Sea surface reverberation: A review

Abstract

Measurements of reverberation from the sea surface and the accompanying development of theories to explain the experimental results are reviewed from a historical perspective. Because of the relative ease of measurement, significant amounts of sea surface reverberation data have been amassed that yield a clear picture of the dependence of measured reverberation levels on acoustic frequency, grazing angle, and wind speed. The evolution of theoretical treatments of rough surface scattering and scattering from the layer of wind-generated microbubbles that exist below the surface is also reviewed, as are the empirical models that describe ocean surface roughness and subsurface bubble distributions. For acoustic frequencies of 3–60 kHz, the data are consistent with the view that the reverberation is due to rough surface scattering at moderate to high grazing angles, and at low grazing angles to scattering from resonant subsurface microbubbles. At frequencies below 1 kHz, measured reverberation levels at low grazing angles exceed those expected from rough surface scatter alone. However, the source of the anomalous reverberation is in this case not clear. For both of the frequency ranges considered, disparities of an order of magnitude exist between measurements performed under similar environmental conditions. At high frequencies, disparately high reverberation levels are observed in coastal areas; at low frequencies the source of such disparities is unknown.