Response characteristics of water-bearing goafs for surface-to-underground transient electromagnetic method

Many of most serious disasters in coal mines around the world are attributed to underground flooding from water-bearing goafs. To ensure the operational safety in the deeper coal seams, water-bearing goafs must be clearly mapped to prevent water inrush accidents. Existing geophysical methods cannot fully meet this requirement. Building on the merits of the transient electromagnetic method (TEM) at the surface and in the mine, we propose a surface-to-underground transient electromagnetic method (SUTEM). We first analyze the differences between the surface TEM and SUTEM with a uniform half-space model. Secondly, we establish a geophysical model for water-bearing goafs and simulate the response signals at different depths with a vector finite-element method. Finally, we conduct a field experiment of SUTEM. The results of the simulation and experiment show: 1) When the surveying line is below the water-bearing goaf, the abnormal response information is mainly focused on the ascending segment of the electromotive force (EMF) curve, and the relative difference (RD) increases with the value of h/z decrease; 2) Applying SUTEM, we can accurately map the lateral position of the water-bearing goaf with the RD.