Analysis of the Karst Development Law Based on Multiple Exploration Technologies of Cambrian Limestone

Coal mine oor limestone aquifers are a major source of water inrush from the coal seam oor and a serious threat to the safety of coal mining. In order to reduce and avoid the occurrence of water inrush within the coal mine, we use multiple detection techniques, which are geophysical exploration technology , drilling technology, water inrush accidents and tracer test, to develop a multi-faceted exploration of karst development and analyze its development characteristics in the Chaochuan mine No. 1 well of Pingdingshan Coal Co.Ltd, Henan Province, China. The results show that, the Cambrian limestone (CL) karst water is poor; there is a certain hydraulic connection. Near faults F 5 , F 1 , and SF 28 , the area is less water-rich area, the water is weak, and the deep karst water forms a closed area; 61.54 % of shallow water inrush accidents in the Taiyuan limestone and CL karsts were caused by large tectonic and nearby shallow faults. The karst vertical zonation is shallow; the shallow water level decreased more in the West Wing of the No. 1 well than in the East Wing at elevations above 140 m and below -150 m. The F125 level decline is greater than that of the east, and the west fault is more than 170 m due to the hydraulic connection intrusive barrier wings of the karst water. the East Wing below -150 m F125 fault was weak and uneven on both sides of the hydraulic connection.


Introduction
Water inrush from the coal seam oor has always threatened safe production in mines. The water inrush is not only sudden, but also has a strong impact. In a short time, it can ood wells and bring huge economic losses [1][2][3].
Limestone is often hidden at the base of the coal seam. Due to its karst development, strongly water-rich with good connectivity, limestone is the main water source for the coal seam oor under mining conditions, seriously threatening safe mining of coal [4][5][6].
The karst development of the coal seam oor is affected by many factors and has very complex features such as hydraulic connections and water inhomogeneity. How to accurately depict the characteristics of karst development and the water-rich law is a major issue in the study of mine hydrogeology [7,8]. Wang [9,10] have studied the water rich characteristics of the Cambrian limestone in the No. 2 well of the Pingdingshan coal eld and formulated the related countermeasures for water control by using geological drilling, eld connectivity tests, water drainage drilling, water temperature eld monitoring, and transient electromagnetic exploration. Dai [11] studied the in uence of geological structures on karst development in the Hancheng mining area and concluded that: (1) the structural ssures are dominant in karst development, and (2) the vertical karst development shows a zonal distribution where upper and lower aquifer connectivity is strong. Hao [12] analyzed the karst development characteristics and in uencing factors of the Ordovician limestone in the Gujiao mining area by using the hydrogeological drilling data and their core characteristics, and concluded that the karst development is mainly controlled by the lithologic rhythm combination, resulting in karst phenomenon between the strong and weak phases, where the ground structure raises the stratum and controls the degree of karst development. Hu [13] applied chemical tracer tests to explore the characteristics of deep karst development in the Qiuji mine and concluded that the karst development in the mine eld is not uniform.
Most of the areas are strong karst regions, and the karst development in a few regions is mainly characterized by ssure and weak karst development. The above research on the characteristics of karst development and the achievements made in the typical coal mine have laid a scienti c foundation for the identi cation of water inrush sources and the probability of water in-rush occurring, and previous studies have also helped determine the direction for prevention and control of the coal mine water damage, which has important theoretical and practical signi cance. This paper takes the No. 1 well in the Chaochuan mine of Pingdingshan Coal Co.Ltd, Henan Province, China as an example (abbreviated as the No. 1 well). Based on the analysis of the hydrogeological conditions of the mine, geophysical exploration, hydrogeological drilling, water in-rush feature analysis, underground water discharge engineering, dynamic changes of groundwater level, and tracer tests were adopted. A comprehensive analysis of the Cambrian limestone (abbreviated as CL) karst development characteristics of the No. 1 well coal seam oor was performed, and then the water strength in order to provide a bene cial reference for the prevention and control of mine water hazards in the future and reduce the probability of water inrush accident in karst aquifer of coal seam oor.

Geological Overview
Chaochuan mine is located about 15 km south of Chenzhou City in Henan Province. The mine measures about 10 km from east to west and 4 km from south to north, and the mining area is about 21 km 2 . The No. 1 well is located in the middle of the Chaochuan mine, with fault F 5 in the north, coal outcrop in the south, fault F 104 in the southwest, and fault F 1 in the northeast. It accounts for 34.4% of the total area of the Chaochuan mine (Fig. 1).
The main faults of the No. 1 well are located on the northern, western, and southern boundaries of the well eld, and a series of northwest (NW) and near east-west (E-W) faults play an important role in the burial and distribution of the CL aquifer on the coal seam oor of the mine eld and the discharge of groundwater recharge and runoff (Fig. 2). The

Transient Electromagnetic Prospecting
In the main body of the paper, three different levels of headings (for sections, subsections, and subsubsections) may be used. The typesetting style for these headings is presented in the next section.
In order to discover the water-rich anomalies of the CL aquifer in the No. 2 − 1 coal seam, ground transient electromagnetic [14,15] exploration was carried out within the No. 1 well range. The following exploration range was used: From the F 5 reverse fault southward to -250 m water storehouse, and west from the 13th exploration line eastward to the 5th exploration line. The total area was 3.1 km2, with a survey line spacing of 40 m * 40 m and a total of 3,184 physical exploration points. The detection depth was set at 80 m in the CL aquifer. According to the transient electromagnetic apparent resistivity, the aquifer was usually divided into 2 levels, rich in water and weak in water [9,10]. According to the exploration situation in this mine, in the top range of the CL (depth 0 ~ 40 m), an apparent resistivity of less than 85 Ω•m was de ned as a strong water-rich zone; and in the middle of the CL depth (depth 40 ~ 80 m), apparent resistivity less than 90 Ω•m was de ned as a strong water-rich area. The survey results are shown in to 80 m CL has been delineated in 16 low resistivity zones. The proportions of the areas occupied by the survey were 17.94% and 18.42%, respectively. In addition, the distribution of the low resistivity anomaly areas was sporadic and beaded, and the range of a single low resistivity anomaly was small, which indicated that the CL aquifer in the No. 1 well range had poor water-richness as a whole. In the 0 ~ 40 m and 40 ~ 80 m depths from the top interface of the CL, there were overlapping regions in the low resistivity anomaly area, which indicated that there was a certain hydraulic connection between the groundwater in the top and the middle sections of the CL aquifer.
In addition, near F 5 , F 1 , and SF 28 , the low-resistance anomaly area was sporadic, indicating that these faults were weakly conducting water, which was consistent with the actual situation revealed during the construction of the mine roadway. In addition, due to the weak water conductivity of F 5 , F 1 , and SF 28 , the hydraulic connection between the deep and shallow parts of the CL was weak on both sides of the fault. A relatively closed hydrogeological unit of the No. 1 well CL aquifer was formed.

Hydrogeological Exploration
During the No. 1 well exploration, 10 geological holes were drilled from the target layer to the CL ( Table 1). The drilling depth was between 16.64 and 652.33 m, and the mean value was 342 m. The nal borehole elevation was within the range of 250.94~ -415.21 m with a mean value of -106.17 m. The water leakage in all 10 boreholes indicates that the karst ssures in the shallow CL aquifer were generally developed and rich in water.
In the No. 1 well mining process, in order to monitor the groundwater level of the CL aquifer, eight hydrogeological boreholes were constructed at different stages ( Table 2). The drilling depth was between 420.20 m and 874.60 m, and the mean value was 555.30 m. The nal hole elevation in the borehole ranged from − 242.00 m to -693.00 m with an average of -355.69 m. There is no leakage phenomenon in the 8 boreholes, which indicated that the CL karst in the deep part of the area was weak and not strongly water-rich.
Comparing the boreholes in Tables 1 and 2, we see that the borehole depth of the former was much smaller than that of the latter, and the elevation of the nal borehole was much higher than that of the latter. For the exposed CL thickness, the former was 44.51 m, and the latter was 40.84 m. The difference between the two was not large, but the leakage situation is very different. This is due to the large amount of groundwater being discharged during the excavation process of the mine, resulting in a drastic decline in the groundwater level of the CL aquifers, which made it impossible to see groundwater in the hydrogeological boreholes of later construction.  For the 13 inrush water incidents with No. 1 well water volume exceeding 10 m 3 /h, eight were caused by geological structure (fault and fold), accounting for 61.54%. For the eight events, seven were water in-rush from the fault and one from the syncline. Figure 5 shows that most of the water in-rush points associated with the fault were located near the fault facing E-W direction, indicating that the structural fracture zone was a zone of karst ssure, which not only controlled the enrichment of the groundwater but also affected the direction of the runoff. Therefore, it was the most prone position for water inrush in the bottom limestone aquifer.
Secondly, as shown in Fig. 5, there were 18 cases for the water inrush of the limestone aquifer of the No. 2 − 1 coal seam above − 200 m, accounting for 81.82% of the total water inrush accidents, indicating that the shallow limestone aquifer had more water inrush than the deep part. The incidents were also more concentrated in the vicinity of small faults, which was mainly due to the fact that shallow small faults were more developed than deep ones. In general, the large faults mainly caused the breakage of the regional rock formations, while the minor faults made the surrounding rock mass more broken, resulting in the development of karst fractures.
In addition, the fractured rock layer had a large CO 2 content, making the erosion ability stronger and the groundwater circulation in the karst fracture faster, which led to stronger CO 2 erosion ability [16]. With the increase of depth, the CO 2 recharge in the water was not su cient, and the groundwater circulation slowed down, which caused the erosion ability of CO 2 to decrease, so that the karst development in the mine eld eventually led to zoning in the vertical direction. That is, the shallow karst development was stronger than the deep part [17,18].
Based on the analysis of the water inrush frequency, the shallow karst fracture was more developed than the deep part, which was consistent with the third part.

Downhole Drainage And Decreasing Pressure Drilling
In order to reduce the underground water level of the limestone aquifer in the Cambrian, which will reduce the threat of water inrush from the oor of the coal seam mining, the No. 1 well-constructed special discharge roadways (Fig. 6) according to the mining progress, and built drainage and decreasing pressure drilling in the roadway (Table 3).
According to Table 3

Analysis Of Time And Space Changes In The Water Level
The location of the groundwater level observation hole for the CL and its water level dynamic changes are shown in

Analysis Of Time And Space Changes In The Water Level
In order to ascertain the hydraulic connection of the CL groundwater on the east and west sides of the F 125 fault below the No. 1 well East Wing − 150 m level, four tracer tests were carried out. The tracers were all potassium iodide [20][21][22][23]. The interval between the two tests was more than 2 weeks. The input points were 4-99, S1, S9, and S2 on the east side of the F 125 fault (Fig. 8), and the receiving point was the 3 − 1 water level observation hole on the − 250 m East Wing discharge roadway (Fig. 8). Water samples were collected every 2 h at the receiving point and the potassium iodide content was measured on-site with a HI93718 portable iodine analyzer with a test accuracy of 0.1 mg/L. The test results are shown in Table 4. In addition, from Table 4 and Fig. 8, we know that in the four-time tracing test, when 4-99 was the input point, the potassium iodide content was detected at the receiving point, and the other three tests (especially the S2 tracer test closer to the receiving point) did not detect potassium iodide contents at the receiving point, indicating that there was no obvious hydraulic connection between the three input points and the receiving point for the CL groundwater. The above tracer tests showed that the total hydraulic connection of the CL karst on both sides of the F-1 fault below the No. 1 well East Wing − 150 m were weak, and only a weak hydraulic connection was found in the local area, indicating the obvious difference in the karst space development of the CL.

Conclusions
(1)The transient electromagnetic exploration results showed that the No. 1 well water-rich anomaly area was scattered, and the area was less than 20% of the entire exploration area, indicating that the CL aquifer was weakly water-rich as a whole. The buried depths of 0 ~ 40 m and 40 ~ 80 m water-rich anomalies partially overlapped, re ecting a certain hydraulic connection between the shallow and deep groundwater of the CL aquifer. There were few water-rich anomalies near the F 1 , F 5 , and SF 28 faults, which proved that these faults were poorly water-rich and water-conducting, making the deep karst water form an independent closed system.
(2)The comprehensive study of hydrogeological exploration, water in-rush characteristics, and downhole drainage data showed that karst development in the shallow part of the CL aquifer was stronger than that in the deep part, showing vertical zoning. The water inrush in the shallow part at an elevation of -200 m accounted for 81.82% of the total water inrush accidents. The water inrush volume exceeded 10 m 3 /h, the CL aquifer accounted for 84.62%, and the water inrush caused by geological structures accounted for 61.54%, which was mainly concentrated near the small faults.
(3)The spatio-temporal variation of the water level and the tracer test analysis showed that the hydraulic connections of the CL groundwater in the shallow part at -300 m and the west of the F 125 fault were relatively close, but the groundwater hydraulic connection in the deep (elevation − 390 m) CL was not close. The F 125 fault had poor water conductivity overall, and there was a weak hydraulic connection between the CL groundwater found only in the fault zone at -200 m elevation; The CL groundwater in the eastern F 125 fault did not have a close hydraulic connection.