Characteristics and generation mechanism of ULF magnetic signals during coal deformation under uniaxial compression

Low-frequency magnetic signals with the advantages of fewer interfering factors and longer propagation ranges are more useful for improving the accuracy of monitoring and early warning technology for coal or rock dynamic disasters. In this paper, the characteristics of low-frequency magnetic signals generated during uniaxial compression failure of coal are investigated in the time and frequency domain. The generation mechanism of low-frequency magnetic signals is proposed based on the correlation between low-frequency magnetic signals and micro-seismic signals. The experimental results show that (1) abnormal ultra-low-frequency (ULF) magnetic signals can be clearly observed in the process of coal failure. The signals occur as oscillating pulses, with amplitudes between 13 and 156 nT. (2) Both the maximum amplitude and energy of the signals are both positively correlated with the strength of samples and loading speed. (3) Signals in the direction perpendicular to the crack surface are the strongest, and those parallel to the crack surface are the weakest. (4) Propagation and oscillation of the crack surface with charge are the main causes of ULF magnetic signals during coal fracture. After the main rupture, the subsequent rotation and motion of fragments with charge can also produce ULF magnetic signals.