Effect of coil coupling distance on roadway transient electromagnetic field and its correction method

Water damage is one of the major disasters in deep coal mining in China. As a powerful method of water damage detection, the roadway transient electromagnetic method (RTEM) has been widely used in major coal mining areas in China in recent years. However, due to the limitation of roadway space, an unequal coupling distance exists between the coil and the surrounding rock surface, resulting in differences in the RTEM field observed at different measuring points, which in turn affects the determination accuracy of the water abundance of the rock strata. Based on the numerical simulation, the effect of coupling distance on the observed RTEM field with and without metal support in the roadway is studied. The results show that:①When there is no metal support in the roadway, the effect of coupling distance on the RTEM field is mainly concentrated in the early stage. It decreases rapidly with the time delay of the RTEM field, and the effect on the measured RTEM field data can be ignored. ②When there is metal support in the roadway, there is an obvious inflection point in the RTEM field curve, and the corresponding time is prolonged due to the increase of the surrounding rock resistivity; Compared with the RTEM field without metal support, the RTEM field response amplitude before the inflection point is significantly increased by metal's influence, and there is no apparent decay law; After the inflection point, the RTEM field amplitude changes significantly, but the decay trend is consistent; The coupling distance greatly influences the observation of the RTEM field, especially at the observation point directly in front of the roadway excavation, which is more sensitive to the change in the coupling distance; The correlation coefficients of the RTEM field curves obtained under different detection directions, different resistivity of the surrounding rock and different coupling distances tend to be 1, with logarithmic translation between the curves. Accordingly, a translation correction method for the RTEM field data in the late stage is proposed, and numerical verifications are carried out for the model with the low-resistance spherical body in front of roadway excavation. The comparison with the data before the correction shows that the corrected RTEM field data can determine the low-resistance body more accurately, and the influence of the coupling distance on the RTEM field observation is effectively eliminated. The field test further supports the method's reliability, which can provide technical support for preventing and controlling water damage in the roadway.