Observations of the Backscatter of Ultrasonic Waves from a Rough Water Surface

An experiment is described in which measurements are made of the Doppler frequency shift imposed on an acoustic signal by resonant backscatter from a wind-generated rough water surface. In particular, for the case where the water waves have plane wave fronts and move in a single direction across the surface, the effects on the Doppler shift of varying the horizontal angle of incidence of the acoustic beam with respect to this direction of movement are studied. Some simple theoretical concepts are invoked in an attempt to explain the apparent dependence of the Doppler frequency shift on the azimuth angle measured from the acoustic beam radial direction. Because of the analogy which exists between the scattering of acoustic and electromagnetic waves from the sea surface, it is proposed that a model employing a procedure similar to that described here would be of use in interpreting data gained in large-scale ocean backscatter experiments.