A simple shallow water propagation model including shear wave effects

The Perkeris model has proved to be very useful in describing some features of acoustic propagation in shallow water, and as a simple test of ideas in normal mode theory. The basic model consists of a homogeneous layer of fluid overlying an infinite homogeneous fluid half-space of greater sound speed. Here we extend the Pekeris model to handle the case of a fluid overlying an elastic basement in which the shear speed is less than the (compressional) speed of sound in the fluid. This gives rise to leaky modes in which both the mode eigenfunctions and eigenvalues are complex. The model predictions are compared to some measured propagation losses for a shallow water site overlying a chalk bottom, where shear-wave conversion at the water-chalk interface causes large losses. The predictions of the simple model explain the very high losses measured at frequencies less than 200 Hz. At higher frequencies the sound speed profile and a thin sediment layer become important, but then an all-fluid normal mode model is in agreement with the measured results.