Acoustic modal analysis in an anisotropic internal wave field

Abstract

Simulations show that an anisotropic, internal wave field in shallow water can cause significant horizontal redirection of acoustic propagation, requiring true three-dimensional sound modeling. Calculations are presented for a sound speed profile that is perturbed by both a nonlinear internal wave packet, and a linear background internal wave field that is homogeneous and isotropic. A 3D parabolic equation code is used to propagate a 400 Hz signal through the spatially and temporally varying sound speed distribution. A single mode starter field at the source is propagated to a receiver location, and there the field is decomposed into modes to determine the degree of modal coupling. If the compressional wave number vector is perpendicular to that of the nonlinear internal wave packet wave number vector, the modeled acoustic field is nearly adiabatic and can be approximated using horizontal mode decomposition. In this case the pressure wave shows horizontal refraction, and ducting. For propagation paths not perpendicular to the internal wave number vector, the acoustic field can b enonadiabatic, and mode coupling can be significant. Modal analysis shows similar refraction and coupling results for lower frequencies. [Work supported by ONR.]