Multibeam backscatter classification of seafloor properties : Examples using response on e.g. deep-water coral reefs

Bathymetrical mapping performed using multibeam sonar systems is widely used in marine science and for habitat mapping. The incoherent part of the multibeam data, the backscatter data, is less commonly used. Automatic classification of processed backscatter has a correlates well with three sediment classes, defined as fine-(clay-silt), medium- (sand) and coarse- (gravel–till) grained substrates. This relation is used directly as a theme in a modified habitat classification scheme, while a more detailed substrate classification is incorporated as another theme. This theme requires a manual interpretation and comprehensive knowledge of the substrate. This can partly be obtained by a newly developed technique using the backscatter strength plotted against the grazing angle. These plots make it possible to determine the critical angle and thereby calculate the compressional acoustic speed in seabed sediments. Marching a theoretical modeled backscatter curve to the measured backscatter strength at lower grazing angles provides estimates of four additional geoacoustic parameters.