The effects of sediment porosity on acoustic reflection and transmission at the seafloor

The Biot theory of propagation in a porous medium provides a mathematical framework for studying acoustic interaction with the seafloor. The theory considers the two-phase porous nature of marine sediments in contrast to the classical models of wave propagation in the seafloor that consider marine sediments as an extended single-phase fluid or solid. A boundary value problem is set up and solved for a line source in a fluid medium above a poro-viscoelastic halfspace. Expressions for the reflected and transmitted field are given in integral form and asymptotic expansions in the high-frequency, far-field limit. A set of simultaneous equations is solved to give plane wave reflection and transmission (Type I, Type II and shear wave) coefficients. These equations also yield the Scholte, pseudo-Scholte, and pseudo-Rayleigh wave phase velocities and attenuations. The plane wave coefficients and the surface wave velocities and attenuations are compared with commensurate quantities from the single-phase theories.;A number of recent experiments of high frequency transmission through a water-sand interface have indicated anomalously high transmitted energy at angles near the critical angle. The Type II wave (predicted by Biot theory but not the single-phase theories) was suspected as a possible reason for the anomalies. Data from one of the experiments are examined using Biot theory to determine whether or not the Type II wave was responsible for the anomalies. Calculations indicate that the contributions from the Type II wave are negligible and thus not the cause of the anomalies. Much of the anomalous behavior appeared to arise simply as a result of an incorrect compressional wave attenuation in the geoacoustic model.