Abstract
A wind-driven water surface possessing the characteristics of a simple sea (near Gaussian distribution of slopes and a realistic ratio of the upwind-downwind slope to the cross slope) has been studied acoustically by analyzing specular scattering at normal incidence (surface roughness g=16π2σ2cos2σ/λ2 ranging from 0.32 to 26). Theory, based on Kirchhoff approximation, is used to estimate the root-mean-square surface height and the root-mean-square slope. Theory and experiment agree far beyond the restrictions of the Kirchhoff assumption. The acoustically determined value of the root-mean-square slope is 17% less than the optically measured value. As predicted, the form of the distribution of scattered sound varies from Gaussian to Rayleigh for increasing surface roughness. At any instant, the relative specularly scattered intensity decreases monotonically with increasing frequency to a minimum value at g≃3, from which it is possible to get a second estimate of the root-mean-square surface height; above g≃3, the instantaneous scattered intensity is a nonstationary function of frequency. [Hudson Laboratories, Columbia University Informal Documentation No. 114. Work supported by U. S. Office of Naval Research.]
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