Application of Geophysical Exploration for Environmental Earth Sciences and Engineering

Transcription

1 Application of Geophysical Exploration for Environmental Earth Sciences and Engineering Satoru Ohya OYO Corporation, Kudan-kita, Chiyoda-ku, Tokyo , Japan. ABSTRACT The role of geophysics in Environmental Earth Sciences and Engineering is considered. In the developing era since 60 s in Japan, geophysics has mainly contributed to investigation of new constructions such as tunnels, dams and high-rise buildings. In the coming sustainable era, geophysics must be used for maintaining the safe and easy human life. Application examples of geophysics to investigations for disaster prevention, maintenance of existing constructions and soil and groundwater pollutions are presented to demonstrate their role in Environmental Geo-engineering. Future subjects in this field for geophysics and geophysicists are also discussed. INTRODUCTION Environmental problems will be definitely one of major social problems in this century. Many academic and engineering societies including the Society of Exploration Geophysicists of Japan, therefore, have paid their attention to the environmental issues in recent years. I have thought that environmental problems include many problems relating to human life such as natural disasters due to earthquakes and volcanic activities, and various accidents due to existing constructions as well as environmental pollutions. Therefore multi-disciplinary approaches should be taken to solve the environmental problems. I would like to call such multi-disciplinary science and technology as Environmental Geo-engineering. In this paper, I will consider the role of exploration geophysics in Environmental Geo-engineering in the future. HISTORY OF APPLICATION OF GEOPHYSICS IN JAPAN AND ITS FUTURE Contribution of geophysics in the developing era in Japan In the 60 s in Japan, geophysics has been mainly used for exploration of mineral resources. As new development of mineral resources has been declining, geophysics has been employed for development of geothermal energy and groundwater in natural resources exploration. Its applications to development of coal and metal mines, however, have been decreasing. After the year of 1960 in Japan, large budgets have been invested to construct new infrastructures as shown in Figure 1. Geophysics has been employed in such large projects. For example, it has been used in investigation of construction of new dams at rivers, tunnels and man-made slopes for railroads and highways, and nuclear power plants. In Japan, earthquake ground motion must be taken into consideration in the design of construction. Since Niigata Earthquake in 1964 arose liquefaction in - 1 -

2 wide area, soil dynamics and S-wave technology that both are very important to understand liquefaction have been developed. Using such knowledge and technology, the seismic hazard map technique that can predict earthquake ground motion and damage at the surface has been developed 뼬듩뙕먠뱤럱뒂 Private invesment (trillion (뮎?걂 yen) 뚺떎뙕먠뱤럱뒂 Public invesment (trillion 걁뮎?걂 yen) Bubble, Cold war Ceased 11th New plan rd Budgets for roads 4th 5th st Oil Shock 6th nd Oil Shock 7th 8th Fig.1 Annual investment to new constructions and budgets for new road plans After the economic fever called as Bubble and the cold war have ceased in 1990 in Japan, economic recession caused reduction of investments to new constructions. However due to in-balance of trades between U.S.A. and Japan, Japanese government has tried to stimulate its economy through investments to public works such as constructions of new highways and high speed railroads. In these large projects, geophysics has been still applied successfully. 9th 10th Contribution of geophysics in the sustainable era in Japan The Great Hanshin Earthquake occurred in 1995 killed more than 6,000 people. Since the earthquake arose the largest damages after the World War II in Japan, we were forced to reexamine the earthquake-resistant design. Thanks to this earthquake, it was first recognized that deep subsurface structure should be considered for predicting earthquake ground motion. Then deep seismic reflection and gravity methods have started to be used to delineate deep subsurface structure for that. Until that time, deep seismic reflection has been mainly used for oil and gas exploration. After the earthquake, it has been actively used for investigation of active faults and deep subsurface structure for earthquake disaster prevention. This earthquake, therefore, became a milestone in the application of geophysical methods to earthquake disaster prevention. Recently many volcanoes in Japan such as Unzen-Fugendake, Usu and Miyakejima have erupted and damaged largely. Rock failure and landslide due to heavy rainfall have often occurred. It is not so easy for us to mitigate these natural disasters because Japan has many volcanoes and is mainly - 2 -

3 occupied by mountains. Geophysical methods are expected to be applied to mitigation of natural disaster. As mentioned above, in these thirty years after 1970 in Japan, huge amount of budgets have been spent to construct new roads and railways and also many high-rise buildings have been constructed. In these constructions, huge amount of concrete has been used. 80% of total amount of concrete so far has been used in these thirty years. Large amount of concrete requires large amount of sands. Therefore even low quality sands have been used for making concrete in these years. Such low quality sands accelerate deterioration of the concrete and shorten its durable years. As shown in Figure 2, 15 to 20 years later, most of these concretes must cause any accidents due to their deterioration. Therefore the role of investigation techniques for health of the concrete will become much more important in the future. Geophysical methods are expected to be non-destructive techniques for testing the concrete construction. Number Fig.2 (1) Annual constructions of concrete bridges (after Kobayashi, 1999) Number (x10 4 ) Year Fig.2 (2) Number of concrete bridges reaching their durable years in the future (after Kobayashi, 1999) - 3 -

4 In the second half of 20 century, Japan has been industrialized very rapidly. In this period, Japan has imported materials from abroad and exported the products made by these materials. In the course of production, huge amount of wastes have been created and disposed into the ground in and around the factory. These wastes are getting polluted soils and groundwater around the factory. Investigations for remediation of polluted soils and groundwater have been already carried out at many sites. There may be much larger amount of environmental pollutions in Japanese islands as a negative legacy of 20th century. In solving these problems, geophysical methods should play an important role. If the second half of 20th century in Japan is considered as the developing era or new construction era, 21st century should be the era in which disaster prevention and environmental protection must be much more important for sustaining safe and easy human life. Maintaining the existing construction and technology for its health investigation also become more important in this century. New application and contribution of geophysical methods must be requested in the sustainable era. The Environmental Geo-Engineering must be an integrated science which consists of many related fields such as geology, soil mechanics, rock mechanics, groundwater hydrology, geochemistry, biology, microbiology and so on. To solve environmental geo-engineering problems, these many kinds of technologies should be used synergistically. What contribution geophysics can make is the subject for geophysicists and geophysical societies to consider in the new era. I think that such contribution has been already made in a part of Environmental Geo-Engineering field, but not yet systematically done. It is very difficult to demonstrate it systematically at this moment. I will, therefore, present some actual field examples of applications of geophysics to Environmental Geo-Engineering in order to demonstrate its role in this field. APPLICATION EXAMPLES OF GEOPHYSICS TO ENVIRONMENTAL GEOENGINEERING Geophysical methods enable us 1) to investigate a wide area very efficiently, 2) to investigate non-destructively, 3) to image invisible underground. These features can be very important for Environmental Geo-Engineering applications. For instance, in the investigation for soil and groundwater pollution, we should first pick up heavily polluted areas in a very wide investigation site and then predict the volume and degree of pollution of the polluted soil and groundwater with as non-destructive way as possible. The examples in the following sections demonstrate these features. Applications of geophysics to investigation for disaster prevention As mentioned above, after the Great Hanshin Earthquake, geophysical methods have been actively used in investigation for earthquake disaster prevention. Especially, it has been recognized that deep seismic reflection and gravity methods are almost always necessary for delineation of active faults and deep subsurface structure in investigation for earthquake disaster prevention. Figure 3 shows an example of the deep seismic reflection profiles conducted just after the Great - 4 -

5 Hanshin Earthquake (Endo et al., 1995). This profile shows abrupt change of the depth of the base rock beneath the Kobe city, which caused the large damaged zone called as Seismic Hazard Belt. In investigation of an active fault, trench survey is very important for evaluation of activity of the fault. The length of a trench is usually of several meters. To determine an optimum trench point, shallow seismic reflection can be also effectively used. Figure 4 shows an example of the very shallow seismic reflection profile in the investigation of an active fault along the Median Tectonic Line. This profile clearly shows the location of the fault and deformation of sediments around it due to faulting. Based on the profile, three boreholes and a trench were dug to reveal the history of activity of the fault (Tokushima Pref., 2000). 200m 0 Depth(m) 1000 Fig.3 Seismic reflection profile beneath Kobe city (after Endo et al., 1995) Fault (m) Depth(m) Base rock Sediments Fig.4 Very shallow seismic reflection profile at an active fault (after Tokushima Pref., 2000) - 5 -

6 The Seismic Hazard Belt in the Great Hanshin Earthquake was caused by interference of seismic waves due to the abrupt depth change of the base rock. It reminded us that deep subsurface structure as well as near surface sediments should be considered for predicting earthquake ground motion accurately. Then a nationwide investigation project of deep subsurface structure has started first in three sedimentary basins in Kanto, Nohbi and Kyoto districts. For this kind of investigation, to estimate three-dimensional S-wave velocity structure is most important because S-wave is generally predominant in earthquake ground motion. In these investigations, one- or two-dimensional P- and S-wave velocity profiles obtained by seismic reflection and micro-tremor array measurements were expanded into three-dimensional P- and S-wave velocity structure, being guided by three-dimensional gravity data. Figure 5 shows the three-dimensional subsurface model thus obtained in the Kyoto basin (Kyoto city, 2001) Perspective View of North-South section Fig.5 Three-dimensional subsurface model of the Kyoto basin (after Kyoto City, 2001) Recently the remote-sensing technology using the satellite has been rapidly developed. Especially interferometric SAR (Synthetic Aperture Radar) processing enables us to detect subtle surface deformation due to an earthquake, which has contributed to understand fault movement due to an earthquake. This technology is also applied to detection of ground subsidence due to the large amount of groundwater use. Any effective methods to detect ground subsidence in wide area are required for predicting and preventing it in the future. Nakagawa et al. (1999) has successfully used the interferometric SAR technology for this purpose. Figure 6 shows the SAR interferogram detecting several cm scale of ground subsidence in Northern Kanto Plaines, where many ground subsidences caused by the large amount of groundwater use

7 Fig. 6 Ground subsidence detected by the interferometric SAR (after Nakagawa et al., 1999) Applications of geophysics to investigation for maintenance of existing constructions After the accident of drop of a concrete block at the Sanyo Shinkansen Railway tunnel recently, almost all tunnels of railways and roads in Japan have been inspected to prevent future accidents. Inspection results revealed that the quality of concrete and its inspection method should be reexamined. As mentioned above, 15 to 20 years later, most of concrete constructions will reach their durable years and also the durable years should be extended to use them longer than before because new construction must be reduced in the coming low economical growth era. For that reason, quality control and maintenance of constructions must be much more important than before. For long constructions such as tunnels, quick and inexpensive inspection techniques are required. At present visual inspection and sonic inspection with the hammer are commonly used for inspection of concrete constructions. However such manual inspection techniques need trained engineers whose number will decrease in the future. In addition, these methods are neither effective nor objective. For making periodic inspection more efficient and repeatable, automatic inspection techniques should be developed (Uomoto, 2000). The geophysical method commonly used for tunnel inspection is the GPR (Ground Penetrating Radar). Figure 7 shows a truck-mounted GPR system for tunnel inspection. This system can conduct several km-long profiles a day with continuous measurement using two antennae. Figure 8 shows an example of GPR profiles and its interpretation at a tunnel obtained by this system. From the profile, cavity was detected behind the concrete tunnel wall

8 Fig.7 Truck-mount GPR system for tunnel inspection steel support lining joint distance (m) depth (m) lining concrete cavity rock depth (m) Fig. 8 GPR profile and its interpretation for cavity detection - 8 -

9 For inspection of the concretes of existing buildings and bridges, distribution of reinforced bars and their degree of corrosion are often inspected. In such case, a small and high frequency GPR system is employed. Figure 9 shows 1.5GHz GPR system and its application to detection of reinforced bars in a concrete slab. Such a small and compact GPR system is very useful for detailed inspection of a building. Reflection S Ø ½ŽË g from iron bar Reflection the bottom ê Ê ½ŽË g from (cm) Žž ŠÔ (ns) Antenna Recorder cm D25 D32 D51 40cm D38 50 Measurement line T ªü cm 20cm 20cm º± ² «ÓŠ 120 cm cm ŠÛ ƒó20 Fig. 9 High frequency GPR system and its profile ê ÊA Ê ð h ÁH ƒƒcƒhƒaƒ ªü 28 úœo ߌã u F32N/mm Maintenance of existing dams and river embankments is also very important for safety. Many of old dams have the problems of water leakage. In case of maintenance of a fill-type dam, strain meters and pore pressure meters buried in the dam are commonly used for monitoring deformation or water content of the soil. However only with these point-based measurements, it is not so easy to monitor the whole body of the dam. Mori et al. (2000) has thus proposed the application of time-lapse resistivity tomography using electrodes buried at many locations in the dam. The time-lapse resistivity tomograms can visualize temporal changes of resistivity distribution in the dam, which may indicate temporal changes of water contents and temperature of soils in the dam required for maintenance. Figure 10 shows the temporal changes of resistivity profile in a section perpendicular to the axis of the dam. This profile indicates water invasion into the dam body. For maintenance of constructions, monitoring of temporal changes of the material properties is very important for early detection of deterioration and making an effective repair plan. Time-lapse geophysical methods should be promoted for this kind of application in the future

10 Upstream Downstream Variation (%) West Temporal resistivity change rate on the section perpendicular to the model dam axis. -10 NS-2 面 比 比 比 比 比 East Variation (%) Temporal resistivity change rate on the section -11 EW 面 比 比 比 比 比 parallel to the model dam axis. Fig. 10 Time-lapse resistivity tomogram at a fill-type dam (after Mori et al., 2000) Applications of geophysics to soil and groundwater pollution In investigation of soil and groundwater pollution, geophysical methods can be used as a tool for 1) estimating the polluted area and its degree of pollution, 2) detection of source of pollution, 3) understanding the mechanism of pollution and 4) monitoring of remediation. For instance, there is no effective technique other than shallow geophysical mapping techniques for detection of drum cans containing contaminant in a huge investigation area. To predict flow paths of contaminant in a soil with groundwater, it is crucial to estimate distribution of impermeable layer and flow characteristics of groundwater three-dimensionally, to which geophysical methods can contribute. Self-potential or streaming potential methods have been proposed as a tool for estimating flow characteristics of groundwater. It is well known that electrical potential (streaming potential) is generated when groundwater flows in permeable subsurface layer. Therefore such electrical potential measurements due to groundwater flow can be used for monitoring remediation process of polluted groundwater. Figure 11 presents a field experiment of monitoring groundwater flow using the self-potential method. Streaming potential vectors visualize the groundwater flows around the pumping-up well

11 Observation point Pumping-up well Observation location Streaming potential vector Fig. 11 Visualization of groundwater flow by streaming potential measurements (after Sato et al., 1997) If geophysical methods can directly detect contaminant in soils or groundwater, they must contribute much more to environmental applications. There are a few trial field experiments of detection of oil, organic chemicals and heavy metals using GPR, electric and electromagnetic profiling techniques together with laboratory tests and database analysis. Figure 12 shows an example of field experiments of direct detection of contaminant using the DC resistivity profiling. In this case, a tetrachloroethylene plume was detected as a high resistivity anomaly in the profile (Maekawa and Nakajima, 1998). Fig. 12 Resistivity profile at the contaminated site (after Maekawa and Nakajima, 1998)

12 FUTURE PROBLEMS THAT GEOPHYSICISTS SHOULD TACKLE In general, many problems in earth sciences or geo-engineering are solved through the following three steps. Firstly, a geological or geotechnical model for solving the problem is constructed and its parameters are estimated. Geophysics can contribute very much to this step and has contributed so far. As mentioned in the examples of earthquake disaster prevention, an accurate subsurface model as well as a source model is required for predicting earthquake ground motion correctly. Geophysical methods such as seismic reflection and gravity methods are used to make a subsurface model. Secondly, simulation is done using the model obtained in the first step for predicting phenomena that will arise. There are already many simulators for oil and gas reservoir, wave propagation and groundwater flow. These simulators have been actively used in Environmental Geo-engineering. Finally, the simulated results are verified by observing and monitoring actual phenomena. Geophysical methods can contribute to this step as a monitoring tool. As mentioned above, the self-potential method can be used to visualize groundwater flow. It can help to make the remediation process of groundwater contaminant more effective. In geological or geotechnical investigations, in the developing or new construction era in Japan, geological model at the site is mainly requested for planning and designing constructions. Therefore geophysical methods have been employed mainly to make the geological model. However, in the new sustainable era in the future, in which disaster prevention, maintenance and environmental protection become main issues for us, geophysics will be requested to contribute to all three steps above. For example, in recent years, when a new tunnel is planned, effect of tunnel excavation to groundwater around the route should be studied before construction. In that case, three-dimensional subsurface permeability model around the route is constructed to simulate groundwater flow. The simulation is verified by observation of groundwater at the wells in the site. If influence of excavation to groundwater is predicted, any countermeasures in the construction stage should be proposed. In such case, if geophysical methods can provide a three-dimensional permeability model through interpretation of physical parameters such as resistivity and velocity, and monitoring techniques of groundwater in place of observation wells, the role of geophysics will become much more important in Environmental Geo-Engineering applications. In the future, geophysicists must be requested to understand the objectives of the whole project, to develop technologies required for the application and to apply them as effectively as possible. CLOSING REMARKS Geophysics has been so far mostly used for exploration of natural resources and subsurface structure. For such applications, geophysics is used at the place far away from our daily life. However in the future, geophysics must be requested to contribute to constructing and maintaining the safe and easy human life. In such case, the method must be used at the place much closer to our

13 daily life than before. Therefore geophysicists must be also requested to develop the safe and easy methods. REFERENCES Endo, H., Watanabe, S., Makino, M., Murata, Y., Watanabe, K., and Urabe, A., 1995, Subsurface geological structure with special reference to concealed faults and basement structure in Kobe and Ashiya cities, Hyogo Pref., Japan: BUTSURI-TANSA (Geophysical Exploration), 48, (in Japanese). Kobayashi, K., 1999, Concrete in danger: Iwanami, Tokyo (in Japanese). Kyoto City, 2001, Investigation of subsurface structure beneath Kyoto City: Abstract of the 2 nd review meeting of investigation of subsurface structure beneath the sedimentary basin, (in Japanese). Maekawa, T., and Nakajima, M., 1998, Present status and future direction in investigation and measurement of groundwater pollution: Geology and Investigation, 3, (in Japanese). Mori, M., Natsuka, I., Hatakeyama, M., Toshioka, T., and Sakurai, K., 2000, Planned new safety control system of the fill-type dam at Takashiba: Soil and Water, 123, (in Japanese). Nakagawa, H., Murakami, M., Fujiwara, S., and Tobita, M., 1999, Land subsidence of the Northern Kanto Plains detected by JERS-1 SAR interferometry: Journal of the Geodetic Society of Japan, 45, (in Japanese). Sato, H., Shima, H., and Sakurai, K., 1997, A study on generation of streaming potential due to pumping-up of groundwater: Proc. of the 97 th technical conference of SEGJ, (in Japanese). Tokushima Prefecture, 2000, Investigation report of active faults in Tokushima Prefecture (in Japanese). Uomoto, T., 2000, Maintenance of concrete structures in the 21 century: BUTSURI-TANSA, (Geophysical Exploration), 53, (in Japanese)