Experimental study of three-component down-hole TEM for detecting water-filled goaf

The borehole geophysical prospecting method is a research hotspot in recent years. Considering that the down-hole transient electromagnetic method used in metal mines has the characteristics of long radial detection distance and less susceptible to surface electromagnetic interference, it is applied to the exploration of water-filled goaf in coal mines. Three-dimensional finite-difference time-domain(FDTD) algorithm is used to simulate the total field response and abnormal response of the goaf model with different orientations of boreholes. The total field three-component could reflect the water-filled goaf obviously, and the abnormal field three-component shape combination can distinguish the azimuth of the abnormal body. Based on the principle of equivalent eddy current, the spatial location algorithm is studied, which can obtain the position, inclination and scale of anomalous body by using three components of anomalous field. By testing the theoretical models in different directions, the combination of the size and location of the equivalent current loop inverted at different times can reflect the spatial position and scale of the water-filled goaf. Finally, the down-hole TEM detection test was carried out in a mine in northern Shaanxi Province. The location and size of the water-bearing goaf were known in this area. The collected three-component signals are smooth and reliable, and the abnormal response is obvious. The three-component data of anomalous field at different time are retrieved and processed. The inversion results are basically consistent with the actual location and scale of the goaf, which proves that this method can be used as an effective means to explore the water-filled goaf through boreholes in coal mines.