Seismic time-lapse monitoring of gas drainage in an underground coal working face

Gas drainage is a popular method used for preventing gas outbursts in coal seams. However, quantitative assessment of the drainage remains a challenging task. In this study, we conducted a seismic time-lapse monitoring study at the No. 15110 working face in the Sijiazhuang coal mine to evaluate the effectiveness of the gas drainage process. The result reveals that there is a remarkable difference in frequency bands and computed tomography (CT) image, inverted from the attenuation coefficients of the recorded channel waves, before and after the gas drainage. In the thick mylonitic coal zone, gas drainage operation generally decreases the wave frequency and wave attenuation. The comparable roof refractions from the monitored seismogram and CT-inverted velocity distributions demonstrate that changes in the coalbed roof during the gas drainage are minor. The attenuation differences are attributed mainly to the stress-relieving effects of the thick mylonitic coal zone. This observation is supported by the forward modeling of the channel waves. Time-lapse seismic monitoring employed in this study provides a useful tool for monitoring the impact of gas drainage on a coal working face, including its roof, for any coal mine with restively simple geological structures.