# Application of Transient Electromagnetic Method for Investigating the Water-Enriched Mined-Out Area
## 1 Introduction  
China is currently the world's largest coal miner, and it is also the largest consumer of coal in the world [1; 2; 3; 4; 5; 6]. The start of the 21 st century was a golden era for the coal sector, and the rapid expansion of coal firms led to huge profits [7; 8]. However, based on China's existing technologies, the mining coal lead to huge mined-out regions. Mined-out areas of different scales will occur, which results in unstable stress and it hence brings stress concentration. Under the action of overlying rocks, deformation, cracking and dislocation will occur, leading to the formation of caving zone, fracture zone, and bent deformation belt [9; 10]. Mined-out areas can be water filled, when water intrushes or surface water leaks through cracks into excavated areas, surface damage that is caused by subsequent subsidence might appear [11]. A typical collapsed area can be divided into three zones, such as Caving zone, Fault zone, and crooked zone, as shown in Figure 1[12; 13; 11].  
In most coal fields in China, the mined-out regions are enriched by water, which led to frequent accidents and the serious destruction of ecological environment and serious threat to coal-mining practices [8; 15; 16]. Therefore the detection of the mined-out areas, especially the water-filled mined-out area in advance, is one of the most important issues [17].  
Recently, the main exploration technologies in mined-out areas are drilling exploration and geophysical prospecting [17]. Although the former technology is accuracy, its project volume and cost are massive and the controlled range is limited [18; 19]. The geophysical prospecting is a useful, safe, economical, and rapid way to detect the mined-out areas. Therefore, the mined-out areas detection method is generally given priority to with geophysical prospecting, and drilling technical method validation is complementary [16].  
Geophysical method especially Transient Electromagnetic (TEM) technique, as being non-invasive and cost-effective, acquisition of strong signals, deep levels of penetration, easy deployment, and suitability for geological exploration, which remains the first choice to explore the target with conductive characters. TEM has been widely used in various fields, such as engineering geology [20] and environmental geophysics investigations [21], mining [22], urban construction [23], tunnel prediction, and groundwater [24] and geothermal explorations. Due to its sensitivity to conductive geologic body when compared to the surrounding rocks with high resistivity, TEM has received great attention in the mined-out area with water investigations [25].  
In this paper, a novel large loop TEM method that combines the advantages of high signals to noise ratio, large investigation depths has been used to investigate geological structures and electrical characteristics in order to map the water-enriched mined-out areas within the survey region. The survey area was located in a coalfield located in the Shanxi Province, northern China [18]. The large loop TEM technique was employed successfully in detecting the water-enriched mined-out areas at a depth of 300 m. These findings were verified by drillings.