Crack-induced acoustic emission and anisotropy variation of brittle rocks containing natural fractures

The acoustic emission (AE), ultrasonic velocity and stress-strain behaviors of brittle rocks containing natural fractures were simultaneously measured under uniaxial compression. An optimized sparse network of transducers was used to monitor the crack-induced AE activities and changes of velocities in different directions. The variation of rock anisotropy was co-characterized by wave velocity and localization of AE hypocenters. The microscale behaviors of rock failure were investigated by the analysis of AE waveforms and multiple characteristic parameters. A flurry of AEs behaving at low frequency and low amplitude appear even under a low stress level for fractured rocks. With an increase of stress, the initiation of cracks first occurs along the natural fracture accompany with increased dominant frequency, then decreases rapidly after the coalescence of cracks. The natural fracture is the main source of rock anisotropy, which decreases first as the pressure-induced closure of fracture, then increases with the opening of cracks along the fracture. For rocks with a higher inclined fracture, the anisotropy parameter is more sensitive to the initiation, propagation and coalescence of cracks. Unexpectedly, the increase of AEs occurs much earlier than the inflection of velocity, and AE hypocenters based on the time-variable velocity depicts the localization and propagation of cracks well. AE energy increases precipitously after 98% peak strength, accompanied with the sharp increase in the ratio of low-frequency AEs, which is a perfect precursor of potentially unstable nucleation.