Acoustic scattering from a “Pierson-Moskowitz” sea surface

Abstract

An exact integral equation method has been used to study low-frequency (200 Hz) acoustic coherent and incoherent scattering from surfaces that vary in only one horizontal direction. The surface roughness spectrum is obtained from the Pierson-Moskowitz frequency spectrum for a fully developed sea. Comparisons are made with standard theoretical predictions using the Kirchhoff approximation, perturbation theory, and the composite-roughness model. The Kirchhoff approximation is found to be reasonably accurate for near-specular scattering and for the coherent reflection loss at grazing angles as low as 10°. Perturbation theory, when carried to order (kh)4 for the scattering cross section, is in essentially perfect agreement with exact results for bistatic backscattering. This is true for kh values as great as 1.8, where k is the acoustic wavenumber and h is the rms waveheight. The standard composite-roughness model is found to be accurate for backscattering, except at the lowest grazing angles, where it over-predicts the scattering. [Work supported by ONR.]