The effect of the rough air–sea interface on scattering from tenuous bubble clouds.

Abstract

Henyey has developed a theory for low-frequency scattering from tenuous bubble clouds [F. S. Henyey, J. Acoust. Soc. Am. 90, 399–405 (1990)]. The bubbly water is treated as an effective medium, described with a spatially varying index of refraction, and the scattering is found using a perturbation theory approach. This model applies to bubble clouds with sufficiently low air volume fraction that buoyancy forces are negligible; the bubble clouds are thus referred to as tenuous. Henyey simplified his analysis by assuming that the air–sea interface is flat. Here the effect of including surface roughness is examined for the 2-D problem (1-D surface roughness), but otherwise Henyey’s approach is followed. The integral equation method is used to obtain the exact acoustic fields near the rough surface, which are then employed in the bubble scattering calculations using perturbation theory. Results comparing bubble scattering with rough and flat sea surfaces show that the average scattered intensity is not much affected by surface roughness, but the statistical distribution of intensities can be significantly affected, leading in some cases to a hightailed intensity distribution (spikes) when the surface is rough. Implications for the time dependence of the scattering and for the Doppler spectrum will also be discussed. [Work supported by ONR.]