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<p>Results from shallow-water observational studies have shown acoustic field fluctuations in excess  of 10 dB due primarily to non-linear internal solitary waves (ISWs). This work concentrates on three  limitations ISWs pose to shallow-water acoustic propagation: anisotropy, quasi-deterministic-stochastic  nature, and frequency dependence. These aspects are explored for low frequencies of 75 and 150 Hz  through the development of a full-wave three-dimensional parabolic equation model in which a single ISW  is introduced and rotated to achieve differing geometries between the ISW and acoustic transmission path.  Two acoustic scattering regimes were found to exist: A horizontal refractive regime that generates very  intense acoustic energy focusing and shadow zones near the ISW at range when the acoustic transmission  path is nearly parallel to the ISW crest, and a vertical mode coupling regime producing moderate to strong  acoustic energy focusing and shadow zones for all geometries. These three-dimensional patterns are similar  for each frequency, but more intense for 150 Hz. The results dramatically show ISWs focus acoustic  energy, dependent upon frequency and geometry, which may be exploitable in both a sonar performance  and ambient noise modeling sense.
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<br>Subjects: Azimuth; Solitons</div>
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Roush, Douglas L.</div>
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