High-frequency (8--15 kHz) seismic waves that travel between two boreholes are used to probe the detailed structure of rock in the Los Alamos National Laboratory Fenton Hill hot dry rock geothermal energy reservoir. We study travel times of direct P waves and spectral characteristics of waveforms in an effort to determine whether or not large fluid-filled fractures are present in the rock between the two boreholes. Data from two experiments, separated in time by 1 year, are studied. We find dramatic differences between the two experiments in P wave velocity and in spectral characteristics of waveforms that traversed the same region. We also find variations in P wave velocity and spectra as a function of location during one experiment.

Loss of high-frequency content of direct-arriving P waves during the second dual-well seismic experiment is compared with Q, found by using the coda wave technique (Aki and Chouet, 1975), and also with theoretical results on frequency dependence of transmission through a water-filled fracture to determine the minimum number of large fractures in the system. Two to four fractures with thickness about 4 mm and vertical extent of as much as 200 m are required to explain observations during the second experiment. Efficient transmission of high frequencies observed during the first experiment is interpreted as meaning that fracture thickness was less than 1 mm at that time. Increased fracture thickness during the second experiment is probably due to a 75-day period of heat extraction during which in situ temperature dropped by as much as 100 ¿C below the virgin temperature. Since the coefficient of linear thermal expansion of granite is approximately 10-5/¿C, a decrease in temperature of 100 ¿C in a region on the order of 1 m around the fracture is required to explain the increased fracture thickness.