Azimuthal Dependence of Sound Backscattered from the Sea Surface

The dependence on azimuthal angle of acoustic sea surface reverberation is investigated at 60 kHz, utilizing 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 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 on azimuthal angle at wind speeds below about 9 kt, but becomes independent of azimuthal angle at higher wind speeds.