Low-frequency acoustic scatter from subsurface bubble clouds

Abstract

At high sea states, acoustic reverberation from the ocean surface achieves anomalously high levels that cannot be explained by bottomside roughness induced by wind-driven surface waves. It has been hypothesized that bubble clouds generated by breaking or spilling waves are the source of the increased reverberation. Evidently, when the bubble-cloud size exceeds the acoustic wavelength, backscatter from the bubble cloud dominates the surface-scatter contribution. By using a new scatter formalism, we are able to quantitatively evaluate the bubble-cloud hypothesis. Computations have been performed for a rudimentary bubble-cloud model in the presence of a highly rough surface. For a flat surface, the bubble cloud and its negative image tend to cancel one another when they are in near contact, but they can interfere constructively as the bubble cloud moves away from the surface. Surface roughness tends to diminish the cancellation effects whereby the average scattered signal strength lies at or above its level for a freely scattering bubble cloud. Thus a detached bubble cloud can be a very effective scatterer. The paper introduces the mutual interaction method for calculating acoustic scatter from an object near a rough surface; this method provides a rigorous basis for evaluating the bubble-cloud surface reverberation hypothesis.