Abstract:

Microbubbles near the surface of the sea and in the water column can be an important factor limiting acoustic images in shallow water and near the surf zone. Classical theories exist which describe the scattering of sound from individual bubbles and sparse distributions of bubbles provided the damping factors are known. Backscatter and forward-scatter coherence loss can be described for random bubble distributions which are both uniform and stationary in space and time, and this results in phase correlation functions, distances, and variances. The spatial bubble population can be described by the statistical distribution of the turbulent field. This paper briefly outlines the relevant theory and applies it to free field bubble distributions at high frequencies. The impact on array processing is considered for a representative bubble size and spatial distributions. The expectation of the mean-squared array response is shown to depend on a coherence function and a coherence length proportional to the ratio of the phase correlation distance and variance which is related to the bubble spatial distribution. Extrapolations to what could be expected to be a limiting array resolution are presented for high-frequency side looking and synthetic aperture sonars. [Work supported by ONR.]