Acoustic propagation in the ocean with a poro-elastic bottom

The acoustic normal modes in a homogeneous ocean overlying a homogeneous porous elastic bottom half-space are obtained in closed form. The Biot theory is used to model the bottom. The effects of sediment properties on the dispersion and attenuation of acoustic waves are examined numerically. It is found that the shear modulus and the poro-viscous-frequency number ksf/βν are the two most critical bottom parameters for acoustic attenuation. Here, ks is permeability, f is frequency, β is porosity, and ν is the kinematic viscosity. For sediment and rocks having shear modulus larger than 5×107 N/m2, the shear waves and the slow compressional waves become important in acoustic propagation. For hard porous rocks, such as sandstones and corals, the attenuation of acoustic waves by a porous rock increases with frequency. For a given soft sediment, the acoustic wave attenuation decreases with frequency. For a given frequency, the acoustic attenuation becomes maximum when the sediment properties are such that ksf/βν is about 0.1. Near the critical value ksf/βν=0.1 the energy loss is totally due to the fast compressional waves and the fluid bottom model may be used. However, as ksf/βν deviates from 0.1, the error associated with the fluid bottom approximation increases progressively and becomes unacceptable even for soft sediments.