Principle of using borehole electrode current method to monitor the overburden stratum failure after coal seam mining and its application

Abstract

The overburden stratum of the working face mined using caving method will be damaged after mining. Mastering the development heights of the caving zone and the water-conducting fracture zone is of great significance for the safe mining. Theoretical analysis showed that the electrode current is inversely proportional to the stratum resistivity. Changes of the electrode current can reflect the changes of the stratum resistivity. In this study, we elaborated the principle of electrode current method and proposed the method and construction procedure of using the borehole electrode current method to monitor the overburden stratum damage. Before mining the coal seam of a certain working face, two boreholes with different elevation angles were drilled to the roof of the coal seam working face in the adjacent roadway, and the on-site experiment of overburden stratum deformation and failure caused by coal mining was carried out. Analysis of the measured data showed that the electrode current is very sensitive to the influence of the caving zone with amplitude dropping by more than 90%. The electrode current of the fracture zone changes more dramatically than the apparent resistivity. With the gradual advancement of the working face, the currents of the borehole electrodes in the caving zone and the fracture zone generally show a trend of gradual decrease with good stability. The on-site application of the method shows that the current or current ratio of the borehole electrodes can directly reflect the resistivity of stratum near each electrode, and the height of the caving zone and the water-conducting fracture zone can be determined. Overall, using the borehole electrode current method to monitor overburden stratum damage is effective, and the data changes are stable and easy to explain. The method does not require complex data inversion and has low requirement for data quality. Thus, it is suitable for working face with complex overburden stratum.