Ocean acoustics and expanded equivalent-fluid methods

The effectiveness of sound propagation in the ocean means that acoustic energy provides an important tool as an environmental probe but also constitutes a potential environmental concern. Effective computational modeling plays an important role in both ocean acoustic tomography and the characterization of possible impacts. When there is significant acoustic interaction with the bottom, the model must incorporate a bottom treatment. This can be challenging if relatively long propagation ranges are of interest. Depending on the nature of the seafloor, shear waves in the bottom may be a contributor to acoustic losses in the water column. A physical treatment of this shear propagation is computationally demanding and often not necessary if it is solely the acoustic signal in the water column that is of interest. "Equivalent fluid" methods have been used extensively to model bottom reflection, often incorporating a complex number for the "density" parameter to easily incorporate this loss mechanism. Previous approaches in complex-density equivalent fluids typically examine the limit of low shear speed and low grazing angle. The method can be expanded to construct equivalent-fluid parameters for additional quantities in order to mimic the reflection coefficient of the actual elastic solid for the grazing angle interval of interest. Test cases involving near-source propagation over volcanic basalt and high grazing angle configurations will be presented. Refinements to the method will be addressed.