Low-frequency acoustic propagation loss in shallow water over hard-rock seabeds covered by a thin layer of elastic–solid sediment

Shallow-water seabeds are often varied and complex, and are known to have a strong effect on acoustic propagation. Some of these seabeds can be modeled successfully as fluid or solid half-spaces. However, unexpectedly high propagation loss with respect to these models has been measured in several regions with rough, partially exposed, hard-rock seabeds. It is shown that the high propagation loss in these areas can be modeled successfully by introducing a thin layer of elastic–solid sediment over the hard-rock substrate. Propagation loss predictions using the safari fast-field program exhibit bands of high loss at regularly spaced frequencies. Normal-mode calculations show resonance phenomena, with large peaks in the modal attenuation coefficients at these same frequencies, and with rapid changes in the mode wavenumbers. Bottom reflection loss calculations indicate that the high propagation loss is due to absorption of shear waves in the sediment layer.