Recent developments in modelling acoustic reflection loss at the rough ocean surface

Abstract

The transmission of sonar signals in a surface ducted environment, or in a shallow ocean, is affected by reflection losses at the ocean surface, when wind action or swell causes the surface to be roughened. Under these circumstances, the amplitude of the specular reflection of sound at the ocean surface is reduced by a number of complex phenomena, including: the sea surface shape; acoustic shadowing of parts of the surface to sound incident at small angles; diffraction of sound into the shadow zones; and bubble formation from white-caps. Recent work has shown that the inclusion of these effects within a ray model of transmission is a formidable prospect, as ray theory cannot describe all the phenomena explicitly, and the inclusion of acoustic wave effects in combination with a ray model is required. This paper addresses several of the complexities, in the search for a comprehensive solution to this modelling issue. In particular, the appropriateness of the Small-Slope Approximation roughness model used by Williams et al, (JASA, 116, Oct. 2004) is investigated, using a Parabolic Equation (PE) model, Also, the refraction near the ocean surface caused by wind-induced bubbles (e.g. Ainslie, JASA, 118, Dec. 2005) is investigated using the PE model. Lastly, the surface loss values obtained for received coherent sound pressure are compared with those relevant to received root-mean-square sound pressure. The paper speculates on the prospects for the future development of a surface loss model that includes all relevant effects.