A Geophysical-Drilling-Hydrochemical Coupled Method for Accurate Detection of Concealed Water-Conducting Faults in Coal Mines

Abstract

The detection of concealed water-conducting structures is essential for preventing water inrush disasters. Aiming to mitigate the limitations inherent in using any single technique, a comprehensive approach that combines integrated mining geophysical exploration, hydrogeological drilling, and hydrochemical exploration (GDH) is proposed for the exploration of concealed water-conducting structures. By conducting a thorough analysis of the background geological data obtained through surface exploration, potentially concealed water-conducting structures can be predicted. Then, a combination of the seismic reflection method (SRM) and mine transient electromagnetic method (MTEM) can be used to detect the location and water-bearing properties of the target structures. Afterwards, the target drilling areas are defined by the anomalies detected by the integrated mine geophysical technique, and the drilling method can directly acquire the hydrogeological information of water-conducting structures and verify the results of the geophysical methods. By means of hydrochemical analysis, inrush water sources and their runoff conditions can be identified, and the spatial relationship betweenof the source aquifers and mining space can be determined; hence, the properties, scale, and configuration of the water-conducting structures can finally be evaluated. Employing a water-conducting fault in a mine as a case study, we verified that the integrated method overcomes the limitations and possible biases of each method, providing a multiple-method solution that can accurately detect concealed water-conducting structures to help prevent water inrush disasters.

1. Introduction

China is the largest coal-producing country in the world. However, due to its extremely complex geological structural conditions, it is also one of the countries suffering from the most serious coal mine water disasters. Statistics indicate that from 2000 to 2023, a total of 1220 water hazard accidents took place in coal mines across China, leading to 5097 fatalities. As shown in Figure 1, over the past decade, China’s coal output has been on a gradual rise, and mine water accidents have been effectively curbed. Nevertheless, there are still dozens of such accidents occurring every year. Particularly in the past five years, the number of fatalities caused by mine water accidents shows an upward trend [1], highlighting the significance of mine water disaster prevention.

Coal mine water inrush accidents are typically induced by the excavation and disclosure of unexpected concealed water-conducting structures, such as faults, fissure zones, and karst. For instance, 80% of floor water hazard accidents are associated with hidden water-bearing faults [2]. Upon disclosure during excavation or connecting by drilling, these structures may channel high-speed water toward the tunnel from various sources, causing disastrous consequences [3,4]. Such structures undermine the integrity and stability of the rock mass and regulate the distribution and inrush of water; thus, determining the accurate location and water-bearing state of concealed water-conducting structures is an essential precondition for preventing water disasters and implementing control measures [5,6,7].

At present, research on the detection of concealed geological structures mainly focuses on assessing and improving the effects of geophysical, geochemical, and drilling methods [8,9,10]. Considerable efforts have been made to detect concealed structures and estimate their water yield properties using the isotope tracing method, hydrochemical analysis [11,12], DC electrical method [5,13], mine transient electromagnetic method [14,15,16], seismic exploration [17,18,19], hydrogeological drilling, and borehole television. However, there are limitations and potential biases in interpreting water-conducting structures using a single method or technique. For instance, there is ambiguity in water source recognition, multiplicity in geological methods, and limited vision in drilling methods [20]. By taking advantage of each method, proposing a multimethod prospecting system becomes an inevitable choice. Unfortunately, such a multimethod prospecting system has not been well developed [21].

In the present study, through an in-depth analysis of the advantages and limitations of each method, a geophysical-drilling-geochemical coupled method for the accurate detection of concealed water-conducting faults in coal mines is first proposed. Then, the proposed method is applied to a real case of water-conducting fault detection in a mine in Xuzhou, China. At last, the limitations of the proposed method is are discussed.

2. Proposal of a Geophysical-Drilling-Geochemical Coupled Method

Exploration conducted on the surface has successfully identified the major geological structures within the mining area. However, smaller concealed structures often pose a greater threat to coal mine safety. These structures, which lack obvious surface exposure or distinguishing features, are typically small-scale formations within the rock and are characterized by high concealment and significant disaster-causing potential. Additionally, the detection of these concealed structures usually occurs concurrently with mining activities, necessitating strict accuracy and timeliness in exploration. The tasks involved in exploring these hidden structures are comprehensive, covering aspects such as the presence of water, water-conducting channels, and sources of water.

Geophysical exploration, drilling, and geochemical analysis are commonly used for advanced detection, with geophysical exploration and drilling being the most prominent. While these methods can partially address geological challenges, they each have significant limitations when used independently. Geophysical exploration relies on interpreting detected geophysical field characteristics; however, as an indirect method, it faces the issue of multiple solutions—different geological conditions can produce similar geophysical characteristics. Additionally, each geophysical technique has its own application constraints. For example, seismic methods offer high positioning accuracy but are not sensitive to water, whereas electrical methods are effective for water detection but have limited positioning accuracy due to the volume effect. Drilling, on the other hand, has a limited control range and blind spots in exploration. If a borehole does not reach a water-bearing body, it only confirms the absence of water at that specific location, necessitating multiple boreholes to accurately detect the water distribution across the entire mining face. This approach requires a long construction period and substantial investment. Geochemical exploration, particularly hydrochemical analysis, is often used to infer geological environments, water sources, and potential anomalies. However, it is a reactive method, often requiring contact with groundwater, which means that water has already surfaced or samples must be obtained through drilling. As a result, in coal mine water detection, geochemical exploration typically serves as a supplement to geophysical exploration and drilling. Therefore, no single method is sufficient for the refined detection of concealed water-conducting structures.

Geological structures have specific physical and chemical characteristics that distinguish them from the surrounding rocks. By employing a multimethod approach to study these properties, we can gather substantial information. Thus, we propose a combined method of geophysical exploration, drilling, and geochemical exploration (GDH). This method should follow a sequence of prediction, detection, verification, and identification steps to accurately detect concealed water-conducting structures. First, using basic geological information, we can predict the areas where these structures might develop. Then, using the integrated mining geophysical method, the location, boundary, and water yield properties of geological structures can be further detected. Directional drilling, guided by geophysical data, allows for the direct detection and verification of the structure’s form and water characteristics. Finally, by utilizing strata information and identifying the source aquifer through geochemical exploration, we can estimate the properties and scale of the structures (Figure 2).

(1)

Prediction of potential concealed water-conducting structures through data analysis aided by surface exploration

By thoroughly analyzing geological data, including mine strata, structures, and hydrology, we can examine the relationship between the main coal seam and the sedimentary structure of aquifers, as well as identify water sources and conduction channels. Evaluation methods are employed to assess the characteristics of water hazard threat zoning and predict suspicious areas surrounding the production area, especially in the vicinity of large geological structures interpreted by surface exploration and areas with low exploration accuracy. These zones may contain undetected or associated geological structures. When mining activities approach these areas, the detection of concealed geological structures is essential.

(2)

Detection of the location and water abundance of water-conducing structures by integrated geophysical exploration (IGE)

As a deep-reaching and non-destructive technology, mine geophysical methods have been widely employed in the detection of geological structures and their water-bearing properties [22]. Geological structures often disrupt the continuity and integrity of the rock mass, leading to fractures and low cementation in tectonic belts. The lithology in these areas may be altered or water-filled, resulting in variations in resistivity and wave impedance (velocity and density). These anomalies in resistivity and wave impedance form the basis of electrical and seismic exploration in mining. Electrical methods (EM), including direct current methods and electromagnetic methods, are sensitive to the water-bearing properties of structures. However, some methods may magnify the volume of low-resistivity anomalies due to the volume effect [15]. However, the seismic method (SM), which employs the travel time characteristics of elastic waves, can detect the location and boundary of geological structures more accurately. Integrated geophysical exploration (IGE) involves adopting at least two underground geophysical exploration methods and taking advantage of the strengths of each method to complement and validate one another. As a result, IGE can provide reliable information on the formation of concealed geological structures and even enable the quantitative detection of water volume within these structures. This information converges in the target area and provides important data for planning hydrogeological drilling.

(3)

Verification of the location and parameters of geological structures by hydrogeological drilling

Hydrogeological drilling is the most efficient method for water detection and drainage in mines; it is also the most direct way to obtain actual geological data and assess the mechanical characteristics of an unexplored area. By observing the water inflow in the drill hole, rock strength, core recovery, and changes in water yield, we can determine the lithology of the aquifer, distribution of geological structures, and water pressure, among other parameters. However, the information provided by drilling is limited to the area surrounding the drill hole, while geophysical exploration results can provide a targeted zone for drilling design and set a more relevant aim for drilling construction. Additionally, the analysis of drilling samples can serve to verify the results obtained from geophysical exploration.

(4)

Identification of water source and potential volume through hydrochemical tests

The properties of a water source are crucial for detecting and managing concealed water-conducting structures. Underground water formation is influenced by various factors, including the sedimentary sequence, host lithology, geochemical environment, and geological structures[23]. The chemical components and hydrochemical characteristics of underground water vary in different aquifers. By analyzing hydrochemical data, we can identify the recharge source and runoff conditions of underground water samples, determine the spatial relationship between source aquifers and the mining area, and thereby deduce the properties, scale, and configuration of water-conducting structures. These results provide a foundation for subsequent water prevention and control measures.

3. Application of the GDH Method

3.1. Background Information

As shown in Figure 3, the study site is a coal mine located in the northern region of Xuzhou, China. The mining area lies within the coal-bearing strata of the Permian-Carboniferous system, with the primary coal seam being the No. 7 coal seam of the Shanxi Formation. The mine contains five main aquifers: a bottom aquifer of the Quaternary system, a sandstone aquifer on the roof of the No. 7 coal seam, a sandstone aquifer on the roof of the No. 8 coal seam, the No. 4 limestone aquifer of the Taiyuan Formation, and the Ordovician limestone aquifer. In Figure 3, only the sandstone aquifer in the roof of coal seam No. 7, the sandstone aquifer in the roof of coal seam No. 8, and limestone aquifer No. 4 are presented. Their rock names are displayed in blue. Among these, the No. 4 limestone aquifer of the Taiyuan Formation has the greatest impact on mining operations due to its thickness of up to 10 m and its proximity to the seam, which is approximately 66 m below the No. 7 coal seam. The Quaternary aquifer and the Ordovician limestone aquifer are farther from the mining area and generally have minimal impact on mining activities unless there is an extremely large fault.

Fractures, particularly tension normal faults, are the major geological structures in this mining area and are known for their significant water abundance and conductivity. Faults tend to reduce the thickness of the aquiclude and decrease the distance between the coal seam and aquifers. Additionally, tensile stress can enhance the development of fissures in limestone, which promotes water enrichment. According to the three-dimensional seismic data and actual exposure information from roadways, a normal fault with a displacement of approximately 50 m has been identified to the west of the shaft station (Figure 4). There may be unexpected associated faults ahead of the mining face. To ensure safe roadway tunneling, the proposed GDH method is applied to detect potential water-bearing faults.

3.2. Accurate Detection Using GDH Method

3.2.1. Integrated Geophysical Method

(1)

Mine seismic exploration

To determine the geological conditions ahead of the roadway, the seismic reflection wave method (SRM) and the mine transient electromagnetic method (MTEM) are employed. The seismic survey line was placed on the right side of the roadway, and the measurement points are shown in Figure 5. A 12-pound copper hammer served as the source, and velocity geophones with a natural frequency of 100 Hz were used as the receivers. The intervals between the geophones and sources were both 2 m.

After processing the data through static correction, filtering, and automatic gain control, seismic records with nearly zero offset, excited by sources No. 8 to No. 24, were extracted to form a time profile. Compared to other seismic methods, zero-offset seismic exploration reduces the influence of surface and refraction waves. Using a minimal offset helps prevent energy loss from wave mode conversion, allowing more reflection wave energy to be utilized, thereby improving the signal-to-noise ratio for data analysis. As shown in Figure 6a, an oblique reflection interface can be observed to the west of the driving face. Subsequently, all seismic records were used for diffraction stacking migration, which converts time records into spatial distributions. In Figure 6b, the axis of the oblique reflection interface extends from (6, 24) to (26, 43). However, due to the large angle between the reflection interface and the survey line, a discontinuous reflection interface appears in the migration imaging.

(2)

Mine transient electromagnetic method

The mine transient electromagnetic method (MTEM) was employed to detect the size and water-bearing properties of the oblique reflection interface. This method is based on Faraday’s law of electromagnetic induction. When current flows through the transmitter coil, the induced primary electromagnetic field propagates into the medium ahead of the coil. Upon switching off the power, good conductors generate a secondary electromagnetic field to counteract the decrease in the primary electromagnetic field. A receiver coil or magnetic probe is used to observe the change in the secondary electromagnetic field over time, allowing hydrogeological information at various depths to be obtained through time-depth conversion. Compared to other geophysical methods, MTEM offers numerous advantages for detecting water-bearing media, such as being non-contact, having good directivity, and high sensitivity to water.

For data acquisition, a square overlapping loop with a side length of 2 m was employed. The measurement points on the right side of the roadway were arranged at intervals of 4 m. To detect the area in front of the heading, the measurement points were increased in density at intervals of 15 degrees at the corner of the driving face. The measurement points and detection direction of the MTEM are shown in Figure 5. Figure 7 presents the contour lines of the apparent resistivity; the blue area represents a structure with lower apparent resistivity. Due to the impact of turn-off time, the MTEM has an exploration blind area of approximately 10 m in the shallow layer. Hence, this region has been subjected to whitening processing. The low apparent resistivity occupies the area ahead of the driving face (Y-axis: 15–32 m) and extends to the right lateral area in front of the roadway. The minimum apparent resistivity is less than 8 Ω∙m. Beyond this area, the contour lines of the apparent resistivity gradually increase, indicating that the rocks in these areas are mostly uniform and contain little water. Although this method amplifies the abnormal area due to its volume effect, the low-resistivity zone corresponds well to the oblique reflection interface identified by the seismic method.

The results of these two geophysical methods are consistent and complementary, allowing for indirect determination of the location and water-bearing properties of the structures. The hydrogeological parameters provided by the integrated mine geophysical method can inform management measures. However, due to the complex geophysical factors involved, these methods cannot directly identify whether the structure is a fault or another type of geological formation. Therefore, hydrogeological drilling is still necessary to confirm the properties of concealed water-conducting structures and to obtain lithological characteristics and hydrogeological parameters directly.

3.2.2. Hydrogeological Drilling

To identify the properties of the concealed water-conducting structure, three hydrogeological boreholes were drilled to investigate the target region detected by the integrated geological method. Full-section coring was carried out for all boreholes. As shown in

Table 1. The borehole parameters and water inrush.

Borehole No.	Azimuth (°)	Dip (°)	Diameter (mm)	Depth (m)	Maximum Water Inrush (m3/h)
1	174	−0.5	75	60	30
2	144	12	75	120	70
3	188	17	75	150	50

, the No. 4 limestone aquifer was encountered at a depth of 46 m in borehole No. 1. The core recovery rate between 46 and 48 m was low, and the rock core was highly fractured. Meanwhile, groundwater surged into the borehole at a rate of roughly 30 cubic meters per hour and was subsequently pumped out. In borehole No. 2, the No. 4 limestone aquifer was reached at 48.9 m. There was a small initial water inflow from the borehole. The water inflow gradually increased, with a significant rise occurring at 56.5 m, reaching a maximum of approximately 70 cubic meters per hour. In borehole No. 3, the No. 4 limestone aquifer appeared at a depth of 2.5 m. The water inrush peaked at 50 cubic meters per hour between 3.7 and 5.7 m, where numerous fissures were present.

Through analyzing the core data and stratigraphic succession, the structure detected by the integrated geophysical method was identified as a normal fault with a displacement of approximately 8 m. As shown in Figure 8, there is a slight deviation between the locations detected by drilling and those identified by the integrated geophysical method. Nevertheless, the correlation is satisfactory and improves the location precision compared with using a single method.

3.2.3. Hydrochemical Analysis

The inrush water sampled from borehole No. 3 at the driving face was tested to determine its source. The data were compared with the known ionic composition of water samples from the bottom aquifer of the Quaternary system, the sandstone aquifer in the roof of coal seam No. 7, the sandstone aquifer in the roof of coal seam No. 8, the No. 4 limestone aquifer of the Taiyuan Formation, and the Ordovician limestone aquifer. In the known water sources, except for the water sample of the Ordovician limestone aquifer, which was taken from the surface hydrological borehole, the water samples of other aquifers were all actually exposed during excavation work. As the major anions and cations in groundwater, the following seven ions were analyzed: Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃⁻, Cl⁻, and SO₄²⁻ [24,25]. The average ion contents of the known water sources and water inrush are presented in

Table 2. Ionic composition results in known water sources and inrush water samples.

Water Aquifer	Sample Number	Concentration (mg/L)
		Ca2+	Mg2+	Na+ + K+	HCO3−	Cl−	SO42−
Sandstone aquifer in the roof of coal seam No. 7	1	290.20	86.92	216.42	201.97	212.90	1029
	2	306.21	84.03	326.83	211.13	264.80	1223.28
	3	252.90	86.79	268.41	180.61	226.64	1061.52
Sandstone aquifer in the roof of coal seam No. 8	1	372.94	90.32	264.96	200.75	270.45	1280.9
	2	328.05	80.84	229.54	206.85	256.72	1075.31
	3	357.35	92.5	333.73	211.13	271.42	1425.78
Limestone aquifer No. 4	1	88.38	26.62	101.75	319.12	52.34	165.87
	2	25.53	52.28	92.28	295.84	59.72	183.99
	3	23.57	52.28	90.87	270.11	68.74	132.54
	4	47.13	52.28	90.87	270.11	68.74	132.54
Ordovician limestone aquifer	1	335.23	126.44	509.29	237.17	373.25	1674.39
	2	290.66	108.13	604.39	195.32	335.72	1677.68
Sample of water inrush		27.25	38.45	146.97	299.24	74.56	233.79

.

Using the data in Table 2, a Piper diagram was created to represent water from various aquifers. In the Piper diagram, the distance between two data points reflects the similarity between different water samples. Figure 9 illustrates that the inflow water samples are most similar to the samples from the No. 4 limestone aquifer of the Taiyuan Formation, indicating that the inrush water likely originates from that aquifer. This suggests that there may be a structure conducting water from the No. 4 limestone aquifer into the roadway, as the aquifer is approximately 10 m below the roadway according to the normal formation mapping. Once it is determined that the water inrush originates from the No. 4 limestone layer, taking into account that the connectivity of the No. 4 limestone layer is relatively weak and it is a drainable aquifer, a discharge treatment measure is implemented. In total, approximately 24,000 cubic meters of water is discharged from three boreholes.

4. Discussion

Exploring concealed geological anomalies is critical for effective mine water prevention and control. We propose a coupled approach that integrates geophysical exploration, drilling, and geochemical analysis for the accurate detection of water-conducting structures. This approach follows a systematic process of prediction, detection, verification, and identification. The exploration strategy is structured with geophysical methods leading the way, followed by drilling for verification, and supported by geochemical exploration. Geophysical exploration has lower costs and higher efficiency. It makes drilling construction more targeted. Thus, geophysical exploration is the initial step in water hazard detection. Drilling, which has a higher cost in terms of time and money, is employed to verify the results of geophysical exploration and obtain hydrological parameters. Hydrochemical analysis is used to identify water sources. Its more significant role is to provide a foundation for subsequent treatment measures. By combining these methods, we can mitigate the limitations inherent in using any single technique. This multimethod approach not only improves detection accuracy but also allows for both qualitative and quantitative assessment of geological anomalies.

Despite the effectiveness of this comprehensive exploration system, challenges remain in its application. The first challenge lies in the selection of an appropriate geophysical method, which is crucial as the initial step of exploration. The diverse characteristics of concealed geological structures, coupled with the complexity of the roadway construction environment, complicate this choice. With numerous geophysical methods available, determining the most suitable method is a key factor affecting the detection accuracy. Additionally, while combined detection methods enhance accuracy, they also increase costs and time and can significantly impact coal mine production. Traditional short-distance exploration and excavation techniques are not aligned with the demands of modern intelligent and rapid excavation practices[26]. Therefore, there is a pressing need for new detection methods and technologies that offer faster detection speeds, longer detection ranges, and higher accuracy.

5. Conclusions

This research shows that the accurate detection of concealed water-conducting structures can be achieved by employing a geophysical-drilling-hydrochemical coupled method.

(1)

The integrated geophysical method, combining seismic exploration and electrical prospecting, accurately detected the location and water content of concealed geological structures. The results of the geophysical exploration provided a target area for hydrological drilling, thereby making the drilling work more efficient and focused.

(2)

Hydrogeological drilling provided rock characteristics and hydrogeological features for both the analysis of the properties of concealed water-conducting structures and the verification of geophysical prospecting results. Meanwhile, it directly exposed the groundwater and obtained water samples for hydrochemical analysis.

(3)

Hydrochemical analysis was employed to analyze the inrush water sources and runoff conditions. According to the spatial relations between the source aquifer and mining spaces, the vertical extension range of the concealed water-conducting structures was roughly estimated.

By combining different methods, we can obtain various parameters and information directly or indirectly and analyze the state and hydrogeological characteristics of concealed water-conducting structures from different perspectives. Thus, the limitations and possible bias in the interpretation of a single method or technology can be avoided. This approach can improve the methods for detecting concealed water-conducting structures from the initial location estimation stage through deduction to qualitative exploration and even quantitative detection, which is significant for the safety and production efficiency of coal mining.