Stress characterization of surrounding rock during extraction of ultra-thick coal seam based on borehole electromagnetic radiation

Electromagnetic radiation monitoring technology is currently used to conduct non-contact directional measurements in roadways, which enables large-scale regional scanning, but in the practice of ultra-thick coal seam extraction it suffers from poor anti-interference performance and the inability to penetrate into the coal-rock mass. To address these problems, a borehole electromagnetic radiation monitoring system suitable for mobile monitoring inside boreholes was developed. The system employed a borehole electromagnetic radiation antenna with a small cross-sectional area and low inductance magnetic core, which exhibited optimal response characteristics in the frequency band of 0-20 kHz and effectively overcame the challenges of complex underground electromagnetic environments and signal attenuation during long-distance transmission, enabling accurate capture of weak electromagnetic radiation signals from deep coal mass. Field measurements of electromagnetic radiation were conducted using the borehole electromagnetic radiation monitoring system at the 1101 working face of Zhundong No. 2 Mine of State Grid Energy Xinjiang Zhundong Coal Power Co., Ltd. along the coal seam dip direction, the working face strike direction, and during the pressure relief process of large-diameter boreholes. The results showed that the intensity and counts of borehole electromagnetic radiation were significantly correlated with the distribution of surrounding rock stress. The spatial distribution patterns of borehole electromagnetic radiation signals along the coal seam dip direction accurately characterized deep surrounding rock stress concentration zones, and the distribution patterns along the working face strike direction effectively delineated the spatial boundaries between the in-situ stress zone and the mining-affected zone, enabling accurate inversion of the surrounding rock stress state during ultra-thick coal seam extraction. The temporal evolution of borehole electromagnetic radiation during the pressure relief process corresponded to three stages, namely the pressure relief silence stage, the stress adjustment and fluctuation stage, and the rheological damage active stage, which effectively characterized the stress evolution of surrounding rock during the pressure relief period.