Experimental Investigation at 60 kHz of the Azimuthal Dependence of Sea Surface Backscattered Sound

Abstract

The dependence on azimuthal angle of acoustics sea surface reverberation is investigated at 60 kHz with a three-axis rotationally stabilized conical beam transducer. Measurements of backscattering strength in the upwind and crosswind directions are compared for a range of grazing angles from 10° to 40°. Polar plots of scattering strength versus azimuthal angle are presented, and the effect of wind speed on these plots is shown for wind speeds from 4 to 13 kt. An attempt is made to correlate the backscattering measurements with swell and local wind-driven waves by means of aerial sea-state photographs taken during the test. Data indicate that surface reverberation has a pronounced dependence or azimuthal angle at wind speeds below about 9 kt, but becomes indepndent of azimuthal angle at higher wind speeds. For the lower wind speed régime, variations in backscattering strength with azimuthal angle appear to be correlated with swell direction. At higher wind speeds, increased surface disturbance causes the reverberation to increase for all azimuth angles, consequently masking the anisotropic backscatter that prevailed at lower wind speeds.