Relationship between anisotropy of P and S wave velocities and anisotropy of attenuation in serpentinite and amphibolite

Abstract

Velocities and Q values of P and S waves as functions of pressure and temperature (at 100 and 600 MPa) are presented for a serpentinite and an amphibolite. Both rocks exhibit a strong lattice preferred orientation (LPO) of the major mineral phases antigorite and hornblende, respectively. Velocities and Q values increase with pressure; the rate of increase is different in the three orthogonal directions (normal and parallel to foliation and lineation) and closely related to progressive closure of microcracks. Increasing temperature decreases velocities and Q values only slightly as long as thermal cracking is prevented by the applied confining pressure. Substantial anisotropy of velocities and Q in P and S waves is observed in both rocks but is found to be different in origin. Anisotropy of P and S wave velocities is highest at low pressure and basically caused by constructive interference of effects related to oriented microcracks and to the LPO of major minerals. Increasing confining pressure decreases velocity anisotropy at a smaller and smaller rate. The residual anisotropy of P and S wave velocities (shear wave splitting) at high confining pressure is mainly a result of preferred mineral orientation. By contrast, anisotropy of Q is very low at low confining pressure and markedly enhanced as pressure is increased. At high confining pressure, substantial anisotropy of Q in P waves is apparent but reversed from that of P wave velocities: Qp is highest in the direction normal to the foliation plane whereas Vp (and VS) is lowest in this direction. The generation of a pronounced anisotropy of Qp by increasing pressure is due to a directionally dependent increase of contact areas on the oriented grain boundaries of the platy minerals defining the foliation. The increase of Q with pressure in the direction normal to foliation is mainly caused by the decrease of energy loss due to compressive strain relative to shear strain. The reverse is true for the X and Y directions (serpentinite) and X direction (amphibolite) parallel to the foliation plane.