High-frequency forward scattering from the sea surface: The characteristicscales of time and angle spreading

Forward scattering from the sea surface is discussed in the context of a fo rward bounce path, or channel, through which high-frequency sound energy is transmitted. Such a channel might be used in an underwater communication o r imaging task. Both time and angle spreading are inherent to the process o f forward scattering by a roughened sea surface. Spreading in each domain r elates, via Fourier transform, to a conjugate or coherence separation varia ble, e,g,, angle spreading and spatial coherence. The measurement and modeling of time and angle spreading are discussed, wit h the modeling incorporating the bistatic cross section of the sea surface. A characteristic scale for each spread variable is defined: L for the time spread and sigma (thetah) and sigma (thetav) for the horizontal and vertic al angular spread, respectively. Simplified expressions for these character istic scales as a function of array acquisition geometry and sea surface co nditions are also obtained. Data from two field experiments are discussed, one conducted in shallow wat ers of 30-m depth, and one conducted in deep, pelagic waters of 4000-m dept h. Both experiments utilized frequencies greater than or equal to 20 kHz, T he role of bubbles in forward scattering is illustrated using measurements from the deep-water experiment. It was demonstrated that bubbles can attenu ate the forward-scattered signal, but otherwise have little effect on L and sigma (thetah,v) until their concentrations approach those necessary to ne arly extinguish the signal scattered from the air/sea interface.