Modelling the acoustic reflection loss at the rough ocean surface

A description of the reflection of underwater sound incident upon a real ocean surface boundary is a necessary component of a sonar transmission model. At low frequencies, the sea surface may be regarded as smooth, with total reflection of intensity at the specular angle. At mid-frequencies (over about 2 kHz), this is no longer the case and an intensity reduction must be included to account for the sound scattered from the surface at non-specular angles. Complications include whether roughness alone causes the acoustic loss effects, or whether near-surface bubbles play a role. In addition, it may be necessary to consider whether the sound reflected in the specular direction consists entirely of a coherent component, or whether an incoherent component is assumed to exist. Due to these complexities in the relevant phenomena, and the non-uniformity of the surface state, the modelling of surface loss remains as an area yet to be mastered. In response to this unresolved situation, DSTO, Thales Australia and the Centre for Marine Science and Technology at Curtin University have compared the performance of surface loss models in their possession, both against each other and against at-sea data for a number of ocean scenarios, including several for which data have not been published previously. In addition, the output of these models has been compared with that obtained from a small-slope approximation model made available for this purpose by the Applied Physics Laboratory of the University of Washington, Seattle. Here, comparisons have been made of the various predictions of surface loss per bounce, as well as comparisons between predictions of transmission loss based on the use of these surface loss models, including comparisons with at-sea measurements of transmission loss. This paper discusses aspects of these surface loss models and the differences between them which have been revealed.