Marine: Bottom Classification |
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Scientific hydroacoustic systems and specialized software provide researchers and environmental contractors with a cost effective method of seabed classification. A transducer is directed toward the seabed either from the surface during a mobile survey in shallow environments or from a deep tow body or ROV in deep-water applications. The acoustic signal that returns from the seabed is analyzed and can be translated into seabed type. This method of bottom typing is far more cost effective than obtaining grab samples or deploying video equipment and is completely unobtrusive.
By relating seabed type with the presence of certain fish species, scientists use hydroacoustics as a crucial tool to help delineate critical habitats. Environmental and Dredging Contractors commonly use seabed classification techniques along with bathymetric studies to map out near shore environments to assess and monitor sediment deposition.
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Case Studies
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Acoustical classification of the bottom sediments in the southern Baltic Sea |
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The basic relationship is described between the parameters of backscattered acoustic signal and the seabed type in the Polish Economic Zone of the Baltic Sea. The properties of the bottom backscattered signal were investigated using a single frequency narrow beam echosounder working at a frequency of 30 kHz. For approximately 15,000 averaged echo pulses acquired in the survey area, calculations of integral backscattering strength, spectral width and Hausdorf fractal dimension were performed and the correlation of the results with bottom characteristic features were evaluated. The chosen set of echo parameters is a good descriptor of the sea bottom sediments and may be used for classification procedures. Having applied cluster analysis to the data sets of averaged echo envelopes, it was possible to classify them into four groups. The groups corresponding to the four cluster centers were assigned to a given type of sediment. The surficial sediment types most often occurring in the southern Baltic are: muddy deposits (deeper part of the study area), clay, fine and medium grained sand, coarse sand or gravel. |
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Acoustical classification of the bottom sediments in the southern Baltic Sea
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Bottom Substrata Classification in Lake Washington, Seattle |
Recent developments in classifying bottom substrates and quantifying underwater vegetation make hydroacoustics an effective tool for monitoring and mapping habitat parameters in aquatic ecosystems. This study incorporated hydroacoustic sampling for bathymetry, substrate type, underwater vegetation, and fish distribution in Lake Washington, Seattle, USA. Data were collected using three independent echo sounder systems, to maximize data accuracy and vessel use, and geo-referenced using a Differential Global Positioning System enabling the acoustic data to be used in a Geographic Information System. Results indicated that four different bottom substrates were present in the sections of the lake tested. This method proved to be an effective way to study various habitat influences on fish distribution, as well as map and monitor important physical and seasonal habitat parameters such as bathymetry, bottom character, and aquatic vegetation distribution. Researchers collected data on bathymetry and substrate type using a BioSonics DE 70 kHz, 6º single-beam echo sounder set to 0.4 ms pulse-width, 5 pings s-1, and a threshold of 70 dB. Two recently developed software packages, BioSonics EcoSAV® and VBT Seabed Classifier, were used for data analysis. The VBT Seabed Classifier software is designed for use with data collected from a single beam digital echosounder. VBT can process and analyze data files of the digitized echo envelope and classify bottom types by comparing an unknown sample to a verified reference sample (e.g. gravel). VBT Seabed Classifier was used to determine bottom type, using the Fractal Dimension method. After completion of the bathymetric analysis and mapping, regions of potentially different bottom substrate were identified based upon assumptions regarding bathymetrically related substrate distribution. Survey transects were selected and then examined using VBT without any pre-determined substrate categories. Areas of similar depth, location, and closely clustered acoustic characteristics (energy and fractal dimension of the first bottom echo) were assigned classification identifications. This method approximates the concept of unsupervised classification, where areas producing similar measurements are classified as a bottom type, without assigning specific physical characteristics to the class. Examination of several regions resulted in the selection of four common sediment types, which were observed in regions where submersed aquatic vegetation was absent. Overall, the study demonstrated that the various echo sounder systems could be combined to yield a wealth of data on aquatic habitat and fish distribution. Integrating the data in GIS was an effective way to evaluate fish distribution in regards to bathymetric features, substrate types, and patches of aquatic vegetation. |
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Document Links:
Digital Acoustic System to Monitor & Map Ecosystem
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Product Link:
VBT-Bottom Classifier
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Comparison of Hydroacoustic Measurement Techniques in Dense Aquatic Vegetation |
The basis for acoustical bathymetric surveys is detecting and timing the echo from a short, vertically oriented pulse. The exact detection process may vary from system to system but is usually based on exceedence of some minimum threshold intensity and peak width. For bathymetric surveys of navigation channels, this approach usually works well. A typical navigation channel consists of open water above a distinct sediment interface, leading to no ambiguity in relating the time of the echoed pulse to the exact depth of the sediment interface. A decided exception to this occurs when the bottom is colonized with submersed aquatic vegetation.
Under these conditions, the acoustical reflectivity of the gas-filled plant stems or blades generates an echo that arrives at the receiver before the true bottom echo. Depending on plant type, height, and density, these plant-generated returns may pass the test for the detected bottom and be declared as the bottom, underestimating the true depth. If undetected, this condition can lead to erroneous surveys of channel depth and overestimates of dredging quantities required to keep the channel at its authorized depth. While this occurs in only a small percentage of the channels maintained by the U.S. Army Corps of Engineers, it is sufficiently common in certain regions to represent a major operational problem. A common offending plant species is
Zostera marina (eelgrass), which occurs in cool, clear, shallow saltwater locations along much of the northeastern and Pacific coastline of the United States. Approximately 60 small boat harbors within the Corps New England District have eelgrass established within the project bounds. Hydrographic surveying within these areas requires extra field work to properly identify the true bottom. Additional data processing and field checking are necessary to verify the existence of the eelgrass and to ascertain that the bottom has been successfully tracked. This simply causes extra work at locations which have a known history of eelgrass. The major concern occurs at locations where eelgrass presence is not suspected. Here, eelgrass presence may go undetected and can cause both an environmental problem and errors in estimated dredging quantities. During the summer of 1998, a bathymetric condition survey in an eelgrass-infested channel (Wood Island Harbor) was conducted simultaneously using two very different hydroacoustic depth measurement systems. The first was an Odom EchoTrac 3200 MKII (Odom Hydrographic, Baton Rouge, LA) with a 200-kHz, 8-deg transducer, a widely used hydrographic system. The second system was the Submersed Aquatic Vegetation Early Warning System (SAVEWS), which uses the Biosonics DT Series single beam digital echosounder (Biosonics Inc., Seattle, WA) with a 420-kHz, 6-deg transducer. SAVEWS (Sabol and Burczinski 1998) is specifically designed to detect submersed vegetation and measure canopy density and height. Analyses of the resulting data showed good agreement between depth estimates from the two systems in unvegetated areas but increasing disagreement as eelgrass density increased. This disagreement was thought to be the result of primarily the differing signal processing approaches used. A short exploratory study was conducted of alternative processing approaches using a sampling of the digital DT Series Echosounder data. Each of these aspects is discussed in the full report (see document link below) and evidence is presented that improved bottom tracking within vegetated areas can be achieved using existing sensor hardware with a modified signal processing approach.
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Document Links:
Methods for Bottom Tracking in Aquatic Vegetation
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Product Link:
EcoSAV
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Coral Remote Sensing Workshop - Proceedings and Recommendations |
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Coral reefs exist in warm, clear, and relatively shallow marine waters worldwide. These complex assemblages of marine organisms are unique, in that they support highly diverse, luxuriant, and essentially self-sustaining ecosystems in otherwise nutrient-poor and unproductive waters. Coral reefs are highly valued for their great beauty and for their contribution to marine productivity. Coral reefs are favorite destinations for recreational diving and snorkeling, as well as commercial and recreational fishing activities. The Florida Keys reef tract draws an estimated 2 million tourists each year, contributing nearly $800 million to the economy. However, these reef systems represent a very delicate ecological balance, and can be easily damaged and degraded by direct or indirect human contact. Indirect impacts from human activity occurs in a number of different forms, including runoff of sediments, nutrients, and other pollutants associated with forest harvesting, agricultural practices, urbanization, coastal construction, and industrial activities. Direct impacts occur through overfishing and other destructive fishing practices, mining of corals, and overuse of many reef areas, including damage from souvenir collection, boat anchoring, and diver contact. |
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Document Links:
Coral Remote Sensing Workshop - Proceedings and Recommendations
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Product Link:
DT-X
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New applications of hydroacoustic methods for monitoring shallow water aquatic ecosystems: the case of mussel culture grounds |
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The conflicts engendered by the multiple uses of coastal
ecosystems (fisheries, tourism, aquaculture, etc.) are becoming
a major challenge for the environmental friendly development
and exploitation of these areas. Since 1988, the
development and management of several artificial reefs (Lacroix
et al., 2002) and mussel culture fields in open sea along
the French Mediterranean coastline have become a major
economical activity (Loste and Cazin, 1993). First experimented
in 1976 near Sète (the most important fisheries harbour
along the French Mediterranean coastline), these developments
have induced several changes in the ecosystem.
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Document Links:
New applications of hydroacoustic methods for monitoring shallow water aquatic ecosystems: the case of mussel culture grounds
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Product Link:
DT-X
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SONAR BACKSCATTER DIFFERENTIATION OF DOMINANT MACROHABITAT TYPES IN A HYDROTHERMAL VENT FIELD |
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Over the past 20 years, sonar remote sensing has opened ways of acquiring new spatial information on seafloor habitat and ecosystem properties. While some researchers are presently working to improve sonar methods so that broad-scale high-definition surveys can be effectively conducted for management purposes, others are trying to use these surveying techniques in more local areas. Because ecosystem management is scale-dependent, there is a need to acquire spatiotemporal knowledge over various scales to bridge the gap between already-acquired point-source data and information available at broader scales. Using a 675-kHz single-pencil-beam sonar mounted on the remotely operated vehicle ROPOS, 2200 m deep on the Juan de Fuca Ridge, East Pacific Rise, five dominant habitat types located in a hydrothermal vent field were identified and characterized by their sonar signatures. The data, collected at different altitudes from 1 to 10 m above the seafloor, were depth-normalized. We compared three ways of handling the echoes embedded in the backscatters to detect and differentiate the five habitat types; we examined the influence of footprint size on the discrimination capacity of the three methods; and we identified key variables, derived from echoes that characterize each habitat type. The first method used a set of variables describing echo shapes, and the second method used as variables the power intensity values found within the echoes, whereas the last method combined all these variables. Canonical discriminant analysis was used to discriminate among the five habitat types using the three methods. The discriminant models were constructed using 70% of the data while the remaining 30% were used for validation. The results showed that footprints 2030 cm in diameter included a sufficient amount of spatial variation to make the sonar signatures sensitive to the habitat types, producing on average 82% correct classification. Smaller footprints produced lower percentages of correct classification; instead of the habitat types, the sonar data responded to intrapatch roughness and hardness characteristics. The sonar variables used in this study and the methods for extracting and transforming them are fully described in this paper and available in the public domain. |
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Document Links:
SONAR BACKSCATTER DIFFERENTIATION OF DOMINANT MACROHABITAT TYPES IN A HYDROTHERMAL VENT FIELD
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Product Link:
DT-X
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Star - 2000 Cruise |
Winners of the Alaska Ocean Sciences Bowl joined University of Alaska Fairbanks students, technicians, faculty, guests from the Alaska SeaLife Center and the Alaska Department of Fish and Game, and Chancellor Marshall Lind and his wife for a 24-hour cruise aboard the R/V Alpha Helix.
The students were participating in the anual spring semester course Shipboard Techniques (MSL 625) which is designed to give them at-sea experience aboard the R/V Alpha Helix. In addition to lectures and demonstrations at the Alaska SeaLife Center in Seward, the course included a cruise to Aialik Bay, a tidewater glacier fjord.
According to Dr. John Kelley, the purpose of the cruise is to familiarize students with a variety of oceanographic sampling techniques. This year a new portable BioSonics, Inc. sonar was also tested prior to its actual deployment in the Bering Sea by Dr. Michaela Dommisse.
Dr. Kelley presented a lecture on Bioacoustics at the Alaska SeaLife Center. After the lecture, the students and guests toured the laboratories and service areas of the center.
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Document Links:
Star - 2000 Cruise
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Product Link:
DT-X
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Tidally Generated Turbulence over the Knight Inlet Sill |
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Very high turbulent dissipation rates (above = 10−4 W kg−1) were observed in the nonlinear internal lee waves that form each tide over a sill in Knight Inlet, British Columbia. This turbulence was due to both shear instabilities and the jumplike adjustment of the wave to background flow conditions. Away from the sill, turbulent dissipation was significantly lower ( = 10−7 to = 10−8 W kg−1). Energy removed from the barotropic tide was estimated using a pair of tide gauges; a peak of 20 MW occurred during spring tide. Approximately two-thirds of the barotropic energy loss radiated away as internal waves, while the remaining one-third was lost to processes near the sill. Observed dissipation in the water column does not account for the near-sill losses, but energy lost to vortex shedding and near-bottom turbulence, though not measured, could be large enough to close the energy budget. |
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Document Links:
Tidally Generated Turbulence over the Knight Inlet Sill
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Product Link:
DT-X
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Mapping of bathymetry and bottom characteristics at the WWF-Miramare Marine Reserve (Gulf of Trieste, Italy) was achieved using a combination of a BioSonics digital echosounder system and GRASS GIS software.
