Surface scattering measurements using broadband explosive charges in the Critical Sea Test experiments

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Extensive measurements of low-frequency (70–1000 Hz) sea surface backscattering strengths have been made as part of the Critical Sea Test (CST) experiments. These measurements were made during CST-1 through CST-5 for a variety of wind speeds from 1.5 to 13.5 m/s and for mean grazing angles from about 5 to 30 deg. These measurements have revealed several regimes in a frequency versus wind speed domain that probably correspond to at least two distinct scattering mechanisms. For relatively calm seas at high frequencies and for all wind speeds at lower frequencies, perturbation theory is found to give an adequate description of surface scattering. For rougher seas and higher frequencies, the Chapman–Harris empirical curves are adequate predictors of the levels of surface scattering. In between these two regimes is a transition region where the scattering strengths are more difficult to predict, as they depend on the details of the surface and wind characteristics. These observations lead to the idea that there are two mechanisms that dominate the scattering of sound from the surface. In the perturbation theory regime, air–water interface scattering is the dominant mechanism; in the Chapman–Harris regime, another mechanism such as scattering from subsurface bubble clouds must dominate the scattering process. The transition region is then the part of the frequency and wind speed domain where the two effects are competing.