Transient Electromagnetic Signal from Impact Damages of an Inherent Gas-containing Coal Seam

Methane is a companion of coal formations, and it has a complex interaction with the coal seam and its surrounding rock. Methane not only has characteristics of weakening coal strength but also influences the characteristics of electromagnetic radiation (EMR) during coal damage. In this paper, a drop hammer impact test method was introduced to conduct dynamic loading on gas-containing coal samples with different pore pressure treatments. Then, a typical high gas and outburst coal mining workface was selected to carry out a long-term field site experiment. The EMR signal features collected during both the experimental tests and field site tests were analyzed to investigate the influence of methane. The results show that: i) there is a clear correlation between the pore pressure and the transient EMR signal characteristics during the failure of the gas-containing coal sample; with the increase in the pore pressure, the overall intensity, duration and total energy of the EMR signal decreases accordingly; ii) the EMR signals measured in the two directions at the site have great consistency; these signals are cluster pulses with energy in the range of 0.1 mJ–2 mJ; iii) the energy, pulse counts, and duration of the EMR signal have good positive correlation with the blasting charges; and iv) with the increase in the gas content in the coal seam, the amplitude and energy of the EMR signal induced by the blasting vibrations are significantly decreased; however, the main frequency of the signal is not substantially affected. This study provides a basic demonstration to prove the effect of methane on the dynamic rupture of coal and the features of the produced EMR, which are of great significance to the application of EMR to conducting early warning of outbursts.