Specular Scattering of Underwater Sound from Anisotropic Sea and Swell Surfaces

Abstract

The surface of a large, anechoic tank was excited by centrifugal fans and a mechanical swell maker to produce several different anisotropic “sea” and “swell” surfaces characterized by “typical” but distinctive frequency spectra, non-Gaussian height correlation functions, and wave-height probability densities. Surface roughness, g(=4k2σ2 sin2θ), was varied from zero to five by a wide selection of sound propagation constants: k, rms wave heights, σ, and grazing angles of specular scatter, θ. Using scattering surfaces that were, variously, swell alone, wind waves alone, and a combination of the two (producing a dual-peaked spectrum), continuous, simultaneous analog data were recorded, displayed, and analyzed for instantaneous wave height, instantaneous scattered pressure, and 20-sec integrations of the squared wave height, the total scattered squared sound pressure, and the variance of the scattered pressure. The steady component of the forward scattered squared pressure for both low-roughness swell and wind-driven surfaces is found to vary as e−g. However, compared with a broad spectrum sea, the swell surface produces a larger time-varying component (at the swell frequency). Results obtained are compared with theories by Parkins, which ignores shadowing, and Wagner, which overcorrects for shadowing. [Work supported by Naval Ship Systems Command.]