Effect of High Temperature and High Confining Pressure on Compressional Wave Velocities in Rocks
Compressional wave velocities have been measured in granite, granulite, amphibolite, and peridotite specimens under temperatures up to 700 °C and confining pressures up to 6 kbars. In general, velocity increases with pressure and decreases with temperature. Quartz-bearing rocks show an anomalous behavior of their compressional wave velocities. The velocity-temperature relations exhibit a velocity-minimum due to the high-low inversion of the constituent quartz crystals. The intrinsic effect of temperature on velocities is hard to determine, due to thermal expansion and the consequent loosening of the structure. The opening of new cracks and the widening of old cracks causes a large decrease in compressional wave velocities. The effects of grain boundary characteristics on elastic wave propagation are progressively eliminated as the pressure is raised and the loosening of structure in the rocks caused by the effect of temperature is balanced by the confining pressure. The minimum pressure to prevent damage at a given temperature should, therefore, be about 1 kbar/100 °C. The values obtained under these conditions are considered to be as correct as the intrinsic properties of the compact aggregates. Velocity anisotropies at high confining pressures and high temperatures correlate with preferred lattice orientation of the constituent minerals. The effect of dimensional orientation and microcracks on seismic anisotropy seems to be of minor importance in dry rocks. The higher the confining pressure the less important it becomes. Seismic anisotropy induced by preferred lattice orientation of the constituent minerals, however, remained unaffected even at pressures of 6 kbars and temperatures of more than 700 °C.