Richard S. Keiffer
NRL-SSC, Code 7181, Stennis Space Center, MS 39529-5004
It has been suggested [Erskine et al., J. Underwater Acoust. 42, 217--280] that the ability at low frequencies (<300 Hz) of first-order perturbation theory to model experimental data implies that scattering at the sea surface is the dominant contributor to backscatter. If one makes reasonable assumptions on the azimuthal dependence of the fully developed sea surface spectra, or if one uses a numerically derived spectra, then a strong azimuthal dependence in the monostatic backscatter is an almost unavoidable aspect of the acoustic modeling. On the other hand (though not directly looking for the azimuthal dependence at these frequencies), the data collected show an apparent lack of azimuthal dependence. In this paper, the second-order symmetric operator expansion method, which has been shown to be accurate for scattering from the sea surface, is applied to numerically derived 2-D sea surface spectra from CST-4. The objective of this investigation is to use the numerically derived 2-D sea surface spectra to obtain an understanding of the azimuthal dependence on the backscatter that is free of specific assumptions concerning the state of development or directionality of the sea surface.