100 Hz-10 kHz Bottom Backscattering Model: Generalized Treatment of Sediment Sound Propagation, Sediment Volume Scattering, and Interface-Roughness Scattering.
WASHINGTON UNIV SEATTLE APPLIED PHYSICS LAB
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A model for bottom backscattering strength in the frequency range 100 Hz to 10 kHz is presented. This model generalizes an earlier low-frequency model intended for the smaller frequency range 100-1000 Hz. The new model matches the predictions of an existing high-frequency model 10-100 kHz but offers new insights into the effect of fine-scale layering. Scattering from both sediment volume inhomogeneity and interface roughness is included in the model. For silt and clay sediments, sediment volume scattering is usually the dominant process. The model requires profile functions for average sound speed, mass density, sound absorption, and volume scattering strength in the sediment. In addition, two parameters describing the spectrum of interface roughness are required. Use of the small-roughness perturbation approximation prevents application of the model to extremely rough bottoms e.g., gravel and rock. Preliminary comparisons with data show that the model can reproduce an interesting frequency dependence often seen in bottom backscattering. This includes a decrease in scattering strength with increasing frequency in the 100-1000 Hz range, an increase as frequency approaches 10 kHz, and approximate constancy or a slow rise above 10 kHz. The increase in sound speed with depth typically observed in sediments causes upward refraction which, in some cases, leads to enhancements of scattering by 6-8 dB. MM