Peter H. Dahl
Appl. Phys. Lab., College of Ocean and Fishery Sci., Univ. of Washington, Seattle, WA 98105
Intensity and coherence properties of high-frequency sound forward scattered from the sea surface are discussed. Propagation of high-frequency acoustic energy often involves a sea surface bounce path. In deep water, and for many sonar acquisition geometries, a single surface bounce may be the best path to the target, because the direct path has faded owing to the sound speed structure. In shallow water, interaction with both the sea surface and bottom is often the rule. For frequencies above 10 kHz, incoherent surface scattering in the forward direction will usually dominate over coherent reflection because of the large sea surface roughness. The mean total energy of the scattered field is reduced by the extinguishing effects of the near-surface bubble field through which surface bounce paths must traverse. Coherence properties of the scattered field, e.g., spatial and temporal, can be related to the sea surface slope distribution and are also influenced by near-surface bubbles. Performance of detection and classification systems will depend on coherence properties, and in particular their degradation as a function of sea surface environmental conditions. Published theories on acoustic scattering from the sea surface in the high-frequency limit are discussed in the context of recent experimental measurements of sea surface forward scattering performed by the Applied Physics Laboratory in both inland water (Whidbey Island, Puget Sound) and open ocean conditions (R/P FLIP off California coast).