A NORMAL-MODE MODEL FOR ACOUSTO-ELASTIC OCEAN ENVIRONMENTS

A normal mode method for propagation modeling in acousto-elastic ocean waveguides is described. The compressional (p-) and shear (s-) wave p ropagation speeds in the multilayer environment may be constant or hav e a gradient (1/c(2) Linear) in each layer. Mode eigenvalues are found by analytically computing the downward- and upward-looking plane wave reflection coefficients R(1) and R(2) at a reference depth in the flu id and searching the complex k plane for points where the product R(1) R(2)=1. The complex k-plane search is greatly simplified by following the path along which \R(1)R(2)\=1. Modes are found as points on the pa th where the phase of R(1)R(2) is a multiple of 2 pi. The direction of the path is found by computing the derivatives d(R(1)R(2))/dk analyti cally. Leaky modes are found, allowing the mode solution to be accurat e at short ranges. Seismic interface modes such as the Scholte and Sto nely modes are also found. Multiple ducts in the sound speed profile a re handled by employing multiple reference depths. Use of Airy functio n solutions to the wave equation in each layer when computing R(1) and R(2) results in computation times that increase only linearly with fr equency. (C) 1996 Acoustical Society of America.