Geophysical surveying for imaging near-surface structure and for characterizing its geotechnical properties

Transcription

1 Geophysical surveying for imaging near-surface structure and for characterizing its geotechnical properties INAZAKI Tomio Geology and Geotechnical Research Group, Public Works Research Institute Abstract High-resolution geophysical surveying is helpful to Image the near-surface structure and to characterize the geotechnical properties of the near surface. However a special attention should be paid to utilize suitable methods and to adopt appropriate parameters for the imaging and characterization of the near surface, because the near surface is inherently heterogeneous, which strongly affects the quality of survey results. The author has conducted research and development on the near-surface geophysics over the last 30 years to obtain high quality data and to provide high reliable results. Recent major case studies are demonstrated in this paper focusing on the geotechnical characterization of the near surface by means of geophysical surveying 1. Introduction Japan is one of the world s most earthquake-prone nations, and has been struck by disastrous earthquakes year after year. Actually, strong earthquakes with those magnitude larger than 7 attacked the Japanese Islands 7 times during the past 5 years, including the East Japan Earthquake on March 11, It was the largest earthquake Japan ever had in its history, and caused a devastating tsunami disaster in the northeastern part of Japan. As well known, the tsunami attacked ill guarded Fukushima-1 Nuclear Power Plant and triggered fatal nuclear accident. The hazardous radioactive emission from the Plant forced more than 200,000 local residents to evacuate to the outside of the 30 km exclusion circle zone. The accident anew reminded us the importance of scientific risk assessment instead of political risk assessment, based on detailed geological and geophysical investigations even for the existent infrastructures. Recently, performance-based seismic design (PBSD) has become a standard procedure to assess the seismic risk of facilities (e.g. FEMA 445, 2006). In this method, performance is calculated in terms of the amount of structural damage when affected by earthquake ground motion. Note that the design earthquake ground motion to be taken account is a function of the near-surface soil conditions, which means delineation of near-surface geological structure is essential for the appropriate PBSD. Notice should be paid to the ground failure associated with an earthquake as well as strong motion. However, only liquefaction potential has been assessed as the ground failure, but little account of the ground failure by nonlinear deformation of soft unconsolidated sediments.

2 In addition, wide distribution of sedimentary basins, where thick unconsolidated sediments are deposited, is one of the geologic features of Japan. Because most of large urban cities in Japan are situated on the basins, delineation of sedimentary structure especially near-surface structure is indispensable not only for the near-surface development but also for the disaster prevention of large urban areas. Geotechnical drilling has been widespread as the standard survey method to the near-surface geology. However, it is hard to interpret lateral changes or local anomalies only from drilling data, since they are characterized inherently as vertical line data and in many cases are inadequate or too sparse for implementing geological correlation. In contrast, near-surface geophysics can image near-surface structure as a 2D profile. High-resolution surveys that target the near-surface structure at engineering sites are essential. However, imaging of the near-surface structure and characterization of the geotechnical properties of the near-surface has been a sort of Herculean task, because the shallower the imaging depths we target, the larger the influence of heterogeneity we suffer. Thus special attention should be paid to utilize suitable methods and to adopt appropriate parameters for the imaging and characterization of the near surface. The author has conducted research and development on the near-surface geophysical survey methods to obtain high quality data and to provide high-resolution results suitable for engineering purposes. Recent major case studies are demonstrated below focusing on the geotechnical characterization of the near surface by means of geophysical surveying 2. Recent cases of geophysical surveying 2.1. Integrated geophysical surveying for safety assessment of levee systems Recent increase in water-related disasters resulting from the global warming has led us to remind the importance of vulnerability assessments of the existing levee systems. Flood risk potential is still high even in developed countries. Since 2005, we have conducted integrated geophysical surveying for the safety assessment of levee systems at 26 actual levee sites (e.g. Inazaki and Sakamoto, 2005). Integrated geophysical investigation for the safety assessment of levee systems comprises multiple methods applied to the same target at different stages. It enables to identify anomalies in the levee body and underlying substrata by combining individual survey results. We have adopted S-wave velocity and resistivity as the geophysical properties based on their relatively high correlation with geotechnical properties of the target soils and sediments. Geophysical survey methods for the levee safety assessment, therefore, are required to measure S-wave velocity and resistivity. We then employed a high-resolution surface wave method (Fig.1(a)) using Land Streamer (Inazaki, 1999, Hayashi & Suzuki, 2004) for the S-wave measurement (LS_SW), capacitively-coupled resistivity (CCR) survey (Fig.1(b)) using OhmMapper (Geometrics, 2001), and supplemental Slingram electromagnetic (EM) survey method for the resistivity measurement.

3 Figure 1 (a). Schematic illustration of the high-resolution surface wave survey tool. Figure 1 (b). Layout of a capacitively-coupled resistivity tool (OhmMapper). In principle, each geophysical method provides us the spatial distribution of single geophysical property, such as seismic velocity, resistivity, or gravity potential. Further, each obtained geophysical property is a function of many physical properties. For instance, resistivity is a function of porosity, pore fluid conductivity, water saturation condition, and grain size (complement to pore size) as described as Archie s equation. For the unconsolidated porous sediments, resistivity is expressed as a function of porosity when assuming the fluid conductivity and water saturation are constant. We can therefore assume that the resistivity of unconsolidated sediments represents mainly the soil types. On the other hand, it is well known that S-wave velocity of the soils has close relation with stiffness. For the safety assessment of levee systems, two major geotechnical parameters, vulnerability on seepage and seismic resistance are required to be evaluated along the levee. As schematically illustrated in Figure 2, seepage characteristics of unconsolidated soil materials and bearing layers is mainly influenced by grain size and stiffness. That is, the coarser in grain size and the looser in stiffness, the more unsafe in seepage vulnerability. Similarly, seismic resistance is also characterized by grain size and stiffness but in different way. Namely, the softer in stiffness and the finer in grain size, the more unsafe in seismic resistance vulnerability. Then when the relationship between these physical properties and

4 geophysical properties of soils and sediments is clarified, we can directly estimate the vulnerability condition from the geophysical properties. Figure 2. Schematic diagram assessing seepage vulnerability (left) and seismic resistance (right) based on general relationship between geophysical and soil properties on crossplot of S-wave velocity and resistivity data (Inazaki, 2011). Figure 3. An integrated geophysical survey result along the levee of Kokai River, about 300 km away from the epicenter of the 2011 East Japan Earthquake. Field survey was conducted in 2005, and had depicted anomaly zones in levee body. Levee failure took place at the shaded part about 60 m in width. (a): S-wave velocity structure reconstructed from LS_SW data; (b): Resistivity profile along the levee inverted from CCR data; (c): Seismic resistance vulnerability section classified into 4 categories based on S-wave velocity and resistivity data; (d): Landform along the levee interpreted from aerial photos. Note that the levee failure occurred just on an abandoned channel (revised from Inazaki, 2007).

5 The East Japan Earthquake caused severe damage to the levee systems situated in Kanto Region even though located more than 200 km far from the epicenter. Among them, two sites had been surveyed before the Earthquake and levee failures took place just at anomaly parts delineated as the zone characterized by low S-wave velocity and low resistivity both in levee body and in substrata. Figure 3 exemplifies a geophysical interpretation of the levee failure caused by the Earthquake. The failure occurred on the left bank of the levee along Kokai River, about 300 km away from the epicenter of the earthquake. Top of the levee settled down about 1 m with slope failure. Fissures and accompanying liquefaction were observed on the ground adjacent to the levee in and around the damaged zone. S-wave profile along the levee is characterized as low in levee body about 140 m/s and moderate but partly low in the substrata (Fig.3 (a)). Compared with S- wave profile, resistivity section shows significant heterogeneous structure in the levee body (Fig.3 (b)). This indicates that coarse or potentially permeable materials were used partly to embank the levee body. As clearly shown in Fig. 3 (d), the levee failure just occurred on an abandoned channel at 35.0 K. However, no clear sign on the failure was observed at other two zones where abandoned channel underlay. Figure 3 (c) is an interpreted section on the vulnerability of seismic resistance along the levee. The section is classified into 4 categories based on S-wave velocity and resistivity value. Here threshold values of 140 m/s in S-wave velocity and 100 Ω-m in resistivity were adopted for the classification empirically. Consequently, the failure part was clearly distinguished as low S-wave velocity and low resistivity zone both in levee body and substrata. This interpreted section demonstrates the advantage of crossplot analysis but also the capability of integrated geophysical surveying. This suggests a physical model that nonlinear loosening of clay layers had caused the ground failures and resulted in the damage of levee systems Detailed imaging of near-surface faulting structure of a concealed active fault using S- wave type Land Streamer An inland earthquake inherently originates from an active fault, or an active fault yields its own specific earthquake recurrently. Consequently, each active fault can be a useful indicator for a future inland earthquake. However, its average recurrence interval is too long about several thousand years to read and write on historical archives. Alternatively, we can clarify fault behavior from the near-surface sediments by means of sedimentological analysis. Actually, a combination of exploratory trenching and drilling has been widely adopted to reveal the near-surface deformation in the paleoseismological studies of active faults in Japan. Trenching survey is the most common and direct technique to reveal the recent behavior of an active fault. The technique is however, valid at near-surface down to 10 m and at the site where fault trace is recognized. Dense drilling is also the common method to clarify faulting history along with the near-surface deformation structure. Detailed sedimentological facies analysis, based on grain size distribution and AMS 14 C dating to the

6 drill cores, can detect faulting events and their ages. However, we would have to note that it was still difficult to delineate the detailed faulting structure by such pinpoint surveying. In contrast, high-resolution shallow seismic reflection surveying is capable to provide detailed information, surpassing that of drilling. The author developed and adopted Land Streamer (Inazaki, 1992, 1999) to high-resolution active fault survey and successfully imaged detailed structure of faulted zone down to 100 m in depth. A recent case study of active fault surveying is described below. The Kakuda-Yahiko fault, described as a 25-km long reverse fault with upthrow of the western side, is one of the behavioral segment members of the Western Marginal Fault System along Niigata Plain with NNE-SSW trending about 70 km in total length. The Kakuda- Yahiko fault is inferred to displaced lower Pleistocene layers by 3,000 m based on drilling data. However, no clear geomorphological evidence was recognized on the surface ground where the fault was presumed to extend due to the recent high sedimentation rate. Recent increase in seismic activities in the Niigata Plain and necessity of seismic risk assessment for the adjacent nuclear power plant in Japan requested us urgent but detailed survey of this fault. We then conducted high-resolution shallow seismic reflection surveying using Land Streamer in Niigata City to image on- and off-fault deformation structure in a faulted zone (Inazaki, et al., 2011). We employed a newly assembled S-wave type Land Streamer (Fig.4; left) and an S-wave generator (Fig.4; right) for the survey intending to delineate paleoseismic deformation events caused by recent activities of the Kakuda-Yahiko fault. Figure 4. Photos showing the S-wave type Land Streamer deployed in this survey. A 120-channel geophones are hooked at 50 cm intervals on steel wires as a towing member (left). A shear wave generator powered by compressed air (right).

7 2011 Korea-Japan Joint Symposium Two seismic lines, 1100-m long GS_AK_SLS1 and 900-m long GS_AK_SLS2, for Swave surveying were set parallel each other to intersect inferred faulting location. Drilling targeting the near surface and detailed core analyses were combined to the geophysical survey. Figure 5 shows a CMP stacked migrated section (upper) and its interpreted depth section with a lithology column superposed (lower). Note that off-fault primary faulting (FF1 to FF3) as well as the major on-fault flexure (MF1) structure is clearly delineated in the near surface down to 120 m in depth. Notice should be also paid that displacement for the picked horizons caused by the faulting increases with depths. This means the displacement Figure 5. A stacked depth section along GS_AK_SLS1 line (upper) and an interpreted depth section (lower) along the line with superposition of a lithology column constructed from drill core analyses. Note that deformation structure at on- (MF) and off-fault zones (FF1 to FF3) is clearly delineated. The major horizons are also picked from H1 to H7 (Inazaki, et al., 2011).

8 accumulation on the active faulting. We therefore can calculate average deformation rate of the Kakuda- Yahiko fault in combination with depositional age estimated from 14 C dating data labeled as figures beside the column in Fig. 5. Figure 6 plots the relative displacement and AMS 14 C dating data of each horizon for the main flexure MF and a frontal fault FF2, and average deformation rate is estimated about 1.0 m/kyr for MF and 0.5 m/kyr for FF2, and the total rate as at least 2.0 m/kyr within the section. As a result, high-resolution reflection surveying revealed that fault deformation took place not only at the major faulted zone but also at off-fault or in the frontal footwall zone. This Figure 6. A cumulative deformation curve suggests possible underestimation of regarding the activities of the major fault deformation rate of an active fault only (MF) and a frontal fault FF2. The average based on the evidence at major deformation rate was estimated 0.5 m/kyr faulted zone. In addition, highresolution seismic reflection surveying al., 2011). for FF2 and 1.0 m/kyr for MF (Inazaki, et is capable to image the near-surface deformation structure of an active fault, and is helpful to provide valuable information regarding seismic zoning near active faults for earthquake disaster prevention of infrastructures. References FEMA, 2006, Next-Generation Performance-Based Seismic Design Guidelines: Program Plan for New and Existing Buildings, FEMA 445 Report, 154p. Geometrics, 2001, OhmMapper TR1 operation manual, Geometrics Inc., 147p. Hayashi, K. and Suzuki, H., 2004, CMP cross-correlation analysis of multi-channel surfacewave data, Exploration Geophysics, 35, Inazaki, T., 1992, Development of subsurface survey methods: in Final Report of Research & Development of Utilization of Underground Space, vol. 3, 2 26, Ministry of Construction. (in Japanese).

9 Inazaki, T., 1999, Land Streamer; a new system for high-resolution S-wave shallow reflection surveys, Proceedings of the 12th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP1999), p Inazaki, T., and Sakamoto, T., 2005, Geotechnical characterization of levee by integrated geophysical surveying, Proceedings of the International Symposium on Dam Safety and Detection of Hidden Troubles of Dams and Dikes, CD-ROM, 8p. Inazaki, T., 2007, Integrated Geophysical Investigation for the Vulnerability Assessment of Earthen Levee, Proceedings of the 20th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems, CD-ROM, Inazaki, T., 2011, Role of integrated geophysical surveying for the risk assessment of levee systems: Lessons from the East Japan Earthquake, Expanded Abstracts of 81st SEG Annual Meeting, 4p. Inazaki T., Miyachi Y., Urabe A., and Kagohara K., 2011,Near-surface deformation structure of the western marginal fault of the Echigo Plain at Yotsu-goya and Akatsuka District, Niigata City delineated by Land Streamer seismic reflection surveying, Digital Geoscience Map Series S-2, Seamless geoinformation of coastal zone Coastal zone around Niigata, Geological Survey of Japan, AIST, 35p. (In Japanese with English abstract).