# Electromagnetic Field Distribution and Data Characteristics of SUTEM of Multilayer Aquifers
## 1 Introduction  
Although energy transition is the core task of global sustainable development and combating climate change, the international demand for coal continues to increase [1]; therefore, coal has remained the \"mainstay\" of international energy in recent years. However, mining operations are threatened by various kinds of groundwater during coal extraction [2]. The safe extraction of coal resources has always been a major concern, and geophysical exploration is commonly used to gather geological information about coal seams as well as roofs and floors [3] to ensure the safe mining of coal. Among these, the methods used for coal mine water disaster detection include the mine DC resistivity method [4], transient electromagnetic method [5], wireless electromagnetic wave perspective detection technology [6], etc. Mine TEM has become the mainstream method of coal mine water disaster prevention and control by virtue of its advantages of close distance to the target body and high construction efficiency [7]. However, due to the limitation of coal mine roadway space, the mine TEM can comprise only a multiturn small loop for detection, with a side length not more than 3 m \(\times\) 3 m, which can effectively detect a hidden water inrush source within only 100 m [8]. For a target body far from the working face, a large transmitting loop [9] is usually laid on the surface, and then ground TEM is used to increase the detection depth. However, ground TEM is not sensitive to the deep geological body, especially when there exists a shallow aquifer. The ensuing detection results have difficulty meeting the current requirements of coal mine water prevention and control. Based on these challenges, to improve the depth and accuracy of the TEM in water detection, prevention, and control in coal mines with reference drill-hole TEM [10], in [11] the authors combined the advantages of the ground TEM (e.g., the emission source is not limited by space) and the characteristics of the mine TEM (e.g., it is close to the target body) to propose SUTEM detection technology. For many years since, researchers have continuously studied and improved SUTEM technology. For basic theoretical research, the problem of the SUTEM field source was analyzed [12], the late apparent resistivity of the SUTEM was defined [13], and the interpretation method of the SUTEM data vector intersection was realized [14]. To address abnormal response characteristics, a large number of numerical simulations were carried out considering common hidden geological bodies [15]. Nevertheless, the coal strata belong to the class of sedimentary strata [16; 17], and there are usually multiple aquifers above the coal seam. Among them, the source of water intrusion disasters affecting coal seam mining is mainly deep aquifers. The accurate detection of deep aquifer depth and other information is helpful to the safe mining of coal seams. In [18] the authors studied the response of ground TEM to detect double-layer water-filled goat in coal mines. However, when the TEM emission source is located on the surface, due to the existence of the shallow, low-resistivity aquifers, the secondary field propagation generated by deep aquifers or geological bodies is limited, which dramatically reduces the effectiveness of ground TEM for detecting deep targets. At present, there are relatively few studies on the detection capability of TEM in deep aquifers under the influence of shallow aquifers or how these dynamics affect the data characteristics of SUTEM. However, research on the distribution characteristics of TEM and the data characteristics of SUTEM under the influence of multiple aquifers can provide a theoretical basis for the interpretation of SUTEM.