Bottom backscatter mapping: the effect of secondary multipath interactions

Abstract

It has been shown that good correlation exists between large-scale, high level signal returns of bottom acoustic reverberation and bathymetric ridge structures. The ultimate goal of this type of analysis would be the removal of propagation effects resulting in a large-scale mapping of scattering strengths. Furthermore, analysis with a quasi-CW propagation model suggests a strong correlation between the small-scale fluctuations in the reverberation signal and the bottom acoustic ensonification, thereby suggesting that uncertainties in the predictions of forward propagation may limit the resolution of such mapping. By employing broadband modeling techniques, a valid representation of the complete time domain forward propagation is provided. Diffuse secondary bottom interactions appear to affect the resolution of the primary, direct-path interaction at ranges beyond a few water depths. Analysis of data recorded by near- bottom vertical line arrays (VLA's) confirms the existence of these secondary, multipath interactions in the forward propagation. The exact, two-way travel times from all bottom interactions are modeled and the influence of the secondary interactions is quantified. Possible ramifications for general sonar system performance are discussed.