Evolution of Elastic Wave Velocities and Amplitudes During Triaxial Deformation of Aji Granite Under Dry and Water‐Saturated Conditions

Elastic wave velocities and amplitudes in fine‐grained Aji granite are measured during a series of deformation experiments under both dry and fluid‐saturated conditions, where compressional and shear waves traveled in a direction normal to the compressional axis, and shear wave was polarized normal to the compressional axis. In the early stages of deformation, both V P and V S increase slightly due to closure of preexisting cracks and then decrease due to nucleation and crack growth as the sample approaches failure. During deformation, the different sensitivities of compressional and shear waves to fluid‐filled cracks mean that the ratio V P / V S tends to increase for wet experiments and decrease for dry experiments prior to failure. The relative changes in elastic wave amplitudes are also related to deformation and fluid saturation: amplitude decreases at high differential stresses due to the opening of cracks oriented subparallel to the axis of maximum compression, whereas a relatively minor change in amplitude was observed under fluid‐saturated conditions. Thus, the changes in velocity and amplitude during the deformation experiments are linked to crack development and geometry. However, this sensitivity differs between dry and fluid‐saturated conditions. The monitoring of fluid injection volumes during fluid‐saturated experiments shows that decreasing elastic wave velocities and amplitudes coincide with increasing fluid injection. These systematic relationships among elastic wave velocity, amplitude, crack development, and fluid saturation suggest that understanding changes in seismic wave properties can offer significant insights into the temporal changes in the structure of fracture zones.