Acoustic Emission Associated with Self-Sustaining Failure in Low-Porosity Sandstone Under Uniaxial Compression

Abstract

Two sets of uniaxial compression tests were conducted on a brittle sandstone under a constant circumferential strain rate (2 × 10−6 s−1) and a constant axial strain rate (2.5 × 10−6 s−1), respectively. A combination of active and passive ultrasonic techniques was implemented to study the effect of the control method on mechanical deformation, ultrasonic P-wave velocity, acoustic emission (AE) characteristics, and the ultrasonic amplitude spectrum. During each test, active surveys were performed at regular time intervals. P-wave velocity was found to be strongly anisotropic and was used for the construction of a time-dependent transversely isotropic velocity model for each specimen. AE data were continuously acquired and digitized at 10 MHz and 16-bits for the duration of each test where four channels were amplified 30 dB and the rest 50 dB. Discrete AE events were harvested from the continuous waveforms and were then used for source location analysis based on the constructed velocity model and the collapsing grid search routine. An analysis of the ultrasonic amplitude spectrum was also performed to relate attenuation to the formation of macroscopic fracture. In addition, the post-peak energy balance was quantitatively estimated by calculating the rupture energy, surplus energy, and residual elastic energy, suggesting a typical self-sustaining failure. Differences in the post-peak energy balance between the two sets of tests are also reflected in the AE magnitude distribution in addition to the failure modes. Finally, the reason for the large amount of missing AE data associated with eventual rupture was investigated, with the conclusion that multiple gain levels should be adopted during brittle failure of rocks.