Nonlinear elastic properties of intact and fractured Westerly granite

The objective of this work is to unravel the physical mechanisms responsible for the nonlinear elastic response of rocks and, in particular, to understand the role of fractures. Cylindrical samples of Westerly granite, either intact or transected perpendicular to the long-axis by a mated fracture, are subjected to a pump-probe protocol. This protocol applies axial and confining stress oscillations of amplitude ±0.5 MPa and pump frequency f0 ≈ 8.3 mHz, while the elastic state of the sample is probed with S-wave, 500-kHz transducers at each end of the sample. The quasi-static axial stress is also varied stepwise, between 1 and 17 MPa, both upward and downward, while the confining pressure is kept constant (4 MPa). Two nonlinear elastic parameters are extracted: the offset R0, tracks the average wavespeed change induced by the oscillation, while R1 tracks wavespeed changes at f0. Congruent with a previous study, and counterintuitively, R1 is smaller for the fractured samples. R0 is positive (average stiffening) during axial stress up-steps, and negative (average softening) during down steps. Moreover, this effect is more pronounced for the fractured samples, that is, R0 is larger in absolute value for fractured samples. These results are discussed in light of previous measurements conducted on fractured rocks.