Cross‐hole seismic surveys: Applications for studying subsurface fracture systems at a hot dry rock geothermal site

Abstract

The use of cross hole seismic surveys for delineating the location and size of subsurface fracture systems is investigated. The radiation pattern for P and S waves emitted by a seismic source in a borehole is derived. Experimental work in relatively homogeneous granite suggests that the derived relationship adequately describes the radiation pattern for both explosive sources and acoustic transducers placed in fluid‐filled boreholes. Using the above functional expressions for the S‐ and P‐wave amplitudes, we have developed a technique to estimate Q and locate discrete fractures in crystalline rock that compose the Hot Dry Rock Geothermal Reservoir at Fenton Hill, New Mexico. To calculate Q, we measure the P‐ and S‐wave amplitudes as a function of distance from the source, compensate for the radiation pattern and geometrical spreading effects, and match the relative attenuation to a function of the form [Formula: see text] using a least‐squares regression technique. For undisturbed parts of the reservoir, we obtain values of Q on the order of 160 for P waves and from 170 to 150 for S waves. Using our method we are able to detect a decrease in the average Q due to extensive fracturing following heat extraction from the Fenton Hill reservoir. We have also been able to locate discrete large‐scale fractures in the reservoir by noting regions characterized by a sudden change in signal amplitude, wave form or frequency content that cannot be explained by radiation pattern effects or increases in source‐receiver separation. These localized attenuating regions in the Fenton Hill Hot Dry Rock reservoir are probably open fractures.