Ocean Surface Reverberation Mechanisms and Their Laboratory Simulation

Abstract

Ocean surface roughness is generated by a combination of gravity and wind driven waves. These water/air interface perturbations are directly related to water motion just below this interface. This circular motion of the water may extend down to a depth of up to approximately two times the trough to crest measure of such waves. The ocean reverberation analyses to date have employed an interface description in the form of wave height measurements taken at a point and/or its associated spectrum, with very reasonable results. The experimental reverberation data has not been fully exploited to determine the significance of the said underlying water motion as the major contributor to the acoustic signal intensity scattered into the water body. It is this aspect that is investigated both theoretically with a model of the moving surface as well as the water mass underneath it, and experimentally in a laboratory tank with controlled and repeatable conditions using 1 Mc/sec carrier pulse of widths 4–6 μsec. The water motion was simulated by forced flow of water just underneath the air/water intersurface. The position of a small specular reflecting test plate was varied vertically between the transmit-receive transducer and the air/water interface in order to identify and separate subsurface volume and water/air interface reverberations. Various mechanisms generating the frequency spread of the acoustic signal incident on the surface are also discussed. [This work is sponsored by the Office of Naval Research, under Contract.]