Frame bulk modulus of porous granular marine sediments

Abstract

Frame bulk modulus is important for analyzing the acoustic wave propagation in porous water-saturated marine sediments such as sands. Previous measurements of the longitudinal wave velocities in air-saturated glass beads of uniform grain size showed that the longitudinal wave velocity increased with the grain size. This result cannot be explained by using a classical contact theory such as the Hertz-Mindlin model. It was speculated that this phenomenon is due to the effect of air elasticity between the grains. In this study, the longitudinal and shear wave velocities in samples of vacuum-, air-, and water-saturated glass beads as well as beach sands at a lower stress were measured. The results obtained were used to estimate the corresponding values of the frame bulk modulus. In the water-saturated samples, these values are about 109Pa at a frequency of 500kHz and about ten times greater than those in air-saturated samples at a frequency of 11.8kHz. The grain size dependence was also observed. These measurements are explained in terms of the effect of fluid elasticity at the grain-to-grain contact in the context of a modified gap stiffness model.