ACOUSTIC BACKSCATTER DATA AS AN INDEX OF SEAFLOOR CLASSIFICATION

Multibeam Echosounders were created to enhance the capabilities of coastal hydrographic surveying platforms. Recently, the scientific community has realized alternate applications for multibeam data, particularly with respect to marine biology. The data can give geographic extents and vertical relief of seafloor features to within sub-centimetric resolution. Most multibeam echosounder systems record intensity values that are associated with the acoustic return, and these intensity values are known as backscatter. Within a dataset, a given seafloor area with a higher backscatter is deduced to be harder substrate than an area with a lower backscatter. Backscatter is only useful as a relative measurement, so that separate datasets cannot be compared using backscatter values. For instance, if a rock outcrop near Charleston, in 10 m water depth gives an average backscatter of 150 db, a similar feature elsewhere, in comparable conditions may give a considerably different value. Also, if a survey were repeated over the same outcrop, but the data were acquired in a direction orthogonal to the original track lines, the backscatter may vary. This poster will describe backscatter data acquired with a Kongsberg-Simrad EM1002 multibeam echosounder from two distinctly different survey areas off the coast of Charleston, SC. Using bottom sediment samples, ROV video, and other available ground truthing data from within the survey areas, we will show how backscatter can be used as a relative measuring tool for seafloor classification, as well as its shortcomings as an absolute measurement of seafloor composition.