Velocity and Q measurements on rock-water system at high pressure and temperature using a large volume cylinder

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Seismic velocities in dry and water-saturated rocks were measured by acoustic pulse transmission technique. We are able to observe not only first transmitted echoes (first arrivals), but also reflected echoes (second arrivals) generated by the acoustic impedance contrast between the rock sample and the transducer device. We, then, determine accurate seismic velocities and quality factors (Q) in rock samples from both the first and second arrivals.Rock samples used here were a serpentinite, a granite and a pyrophyllite. Both velocity and Q in rock samples decrease with the addition of water. While velocity decreases at most 2%, Q decreases more than 50%. Q is very sensitive to the presence of fluid. The results mean that the seismic Q is a powerful tool to study fluids in the Earth's low Q zone.We have also determined velocity and Q in amphibolite to 800℃ at 1 GPa. The solidus of the amphibolite sample is 700℃ at 1 GPa ; therefore, the effect of partial melt on velocity and Q was studied. Above the solidus temperature, increases in melt fraction cause rapid decreases in both velocity and Q. The results indicate the presence of partial melt in the lower crustal low velocity and low Q zone beneath the volcanoes.