Acoustic classification of a shell-covered seafloor

Abstract

Acoustic scattering by the seafloor is sometimes influenced, if not dominated, by the presence of discrete volumetric objects such as shells. Classification of a shell-covered seafloor requires understanding of the acoustic scattering properties of the shells. A series of measurements of target strength of a type of benthic shelled animal and associated scattering modeling have recently been completed. In addition, a simple formula has been derived that expresses the area scattering strength of the seafloor in terms of the reduced target strength (RTS) of the discrete scatterers and their packing factor (where RTS is the target strength normalized by the size of the scatterers). The formula shows that, to first order, the backscattering at high acoustic frequencies by a dense layer of shells (or other discrete bodies such as rocks) depends principally upon material properties of the objects and packing factor, and is independent of size and acoustic frequency. There are reasonable comparisons between scattering data from shell-covered sections of seafloor and predictions using this formula and the measurements/modeling of shell target strength. The functional dependence of this and other seafloor scattering formulas upon shell and shell-bed characteristics are discussed to assess one’s ability to classify a shell-covered seafloor. [Work supported by ONR.]