Shallow Water Propagation and Surface Reverberation Modeling
SCRIPPS INSTITUTION OF OCEANOGRAPHY LA JOLLA CA
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The primary long-term goal is to measure and model high-frequency acoustic propagation in the presence of surface gravity waves and breaking waves to better understand the effects of surface reverberation on shallow water, underwater acoustic communications ACOMS. Secondary long-term goals are to exploit measurements of breaking wave noise to infer bubble cloud populations at the sea surface and their effect on reverberation, and to model high-frequency, forward scattering from sea ice. The primary goals of the research are to 1 measure the amplitude, time delay and Doppler shifts associated with high-frequency, forward scattering from surface gravity waves and 2 continue the development of the Wavefronts time-domain propagation code to model surface scattering. The standard approach to modeling high-frequency, forward scatter from the ocean surface is to use statistical methods. Surface arrival intensities, for example, are often characterized in terms of probability density distributions. This approach has the advantage that deterministic details about the physical properties of the surface wave field do not need to be known. However, this lack of knowledge can also be a disadvantage if propagation models and underwater acoustic communications systems algorithms do not incorporate all the relevant scattering physics. For example, the transient focal regions created by surface swell over short ranges contain micro-paths with regular patterns of significant, time-varying Doppler shifts, which introduce errors into channel equalizers. The result is a decrease in ACOMS performance in what would appear to be a benign environment short propagation range with swell and low wind speed. These micro-path properties only become obvious when individual wave-focused arrivals are studied.