The effect of varying damage history in crystalline rocks on the P and S wave velocities under hydrostatic confining pressure

Cracks play a very important role in many geomechnical issues and in a number of processes in the earth's crust. Seismic waves can be used as a remote sensing tool for determining crack density. The effect of varying crack density on the P and S wave velocities and dynamic elastic properties under confining pressure has been quantified. The evolution of P and S wave velocities were monitored as a suite of dry Westerly granite samples were taken to 60%, 70%, 80% and 90% of the unconfined uniaxial strength of the sample. The induced microcrack damage samples were then subjected to hydrostatic confining pressure up to 200MPa to quantify the effect of varying crack density on the P and S wave velocities and elastic properties under confining pressure. Strong stress-dependency of P and S wave velocities, Vp/Vs, Young's modulus and Poisson's ratio exist during uniaxial loading and unloading due to closure and opening of microcracks. The opening and propagation of microcracks predominantly parallel to the loading direction caused the S wave to decrease dramatically while the P wave decreased at a near constant rate. This resulted in a rapid decrease in the Young's modulus and a rapid increase in the Vp/Vs and Poisson's ratio. The results show that anomalously high Poisson's ratio and Vp/Vs and low Young's modulus can serve as a sensitive indicator of the amount of crack damage in the upper crust. Kachanov's (1994) non-interactive effective medium model was used to invert elastic wave velocities and infer crack density evolution. Comparing the crack density results with experimental data on Westerly granite samples shows that Kachanov's (1994) model gave interpretable and reasonable results at least up to crack density of approximately 0.5. Changes in crack density can be interpreted as closure or opening of cracks and crack growth. Fractures within rocks are closed when they are subjected to increasing hydrostatic confining pressure. We found that fractures without fluid and pressure are completely closed at 130MPa confining pressure in crystalline rocks. We observed an overall 6% and 4% reduction in P and S wave velocities respectively due to an increase in the fracture density at low confining pressures. The overall reduction in the P and S wave velocities decreased to 2% and 1% at 50MPa. Consequently, assessing the degree of fracture between 50MPa and 130MPa confining pressure using seismic wave velocities may be difficult.