Velocities for both compressional and shear waves in an Alpine gneiss have been determined along its principal fabric directions under dry, saturated, and high-pressure conditions. Both saturation and pressure introduce large anisotropic changes in velocities, suggesting that the rock contains microcracks that are preferentially parallel to mesoscopic fabric directions. The ratio between compressional and shear velocities in all directions increases with saturation, as can be expected. The changes due to pressure are more complicated: along the direction perpendicular to the oriented microcracks the ratio increases significantly; along directions parallel to the oriented microcracks, however, the ratio changes only slightly; and it either increases or decreases depending on the direction of polarization of the shear waves. |