UC San Diego

During multiple experiments in shallow water, low- frequency, narrowband acoustic tones were transmitted from stationary sources to hydrophone arrays about 1 km away. The spectrum of the received field depended strongly on ocean surface wave conditions. Doppler-shifted sidebands invariably are observed in the received spectra, with spectral content and levels depending on the prevailing ocean surface wave spectrum. Moreover, through the use of horizontal hydrophone arrays, the directionality of these scattered sidebands is also shown to depend on the directional surface wave spectrum. At times when the ocean surface waves were small, increased levels of higher order sidebands were observed. First-order and higher-order perturbation theories based on normal mode propagation are explored in an effort to understand the observed dependence is shown to predict the deviations in angle of arrival of the scattered sidebands. Higher order perturbation is explored as a possible explanation of the higher order sidebands, but it is shown to be insufficient to predict all the features observed in the measured spectra. A simplified model is developed that reproduces the increased levels of the higher order scattering by modeling the phase deviation that arises from path-length time-dependence caused by motion of the upper, reflective surface. This model is useful for predicting the effects of multiple interactions with a moving reflective ocean surface