Acoustic propagation in very shallow water

In the 1970´s and 80´s there was a great deal of progress in computational ocean acoustics, largely oriented to passive sonar applications at frequencies below 1 kHz. This led to a variety of propagation models based on four principal approaches: (1) normal modes, (2) ray/beam tracing, (3) parabolic equations, and (4) wavenumber integration. In this paper we discuss the application of these techniques to very shallow water, of interest for applications in waterside security. As a particular application of interest, we consider acoustic modems which may be used to provide communications links with underwater vehicles. We discuss the basic physics, the modeling approaches, and the implications for modem performance.