Attenuation and velocities of P- and S-waves in dry and saturated crystalline and sedimentary rocks at ultrasonic frequencies

Abstract

Velocities and Q-values for P- and S-waves and their directional dependence have been determined for several silicate rocks (dry and saturated) exhibiting different porosities. Measurements were done at confining pressures in the range 20 to 600 MPa. Velocities and Q-values exhibit a non-linear steep increase with increasing confining pressure, due to progressive closure of microcracks. Q increases steeply normal to foliation as pressure is increased, and the values at high pressures are higher than those observed parallel to foliation. The reverse is true for the velocities. Both, increase of confining pressure and saturation of pore space, reduce the anisotropy caused by oriented microcracks. Saturation affects Q and velocity more markedly in the high-porosity rocks. Basing on the laboratory data, we might conclude that velocity changes and changes of Q-values are, at least in part, different in origin. The simultaneous measurements of velocities and Q provide information on the mechanisms of attenuation, especially for an assessment of whether the predominant damping mechanism is in shear or in compression.