Innovative Surface-Borehole Transient Electromagnetic Method for Sensing the Coal Seam Roof Grouting Effect

With the purpose of monitoring coal seam roof grouting, an innovative surface–borehole transient electromagnetic (TEM) method to compare the Earth resistivity distribution of the coal seam roof before grouting and after grouting is proposed. The TEM secondary field time–decay curve of the proposed surface–borehole TEM method is compared with that of the traditional surface TEM method. It is found that the time–decay curve of the former is very different from that of the latter. For the first time, forward modeling algorithm has been proposed to evaluate the TEM response characteristics of Earth, with an inversion algorithm developed to invert the measured data into resistivity distribution. A field work has been implemented with the customized equipment to evaluate the surface–borehole TEM method. In the field work, the proposed surface–borehole TEM measurement is performed twice, i.e., before and after the coal seam roof grouting. The analysis results demonstrated that the resistivity distribution had a distinct change near the depth of grouting with low resistivity distribution replaced by high resistivity distribution, which validates the effectiveness of the proposed surface–borehole TEM method.