Acoustic mode coupling induced by shallow water nonlinearinternal waves: Sensitivity to environmental conditionsand space-time scales of internal waves

摘要

While many results have been intuited from numerical simulation studies, the precise connectionsbetween shallow-water acoustic variability and the space-time scales of nonlinear internal waves(NLIWs) as well as the background environmental conditions have not been clearly establishedanalytically. Two-dimensional coupled mode propagation through NLIWs is examined using aperturbation series solution in which each order n is associated with nth-order multiple scattering.Importantly, the perturbation solution gives resonance conditions that pick out specific NLIW scalesthat cause coupling, and seabed attenuation is demonstrated to broaden these resonances,fundamentally changing the coupling behavior at low frequency. Sound-speed inhomogeneitiescaused by internal solitary waves (ISWs) are primarily considered and the dependence of modecoupling on ISW amplitude, range width, depth structure, location relative to the source, and packetcharacteristics are delineated as a function of acoustic frequency. In addition, it is seen thatsignificant energy transfer to modes with initially low or zero energy involves at least a second orderscattering process. Under moderate scattering conditions, comparisons of first order, singlescattering theoretical predictions to direct numerical simulation demonstrate the accuracy of theapproach for acoustic frequencies upto 400 Hz and for single as well as multiple ISW wavepackets.