Nonlinear elastodynamic behavior of intact and fractured rock under in-situ stress and saturation conditions

We probe mechanisms controlling the nonlinear elastodynamic response of intact and fractured rock under both fluid-saturated and dry conditions. We present the results of dynamic acoustoelastic testing (DAET) on Westerly granite in three states: dry-intact, dry-fractured and saturated-fractured to study the influence of in-situ stress, fracture and saturation state on nonlinear elastodynamic behavior. Each sample is tested at effective normal stresses from 10 to 20 MPa. Dynamic stresses of prescribed amplitudes (0.2 – 1.0 MPa) and frequencies (0.1, 1 and 10 Hz) are superimposed on the static stress field, while pulsed ultrasonic waves transit the sample and monitor stress-induced changes in wave velocity and spectral amplitude to infer sample's elastodynamic nonlinearity. Surprisingly, dry-intact rock exhibits higher nonlinearity than dry-fractured sample at all stress levels. We use numerical simulations to argue that the reduced nonlinearity is a result of the non-uniform strain field post fracture with highly strained regions on the fracture plane and weakly-strained regions in the surrounding host-rock, both leading to a nonlinearity reduction. In addition, we demonstrate that the saturated sample is less nonlinear than the dry intact except at 10 MPa normal stress, where the fracture is fairly open. The expected decrease in nonlinearity is due to increased fracture stiffness by the presence of fluid within the fracture interface. The reported in-situ measured nonlinear elastic properties along with their frequency dependencies will be of practical importance in predicting poromechanical properties of rock masses and will facilitate comparisons between the observations made in the laboratory and field scales.