Interpretation of seismic data from hydraulic fracturing experiments at the Fenton Hill, New Mexico, hot dry rock geothermal site

An attempt was made to synthesize the results of active seismic experiments carried out by the Los Alamos National Laboratory's Hot Dry Rock Project staff for determining the geometrical and physical properties of the fracture system produced by hydraulic fracturing in a hot, low-permeability rock. Interpretation of data from several reflection, transmission, and attenuation experiments using seismic probes in the frequency range from a few hundred to more than 10/sup 4/ Hz led us to postulate the existence of a highly complex fracture system consisting of major discrete vertical cracks intersected by several inclined joints which are surrounded by a large volume of rock containing small-scale cracks. We found an interesting coincidence between the mean square fluctuation of P arrival time and Q/sup -1/ of coda waves. Both are increased by nearly the same amount (35approx.40%) when fluid pressure in the reservoir is increased. Another coincidence is the scale length of the inhomogeneity of roughly 3 m obtained from the frequency dependence of attenuation and also from the spatial variation of P arrival time. These results suggest that the seismic attenuation in the fractured region is caused primarily by scattering.