| Water accumulation in goaf is one of the main causes of serious accidents in coal mines in China. With the increase of the depth of coal mining, many coal mines are faced with the problem of detecting multilayer goaf. With the advantages of ground transient electromagnetic method and mine transient electromagnetic method, the ground-tunnel transient electromagnetic method(ground-tunnel TEM) is expected to be one of the effective methods for the detection of multilayer goaf. In this paper, theoretical derivation, numerical simulation and field experiment are used to analyze the feasibility of ground-tunnel TEM research used in detecting multilayer goaf.Firstly, this paper briefly introduces the working method of ground-tunnel TEM and the theoretical formula based on one dimensional layer.In addition, the numerical simulation method and the simulation flow of ground-tunnel TEM are analyzed in this paper.Based on the geological features of goaf area, the corresponding geophysical models are established, The potential response characteristics of the ground receiving and the underground tunnel receiving are simulated.(1) For the single layer goaf area, whether it is filled with water or not(high resistance and low resistance), the abnormal response amplitude of the underground tunnel is higher than that of the ground detection, the resolution of ground-tunnel TEM is better than that of ground TEM.(2) For the double layer goaf, as well as the single layer goaf, the resolution of of ground-tunnel TEM is also better than that of ground TEM; the detection effect of ground-tunnel TEM for double layer high resistance goaf is worse than that for single layer high resistance goaf, howerer the detection effect of ground-tunnel TEM for double layer low resistance goaf is better than that for single layer low resistance goaf; for the double layer goaf consists of high resistance one and low resistance one, the ground-tunnel TEM is better for the detection of low resistivity one than low resistivity one. In summary, ground-tunnel TEM can effectively distinguish the double layer mined out area.(3)With the increase of electrical difference between the goaf and the surrounding rock, the detection effect of ground-tunnel TEM is gradually enhanced; the detection effect of the ground-tunnel TEM gets better with the increase of goaf thickness, this method has better effect on the high resistance goaf with thickness larger than 20 m and low resistance goaf thinner than 50m;for ground-tunnel TEM,the recognition degree of the layer position of the high resistance goaf is lower than that of high resistance goaf.The field test of ground-tunnel TEM and ground TEM is carried in a coal mine in Shuozhou, Shanxi.Comparing the experimental results with the simulation results, it is proved that the ground-tunnel TEM is feasible to detect the goaf. It also shows that the detection effect of ground-tunnel TEM is better than that of ground TEM. |
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