hard ground; mixed mud, sand and hard ground; and mostly hard ground. Herring schools were identified from the echo-sounder record. The substrate was mapped using the RoxAnn seabed classification system interfaced to the same echo-sounder. The relationship between herring distribution and substrate was examined at two different levels; map-based, using contour plots of the important variables, and track-based, using individual herring schools in relation to the specific substrate type found on the survey track under the schools. Data are presented which show a strong tendency for schools to be found preferentially over hard seabeds, particularly in the track-based analysis. There also appears to be a strong relationship between herring aggregations and particular topographic features. This is discussed in relation to specific features, a low ridge and two escarpments, identified in two of the study areas.

Remote Acoustic Habitat Assessment Techniques Used to Characterize the Quality and Extent of Oyster Bottom in the Chesapeake Bay

Underwater acoustic technologies may be used for remote sensing of estuarine and coastal benthic habitat in addition to more traditional methods of assessing the seabed. These systems use reflected sound energy to identify surface objects, texture, and subbottom density discontinuities, and to classify benthic habitat. We evaluated the ability of subbottom profiling systems, side scan sonar, and acoustic seabed classification systems (ASCS) to assess oyster habitat in the Chesapeake Bay. Our criteria were the systems' abilities to assess the quality and quantity of oyster shell resources and to integrate with Geographical Information Systems (GIS) technology for the charting of the seabed. Although all systems examined have previously been used for benthic habitat assessment, we found that ASCS offered the most promise as a stand alone system for mapping complex and heterogeneous habitat typical of oyster bottoms. The results from this study suggest that ASCS technology is highly suited for the identification and charting of oyster shell as well as distinguishing among different combinations of shell and fine sediments. Such systems also offer excellent linkage with GIS display and analysis capability.

Non-Energy Resources, Connecticut Coastal Waters, Year 9 and 10 Activities

The Connecticut effort in year 9 of the Continental Margins Program concentrated on a 13.6-mi2 area south of the Housatonic River. This area was chosen for a side-scan sonar survey because it lies on and adjacent to the bathymetric expressions of two fairly large, subcropping, potential sources of coarse material. Previous seismic work in the area indicated that outcrops or subcrops of these potential source deposits could be delineated using their bathymetric expression. Owing to the limited resolution of the seismic data, a correlation between bottom type and underlying source deposits could not be made with the seismics alone. Results from the November 1993 side-scan survey show that although the source deposits have discernible bathymetric expressions, they are not cropping out as much as expected. As a result, bottom type is not necessarily determined by subcropping deposits in this particular area. Year 10 work was concentrated in Fishers Island Sound, where three areas of potential interest for near-shore gravel resources had previously been identified. These areas were surveyed, during the spring of 1996, using the RoxAnn Seabed Classification System. A small video camera and a Van Veen grab sampler were used to calibrate and verify the RoxAnn data. Although previous sampling had indicated the presence of gravel or gravelly sediment in all of the survey areas, the RoxAnn results showed less gravel than anticipated. Vibrant eelgrass beds and other habitat indicators were detected in all of the survey areas. Given the variable sediment results, and the high habitat potential of the areas surveyed, the likelihood of developing a sand and gravel supply from the near shore of Fishers Island Sound appears quite low.

Broad-scale benthic community analysis in the Greater Minch Area (Scottish west coast) using remote and nondestructive techniques

A number of surveys were carried out in the Greater Minch Area to investigate the variety and distribution of benthic habitats and their resident faunal communities. RoxAnnTM, an acoustic seabed classification system, was used to map the different substrata whilst the benthic communities were investigated using both photographic and underwater television techniques. A variety of substrata were identified ranging from bedrock and boulders to fine silty mud. Statistical analysis of the faunal data revealed the presence of four distinct benthic communities which could be related to the substratum type. The application of acoustic and underwater television techniques to broad-scale mapping of benthic communities is discussed.