Sensitivity of modal attenuation coefficients to environmental parameters

In waveguides that are nearly independent of range, modal attenuation coefficients (MACs) convey the influence of intrinsic sediment attenuation to propagating modes in the water. The intrinsic attenuation in the upper sediment layer in sandy-silty areas has been recognized as having a nonlinear dependence on frequency f. The frequency dependence of the MACs is particularly interesting. The MACs in a Pekeris waveguide decrease like f^-1, even though the upper layer intrinsic attenuation increases like f^-2. These results are consistent with calculations in a classic paper by Ingenito, For adiabatic propagation a perturbation formula is useful for calculation of the MACs. The formula demonstrates that downward refracting sound speed profiles (SSPs) dramatically change the behavior compared with a Pekeris waveguide. For example, depending on the strength of the downward gradient, the high-frequency power law of the MACs varies from about zero to one. Sensitivity of the MACs to various environmental parameters and profiles will be discussed, as well as their consequences for the average rate of decay with range of transmission loss. A recent experiment off Nantucket Island provides the opportunity for comparisons of experimentally determined MACs with those calculated from the perturbation approach