Bistatic Surface Scattering Strength at Short Wavelengths.

Abstract : A series of measurements of the scattering strength of a wind driven water surface at 1.3 mHz have been made. The measurements have been made at 12 degree and 17 degree grazing angles, bistatic azimuths ranging from 0 degree to 180 degree, and two different surface roughness conditions. Orientations with respect to the wind over a range of 180 degree were used. The measurements were made with short, relatively narrow band probing signals under computer control. The data were converted to scattering strength using a flat surface reference and a compensation for beam patterns and source and receiver timing. The results of these measurements indicate that wind direction is an important factor in determining reverberation from the surface. Surface roughness was found to be important, but the scalar Rayleigh Parameter is found not to be a useful indicator of reverberation level. Grazing angle and acoustic wavelength were found to be relatively insignificant variables. The results obtained in this series of experiments indicate that present theory will not predict scattering strength accurately without a detailed knowledge of the second order spatial statistics of the surface. Efforts to obtain measurements of surface slopes of the required precision have met with very limited success but will continue. (Author)