Evaluation of equivalent-fluid geoacoustic seabed models

Abstract

Sound reflection from the seabed is very important in the study of sound propagation in shallow water. As most seabeds support both compressional and shear waves, seabed rigidity affects the reflection loss and phase shift of the bottom reflection. In this case the problem of modelling sound reflection from the seabed becomes more complicated. An approximate method is to replace the solid with a fluid by choosing suitable seabed parameters. This replacement fluid is termed the equivalent-fluid seabed. An equivalent-seabed model is an approximate method to simplify the mathematical analysis and reduce the calculational expense in modelling water-borne shallow water sound propagation, taking seabed shear-wave effects into account. The objective of this work is to develop and evaluate equivalent-fluid seabed models. Special attention is paid to the models of Tindle and Zhang (1992) and Zhang and Tindle (1995). Shear-wave effects on reflection have been studied via prediction of the reflection coefficient. A new effective-seabed model is proposed from the calculation of the effective impedance of the seabed. Comparison of the new model with existing models shows that the new model agrees better with the solid seabed. Furthermore, grazing-angle-dependent parameters of the equivalent-fluid seabed are proposed. It is shown that the use of grazing-angle-dependent parameters gives the best agreement with the values for the real seabed in the prediction of the reflection coefficient.