Shallow water propagation in the presence of a rough sea surface and the associated bubble clouds

Abstract

Propagation models in underwater acoustics usually incorporate the sea-surface roughness as a perturbation, (i.e., a loss mechanism through an additional attenuation factor) based on coherent loss in the specular direction. In addition, scattering kernels are generally derived assuming a homogenous medium underlying the sea surface, an assumption incompatible with a realistic environment. Using a numerical model [Norton et al., 3018(A) (1994)] that combines a high-fidelity parabolic equation propagation model with the conformal mapping technique to handle surface roughness in a marching algorithm developed by Dozier [L. B. Dozier, 1415–1432 (1984)], forward propagation under a rough surface can be modeled in a mathematically consistent way. The technique is applied to the problem of shallow water propagation in the presence of an inhomogeneous bubble distribution. The ocean environment consists of bubble plumes in different stages of development. The effect of the bubbles are introduced through the complex index of refraction. The model is exercised for frequencies between 20 and 40 kHz. The combined effect that the rough surface and the inhomogeneous environment has on amplitude and phase fluctuations and on transmission loss is examined. [Work supported by ONR.]