A MODEL DATA COMPARISON FOR LOW-FREQUENCY BOTTOM BACKSCATTER

This article describes a model for bottom backscattering strength appl icable over the frequency range 100-1000 Hz. The model includes backsc atter due to both a rough water/sediment interface and internal sedime nt inhomogeneities-i.e., sediment-volume scattering within an upward-r efracting sediment. The net result is predictions for bottom backscatt ering strength that depend on the following: frequency, sound speed of the water near the water/sediment interface, density ratio, sound abs orption within the sediment, near-surface sound-speed gradient within the sediment, two interfacial roughness parameters, and a parameter pr oportional to the scattering cross section of a unit volume of sedimen t. The sediment sound-speed profile is specified in terms of near-surf ace values for the sound-speed ratio and gradient. Salient features of the modeled backscatter strength include marked departures from Lambe rt's law for sediments with sound speed ratios greater than 1; 1-5-dB oscillations as a function of grazing angle, due to the complex struct ure of the sound field in the sediment; the dominance of sediment-volu me scattering over rough surface scattering for most of the grazing an gles, frequencies, and sediment types considered; and, for soft sedime nts, a decrease by about 8-10 dB in the overall level of backscatter a s frequency increases from 100 Hz to almost 1000 Hz.