Application of waveguide invariants to analysis of spectrograms from shallow water environments that vary in range and azimuth

The waveguide invariant summarizes in a single scalar parameter the dispersive propagation characteristics for a given environment. In this paper, an expression is derived for the invariant in shallow water environments where the bottom bathymetry varies with range and azimuth. It then is applied to the interpretation of broadband, single-element spectrograms collected during SWellEx-3, a shallow water acoustics experiment off the coast of San Diego. The unusual behavior of the interference patterns seen in the spectrograms from this experiment during the transmission of a 75–150-Hz pseudo-random noise sequence by a source towed over range-varying bottom bathymetry is well matched by the model predictions.