Three-dimensional analysis of Azimuthal dependence of sound propagation through shallow-water internal solitary waves

Abstract

Results from shallow-water observational studies have shown acoustic field fluctuations in excess of 10 dB due primarily to non-linear internal solitary waves (ISWs). This work concentrates on three limitations ISWs pose to shallow-water acoustic propagation: anisotropy, quasi-deterministic-stochastic nature, and frequency dependence. These aspects are explored for low frequencies of 75 and 150 Hz through the development of a full-wave three-dimensional parabolic equation model in which a single ISW is introduced and rotated to achieve differing geometries between the ISW and acoustic transmission path. Two acoustic scattering regimes were found to exist: A horizontal refractive regime that generates very intense acoustic energy focusing and shadow zones near the ISW at range when the acoustic transmission path is nearly parallel to the ISW crest, and a vertical mode coupling regime producing moderate to strong acoustic energy focusing and shadow zones for all geometries. These three-dimensional patterns are similar for each frequency, but more intense for 150 Hz. The results dramatically show ISWs focus acoustic energy, dependent upon frequency and geometry, which may be exploitable in both a sonar performance and ambient noise modeling sense.