Introduction
In recent years, due to coal mining, a large number of underground water has been pumped out, and the bearing capacity of the upper bottom of the mine is reduced. In addition, most of the small coal mines do not take preventive measures such as reserving coal pillars during the mining process. Some small coal mines even arbitrarily excavate and destroy the reserved coal pillars of state-owned coal mines, resulting in a large number of underground goafs leading to geological disasters such as stratum dislocation and surface subsidence. If the waste water is discharged without treatment, it will seriously pollute the surface water, block the river and farmland channels, and cause soil hardening, resulting in increasingly prominent environmental pollution and ecological damage. Because of its high efficiency, low cost and high precision, geophysical prospecting plays an increasingly important role in goafs exploration.
1. The premise of geophysical work
1.1 Geological characteristics of goafs in coalfield
In the process of coal mining, a large amount of groundwater is pumped out, which leads to the decline of groundwater level and the decline of surface water. The "three zones", namely the collapse zone, the fracture zone and the bending subsidence zone of the coal mine strata are formed by the dislocation and subsidence of the strata. The collapse zone is formed because the coal in the lower part of the immediate roof is mined out and the immediate roof is crushed and sunk under the pressure of the upper strata; the fracture zone is formed due to the fracture of the main roof caused by the pressure of the overlying strata after the crushing and sinking of the immediate roof; the bending subsidence zone is formed due to the bend and sinking of a part of the upper strata of the main roof caused by the pressure of the overlying strata after the main roof is fractured and sunk. The subsidence, separation and destruction of overlying rocks in the fracture zone develop from occurrence, rise, maximum height, falling back and remaining stable with the coal mining. The final shape of the fracture zone must be formed by the destruction and movement of overlying rocks caused by the separation of the caving gangue and the compaction of the fracture when a certain degree in the mining area is reached and the corresponding position is pushed over for a certain distance. The fracture zone is gradually formed above the caving zone. It is a subsidence basin in the surface when a large number of fractures appear in the rock mass, and their integrity is seriously damaged (SONG Hong-wei et al. 2018; MA Ya-jie et al. 2012; LIU Guo-hui et al. 2007).
1.2 Geophysical model of goafs subsid-ence area
The coal strata are mainly layered strata, mainly composed of mudstone, shale and argillaceous sandstone. The resistivity of clayish strata is generally low, ranging from one to dozens of Ω·m; the resistivity of argillaceous sandstone is high, ranging from tens to thousands of Ω·m; because the coal seam is relatively loose, the resistivity is also tens to thousands of Ω·m. These data can be used as the reference value of geological background and the premise of establishing geophysical models. After coal mining, due to the deformation and destruction of the overlying strata, the collapse zone, the water diversion fracture zone and the bending subsidence zone are successively formed from the bottom to the top. In the collapse zone, loose rocks are compacted in a certain time after mining, and the resistivity is much higher than the normal value. Due to the three zone effect of coal seams, the subsidence of the overlying strata caused by the goafs often makes the continuous water body formed by the surface flowing to the ground along the fracture generated by the subsidence in the shallow fracture zone present the relatively discontinuous and other abnormal morphological characteristics in the shallow strata due to the increase of the subsidence degree. The high-density resistivity method aims at the characteristics of large electrical difference between the collapse zone and the fracture zone caused by the deformation and destruction of the overlying strata in the coal mine goafs. It indirectly infers the location and direction of the goafs under the shallow aquifer by detecting the discontinuity of local drainage zone and aquifer caused by the collapse zone, which has a high accuracy (LI Jin-ming, 2005; SONG Hong-wei et al. 2012; SONG Hong-wei et al. 2015; LIU Yun-zhen et al. 2006).
2. High-density resistivity method
2.1 Principles of high-density resistivity method
The basic working principle of high-density resistivity method is basically the same as that of conventional resistivity sounding method. It is also a method based on the difference of electrical properties of rock and soil. Through electrodes A and B, the electric current I is powered to the underground, and then the potential difference △V is measured between electrodes M and N. Using a geometric coefficient K, the apparent resistivity value of this point (between M and N) can be obtained as follows: ρ s = k ·△ V / I.
The detection depth increases with the increase of the distance between electrodes A and B. When the isolation coefficient n increases gradually, the A-B electrode distance also increases gradually, the current transfers deeper to the underground, and the reflective ability to the deep underground medium also increases gradually. High-density resistivity method can also be regarded as a resistivity profile made of multiple resistivity sounding points, with higher efficiency and resolution. Based on the measured apparent resistivity profile, the resistivity distribution in the underground strata can be obtained by calculation and analysis, thus the strata can be divided and anomalies can be determined. However, the measuring depth is limited by the power of high-density instruments, and the distance between electrodes A and B cannot reach the required length theoretically, which often fails to meet the requirements for measuring deep geological bodies (XIA Fan et al. 2019; SONG Hong-wei et al. 2011; LIU Xiao-dong et al. 2001).
Therefore, the high-density method is theo-retically unable to directly detect deep geological bodies, but it is very accurate to react to geological anomalies within 100 m depth.
2.2 Ultra-high density resistivity in-strument
Ultra-high density direct current exploration inversion system is a new type of direct current exploration instrument which has just been introduced into China in recent years. It is different from traditional high-density resistivity method in data acquisition. In actual measurement, electrode system is arranged at one time, acquisition para-meters of the instrument are set, and the selection of dipole, wenner and other devices is ignored. Fully automatic and free combination data acquisition is adopted. In this way, the main observation parameters are the potential difference between different combinations of electrodes, so that with the same number of electrodes, the amount of data collected will exceed 40 times of the conventional method, regardless of the advantages and disadvantages of various data acquisition devices. At the same time, the multi-channel operation mode can collect 61 sets of V/I data simultaneously, so the acquisition speed will not be affected by the large set of data. The collected data of potential difference is automatically converted into the measurement and storage record of apparent resistivity value by instrument. The data in the instrument are replaced to the computer in the laboratory. The potential difference value is converted to the resistivity value by the instrument' s own processing software, and the geographic electrical image (inverted resistivity section) is obtained by inversion mapping. The abnormal shape is clear. The resistivity section can clearly represent the characteristics of the changing resistivity value on the line section. Based on the physical characteristics in the known measuring area, the occurrence state of various underground geological bodies can be judged in detail (SONG Hong-wei et al. 2019; LIU Zhao-ping et al. 2010; ZHANG Guang-bao, 2007).
3. Analysis of detection results
3.1 Geological overview
According to the existing geological data and ground investigation and analysis, the geological conditions of the detected area are as follows: (1) Villages and coal mines are located in hilly areas, with a lot of arable land on the surface and mainly composed of clayey silt, mild clay and gravel; (2) Quaternary aquifer is mainly composed of modern riverbed and gravel layer, and its thickness changes greatly, within the range of 40~80 m. The terrain is relatively flat, generally low in the northwest and high in the southeast; (3) Lithological underlying bedrocks are archeozoic, middle-upper proterozoic, lower paleozoic cambrian and ordovician, upper paleozoic carboniferous and permian from old to new; (4) The coal mine is located in the northwest of the village and has been exploited for many years. Uneven surface subsidence has occurred in the last two to three years, resulting in cracks in housing of many households in the village in the detected area. The location of cracks has been identified in Fig. 1.
3.2 Layout of the geophysical pro-specting work
According to the on-site investigation, we can judge the general direction of fractures and the subsidence of houses. Combined with the geological, topographical and geomorphic conditions of this area and the distribution of goafs provided by the miner, four geophysical prospecting sections covering the whole area are arranged in the shape of a Chinese character "井", among which Line 1 is parallel to Line 2 and Line 3 is parallel to Line 4, and the starting point is basically in the same place, which basically controls the distribution of underground goafs in the village and the influence of their subsidence. The layout of the lines is shown in Fig. 1.
3.3 Analysis of geophysical prospec-ting results
According to the data collected by the on-site instrument, the inversion analysis is carried out, and the electrical characteristic diagram is obtained by the geophysical prospecting interpretation of four high-density lines (Fig. 2-Fig. 5).
Enlarge this image.Fig. 2. Electrical characteristics diagram of high-density resistivity profile 1 and profile 2
Enlarge this image.Fig. 3. Electrical characteristics diagram of high-density resistivity profile 3 and profile 4
The profiles obtained from the four Lines are analyzed as follows:
The dotted line in the figure is the high-resistivity area formed by the drainage of the fractured aquifer, and the dash-dotted line is the line of the same measuring point corresponding to the electrical abnormal area on the speculated high-density electrical profile. It can be seen that the positions of the detected abnormal areas under different parameter settings are basically the same.
As shown in Fig. 2, the resistivity isoline of Line 1 has several low resistivity closed loops at a depth of about 50 m, and the curves are discontinuous horizontally, which are cut off by high resistivity body. The corresponding geological phenomenon is that the aquifer on the contact surface of quaternary overlying layer is pinched out. It is inferred that three relatively high resistivity areas are formed after the groundwater is drained, which are located at 70 m, 390~450 m and 570 m respectively.
As shown in Fig. 2, there are several low resistivity closed loops on the resistivity isoline of Line 2 at a depth of about 50 m. The horizontal curves are discontinuous and cut off by the high resistivity body. The corresponding geological phenomenon is that the aquifer on the contact surface of quaternary overlying layer is pinched out. It is inferred that four high resistivity areas are formed after the groundwater is drained, respectively at 120 m, 390~450 m, 870~930 m and 1 080 m.
As shown in Fig. 3, there are several low resistivity closed loops on the resistivity isoline of Line 3 at the depth of 40~50 m, and the horizontal curves are discontinuous, which are cut off by high resistivity body. The corresponding geological phenomenon is that the aquifer on the contact surface of quaternary overlying layer is pinched out. It is inferred that four high resistivity areas are formed after the groundwater is drained, which are located at 30~180 m, 390~540 m, 780~840 m and 1 080 m respectively.
As shown in Fig. 3, the resistivity isoline of Line 4 has several low resistivity closed loops at a depth of about 50 m. The curves are discontinuous horizontally and cut off by high resistivity body. The corresponding geological phenomenon is that the aquifer at the contact surface of quaternary overlying layer is pinched out. It is inferred that five high resistivity areas are formed after the groundwater is drained, which are located at 270 m, 450~510 m, 600~660 m, 780~840 m and 1 050 m respectively.
According to the underground geological structure of the detected area, the resistivity of the shallow fractured aquifer is relatively low compared with its surrounding geological body, so the high-density resistivity method has a higher resolution for the detection of the resistivity anomaly of the relatively shallow aquifer. Finally, except for 70 m from line 1, at 120 m and 1 080 m from Line 2, 30~180 m from Line 3, 1 050 m from Line 4, the high resistivity anomalies in the shallow drainage area measured by geophysical prospecting cannot be determined whether there are goafs or not, other anomalies are confirmed to be the reflection of the existence of goafs. At the same time, there are cracks in the houses where the geophysical anomalies are reflected, and the direction of the cracks is basically consistent with the inference of geophysical prospecting. It can be proved that the hiatus and fracture of aquifer are directly related to the subsidence of underground goafs, and the abnormal shape of aquifer has a high accuracy in judging the position of goafs and the collapse zone.
By combining the results above with the subsidence data, we find that the miners have mined beyond the approved areas. Through the geophysical prospecting results, the influence area of subsidence can be roughly drawn. At the same time, it is shown that the high-density resistivity method can accurately reflect the subsidence effect of the underground mining area, and its effectiveness meets the requirements of engineering geological exploration.
4. Conclusions
(1) Through this study, it can be seen that in the densely populated areas of villages, compared with other geophysical methods, high-density resistivity method has the advantages of low cost, high efficiency, strong anti-interference ability and high detection accuracy, which is an effective method for the disaster assessment of subsidence in coal mining.
(2) According to the geological characteristics of the three zones in the coal mine, the lack of water body in the distribution model of the shallow underground fractured aquifer is used as a sign of the existence of underground mining subsidence. The establishment of a reasonable geophysical model is the focus and highlight of this study, which not only overcomes the shortcomings of the limited detection depth of high-density resistivity method, but also gives full play to its sensitive response to the electrical differences in the shallow geological bodies. Combined with the known geological information, interference factors are eliminated, and accurate conclusions are obtained.
(3) Ultra-high density resistivity instrument is a new type of high density resistivity instrument. It adopts the mode of hybrid devices, which greatly increases the amount of data collected, and overcomes the limitation of traditional high density resistivity instrument. With the increase of detection depth and the electrode distance, the observation section inevitably produces "edge loss", that is, the deeper the depth is, the smaller the scope of exploration, often forming the "inverted trapezoid" shape, which is unable to reflect many problems of the deep geological information at the edge of the trapezoid.
(4) At the same time, it needs to be pointed out that due to the dense houses in the village, it is difficult to arrange detection lines in the village, and the influence scope of mining can only be divided by several sections by the village, which inevitably leads to fuzzy boundary.
Acknowledgements
This study was supported by the geological project "Shendong-Jindong Large Coal Base 1:50 000 Hydrogeological Geophysical Prospecting (G201611-4)", and the project "Research on Physical Characteristics of Aqui-ferous Structure in Areas with Serious Water Shortage in Taihang Mountain(SK201303)" with basic scientific research expense.
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Hong-wei SONG 1, 3
Fan XIA 2
Hai-dong MU 1, 3
Wei-qiang WANG 1
Ming-sen SHANG 1
1. The Institute of Hydrogeology and Environmental Geology, CAGS, Shijiazhuang 050061, China
2. Hebei Provincial of Academy of Environmental Sciences, Shijiazhuang 050031, China
3. Key Laboratory of Groundwater Contamination and Remediation, China Geological Survey (CGS) & Hebei Province, Shijiazhuang 050061, China
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