Effect of pressure on 3D distribution of P-wave velocity and attenuation in antigorite serpentinite

We have developed a detailed study on the pressure dependence of P-wave velocities and amplitudes on a spherical sample of antigorite serpentinite from Val Malenco, Northern Italy. Measurements were done at room temperature and hydrostatic pressures up to 400 MPa in a pressure vessel with oil as a pressure medium. The transducer/sample assembly allows simultaneous velocity and amplitude measurements on the spherical sample in 132 independent directions. Three significant directions of the foliated sample were selected to study changes of the directional dependence (anisotropy) of velocity, amplitude, and Q-factor with increasing pressure. Remarkable differences are observed between the changes of velocity and attenuation anisotropy as pressure is increased. Although the velocity anisotropy is quite stable through all pressure levels, the attenuation anisotropy and the Q-factor vary significantly in magnitude and orientation. The variations are probably caused by the closing of microcracks due to acting hydrostatic pressure, so the contact conditions between individual minerals consolidate and the transmitting energy is less attenuating.