Suzanne T. McDaniel
HC-01 Box 62, Spruce Creek, PA 16683
High-frequency sea surface reverberation is reviewed from an historical perspective starting with the first comprehensive set of measurements made during World War II. Basic theories to predict reverberation due to scattering from resonant subsurface microbubbles and sea surface roughness are also reviewed as well as the environmental inputs to these theories: microbubble distributions and the ocean surface wave-number spectrum. Because resonant microbubbles absorb as well as scatter acoustic energy, a saturation effect is predicted, that is, when the bubble density increases beyond that needed to produce saturation, the backscatter remains constant. The reverberation level in this limit is a unique feature of scattering from resonant microbubbles permitting a clear identification of this scattering mechanism. Comparison of theoretical predictions with representative acoustic data demonstrates that reverberation at high grazing angles is due to rough surface scattering, while at lower grazing angles it is governed by scattering from resonant microbubbles. Large variations in backscattering strength are apparent for measurements performed at the same grazing angle, frequency, and wind speed, with reverberation levels in coastal waters being an order of magnitude higher than those in the open ocean. Although the physical mechanisms responsible for high-frequency reverberation are well understood, the relationship between backscattering variability and environmental factors is not.