A model/data comparison for low-frequency bottom backscatter

This article describes a model for bottom backscattering strength applicable over the frequency range 100-1000 Hz. The model includes backscatter due to both a rough water/sediment interface and internal sediment inhomogeneities--i.e., sediment-volume scattering within an upward-refracting sediment. The net result is predictions for bottom backscattering strength that depend on the following: frequency, sound speed of the water near the water/sediment interface, density ratio, sound absorption within the sediment, near-surface sound-speed gradient within the sediment, two interfacial roughness parameters, and a parameter proportional to the scattering cross section of a unit volume of sediment. The sediment sound-speed profile is specified in terms of near-surface values for the sound-speed ratio and gradient. Salient features of the modeled backscatter strength include marked departures from Lambert'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 structure of the sound field in the sediment; the dominance of sediment-volume scattering over rough surface scattering for most of the grazing angles, frequencies, and sediment types considered; and, for soft sediments, a decrease by about 8-10 dB in the overall level of backscatter as frequency increases from 100 Hz to almost 1000 Hz.