Surface-to-Underground Transient Electromagnetic Detection of Water-Bearing Goaves

Water-bearing goaves are one of the main causes for water inrush accidents in Chinese coal mines. At present, both the surface and subsurface geophysical technologies cannot fully meet the technical requirements of high precision detection for coal mines safety development and production. Based on the surface-to-borehole electromagnetic method, we propose a surface-to-underground transient electromagnetic prospecting (SUTEM), in which the large rectangular loop is laid on the surface as the transmitting source and the receivers are laid in the subsurface roadways of the coal mines. First, we describe and discuss the SUTEM signal response using numerical simulation for a homogeneous half space. The numerical results show that the curve of the induced electromotive force (EMF) first increases to a peak, and then decreases gradually till the end of the time window, the shape of the curve closely reassembles a parabolic. So the conductive bodies can be identified with the characters of the EMF curve. Second, we conducted a numerical simulation using the 3-D, unstructured tetrahedral grids vector finite-element total-field electromagnetic forward modeling algorithm for the response of typical 1-D layered models and 3-D models. The results show that the relative sensitivity is no relation with the depth of the abnormal layer when receiving points are under the layer, the deeper the abnormal layer is, the more obvious the advantage of SUTEM is. Finally, the field experiment was carried out at a coal mine, where the positions of water-bearing goaves are known. The SUTEM detection results do not exhibit a clear trend in the variation of electrical properties in the vertical direction of the formation. However, the variation in the lateral electrical properties is clear, the expression of the low-resistivity goaves is much clearer than the results from the surface TEM. The field experiment result is in agreement with the suggestions of the computer simulation and confirms the effectiveness of SUTEM.