A simplified model of frequency distortion in ocean acoustic signals by multiple interactions with a moving ocean surface and its use in oceanographic inversions.

Abstract

Received omnidirectional spectra of low frequency narrowband tones measured in a shallow ocean waveguide exhibit Doppler-shifted sidebands at frequencies whose offsets are integer multiples of the dominant surface wave frequencies. These observations suggest that higher order scattering is important. However, higher-order perturbation theory involving only a single surface interaction does not predict the observed relative sideband levels. In particular, the higher order sidebands were often observed to increase in relative level to the main spectral peak during smaller surface wave, and, therefore, smaller kh, conditions. A simplified model is presented that simulates the surface wave-induced frequency distortions as path-length modulation caused by the vertical motions of the surface. The model incorporates multiple interactions with the moving surface, and quantitatively predicts the effects on the received spectra of the spatial coherence of the surface waves along the propagation path. The model results are identical to those from higher order perturbation with a single interaction. The simplified nature of this model allows it to be readily used to invert for various properties of the ocean surface wave directional spectrum. [Work supported by ONR, Code 321(US).]