... of China Railway, Fuxing road 10, Beijing, P.R. China 100844. |Chengdu University of Technology, College of Information Technology, Chengdu, P.R. China.
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Application of EM Methods for the Investigation of Qiyueshan Tunnel, China Lanfang He*, Minghai Feng**, Zhanxiang He* and Xuben Wang *BGP, China National Petroleum Corporation, BGP, Non-seismic, P.O. Box 11 Zhuozhou, Hebei Province, P.R. China 072751 **Economic Project Institute of China Railway, Fuxing road 10, Beijing, P.R. China 100844 Chengdu University of Technology, College of Information Technology, Chengdu, P.R. China ABSTRACT A successful case history of applying the high-frequency passive source electromagnetic (EM) method and controlled-source audiomagnetotellurics (CSAMT) to investigate the Qiyueshan (Q) Tunnel route is presented in this paper. The high-frequency EM system (EH-4, with frequency range from 90 KHz to 12.8 Hz) and the CSAMT system (V6-A Multipurpose Receiver, with frequency range from 8,192 Hz to 0.125 Hz) were used for the data acquisition. The orthogonal components of the electromagnetic field were measured in the high frequency EM method, while scalar measurements of the electrical and magnetic field components were used in the CSAMT method. The relevant electrical properties of the earth were extracted from the electromagnetic profiles. High frequency EM has high resolution in the shallow earth but a smaller depth of exploration, while the CSAMT method has a powerful signal but a lower resolution in the shallow earth. The integration of the two methods might be effective for the survey of the deep tunnel route. Q Tunnel, located in central south China, has a length of 10 km and a depth of up to 900 m. Half of the tunnel goes through karst terrain, where the geologic structures are very complex, due to cavities, underground rivers and faults. The EM mapping results distinguish the electrical resistivity of different rock formations. Five lowresistivity areas and four high-resistivity areas were found and nine faults were verified by the EM method. These findings were very useful for the later engineering design.
Introduction Most of the early railway tunnels in China are less than 2 km in length. The primary geophysical method used in previous investigations at these sites was electrical sounding. The electromagnetic method is receiving growing interest from geophysicists in China for deep tunnel route investigations. In the past two years, this technique has been successfully used in the prospecting of Qinling Mountain railway tunnel (Compilation committee of History of China Railway Tunnel, 2004). In the route investigation of Yiwan (YW) railway, the mapping of karst terrain proves to be an extremely difficult problem due to its complicated geological structures. To address this problem, an investigation of deeply buried tunnels in the karst terrains was set up. Special attention has been paid to: (1) Mapping the faults, including width, attitude, location, evaluation of their moisture content and effect on tunneling; (2) Mapping the cavities, including their modality, size and depth, evaluating their effect on tunneling; (3) Mapping the strata of the tunnel route and analyzing the lithologic characteristics, especially the watertight strata and their overlying or underlying formations. Q Tunnel lies in western Hubei province, China, where karst geology is prevalent. Cavities and underground JEEG, June 2006, Volume 11, Issue 2, pp. 151–156
rivers are well developed. The underground Qingjiang River, for example, has a length of more than 50 km. In the route of Q Tunnel, Qiqueshan Mountain is a watershed of underground river systems. The eastern side underground river system is dominated by Dayu underground river, which is one of the headwaters of the Qingjiang River. The western side is dominated by the Desheng underground river, which flows into the Changjiang River after traveling scores of kilometers. The major formations in the tunnel route (Fig. 1) can be described as: (1) Permian clastic and carbonate rocks of littoral swamp phase to lagoon phase. It is the major composition of the core formation of Qiqueshan Anticline; (2) Triassic carbonate rocks of neritic phase to lagoon phase. It is the main formation composing the tunnel route and the major formation in which the solution cavities were developed; (3) Jurassic clastic rocks of inland lake phase (Chen et al., 2003). From the aerial photograph interpretations, field surveys, drilling, and earlier geophysical investigations, the route is known to have 9 major faults in the exploration site. All of them have been confirmed by EM methods employed in this project (Wu et al., 1999). The CSAMT method was introduced to China in the early 1980’s. It has been widely used in environmental investigations and oil and gas exploration. The instruments we used for CSAMT surveys include GDP-12, GDP-16
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Figure 1.
Geological map of the working area.
(Zonge Engineering Inc.), and V4 and V5 (Pheonix Geophysics Ltd.). The most popular instruments currently used in China are GDP-32 (Zonge Engineering Inc.) and V6 (Pheonix Geophysics Ltd.). The CSAMT method was first used in the karst region of China for mapping a railway tunnel in 1998. As a new geophysical technique, CSAMT has the advantages of greater depth of exploration and higher working efficiency, because with a single setup of the transmitting dipole, the survey can cover a large area. However, CSAMT has disadvantages in that it is easily influenced by the near-field, topographic and source-shadow effects. These effects restrict the method from being widely used in engineering applications (He, 1990). For the work reported here we used both high-frequency passive EM and CSAMT in a karst area for tunnel route exploration. In contrast to traditional practice, several standards have been modified based on our experience with tunnel exploration. For example, the T-R separation is traditionally chosen to be only 3 times the exploration depth. In our case for karst mapping, we chose it to be at least 8 times the exploration depth. After many applications, we also found that a combined CSAMT and high-frequency passive EM is very helpful for mapping the karst areas.
Date Acquisition and Analysis We carried out the survey at 160 stations for highfrequency passive EM and 180 stations for CSAMT. The sites for both methods coincide. A STRATAGEM EH-4 real-time system for passive source EM and two V6-A Multipurpose Receivers were used for data acquisition, while a T-30 system was used for CSAMT transmitting. The STRATAGEM EH-4 records orthogonal electric and magnetic fields, which are used to obtain the tensor impedance for interpreting 2-D structures (He et al., 2002). It can
Figure 2. Layout of the field survey of CSAMT (a) and EH4 (b). provide electrical conductivity imaging of the subsurface for depths between 10 meters and 1,000 meters. The electric field was measured by two pairs of titanium electrodes, while the magnetic field was measured by highly sensitive magnetic coils (Fig. 2b). The scalar survey mode was used in the CSAMT method. Seven C-C electrodes, a new kind of electrode made primarily from carbon, arranged in a line to cover six field sites, were used to record the data. To avoid the nearfield effect, we chose the transmitter-receiver separation to be larger than 9 km, which is about ten times the maximum depth of the tunnel. The peak current generated by the T-30 was 20 A, and the length of the current dipole was 1 km. The field layout of CSAMT is shown in Fig. 2a. The frequency ranges from 0.125 Hz to 8,192 Hz with 16 frequencies. For the passive source EM analysis, the electric and magnetic field time-series are first transformed into the frequency-domain by a Fourier transform. The results are then corrected for system effects. The EH4 system software defines time windows or center frequencies in a manner that
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Figure 4. Result of comprehensive correction using EAMP. Section (a) shows the data before the correction, while Section (b) shows the data after the correction. (The profile is a part of the section from Site 4100 to Site 5100). the raw data. From Fig. 3 we note that the resistivity values in the higher frequency portion of the curves after correction is the same as that before the correction. However, they are quite different in the lower frequency portion where the value after correction by ‘‘REAR’’ is lower than that before the correction. Before the correction the curves show strong near-field effect (i.e., monotonic resistivity rising with lowering frequency). After correction, however, it disappears. Many anomalies (Fig. 4a), mainly caused by topography or static shift in the section before EMAP processing, have been successfully filtered (Fig. 4b).
Figure 3. Result of near-field correction using the BGP software ‘‘REAR’’. is independent of the sampling rate or starting time for data acquisition (He et al., 2004a). The calculation of impedance and apparent resistivity is similar to that in AMT analysis (Hautot et al., 2002). In CSAMT, only the spectral data were recorded. The data correction using the calibration results is the same as in the high-frequency EM method. The near-field effect was corrected with a program ‘‘REAR’’, developed by BGP. Figure 3 shows the raw data and the data after the near-field correction. We use EMAP (Warren, 1996; Zhang, 1999; He et al., 2004b) to remove both the topographic and static shift effects. Figure 4 shows the results after the correction using EMAP in comparison with
Method Comparison Although the high-frequency passive EM has higher resolution in the shallow earth, it is not suitable for the exploration of relatively deep geological targets especially in conductive areas. First, the lowest frequency in the EM receiver is about 12 Hz, which is too high for deep targets due to high attenuation of high-frequency EM signal. Second, the natural electromagnetic signal is very weak at frequencies around 1,000 Hz, resulting in poor data quality in this frequency range. CSAMT has a powerful transmitting source and a relatively low frequency range (from 8,192 Hz to 0.125 Hz), implying that it has a greater depth of exploration but a lower resolution in the shallow earth. The fact that CSAMT has a higher S/N ratio than highfrequency passive EM is also noticeable. The CSAMT method is, however, more easily influenced by the near-field and topographic effects. Thus, in a less-noisy area with target depths less than 200 m in conductive area, or less than 500 m in karst area, high-frequency passive EM is strongly recommended. Otherwise, as shown later, a combined
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Figure 5. Comparison of high-frequency passive EM data (a) with CSAMT data (b). (The test site is near Site 5200).
passive source EM and CSAMT can offer information on structures over a wide range of depths. Figure 5 shows the results from both passive EM and CSAMT techniques. We chose data from both methods within the same frequency range for comparison. From the figure, one sees that, while the passive source EM (curve A) has a poor data quality within the frequency range shown here, the CSAMT data (curve B) clearly distinguishes a resistive-conductive-resistive earth structure. Figure 6 shows the profiles obtained from the inversion of high-frequency passive source EM (a) and CSAMT (b) data. It is seen that the general earth structure from the two methods is the same, but the detailed earth structures are different. More detailed information about the shallow earth structures is
Figure 6. Comparison of the profiles inverted from the high-frequency EM and CSAMT data. (a) Profile of high-frequency EM (EH-4). (b) Profile of CSAMT.
Figure 7. CSAMT result over a known fault. (a) Depth-resistivity section. (b) Apparent resistivity curves above the fault.
revealed by the high frequency EM (a) than CSAMT (b). The geoelectrical structures of karst look smoother in the CSAMT (b) section with fewer anomalies than that of high frequency EM. However, more detailed information can be obtained from the CSAMT data (b) than high-frequency EM data (a) in deep structure. Mapping the Karst Features Faults and cavities were the key targets of the EM investigation in the survey area. Most of our surveys were conducted in karst terrains with known cavities and faults. Fault detection was based on the attributes of EM data, e.g., the abrupt change in the apparent resistivity between neighboring stations in a profile. Drilling and other geological data were also used to evaluate these features. The detection of cavities was confirmed by the comparison of the survey data to results over known cavities. Figure 7 shows the result of CSAMT obtained over a known fault found by drilling and considered as the most dangerous structure in the tunnel planning. Figure 8 shows the CSAMT results at a known cavity that has been developed for tourism. The cavity is about 50 m high and 100 m wide. There are several
155 He et al.: EM Investigation of Qiyueshan Tunnel, China
Figure 8. CSAMT results over a known cavity. The survey line is perpendicular to the trend of the cavity and crosses the cavity near station 250. The cavity is over 10 km long and the entry is about 3 km from the survey line. The cavity is distinguished as a resistive zone. The inset photo shows the entrance to the cavity. sub-cavities, including one that is air-filled and one that is water-filled. From Fig. 8, it is obvious that the dry cave is characterized by a high-resistivity zone in the depthresistivity section. Based on the known cavities and faults and the corresponding EM responses, we generated Fig. 9 by using shallow results (,500 m) of passive EM (Fig. 6a) and deeper results (.500 m) of CSAMT (Fig. 6b). The combined results are presented in Fig. 9. Due to gridding, the difference between Figs. 6 and 9 was notable. Nine faults were mapped from the low resistivity contour in the section.
Figure 9.
Further, we distinguished five high-resistivity anomalies in the section. Four of them are believe to be cavities, because of sinkholes at the earth surface at these locations. The unmarked high-resistivity anomaly lies in the core of the Q anticline (from 2,700 m to 3,700 m). We already know that the highly resistive formation in this place is responsible for the resistive anomaly. The low resistivity anomalies are important for mapping the karst terrain in the investigation of Q Tunnel, because most of the engineering concerns such as faults, filled cavities, etc., are usually connected with the areas of low resistivity. From the depth-resistivity section in Fig. 8, we distinguished five low resistivity zones, named from left to right respectively as ‘‘ab1’’ to ‘‘ab5’’. Zone ‘‘ab1’’ and ‘‘ab2’’ lie in the entrance of the tunnel. Faults and an underground river occur there, so we believe the major cause of the anomaly is water-filled fractures. Zone ‘‘ab3’’ lies near the core of the anticline. We believe that the low resistivity is associated with fractures due to the pressure. Zone ‘‘ab4’’ is located above ‘‘ab3’’. The major reason for this low resistivity zone is again fractures. Zone ‘‘ab5’’ lies in the contact between calcareous rock and clastic rock, which corresponds to the underground rivers. Thus, the lithology, the faults filled with water, and underground rivers may be responsible for the low resistivity anomaly.
Conclusions Based on the different resolutions and depths of exploration, integrated passive source EM and CSAMT has been successfully used for mapping the deeply buried geological structures in karst terrains in China. For the Q Tunnel exploration, the underground structures were clearly distinguished by interpreting the combined data from the two methods. Resistivity anomalies closely connected with the cavities and faults, were easily identified. This provides geophysical input for the engineering design of the tunnel
Geological interpretation of the combined passive EM and CSAMT data in Fig. 6.
156 Journal of Environmental and Engineering Geophysics route. We believe that a combined passive and controlledsource EM can be a good tool for tunnel route mapping. Acknowledgments Thanks are given to the Natural Science Fund (40074036) of China for the support of the project. We also appreciate the help from Xubing Lu from the Hong Kong Polytechnic University for checking this paper.
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