Theory of scattering and reflection of sound from the sea surface

Abstract

Theoretical work on scattering and reflection of sound from the random timevarying rough sea surface is critically reviewed. It is shown that sound scattering from the sea surface may be treated as a purely acoustic boundary value problem with the sea surface as the given boundary which may be considered as being quasi-static (linear response approximation, neglection of the fluid motion in the surface waves, subsequent lnclusion of timevariations). The formulation of this boundary value problem in Fourier space (plane wave representation of the acoustic wave field) is most suited for the application of perturbation techniques and statistical averaging. The perturbation techniques which can be adapted to various limiting cases show that sound scattering from the sea surface is essentially determined by phase modulation spectra as expected for "phase objects" like the sea surface layer. After statistical averaging the corresponding functions are the Fourier transforms of the characteristic function of the joint probability density of the surface displacements. On the basis of an appropriate 1. order approximation which yields sufficiently good results over a wide range of surface roughness parameters and grazing angles the two-scale (or wave-facet) model for the scattering from the sea surface can be discussed. Shadowing corrections have to be considered separately.