Elastodynamic nonlinear response of dry intact, fractured and saturated rock

Nonlinear elastic properties of fractured rocks carry crucial information on fracture features that can be exploited to forecast flow properties, friction constitutive behavior and poromechanical response. We report on a series of experiments designed to compare the nonlinear elastodynamic response of Westerly granite samples under true triaxial stress conditions in three states: dry intact, dry fractured and saturated fractured. We study the effect of fracturing and saturation in modifying the elastodynamic response of the rock. In order to measure the nonlinear elastodynamic response, a dynamic stress perturbation is applied by oscillating the normal stress. Ultrasonic waves transmitted across the fracture are used to monitor the evolution of wave velocity and amplitude before, during and after dynamic stressing. The nonlinearity of the response is evaluated by measuring the stress-dependency of wave velocity, amplitude and recovery rate. As expected, the saturated sample exhibits less nonlinearity than the dry intact and fractured samples due to presence of interstitial fluid and the resulting increased interface stiffness. Conversely, the dry intact rock shows a higher nonlinearity than that for dry fractured. We hypothesize that the fracture significantly reduces the transmission of strain to the half of the sample remote from transmission, thus resulting in a decrease in the measured elastodynamic nonlinearity.