A model for acoustic backscatter from muddy sediments

Abstract

It is well known that muddy sediments often contain significant amounts of gas of biological origin. Since the scattering cross section of a gas bubble in water is typically 1000 times its geometric cross section, it is reasonable that an acoustic backscatter model intended to work over muddy sediment should contain a bubble resonance scattering component. In this paper a heuristic model is presented, based on scattering from a distribution of suspended bubbles in mud. For acoustic propagation purposes, the mud is treated as a viscous fluid. Since estimates of bubble size distributions in mud are currently unavailable, a bubble distribution similar in shape to that observed in the water column is assumed. The model’s only free parameter is the gas fraction, which can be varied to fit the model to observed data. Small amounts of gas appear to be sufficient to produce observed levels of backscatter. For a homogeneous bubble distribution, the model can be inverted to give an estimate of the gas bubble size distribution from measured backscatter data. A discussion of depth dependent bubble distributions is included. The simplest case is that of a homogeneous bubble half-space buried underneath a finite gasless layer. The thickness of the layer appears to affect the grazing angle dependence of the backscatter significantly.