Detection of Shallow Buried Water-Filled Goafs Using the Fixed-Loop Transient Electromagnetic Method: A Case Study in Shaanxi, China

The fixed-loop transient electromagnetic method (TEM) is used to detect the distribution of goafs in a proposed thermal power plant in the northern part of Shaanxi Province, China. A basic geoelectric model of the goaf was established using borehole data, a pore-skeletal model, and a parallel resistance model. The Arjuna module of the Maxwell software was used to perform 2.5-dimensional forward modeling in order to make the model more realistic, and then the conventional software IX1D was used to perform one-dimensional inversion of the forward results. The model results show that the low-resistivity anomaly composed of the water-filled goaf and the fissures in its roof and floor forms an obvious anomalous response on the resistivity contour section. This abnormal feature can be used as the basis for the interpretation of the goaf. When there are no cracks in the roof or floor and only water-filled goafs, it is difficult to identify low-resistivity anomalies. The processed data collected on site show that the fixed-loop TEM has a clear image of the shallow strata geoelectric structure. The coal-bearing seam is the stable and continuous high-resistivity target layer. According to the results of the fixed-loop TEM, the range of the goaf, the range of the coal-bearing strata with the pores filled by argillaceous materials, and the range of the coal-free strata with the pores filled by calcareous materials are respectively delineated. The TEM results agreed well with the available borehole data. The TEM detection results are reliable and replace the need for extensive drilling.