Seismological Investigation of the Mechanical Properties of a Hot Dry Rock Geothermal System

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 Scientific Labora:ory Fenton Hill geothermal energy reservoir. Because of the unique ability to control fluid pressure in the reservoir, we can make in situ seismic measurements at different fluid pressures and apply the results of pertinent laboratory measurements and theoretical models of rock containing cracks to the velocity and attenuation data obtained. Two theoretical problems involving the interaction of seismic waves with a fluid-filled crack are studied. First the reflection and transmission of plane P and S waves incident on a fluid layer imbedded between two elastic half spaces are studied as a function of fluid layer thickness, fluid viscosity, elastic properties of fluid and solid as well as angle of incident wave. It is found that a water layer as thin as 1 mm can have a strong effect on seismic waves in the kilohertz frequency range. Transmission is freauency dependent in the range 8-15 KHz if layer thickness is greater than 1 mm. Secondly diffraction of plane P and S waves by finite two dimensional empty and fluid-filled (zero viscosity) cracks as a function of wavelength and direction of incident wave is studied. Wavelengths on the scale of the crack length are considered.