Detection of abandoned water-filled mine tunnels using the downhole transient electromagnetic method

Many abandoned water-filled mine tunnels at unknown locations in major coal fields in China pose a hazard to the safe production in modern mines. Geophysical methods with rapid and accurate determination of the location and size of the tunnels are needed. The downhole transient electromagnetic method (TEM) applied in a borehole has significant potential for detection. A 3D finite-difference time-domain (FDTD) modelling method was used to simulate the anomalous response and parameter influence in conjunction with a model of a water-filled tunnel. The results show that the method gives a clear response and has distinct benefits for the detection of water-filled tunnels. Three components of the response signal were collected in an exploration hole in the vicinity of an abandoned mine in Shaanxi Province, China. The three components of the signal, especially the horizontal components, indicated obvious anomalies. We compared three different inversion procedures: (1) using three components of the total (primary + anomalous) field; (2) three components of the anomalous field and (3) using the horizontal components of the anomalous field. The results of the three inversions are almost the same and accord with the exploration result from densely distributed exploration boreholes nearby test borehole. The study showed that the downhole TEM effectively solves the problem of detecting and locating water-filled tunnels in the vicinity of a borehole. This information is a valuable complement to the geophysical detection of abandoned coal mine tunnels.