Abstract
The intrinsic sediment attenuation in adiabatic waveguides affects transmission loss through the modal attenuation coefficients (MACs). For sand-silt bottoms, the non-linear frequency dependence of attenuation in the upper sediment layer significantly affects the frequency variation of the MACs. This variation is shown to depend strongly on the water, and weakly on the upper sediment layer, sound-speed profiles. The MACs are calculated for a layered environmental model that permits good comparisons with values derived from experimental data in the Gulf of Mexico [Ferris, JASA, 82 (1972)]. The comparisons illustrate the dependence on measured water profiles. The rate of increase of transmission loss with range is conveniently measured by an effective attenuation coefficient. The relationship between this quantity and the MACs is known when one propagating mode dominates [Evans and Carey, JOE, 23 (1998)]. We describe the interesting connection for downward refracting profiles at mid-frequencies with multiple propagating modes. [Work partially supported by ONR.]
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