Compressional and shear wave velocities in granular materials to 2.5 kilobars

In spite of the common occurrence of granular materials, both on the earth and on the moon, little is known about their mechanical behavior at pressures corresponding to large depths of burial. This is particularly true for seismic wave velocities. We have measured the compressional and shear wave velocities in silica sand, volcanic ash, and powdered basalt subjected to hydrostatic pressures from 1 atm to 2.5 kb. Simultaneous determination of porosity was also obtained as a function of confining pressure. The results reveal that large velocity (both Vp and Vs) variations are almost completely reversible upon pressure cycling, even though there is an irreversible change in porosity. The amount of permanent change in porosity was found to depend on the original grain size of the sample. The derived effective elastic moduli are not unique functions of pressure. The change in Vp and the computed bulk modulus depends on the initial porosity, whereas grain size controls the behavior of Vs and the shear modulus. The rock powders have very large values for Poisson's ratio (∼0.3–0.4) at high pressures, and it is thus verified that simple cold compaction to a pressure of a few kilobars cannot transform rock powders to solid rocks. These results have important implications for the self-compaction hypothesis, which has been postulated to explain the lunar near-surface seismic velocity variation.