Pressure dependence of Q in selected rocks.

The dissipation factor (Q^-1) for torsional vibration has been measured for several low-porosity crystalline rocks up to 0.5GPa confining pressure. Q was measured at frequencies of approx 10kHz by a resonant bar technique in which cylindrical samples are enclosed in a thin-walled copper sheath welded to copper end plugs, with piezoelectric transducers bonded to the ends. Some of the samples were encapsulated under laboratory conditions (30-40% humidity) whereas others were encapsulated in vacuum (approx 10^-5torr). Q was found to increase by 10³ or more between 0 and 0.5GPa for most of the samples. In some cases, values well over 2000 were found at 0.5GPa. For some of the rocks, including vacuum-dry as well as air-dry samples, Q was found to continue increasing pressure even after most of the cracks were closed. A significant time dependence was also observed. After any increase or decrease in confining pressure, Q was observed to increase slowly for several days at least. Possible attenuation mechanisms and implications for the earth's interior are discussed.-Authors