Dynamic monitoring of water abundance of overlying strata in coal seam by transient electromagnetic method

Large-scale exploitation of coal resources in the upper and middle reaches of the Yellow River basin has caused the continuous deterioration of ecological environment.It is helpful to realize the coordinated development of coal resources exploitation and ecological environment protection to understand the influence law of coal exploitation on groundwater system.Taking the south wing of Ningtiaota Coal Mine in Northern Shaanxi as the study area,the geoelectric model is established by analyzing the geological and geoelectric conditions in this area.Based on the time domain electromagnetic field theory,the diffusion process of transient electromagnetic field and the attenuation characteristics of voltage curves at the center of exciting loop under normal (no water) and abnormal (local water) geoelectric model conditions are simulated and analyzed.The results show that the transient electromagnetic response of water-bearing anomalies within 10 ms is obvious.Moreover,the response time range and amplitude of water-bearing anomaly in different layers are also greatly different.Thus,the feasibility of using transient electromagnetic method to detect groundwater in the study area is demonstrated theoretically.According to the site test conditions and combining with the results of theoretical simulation and field measurements in the study area,the transmission frequency of 25 Hz is determined,which is conducive to the acquisition of transient electromagnetic field data in early and middle stage.Through the dynamic monitoring of transient electromagnetic field of the strata,the characteristics of vertical and horizontal resistivity of strata are compared and analyzed.The laws,that coal mining generally reduces the water bearing capacity of the overlying J2z and J2y5 strata,and the relative water rich areas change from the central and north parts before mining to the northeast parts during the stable period after mining,are revealed.At the same time,it is pointed out that the laws are closely related to the flow direction of groundwater,the change of the stratum structure and the variation of overlying soil water insulation property and so on,which further reflect the significant influence of coal mining on groundwater circulation system.Finally,considering the limitations of the transient electromagnetic monitoring results,it is necessary to further integrate other results to reveal the influence law of coal mining on groundwater system as a whole,so as to provide a reference for ecological environment protection and restoration in the mining area.