Influence of depth-dependent sediment properties on the pressure reflection coefficient at normal incidence using the Biot-Stoll Model

The impact of depth-dependent geophysical parameters on the reflection coefficient is studied using the Biot-Stoll theory in porous marine sediments. The seabed is modeled as a sediment layer with depth-dependent properties on top of a homogeneous half-space, as originally proposed by Stern. Reflection coefficient, phase velocity and attenuation coefficient for the slow, fast and shear waves are computed as a function of frequency and layer thickness. The model is tested assuming normal incidence of plane acoustic waves to the sea floor, which simulates the reflection coefficient measured from classic sub-bottom profilers. Results are determined by the evaluation of boundary conditions at the water-sediment layer interface and the sediment layer-half-space interface. The wave equation is solved as a function of frequency and layer thickness using the Runge-Kutta method. All depth-dependent parameters are linked to the porosity using equations provided by Berryman, Ogushwitz, Hovem and Ingram. Mean grain diameter and porosity are obtained from the Geoclutter experiment. The porosity varies with depth, while the mean grain diameter remains constant. Results are obtained for different types of sediment, from medium size sands to silty clay, and different porosity profiles. Depending on the sediment core, the porosity varied between 32% and 70%, indicating a high water content in every sample and suspension near the top of the layer. The maximum thickness of the layer is limited to 0.5 meters. The frequency range is 100 Hz to 5 kHz. If the porosity is lower than 45%, results show that the reflection coefficient vs. frequency may be very similar for two sediments of very different mean grain diameter but of similar porosity profile. However, a large difference in porosity at any depth between two samples leads to very different reflection coefficient spectra. This work is sponsored by the Office of Naval Research, Code # ONR321CG, Dr. Tom Drake.