Abstract
Even at low frequencies (100–1000 Hz) backscattering levels rise above rough interface scattering levels at higher sea states, most likely due to scattering from bubble clouds near the surface. Because the bubble clouds are convected in the orbital motion of the surface waves, frequency broadening will occur for backscattering of narrow-band signals. However, the moving rough surface leads to another effect. A scatterer below the moving surface is ensonified by the total field: incident plus the surface scattered field. Because the total field vanishes on the surface, the main scattering contribution comes from several meters below the rough surface at low frequencies. The magnitude of this total field will fluctuate with time as the surface waves move by the scatterer. Therefore, even if the scatterer is stationary the scattered field will have some frequency broadening, and this will be combined with the Doppler broadening due the orbital motion for a scatterer free to move. The possible importance of rough surface effects on frequency broadening has been examined using numerical simulations with a one-dimensional surface model. [Work supported by ONR.]
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