Evaluation of geological conditions ahead of a tunnel face using seismic tomography between a tunnel and ground surface

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1 Volume x, Number x, Month 2x, pp.x-x [Summary] Evaluation of geological conditions ahead of a tunnel face using seismic tomography between a tunnel and ground surface Yasuhiro YOKOTA*, Takuji YAMAMOTO*, Keisuke KURIHARA*, Yasuyuki MIYAJIMA* & Tomoaki MATSUSHITA* * Member of ISRM: Kajima Technical Research Institute, Kajima Corporation, Chofu city, Tokyo Japan Received ; accepted dd mm yyyy ABSTRACT Evaluating geological conditions ahead of a tunnel face accurately and quantitatively is essential for a safe and reasonable tunnel excavation. Recently, it has also become important to ensure that geological surveys do not interrupt the daily tunnel excavation cycle. In this research, the authors have developed a new seismic tomography technique between a tunnel and ground surface. In order to conduct an accurate seismic tomography analysis, the source and receiver systems must be synchronized with negligible errors. Furthermore, utilizing the tunnel blasting vibrations as survey sources and the newly developed automatic data acquisition system made it possible to conduct the survey without interrupting the tunnel excavation. The results of applying this new method to the tunnel construction site show that the predicted values were in good agreement with actual geological conditions around the tunnel face. Keywords: Geological survey ahead of a tunnel face, Seismic tomography, GPS, vibration, Automatic data acquisition 1. INTRODUCTION It is difficult to determine precisely the geological conditions ahead of a tunnel face before tunnel excavation. In addition, it is also hard to evaluate them quantitatively. Therefore, various types of geological surveys, such as boring surveys and seismic refraction surveys, are commonly utilized for estimation. However, although the boring survey enables us to observe the rock samples directly, it is not easy to conduct multiple boring surveys because of cost and time constraints. Furthermore, in seismic refraction surveys, when a tunnel cover is deep, it is difficult to obtain an accurate elastic wave velocity for the area near the tunnel, since seismic waves could not reach the tunnel excavation depth. Because of such problems with preliminary surveys, seismic reflection surveys ahead of a tunnel face during excavation have come into widespread use in recent years (Sattel et al., 1996; Ashida et al., 21a, 21b; Yamamoto et al., 23, 21). In seismic reflection surveys, seismic waves are generated near the tunnel face, and waves reflected from geologically discontinuous surfaces are analyzed in order to predict locations of faults, fracture zones, hard rock masses, etc. With this method, it is possible to gain a general understanding of such discontinuous locations; however, it is difficult to make quantitative evaluations. Furthermore, the tunnel excavation cycle has to be interrupted with these surveys for a half or whole day. Therefore, the authors have developed a new survey JSRM All rights reserved. technique using seismic tomography between a tunnel and ground surface (Yokota et al., 215). In this survey, seismic waves generated near the tunnel face are measured at ground surface, and their data are subjected to tomography analysis (see Figure 1). As a result, it is possible to predict ground conditions more precisely and quantitatively. Moreover, by using blasting vibrations for tunnel excavation as survey sources, this survey could be conducted without any work interruption. :Sources :Receivers :Ray paths Vp (km/s) 5. Vp = 4.5 Fault, Fracture zone etc. Vp = 2. (Low velocity zone) Figure 1. Principle of new seismic tomography between a tunnel and ground surface

2 2 Y.YOKOTA et al. / International Journal of the JSRM vol.x (2x) pp.x-x This paper is organized as follows. The verification of applicability of the new seismic tomography to tunnel construction site using numerical simulation is explained in Chapter 2. Chapter 3 discusses the developments of the new survey technique, while the results of application to tunnel construction sites are explained in Chapter 4. Finally, the conclusions are explained in Chapter Bedrock Poor geological zones Ground surface :Receivers :Sources 1 2. VERIFICATION BY NUMERICAL SIMULATION In order to verify the applicability of this new seismic tomography technique to predict geological conditions ahead of a tunnel face, a numerical simulation was conducted as shown in Figure 2. The target profile was located in a bedrock having a P-wave velocity of 4. km/s with four poor geological zones (approximately 3. km/s). As the tunnel face advanced, vibrations were generated at 25 m intervals, and seismic waves were received by geophones installed on the ground surface with 25 m spacing. Figure 3 shows the analyzed P-wave velocity distribution of the ground when the tunnel face advanced at 5 m, 275 m, and 5 m, respectively. As shown in Figure 3(a), when the tunnel face was located at 5 m, a decrease in the velocity was observed in the poor geological zone near the tunnel face. However, because of the small number of wave rays passing through the zone, the decrease in seismic wave velocity can be observed throughout and not only in the poor geological zone. On the other hand, as shown in Figure 3(b) and (c), as the number of passing wave rays increased, the ability to detect both the width of the poor geological zone and the decreased velocity value also increased. In particular, it is clear in Figure 3(b) that the width and velocity of fault fracture zones distributed ahead of the tunnel face can be estimated with high precision. As a result of this simulation, we have confirmed that the new seismic tomography technique between a tunnel and ground surface is applicable for the effective geological survey ahead of a tunnel face Figure 2. Simulated geological profile (a) cutting face : 5 m :Receivers(15 points) :Sources (3 points) Vp(Km/s) :Receivers(15 points) :Sources (11 points) Vp(Km/s) 3. DEVELOPMENT OF THE NEW SURVEY TECHNIQUE 3.1 Outline of the newly developed survey method (b) cutting face : 26 m As shown in Figure 1, first, seismic waves generated near the tunnel face and ground surface are received by geophones installed on the ground surface. Not only manual sources, such as hammer strikes, but also vibrations of tunnel blasting for excavation could be applicable as survey sources. By doing so, this geological survey could be conducted without interrupting any tunnel excavation cycles. Second, by using the obtained seismic wave data, tomography analysis is conducted. Finally, the velocity map ahead of the tunnel face is generated. This velocity map enables us not only to evaluate locations of geological discontinuous zones but also to provide quantitative evaluations. In order to apply this technique to an actual tunnel construction site, the following methods had been employed Ground surface :Receivers(15 points) :Sources (21points) (c) cutting face : 5 m Figure 3. Results of numerical simulation Vp(Km/s)

3 R. MASSEY et al. / International Journal of the JSRM vol.x (2x) pp.x-x Precise time synchronization system In order to conduct tomography analysis accurately, the clocks of the wave-generation system (source system) and wave-receiving system (receiver system) must be synchronized precisely. Generally, seismic tomography surveys are conducted with wired system, which means that the source system connects to the receiver system directly with long cables. However, when this survey is conducted in an actual mountain tunnel construction site, the distance between the tunnel face and ground surface is usually more than several kilometers. This makes it difficult to apply the conventional seismic tomography technique for geological surveys. Therefore, a new wireless time synchronization system has been employed (see Figure 4). As shown in this figure, a GPS receiver and a high-precision GPS satellite time marker are installed in both source and receiver system in order to solve such problems (LS-88 and LS-2K Hakusan Corporation). As GPS data cannot be received inside a tunnel, an optical transmission device, which consists of an optical transmitter, junction cable, and optical receiver, was utilized for receiving GPS signals. As a result of this system, both source and receiver systems could be synchronized within.125 ms error theoretically. In order to verify the accuracy of this wireless time synchronization system, several seismic waves generated by the new system were compared with those generated by the conventional wired system (see Figure 5). Figure 6 shows the typical acquired waveforms. From these results, it was found that no major differences between the waveforms could be observed. This finding indicates that the time synchronization could be done precisely. In these tests, the average difference value was calculated as.2 ms. Although the measured errors were slightly larger than theoretical ones, this value could not decrease the accuracy of the seismic tomography analysis results. 3.3 Measurement during tunnel excavation cycles To conduct the geological survey without interrupting any tunnel excavation cycles, the authors have employed two strategies: one is adapting tunnel-blasting vibrations as survey sources, and the other is developing the automatic data acquisition system Applying tunnel blasting as survey sources In order to utilize the excavation blasting as survey sources, the time when tunnel blasting occurs must be obtained accurately and safely. The general approaches for acquiring the blasting time are briefly described as follows: 1. a method for detecting the time when explosives with electric cables or optical fibers are blasting. 2. a method for measuring directly the electric current or voltage from a blasting machine. However, in the case of adopting the first method, several electric cables or optical fibers have to be attached to the explosives, making these procedures difficult to conduct without interrupting the tunnel blasting cycles. On the other hand, in the case of employing the second option, it is possible to measure the electric current and voltage directly. However, the electric voltage output from a blasting machine Figure 4. Overview of the new wireless time synchronization system Amplitude 振幅 Amplitude 振幅 Figure 5. Experimental conditions Time(second) 時間 (s) :Wireless(GPS) 合成有線式 :Wired Figure 6. Typically acquired waveforms GPS :Wireless(GPS) 合成有線式 :Wired Time(second) 時間 (s) machine Without any changes in blasting patterns and cycles Change in Magnetic field Non-contact Detecting system Current flow timing signal Seismicwave measurement device Seismic wave Figure 7. Schematic block diagram of the noncontact blasting timing detection system

4 4 Y.YOKOTA et al. / International Journal of the JSRM vol.x (2x) pp.x-x is normally more than 5 V, which might damage the measuring equipment. Moreover, in terms of safety, it is not advisable to connect electrical equipment with blasting circuits. Therefore, the authors have developed noncontact blasting timing detection system (Nippon koki Co.,Ltd.) which is a noncontact method for detecting variations in a magnetic field when electric current flows through blasting circuits. Figure 7 shows the schematic block diagram of this method. As shown in this figure, this new method enables us to obtain the blasting signal safely without stopping the tunnel excavation cycles. In order to confirm the accuracy of this new noncontact detecting method, a verification test was conducted at the actual tunnel construction site. Figure 8 shows the operational conditions of the newly developed equipment. Since this equipment is relatively small and does not need a power supply, it could be installed near the blasting machine. Figure 9 shows the seismic waves received by geophones on the ground surface. In this figure, the waveform from the blasting vibration is compared with that of a hammer strike. The test results indicate that there were no arrival time differences between both blasting and hammer strike sources. It was also found that the blasting source could improve the S/N ratio (signal to noise ratio) dramatically. Finally, we conclude that this noncontact method could be used for seismic tomography between a tunnel and ground surface without decreasing the accuracy of this new survey Developing the automatic data acquisition system A number of seismic data must be used for an accurate tomography analysis. In the case of using blasting vibrations as survey sources, if preparation/removal works and measurements works are conducted every time seismic waves are generated, the new seismic tomography method will lack versatility and will be expensive. Therefore, the authors have developed an automatic data acquisition system to solve this problem. There are three difficulties for developing the automatic data acquisition system: 1. how to operate the complicated measurement system automatically, 2. how to secure the power supply for long-term field measurement, and 3. how to protect the measurement system and cables for long-term field measurement. Figure 8. Operational condition of the noncontact blasting timing detection system (Amplitude) Non-contact Detecting system 1No arrival time gap 2Improvement of S/N raito : Hammer strike Figure 9. Typical seismic waves obtained from the blasting vibration and hammer strike Mobile phone Battery machine : High-precision GPS time marker PC Figure 1. Original start-up system using a mobile phone In order to solve the first problem, the original start-up system using a mobile phone has been developed (see Figure 1). When this mobile phone receives a phone call, the system starts up and automatically prepares for data logging. As a result, the measurement can be operated by remote control. To resolve the second problem, a portable solar panel (14 cm 66 cm) is utilized (see Figure 11). Finally, to solve the third problem, geophones and all cables are protected by flexible protecting tubes. These improvements made it possible to obtain the seismic waves generated by tunnel blasting easily over the long term. Solar panel Original Start-up system Figure 11. A portable solar panel and start-up system

5 R. MASSEY et al. / International Journal of the JSRM vol.x (2x) pp.x-x 5 4. APPLICATION OF THE NEW SEISMIC TOMOGRAPHY TO AN ACTUAL TUNNEL SITE 4.1 Overview of the survey Figure 12 shows a geological profile of a tunnel where the new seismic tomography between the tunnel and ground surface was applied. The length of the tunnel is 1637 m and the maximum tunnel cover is 17 m. The geology mainly consists of granodiorite and sandy gneiss. From preliminary surveys, plural faults were estimated to exist near the portal of this tunnel. Therefore, it was necessary to conduct a detailed geological survey in order to excavate the tunnel rationally and safely. In this survey, six excavation blasts were used as survey sources. Figure 13 shows the equipment used to detect the blasting signal, the measuring device inside the tunnel and the cutting face after blasting. 4.2 The survey results When conducting the tomography analysis, the data obtained from this new seismic tomography method were combined with seismic refraction data conducted from the ground surface before tunnel excavation. Table 1 shows the locations of sources and receiving points. Figure 14 shows the diagram of ray paths of obtained seismic waves. From this figure, these ray paths traversed the area ahead of the tunnel face and increased the surrounding area of the tunnel each time that the blasting vibration data were obtained as the tunnel face progressed. Figure 15 shows the distribution of seismic wave velocity from the tomography analysis. The result of the tomography analysis indicated that there were almost no areas with a noticeable decrease in seismic wave velocity except for the slightly low seismic wave velocity area in the upper part of the tunnel at approximately 148 m T.D. (tunnel distance from the portal). From this result, it was verified that plural faults that had been of concern in the preliminary survey were small in scale, and they would have no effect on the efficiency of the excavation works. Therefore, the construction could proceed without increasing the tunnel supports. Moreover, after tunnel construction, it was found that these results were in good agreement with the observed results of the tunnel face. Consequently, we concluded that it was possible to get a detailed understanding of the geological structure at the tunnel depth without interrupting the tunnel excavation cycles by using the new seismic tomography survey between the tunnel and ground surface. 5. CONCLUSION This paper has reported about the newly developed seismic tomography technique between a tunnel and ground surface with a brief description of the exploration and its application in the field. This new survey method makes it possible not only to obtain the location of faults, fracture zones, and hard rock masses, but also to provide quantitative evaluations. This is because this method measures the direct waves that penetrate through fracture zones or hard rock zones ahead of the tunnel EL.45m 4m 35m 3m 25m 2m T.D. ( Distance) TD. 到達側坑口 Portal TD.1637m 受振測線 Receiving ( 水平距離 points (22m) TD.1634m~1414m トンネル延長 length (L= 1637m) L=1637m Fault F3 断層 3 Fault F1 断層 1 F2 Fault 断層 2 F4 Fault 断層 Figure 12. Geological profile of a tunnel where the new seismic tomography was applied Noncontact ( 左 ) 点火信号検出器 machine detecting system ( 右 ) 発破器 Figure 13. Equipment of detecting the blasting signal, measurement device inside the tunnel and cutting face after blasting Table 1. Locations of sources and receiving points Preliminary survey * (Surface) Sources During excavation ** () Figure 14. Ray tracing of the seismic waves < 地質凡例 Geology > Granodiorite 花崗閃緑岩 Sandy 砂質片麻岩 gneiss 発振 Source ( 発破 points ) 位置 Cutting face 発破後の切羽例 after blasting High precision GPS time marker & data 高精度刻時装置 logger (Source) Preliminary survey * (Surface) Receivers During excavation ** (Surface) Range -2~44m 19~33m -2~35m ~22m Number of points Interval 3~9m 1~6m 5m 5m :Receivers (Surrface) :Sources() :Sources(Surface) * Seismic refraction survey before excavation ** Seismic tomography survey during excavation Analysis range = 48m Ground surface EL.

6 6 Y.YOKOTA et al. / International Journal of the JSRM vol.x (2x) pp.x-x REFERENCES Vp(km/s) Figure 15. Distribution of seismic wave velocity from the tomography analysis face where the tomographic analysis is conducted. In order to achieve an accurate seismic tomography the authors have developed the wireless time synchronization system (comprising of a GPS receiver and a high-precision GPS satellite time maker), method for utilizing tunnel blasting vibrations as survey sources, and automatic data acquisition system. In conclusion, by applying this newly developed survey technique, it is now possible to conduct an accurate and quantitative geological survey ahead of a tunnel face without interrupting the progress of tunnel excavation. ACKNOWLEDGEMENTS EL. 39 The authors would like to thank Hakusan Corp. for the development of wireless GPS time synchronization system and also thank Nippon Koki Co., Ltd. for the development of noncontact blasting timing detection system Very poor Poor Very poor 5. Poor or very poor Poor Fair Poor - Preliminary survey result - Fair - Estimated result from new developed survey - Fair - Observation result - Ashida, Y., 21a. Seismic imaging ahead of a tunnel face with three-component geophones, International Journal of Rock Mechanics & Mining Sciences, 38, pp Ashida, Y., Matsuoka, T. & Kusumi, H., 21b. Seismic imaging technique of looking ahead of tunnel face by use of 3 components receivers, Journal of Geotechnical Engineering, 68, pp Kurihara., K., Yamamoto, T., Yokota, Y. & Miyajima Y., 214. Development of a technique of geological survey ahead of tunnel face using tunnel tomography and introduction of site application example, Proceedings of tunnel engineering JSCE, 24, I-27. (in Japanese with English abstract) Sattel, G., Sander, B., Amberg, F. & Kashiwa, T., Predicting ahead of the face, s and ling, 28, 4, pp Yamamoto, T., Shirasagi, Aoki, K. & Descour, J.M., 23. Explore the geological conditions around the tunnel face using the seismic reflective survey, ISRM23-Technology roadmap for rock mechanics, 2, pp Yamamoto, T., Shirasagi, S., Yokota, Y. & Koizumi, Y., 21. Imaging geological conditions ahead of a tunnel face using Three-dimensional Seismic Reflector Tracing System, International Journal of the JCRM, Vol 6, 1, pp Yokota, Y., Yamamoto, T., Shirasagi, S. & Koizumi, Y., 212. Evaluation of Geological Conditions Ahead of Face Using Seismic Reflector Tracing and New Seismic Tomography between and Surface, Ground Engineering in a Changing world, 212 ANZ Conference Proceedings, pp Yokota, Y., Yamamoto, T. & Kurihara, K., 214. Evaluation of Geological Conditions Ahead of Face Using Seismic Tomography between and Surface, Proceedings of 8th Asian Rock Mechanics Symposium. Yokota, Y., Yamamoto, T., & Kurihara, K., 215. A new type of seismic tomography between tunnel and surface, Journal of Japan Society of Civil Engineers Ser. F1 ( Engineering), 71, 3, pp. I_28-I_37. (in Japanese with English abstract)