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Finding More Than FishContributors: John King, GSO; Jon Boothroyd and Brian Oakley, URI Geosciences Many fishermen consider their fish-finders to be an invaluable tool. These instruments use sonar to detect the seafloor and schools of fish in the water column. The side-scan sonar technology used by MapCoast is a high-end version of a fish-finder that also uses sound waves to collect an acoustic image of a lagoon or bay floor to determine benthic (bottom) type. Because the acoustic data are collected simultaneously off each side of the survey vessel (hence the name “side-scan”), the boat can drive back and forth over the area—much like mowing a lawn—to get a full acoustic picture of the ocean bottom. Benthic habitats have features that make them acoustically distinct. A benthic habitat will respond to sound waves differently depending on surface hardness and complexity, and these responses, or backscatter intensities, can be used to identify habitat boundaries. For example, a rocky bottom will have high backscatter intensity, and a muddy bottom will have very low backscatter intensity. Organisms, such as eelgrass or oyster beds, living on the Bay bottom can also affect the backscatter intensity. Habitat restoration specialists can use these acoustic maps to guide their restoration efforts. In order to determine if the acoustically distinct areas really are different habitats, it is important to ground-truth the acoustic map. Ground-truthing is an essential step in creating benthic habitat maps using remotely sensed data. Factors like water depth, bottom slope, and vegetation can change the acoustic signature from place to place. Underwater imagery techniques such as video and sediment profile imagery (see page 6) along with traditional grab samples are used to interpret the acoustic signals and get a closer look at the bottom of the Bay. Because of the excellent spatial coverage generated by side-scan sonar, the acoustic data act as a guide, and ground-truthing surveys can maximize our understanding of different habitat types. Just as aerial photography is an essential source of information when mapping terrestrial soils, side-scan sonar is equally important for mapping ocean bottom conditions. The imagery collected by the MapCoast research team is the basis for geological mapping of the seafloor, benthic habitat mapping, and subaqueous soil delineation. |
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Island Sea Grant |
Coastal Institute |