Application of three-component acoustic emission sensor in rock mechanics experiments

In laboratory rock mechanics experiments, it is common practice to employ single-component acoustic emission (AE) sensors to monitor the rock fracturing process, where the fracture location and fracturing mechanism can be inverted from the single-component AE waveforms. However, since such applications are often based on simplified isotropic hypotheses, accurately capturing the influence of strong anisotropy in rock on particle vibrations at rock surfaces using single-component AE sensors remains challenging. This limitation biases the actual particle vibration amplitude and direction, potentially leading to erroneous estimations in AE waveform inversion mof rock fracturing. Here, we combined three piezoelectric discs to create a three-component AE sensor, allowing us to capture particle vibration along three sensing directions on rock surfaces. Through two calibration experiments and polarization analysis of the three-component waveforms, we verify the new sensor's capability to reliably capture the vibration features of hypocenters that are not easily obtainable using a single-component sensor. We apply this new sensor in uniaxial compression tests on shale, a material known for its strong anisotropy. The results indicate that the AE behaviors monitored by the three-component sensor are consistent with those obtained from the widely used single-component sensor. Moreover, the polarization analysis of the three-component waveforms reveals the presence of coupled hypocenters associated with multiple fractures. This is the first time a three-component AE sensor is used in laboratory rock mechanics experiments, which offers a more comprehensive understanding of the dynamic evolution of fractures in rock formations.