Elastic wave velocity of granite during triaxial compression under controlled pore pressure

Elastic wave velocity is one of important physical properties to investigate structure in the Earth's interior. Because of a markedly change in elastic wave velocity at the presence of fluid, the geothermal fluid reservoir is frequently detected through seismic tomography. Previous laboratory experiments have carried to investigate effect of confining pressure (e.g. Nur and Simmons, 1969), axial stress during deformation (e.g. Lockner et al, 1977), fluid saturation (e.g. Nur and Simmons, 1969). However, there are few studies examining elastic wave velocity change on fracture process under controlled pore pressure. In this study, we examined change of elastic wave velocity and amplitude during triaxial compression under pore pressure as a fundamental research on estimating of artificial geothermal reservoir on hot dry rock system. We used Aji granite with a cylindrical shape. On dry condition, confining pressure was 20 MPa, and on wet condition, we used water as a pore fluid and confining pressure was 20 MPa and pore pressure was 10 MPa. We adopted pulse transmission method for measurements of elastic wave velocity and amplitude. We observed a systematic change of elastic wave velocity possibly due to closure, growth and formation of cracks during deformation. While elastic wave velocity was increased due to closure of preexisting cracks at the primary stage of deformation, it decreased markedly at the late stage of deformation. Vp/Vs tends to increase during deformation on wet condition while it decreases on dry condition. These data are consistent with theoretical model by O'Connell and Budiansky (1974), in which fluid filled cracks increase Vp/Vs but open (dry) cracks have an opposite influence. Based on the theoretical model, crack density tends to be suppressed during deformation under wet experiments. During deformation, amplitude was decreased with increasing cracks in the specimens, in which P wave has relatively small amplitude compared to wet condition because of less scattering of elastic wave at crack surfaces with water. Our experimental results show a systematic correlation between porosity and elastic wave velocity during deformation, suggesting that extent of fluid reservoir can be adequately monitored through seismic velocity.