Prediction Technology of Buried Water- Bearing Structures in Coal Mines Using Transient Electromagnetic Method

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1 Jun Journal of China University of Mining & Technology Vol.17 No.2 J China Univ Mining & Technol 2007, 17(2): Prediction Technology of Buried Water- Bearing Structures in Coal Mines Using Transient Electromagnetic Method JIANG Zhi-hai, YUE Jian-hua, LIU Shu-cai School of Resource and Earth Science, China University of Mining & Technology, Xuzhou, Jiangsu , China Abstract: Buried water-conducting and water-bearing structures in front of the driving head may easily lead to water bursts in coal mines. Therefore, it is very important for the safety of production to make an accurate and timely forecast about water bursts. Based on the smoke ring effect of transient electromagnetic fields, the principle of transient electromagnetic method used in detecting buried water-bearing structures in coal mines in advance, is discussed. Small multi-turn loop configurations used in coal mines are proposed and a field procedure of semicircular sector scanning is presented. The application of this method in one coal mine indicates that the technology has many advantages compared with others. The method is inexpensive, highly accurate and efficient. Suggestions are presented for future solutions to some remaining problems. Key words: mine transient electromagnetic method; advance detection; water-bearing structure; small multi-turn loop; sector scanning CLC number: TD 8 1 Introduction Buried water conducting passages and the water-bearing structures in front of a driving head may easily lead to water bursts in coal mines, greatly affecting the safety and efficiency in coal mine production. At present a method of contact drilling is used to forecast the presence of water in structures. This method, however, is not only expensive and inefficient but also easily results in flooding. Therefore, the development of a new geophysical technology to detect buried water conducting passages and to predict water-bearing structures accurately and instantaneously is of great operational importance [1 2]. Although tunnel seismic prospecting (TSP) is very good in forecasting geological structures, it cannot reflect the water conducting and water bearing situation of such structures. Because it requires expensive instrumentation and is complex in operation, the TSP technology is rarely used in coal mines [3]. Many geophysical methods are used to detect structures in front of the driving head at present, such as the channel wave seismic method, the geological radar method and the mine direct current electric method. Similar to TSP, both the channel wave seismic method and the geological radar method are very good in detecting geological structures exactly, but not good in detecting the water bearing loads of such structures. Based on the difference in rock conductivity, the mine direct electric detection technology has many advantages. It is quick, simple and convenient to operate, it is effective over long distances and sensitive to water. It has been successfully applied in the detection of water conducting and water bearing loads of structures in front of the driving head [4 6]. However, it also has some limitations. For example, the roadway must be longer than the detection distance and cannot have water and metal in its sight [7]. Since the realization of the infrared spectrum, its use as one of the more recent new ways of detecting and predicting water bursts in tunnels, has been variously applied. However, its effective predictive distance is short and Received 10 November 2006; accepted 05 December 2006 Project supported by the National Natural Science Foundation of China, by the Specialized Research Fund for the Doctoral Program of Higher Education and D by the Scientific Research Fund for Youth of China University of Mining & Technology Corresponding author. Tel: ; address: jzh @126.com

2 JIANG Zhi-hai et al Prediction Technology of Buried Water-Bearing Structures in 165 it can easily produce errors when it is affected by water movements in mine construction and interruptted by radiators [8]. Some investigators have already carried out preliminary research on the application of electromagnetic missile detection technology in forecasting geological structures of tunnels and pointed to some possible breakthroughs in support of new technologies [9]. A mine transient electromagnetic method (MTEM) with a small multi-turn loop used in coal mines has recently been developed. Compared with the direct current method, this technology is not limited by the size of the construction site and has no problem of electrode grounding. With its many advantages, such as rapid and convenient operation, the small effect exerted by the volumes involved and its long detection distance, the effect of the MTEM applications have been satisfactory in detecting and predicting the conduct of water conduits of faults, collapsed columns and hydrologic boreholes [10 11]. 2 Principle The MTEM usually adopts two small multi-turn loops: one is used to transmit magnetic signals and the other is used to receive the induced signals. The normal direction of the loop is just the detection direction and the loop is laid out to make the normal direction aim at the object to be detected. The step current flows through the transmitter loop and a primary magnetic field is then generated. Afterwards, the transmitter current is switched off, making the primary magnetic field immediately fall to zero. An eddy current centered on the normal direction of the loop is induced in the rocks surrounding the roadway. Initially the induced current field is only located around the transmitter loop and is the strongest. Over time, the induced current diffuses in the surrounding rocks and the power becomes increasingly weaker. According to research conducted by Nabighian and Macnae [12], the induced eddy current is distributed annularly in the uniform medium with the initial maximum value of the current field in first instance lying in the rock near the transmitter loop. With the passing of time, the maximum zone spreads out along the cone-shaped surface. A second magnetic field formed by the eddy current at the center of the transmitter loop can be equivalent to the field of a loop-line current at any moment. When the step current is switched off, the equivalent loop-line current gets close to the transmitter loop and has the same shape as the loop. Then it diffuses in every direction, gradually turning into a round current loop. The equivalent current is just like a set of smoke-rings blown out of the transmitter loop. So this process of diffusion of the eddy current can be called the smoke ring effect, as shown in Fig. 1. Fig. 1 Sketch map of advance detection by MTEM According to the smoke ring effect, the early transient electromagnetic field is mainly formed by the induced current in the medium in front of the driving head near to the transmitter loop, which reflects the conductivity of the medium over a short distance; while the late transient electromagnetic field is formed by the induced current in the medium in front of the roadway far from the transmitter loop, which reflects the conductivity of the medium over a long distance. So the conductivity of strata in front of the driving head can be detected given the variation of the received magnetic field. By doing so, the water bearing structures can be determined. The basic principle of MTEM is essentially the same as the ground transient electromagnetic method.. The small loop configuration adopted in this technology makes the detection possess directionality and so the loop can be laid out at will according to different target orientations. It must be pointed out that the full space effect has been an inherent problem of MTEM because this technology is constructed in a roadway and the field is distributed over the entire space. 3 Configuration Because of the roadway space restriction a small multi-turn loop is adopted in MTEM. The loop side length is not longer than 3 m. Some configurations can usually be used, such as a coincident loop, an inloop or a displaced loop (dipole-dipole). The transmitter loop of the coincident configuration overlaps the receiver loop (Fig. 2a), its advantages are: 1) high signal levels, 2) sensitivity to abnormity and 3) convenient in operation. However, there is a detection blind zone from 0 to 20 m because the early data is disturbed by the mutual inductance between Tx and Rx (Fig. 2). In the in-loop configuration, the small receiver loop with hundreds of turns is located in the center of the transmitter loop (Fig. 2b). In the displaced loop configuration, the receiver loop is separated from the transmitter loop, so the mutual inductance is small, but the signal received is weak and the operation is not convenient (Fig. 2c).

3 166 Journal of China University of Mining & Technology Vol.17 No.2 (a) Coincident loop (b) In loop (c) Displaced loop Fig. 2 Sketch map of configurations 4 Method Most likely, objects lie in different orientations in front of the driving head. As is shown in Fig. 3, the arrowheads denote the normal direction of the transmitter loop. In turns, it can point to the left of the driving head, to the left side of the working face, to the working face, to the right side of the working face and to the right of the working face in turns. The driving head is scanned in the semicircular sector, so information from all sides of the working face is obtained. This detection mode is called the sector scanning method by some authors. In order to emphasize the object in background, detection stations should be displayed on both side walls of the roadway beside the driving head. For gaining information of the object in the space, the roof, the bedding and the floor can be scanned in Fig. 3 Sketch map of sector scanning turn. The normal direction of the loop is pointed at the roof (Fig. 4a), the bedding (Fig. 4b) and the floor (Fig. 4c) while doing sector scanning. According to the detection results, the isolines of apparent resistivity are drawn in sector. The 3D information of the object can be obtained by analyzing these three sections synthetically. Under some conditions, two of the three sections may normally be enough. Detection direction Transmitter loop 60 (a) detection Transmitter loop Detection direction 90 (b) Bedding detection Fig. 4 Sketch map of detection direction Transmitter loop Detection direction 60 (c) Floor detection 5 Example The roof strata of the 75 # return airway is a Taiyuan formation limestone in a coal mine in Jiangsu Province. According to the geological mine data, there may be a fault in front of the driving head, but the location of the water conducting and water bearing structures are not known. In order to prevent flooding and ensure safety, the water conducting and water bearing structures must be confirmed. The coincident loop is adopted and the length of the loop side is 2 m. Twelve detection stations are arranged at the driving head, as shown in Fig. 3. The angle of the adjacent scanning line is 22.5 from No. 1 to No. 5 and from No. 7 to No. 11 and the roof and the bedding are also investigated. The isoline of the apparent resistivity of the roof is shown in Fig. 5a. The resistivity is 10 Ω m in the range of the scanner line from No. 1 to No. 3 and from No. 9 to No. 11. The low value results from the side walls, bolted with wire mesh. Elsewhere, in the range of the scanner line from No. 4 to No. 8 and a detection distance ranging from 50 m to 80 m, there is an abnormal zone with a low value of 40 Ω m, while the value of other areas is higher than 50 Ω m. It can be concluded that the abnormal zone is just the fault, but the water bearing is weak. Fig. 5b shows the isolines of the apparent resistivity of the bedding and the changing pattern of the apparent resistivity agrees with that of Fig. 5a. The abnormal zone with a low resistivity of 30 Ω m appears in the range of the scanner line from No. 4 to No. 8, where the detection distance ranges from 40 m to 70 m. According to this analysis, the fault lies at 40 m distance from the driving head, but the water conducting and water bearing is weak, so safety measures to prevent a water burst are not needed. The detection result is verified to be valid in the driving direction.

4 JIANG Zhi-hai et al Prediction Technology of Buried Water-Bearing Structures in 167 Distance (m) (a) Of roof Fig. 5 Isolines of apparent resistivity Distance (m) (b) Of bedding 6 Conclusion The detection technology of the mine transient electromagnetic method have a number of merits. These include a rapid and convenient operation, a small effect independent of the volume involved a long detection distance, which avoids the difficulty of grounding the electrodes and the space limitation in the construction site. It has been shown that the application of this technology has been satisfactory in detecting a water conducting and water bearing structure. As a new technology, developed recently, MTEM is still incomplete. The conditions under which it can be applied and its effects have not been completely investigated, while the data processing is still at the 1D level without any development in 2D and 3D. To ascertain the geological unique location of the abnormality in space, it is essential to distinguish the geoelectric abnormality from the effects of the roadway and the entire effect of space. At the same time, the effective detection depth in the bedding and its perpendicular layer must be known. MTEM research on the response regularity of the 3D geoelectric abnormality under different roadway conditions and different observation methods must be developed by numerical simulation and physical modeling. It is believed that MTEM must become gradually more important in the roadway driving of coal mines, in tunnel engineering and underground engineering of cities. Acknowledgements Financial support from the National Nature Science Foundation of China ( ), the Specialized Research Fund for the Doctoral Program of Higher Education of China ( ) and from the Scientific Research Fund for Youth of China University of Mining and Technology (D200409) are greatly acknowledged. Thanks are also due to Feng Meimei for her efforts in English translation. References [1] Liu S C, Yue J H, Liu Z X. Geophysical Technology and Application on Coal Mine Hydrogeology. Xuzhou: China University of Mining & Technology Press, (In Chinese) [2] Wang Q R. Progress in the geophysical exploration research on underground geological hazards. Progress in Geophysics, 2004, 19(3): (In Chinese) [3] Liu Z G, Liu X. TSP application and development in tunnel lead forecast. Chinese Journal of Rock Mechanics and Engineering, 2003, 22(8): (In Chinese) [4] Wang W M. Methods of detecting water-bearing structures. Coal Geology and Exploration, 1996, (6): (In Chinese) [5] Cheng J L, Wang Y H, Yu S J, et al. The principle and application of advanced surveying in roadway excavation by resistivity method. Coal Geology and Exploration, 2000, 28(4): (In Chinese) [6] Liu Q W. Undergroud electrical lead survey method and its application. Coal Geology and Exploration, 2001, 29(5): (In Chinese) [7] Li Y B. Mine electrical method pilot detection technology. Coal Science and Technology, 2002, 30(2): 1 3. (In Chinese) [8] Wang Y, Chen Q. Application of infrared acquisition technology in prediction of water gushing in yuanlingshan tunnel. Chinese Journal of Rock Mechanics and Engineering, 2003, 22(5): (In Chinese) [9] Li X, Xue G Q. Advanced forecasting of tunnel and lane tunneling by electromagnetic missile detecting technology. Journal of Chang an University (Natural Science Edition), 2002, 22(3): (In Chinese) [10] Liu Z X, Yu J C. Application of mine transient electromagnetic method to the exploration of hydrological borehole. Geophysical and Geochemical Exploration, 2006, 30(1): (In Chinese) [11] Fu P H, Zhu Y M. Detection of yielding property of faults in mines by transient electromagnetic method. Coal Mine Technology, 2006, 11(2): (In Chinese) [12] Nabighian M N, Macnae J. Quasi-static transient response of a conducting half-space an approximate representation. Geophysics, 1979, (44):

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