Time-frequency characteristics of acoustic-electric signals induced by coal fracture under uniaxial compression based on full-waveform

The geological conditions of coal mines in China are complicated. In recent years, with the continuous increase of intensity and depth of coal mining, coal and rock dynamic disasters are becoming more and more serious, and an important factor threatening the safety of coal mining. The accurate monitoring and early warning of coal and rock dynamic disasters are of great significance for disaster prevention. A large number of experiments conducted on both laboratory and field scales have demonstrated that the energy accumulated in rock material under loading can be released in the forms of acoustic emission (AE), electromagnetic radiation (EMR), etc. Therefore, AE and EMR, as the real-time, dynamic and continuous geophysical monitoring methods, have been widely used and played important roles in the field of monitoring and early-warning of coal and rock dynamic disasters in mines. To further study the time-frequency characteristics of AE and EMR and the relationships between these characteristics and load, and to provide the experimental basis for the monitoring and early warning of coal and rock dynamic disasters, an acoustic-electric full-waveform synchronous acquisition system of coal and rock fracture under loading was constructed in this paper. Using this system, the full-waveforms of AE and EMR of failure processes of coal samples under uniaxial compression were collected. The correlations among AE energy, EMR energy and load drop were studied, and the spectral characteristics of AE and EMR were analyzed. The results show that, (1) obvious acoustic-electric signals are emitted during the process of coal failure under loading. EMR is the paroxysmal signal, only accompanied by load drop and higher intensity AE. (2) Compared with AE, EMR has a higher correlation with load drop. There are high positive correlations among the cumulative values, relating to the cumulative released energy of coal fracture under loading, of AE energy, EMR energy and load drop. (3) The superiority frequency band of EMR is narrower than that of AE, with the former mainly concentrated in 1-25 kHz and the latter mainly in 1-280 kHz. Influenced by the initiation and propagation of the same crack, AE and EMR have similar low frequency components in the spectrum and main-frequency distribution.