Dispersion and attenuation due to scattering from heterogeneities in the frame bulk modulus of sand sediments

A model that predicts the sound speed and attenuation of Biot fast compressional waves when there is scattering into the slow compressional wave has been developed. The scattering is due to small scale heterogeneities in the sediment properties such as the porosity or the bulk modulus of the frame that arise due to the random packing of the sediment grains. This model may account for the deviation of measured fast wave attenuation from the predictions of Biot theory that have been observed in both ocean and laboratory sediments. To compare the predictions of this theory with sound speed and attenuation measurements, the spatial statistics of the sediment properties must be known. Despite work in the granular physics community to understand the distribution of the contact forces between the grains, no direct measurements have been made on the spatial variations in the bulk modulus for dry or fluid-saturated sediments. In order to estimate these variations, we have performed molecular dynamics simulations of spherical bead packs which may approximate simple glass bead sediments. The progress of these simulations and the techniques used to estimate the bulk modulus on scales approaching the grain diameter will be presented. [This work is supported by ONR.]