ASA 125th Meeting Ottawa 1993 May

1aAO5. An issue (sediment volume fluctuations) and a nonissue (shear wave propagation) in shallow water acoustics.

F. Tappert

T. Yamamoto

Appl. Marine Phys., Univ. Miami/RSMAS, 4600 Rickenbacker Cswy., Miami, FL 33149

A two-way full-wave PE reverberation model has been developed that provides predictions of mean reverberation levels in shallow water to long ranges (hundreds of water depths) due to sediment volume fluctuations as well as bottom and basement interface roughnesses. Using T. Yamamoto's tomographically measured sediment volume fluctuation spectra as one of the inputs, the PE reverberation model has been exercised at several locations on the shelf in the East China Sea over a broad range of frequencies. It has been found that predicted reverberation levels due to both mechanisms are generally comparable in magnitude, and that absolute levels are controlled mainly by total field ensonification of the sediment volume and boundaries, i.e., forward transmission losses. The question whether it is necessary to include shear wave propagation in shallow water acoustic models has been resolved: the answer is definitely negative, and this is no longer an issue. Measured shallow water transmission loss data over a broad range of frequencies with source and receiver in the water column has been successfully modeled using a full-wave range-dependent PE model that includes rough bottom forward scattering (necessary for good agreement), but treats shear wave effects as an equivalent fluid by augmenting the volume attenuation of compressional waves in the sediment layers. The conclusion that it is not necessary to include shear wave propagation when sources and receivers are in the water column has been further confirmed by inter-model and intra-model comparisons using realistic geoacoustic sediment properties as measured by T. Yamamoto's BSMP system [J. Acoust. Soc. Am. 90, 441--456 (1991)]. [Work supported by ONR, 1125OA.]