Coal Mine Hidden Disaster Detection Based on Opposing Coils Transient Electromagnetic Method Open Access

Although the frequent occurrence of coal mine water disaster accidents in China has been alleviated, with the shift of the center of gravity of coal development to the west and the deepening of mining depth, major water disaster accidents still occur sometimes, which makes the incidence of water disaster accidents and the number of deaths caused tend to rise [1-3]. Therefore, it is the key measure to strengthen the prevention and control of coal mine water damage and effectively prevent the occurrence of water damage accidents to carry out the census and management of hidden water disaster–causing factors [4, 5]. According to Sun et al. [6], there were 1103 coal mine water disasters in China from 2001 to 2022, and the number of deaths reached 4667. Chang et al. [7] used the underground transient electromagnetic method (TEM) to detect the water-filled goaf roadway to deal with sudden accidents in coal mines. The results show that the low-resistance area of the apparent resistivity contour can accurately reflect the water filling situation in the goaf. Wang et al. [8] used the opposing coils transient electromagnetic method (OCTEM) to carry out the surface exploration of the water channel in the coal mine in the urban environment. Through the comprehensive analysis of the detection data in the low-velocity region and the low-resistivity region, the distribution map of the water-conducting channel is preliminarily judged and drawn. This result is roughly consistent with the subsequent drilling coring analysis. Ma et al. [9] successfully obtained the characteristic parameters of spontaneous combustion in goaf through high-rise drilling site and corresponding drilling technology. In addition, the coal spontaneous combustion disaster was controlled by using effective means such as grey correlation analysis method, which provided a valuable reference for identifying and properly dealing with similar accidents in goaf. Therefore, the detection of hidden water disasters usually needs to be sensitive to the electrical and magnetic changes of underground media and can quickly obtain high-resolution profiles of underground media. It is suitable for various terrain and geomorphological conditions and has a highly practical method. OCTEM has these characteristics and has a good application effect in detecting hidden water disasters in coal mines.

OCTEM is a new geophysical exploration method with high resolution. Based on the traditional TEM, this method theoretically realizes the high-precision detection of the TEM in the depth range of zero to several hundred meters by eliminating the influence of the primary field of the receiving coil [8-10]. Because of its sensitivity to low-resistivity geological bodies, OCTEM has become an important means to detect hidden water–damaged geological bodies [11, 12]. In this study, geophysical exploration work was carried out on the old goaf and the hidden water hazards of Hengyang Coal Mine in Hunan Province in the planned mining area in the next 5 years. Through the data collected by OCTEM, the inversion result map was generated by Surfer software, and the slice diagrams and three-dimensional shape rendering diagrams were drawn by Voxler software. Combined with the data of on-site geological drilling, the spatial location, buried depth, and water accumulation of hidden water hazards such as air-filled goaf and water-filled goaf under coal mine were detected [13].

The coal mine is located in Hengyang City, Hunan Province, China (see Figure 1), with low mountains and hills. The mountains are arranged in NE-SW direction along the stratigraphic direction. The terrain is high in the northwest and low in the southeast. The valley is deeply cut and distributed along the dip direction of the stratum, forming many isolated peaks and steep ridges. The highest point is 420 m; the lowest point is 153 m; and the general elevation is about 160 m. Daling coal mine exposed strata from new to old. Quaternary (Q): it is mainly composed of subclay and clay, followed by silt, sand, and breccia; Lower Triassic Daye Formation (T₁d): it is mainly composed of blue-gray, dark gray thin to medium-thick layered argillaceous limestone, with fine crystalline limestone, calcareous mudstone, and calcareous siltstone; Upper Permian Dalong Formation (P₂d): The upper part is dark gray medium-thick layered siliceous limestone; the middle part is dark gray medium-thick siliceous rock and gray-black thin layer of siliceous mudstone interbedded; and the lower gray-black thin layer of siliceous mudstone with a small amount of siliceous limestone; Longtan group (P₂l): It is composed of light gray to dark gray, medium-fine grained, fine-grained sandstone, black-gray siltstone, gray-black sandy mudstone, mudstone, and coal seam. According to lithology, coal-bearing property, paleontology, and other characteristics, it is divided into upper and lower sections. The upper section (P₂l₂) is mainly composed of limestone, argillaceous limestone, fine sandstone, mudstone, calcareous mudstone, claystone, carbonaceous mudstone, and coal seam. According to the characteristics of lithology and coal-bearing property, it is divided into six groups from top to bottom: 1 coal group, thin sandstone group, 2 coal group, 5 coal group, loose sandstone group, 6₁, 6₂, and 6₃ coal group. The lower section (P₂l₁) is composed of sandstone, siltstone, sandy mudstone, and thin coal seam.

The aquifer groundwater in the study area is mainly recharged by atmospheric precipitation. Atmospheric precipitation infiltrates into each aquifer through rock strata, weathering fissures, structural fissures, and collapse fissures produced after mining. Due to the large fluctuation of terrain, atmospheric precipitation is easy to form surface runoff flow, only a small part of which infiltrates into the underground to recharge each aquifer. The dynamic change of groundwater is strictly controlled by atmospheric precipitation, which has the characteristics of rainy season recharge and seasonal excretion. The Daye Formation (T₁d) limestone, the Dalong Formation (P₂d) siliceous rock, and the Longtan Formation (P₂l) coal-bearing strata constitute the hills in the area, which are directly recharged by atmospheric precipitation. The groundwater flows to the low-lying valleys on both sides of the watershed and is exposed in the form of springs in the low-lying valleys or structural fissures in the mountains. It is collected in the lowest erosion gully stream in the area and then flows into the stream to the Leishui River. Another part of groundwater seeps into the goaf of old kiln and the sandstone fissures of coal measures strata in the subsidence area and fracture area of Longtan Formation coal measures strata.

The Daye Formation is mainly composed of marl, argillaceous limestone, and limestone. The Dalong Formation is composed of siliceous limestone, argillaceous limestone, thick gravel limestone, and thin layer of siliceous rock, with thin layer of calcareous mudstone at the bottom. The Daye Formation and the Dalong Formation are the overlying high-resistivity layers of the coal-bearing strata. The Longtan Formation is a coal-bearing stratum in this area, and the upper section (P₂l₂) is a coal-bearing section, which is composed of black mudstone, sand shale, and light gray sandstone. The lower section (P₂l₁) does not contain coal and is composed of mudstone, sandy mudstone, and sandstone. The whole coal-bearing strata are low-resistivity layers. The apparent resistivity of different rock layers is different, and the apparent resistivity of coal seam section is lower than that of noncoal seam section sandstone and argillaceous sandstone. The specific formation physical parameters are shown in Table 1, it can be seen that the water content and weak water content of coal and rock strata in the study area have obvious differences in resistivity values, and the resistivity of different geological bodies is quite different. Therefore, the area meets the basic conditions for geophysical exploration of OCTEM.

OCTEM is a new TEM for measuring the attenuation and diffusion of the opposing coils transient electromagnetic field [14-16]. The specific technical ideas and schemes are as follows: using the space–time distribution law of the electromagnetic field that the same set of coils generates the opposing coils electromagnetic after the reverse current is applied to the upper and lower parallel coaxial two sets of the same set of coils as the emission source, on the zero magnetic flux plane of the primary field generated by the double coils source, the measurement of the pure secondary field of the ground center coupling is carried out [17-19] (see Figure 2).

When the double coils radiate the transient pulse electromagnetic field signal to the ground, a group of coils are placed near the ground surface, and the magnetic field superimposed near the ground surface is the most at the moment when the transient pulse is powered off. Therefore, at the same time of change, the maximum value surface of the induced eddy current is mainly located near the ground surface. The strongest magnetic field generated by the induced eddy current is delayed with the interruption interval, and the surface induced eddy current is weakened. After generating a new eddy current maximum surface, it gradually spreads to the deeper and edge diffusion far away from the transmitting coil, which is the “smoke ring effect” of the TEM described by M. N. Nabighian [20-22] (see Figure 3). The diffusion velocity of the eddy current maximum surface and the attenuation velocity of the induced eddy current field value are related to the geoelectric parameters. Usually on nonmagnetic ground, this is mainly related to the conductivity: the greater the conductivity of the earth, the smaller the diffusion velocity and the slower the attenuation. Based on the attenuation law of the eddy current field signal received on the surface in time, the information of the underground conductivity can be obtained, that is, the physical principle of the OCTEM [23-25]. The diffusion depth is:

where $σ$ is the earth conductivity, $s$⁠; $t$ is the decay time, $ms$ ; $μ0$ is the vacuum permeability, $H∙S−1$⁠. When calculating the resistivity, the apparent resistivity calculation method suitable for the whole region is adopted. The calculation formula of the apparent resistivity of the whole region at time $ti$ is:

where $L$ is the coil side length, $m$ ; $z=2πL/τ$⁠, $τ$ is the diffusion parameter.

The high-precision opposing coils transient electromagnetic system used in this study is a HPTEM-28 type equipment jointly developed and produced by Central South University and Hunan 5D Geological Technology Co., Ltd. The system uses OCTEM to eliminate the inductive coupling between the transmitting and receiving coils and uses the dual center coupling principle to improve the lateral resolution. In addition, the system uses a unified standard micro-coil dual magnetic source, high-sensitivity magnetic induction sensor, high-speed 24-bit data acquisition card, and high-density measurement technology to achieve high-precision transient electromagnetic exploration of shallow layers. The HPTEM-28 system includes HPTEM antenna (the shell is made of vacuum FRP, the central main antenna is made of imported PDCPD material, and the connecting rotating bearing is made of ceramic material), instrument host, connecting cable and operating PC (see Figure 4).

Combined with the geological data, drilling data and field survey of the mining area, the four survey lines of this work are basically perpendicular to the strike of the coal seam, and the survey lines are arranged above the mining roadway and the planning area of the coal mine in the next 5 years. The survey line is generally 53° southeast, the length of both lines a and b is 560 m, and the length of both lines c and d is 520 m, the survey line spacing is 80 m, the survey point spacing is 20 m, and the total number of measuring points is 112 (see Figure 5).

Before the OCTEM data acquisition, the instrument host is checked to ensure that the instrument is in a normal state. The field working parameter tests such as emission frequency and superposition times are carried out. Through systematic analysis, the emission current of 20 A is finally selected. The emission frequency is 0.625 Hz; the superposition times are 400 times; and the observation is repeated twice at each point. Such parameters can achieve the expected detection effect.

OCTEM uses the system’s own HPTEM Data Process data processing software to preprocess the field data. First, the field data are removed and denoised, and then the appropriate inversion parameters are determined by parameter analysis, curve type analysis, and apparent resistivity analysis [26]. The inversion method uses the transient relaxation method for inversion. The advantage of this inversion method is that it can perform plane analysis and three-dimensional modeling of data in the postprocessing stage, so as to solve the problem of spatial location distribution of coal mines more intuitively and effectively [27]. Then the inversion result data are derived, and the two-dimensional apparent resistivity inversion result map is drawn by Surfer software, and the slice map and three-dimensional shape rendering map are generated by Voxler software (see Figure 6). Finally, according to the apparent resistivity inversion result map, combined with the known geological data, the comprehensive interpretation is carried out.

According to the analysis of Figure 7, the shallow blue areas of the four survey lines a, b, c, and d are speculated to be the relative low resistivity of surface water after infiltrating into the weathered and broken rock mass. The red area in the middle and upper part is characterized by high resistivity, which is speculated to be the electrical reflection caused by argillaceous limestone of Daye Formation (T₁d) and siliceous limestone, argillaceous limestone, thick gravel limestone, and thin siliceous rock of Upper Permian Dalong Formation (P₂d). The resistivity value is greater than 400 Ω·m, and the local resistivity is about 1000 Ω·m. The middle and lower parts are characterized by medium and low resistance, and the resistivity is 50–400 Ω·m. It is speculated that it is caused by the coal measure strata and sandstone in the upper part of Longtan Formation (P₂l₂), which is the main coal-bearing strata in this area (see Table 2).

In the four survey lines of a, b, c, and d, there are 1–2 local low resistance or very low-resistivity characteristics, and the resistivity is 50–200 Ω·m. Table 2 records the characteristic information of seven low-resistivity anomalies. The orange ellipse in Figure 7 delineates four low-resistivity areas. According to the data analysis, the four low-resistivity areas are all in the east wing of the mine. The normal construction of the existing mining project is speculated to be the goaf of the coal seam. After the goaf, there may be some pore water infiltration to form a low-resistivity anomaly. The pink ellipse in Figure 7 delineates three low-resistivity areas. According to the data analysis, it is speculated that the low-resistivity reflection caused by the water content of some sandstones such as “thin-layer sandstone” and “loose sandstone” at the top of the coal is judged to be water-filled goaf.

The slice diagram clearly and intuitively reflects the resistivity distribution of each survey line, especially the low-resistivity area. Figure 8 shows that the low-resistivity area within the range from survey line a to survey line d is delineated by orange large ellipse. According to the data, it is inferred that this low-resistivity area is a coal seam air-filled goaf; Figure 9, shows that the low-resistivity area in the range from survey line b to survey line d is delineated by pink ellipse. According to the data, it is inferred that this low-resistivity area is a water-filled goaf.

The OCTEM imaging results are sliced every 50 m from the elevation of 200 m to the depth of −400 m, and the horizontal section of about 600 m below the ground of the mining area is detected, as shown in Figure 10. Then the slice map of the four survey lines is superimposed with the depth slice map to more intuitively see the resistivity distribution at different depths, and the air-filled goaf and water-filled goaf are delineated with pink and orange large ellipses (Figure 11).

Using OCTEM data to generate a three-dimensional shape rendering map, a more intuitive display is located in the depth of −220 to −90 m air-filled goaf and the depth of −170 to −110 m water-filled goaf, as shown in Figure 12.

Figure 13 shows that the speculated results after the measurement and analysis of the OCTEM. The orange shadow area in the figure is the speculated coalfield air-filled goaf, which is the normal construction area of mining engineering. The pink shadow area is the presumed coalfield water-filled goaf, which is the mining planning area of Daling coal mine in the next 5 years, so this detection has important guiding significance for the later construction. Through the mutual verification of the detection results between the previous lines, it is shown that the inversion results obtained by OCTEM show a good correspondence between high-resistivity and low-resistivity anomalies, and the detection results are reliable.

The wiring positions of a line and 26 exploration line profile, d line, and -26 exploration line are almost coincident (see Figures 14 and 15,14). By comparing the drilling exploration profile of 26 and −26 with the apparent resistivity profile of a and d lines, it can be inferred that the abnormal area is consistent with the coal seam excavation area in the drilling data, which proves the feasibility of OCTEM.

1. According to the geological data of coal mine and the inversion profile of OCTEM, the middle and low-resistance areas are mainly affected by the coal-bearing strata in the upper section of Longtan Formation. The resistivity of the delineated low-resistance anomaly area is between 50 and 200 Ω·m. It is speculated that it is mainly caused by the water content of some sandstones such as the “thin sandstone” of the coal-bearing strata in the upper section of the Longtan Formation and the “loose sandstone” at the top of the coal, which may be air-filled goaf or water-filled goaf.

2. According to the low-resistance anomaly characteristics of the inversion profile of OCTEM, two low-resistance anomaly areas were determined in the coal mine detection area, which were later confirmed as air-filled goaf and water-filledgoaf by drilling verification.

3. In practical engineering applications, OCTEM has demonstrated excellent response to both high- and low-resistance anomalies, which enables it to effectively identify and analyze internal structural characteristics of mines, including potential rock movement, fracture development, groundwater penetration, and hidden hazards such as gob. It provides a new technical means for mine safety assessment and disaster warning. The introduction of this technology has shown broad application potential in the field of mine exploration and is expected to significantly improve the efficiency and accuracy of mine disaster prevention and management.

The article data are collected in accordance with the requirements of the standard science; there is no fraud; the data are true and reliable, if there is a question that can be emailed.

Financial interests: Jian Xiao declares they have no financial interests. Shili Han has received speaker and consultant honoraria from Company Hunan Coal Science Research Institute Co., Ltd.

This work was supported by the Natural Science Foundation of Hunan Province, China (Grant numbers [2023JJ30506]); the Research Project of Geological Bureau of Hunan Province, China(Grant numbers [HNGSTP202413]). Author Jian Xiao and Shili Han has received research support from Hunan Coal Science Research Institute Co., Ltd.