SAGEEP 2011 Charleston, South Carolina USA http://www.eegs.org

UTILIZATION OF INTEGRATED GEOPHYSICAL SURVEYING FOR THE SAFETY ASSESSMENT OF LEVEE SYSTEMS

Tomio INAZAKI, Public Works Research Institute, Tsukuba, JAPAN Koichi HAYASHI, OYO Corporation, Tsukuba, Japan

and SEGJ Levee Consortium, Society of Exploration Geophysicists of Japan, Tokyo, Japan

Abstract

An integrated geophysical technique we proposed has begun to be utilized in the vulnerability assessment of levee systems in Japan. The integrated geophysics applies multiple methods to the same target and enables to assure to detect anomalies in the levee systems by combining individual survey results. The integrated geophysical technique mainly consists of surface wave method using Land Streamer, capacitively-coupled resistivity method using OhmMapper, and supplemental Slingram electromagnetic survey method. The geophysical properties evaluated by these methods, S-wave velocity and resistivity, are useful to evaluate permeability and stiffness of levee systems. We also demonstrated the usefulness of crossplots of the measured data to classify the target ground and to easily show vulnerable zones in the levee systems for the safety assessment. The surveyed data are correlated with ground truth data or various field test data. That is one of the reasons why we call the technique integrated. Furthermore, an appropriate geophysical technique is planned to be applied in the different phases of the safety assessment of levees, from outline survey for locating vulnerable zones to detailed survey for assessing the safety conditions of each zone.

Thus far we have tested several geophysical methods at 20 actual levees in Japan, and confirmed the advantages of the above methods through the field work, in the viewpoints of cost effectiveness and their capability in detecting anomalies. We could successfully identify anomaly structures in and beneath the levee systems at excavation sites for replacing old hydraulic structures. In conclusion, the proposed integrated geophysical survey is practical and quite useful for the safety assessment of levee systems.

Introduction

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. Levee failures would cause severe flood disaster as exemplified by the Hurricane Katrina disaster in 2005. Flood risk potential is still high even in developed countries. Indeed, Japan lies in an earthquake prone region, however, both economic loss and casualty by water-related disasters in the last decade are greater than those by earthquakes in Japan. Because most earthen levees have been repeatedly mounded and repaired during historic time in Japan, their internal structure is generally inhomogeneous both in a lateral and a longitudinal direction in spite of their similar physical appearance. Levee systems are characterized not only as such historical structure but also as continuous linear structure, which force to be founded even on the problematic ground. It can be easily imagined such inhomogeneous structure and problematic ground conditions would lead to the increase in failure potential of the levee body when attacked by flooding or by a strong earthquake. For instance, a part where coarse materials were used to stack the

Preprint

SAGEEP 2011 Charleston, South Carolina USA http://www.eegs.org levee might have a potential risk of seepage. Underlying paleo-channel would cause piping. Hence, it has been required to clarify the internal structure of levee body and underlying ground, and to characterize their geotechnical properties for the vulnerability assessment of levee systems. Stiffness is one of the important properties for analyzing deformation and failure process of levee systems and substrata. Permeability is also a crucial factor which controls seepage flows and piping underneath the levees.

Geophysical investigation methods are expected to play an important role in the assessment of levees because conventional visual monitoring or spot drilling techniques are ineffective in revealing internal structure of levee. We conducted field tests, and proposed an integrated geophysical technique for the effective tool to assess the vulnerability of levee systems. The geophysical technique mainly consists of a high-resolution surface wave (HRSW) method (Hayashi & Suzuki, 2004) using Land Streamer (Inazaki, 1999), capacitively-coupled resistivity (CCR) survey using OhmMapper (Geometrics, 2001), and supplemental Slingram electromagnetic (EM) survey method.

S-wave velocity obtained by the HRSW method is useful to estimate stiffness of levee systems as well as N-values by Standard Penetration Test. As is well known, S-wave velocity has close relation to stiffness, and commonly used to estimate N-value and vice versa. The CCR or Slingram EM survey provides essential information on permeability. It is also known that resistivity is the function of water content, porosity and grain size characteristics of the target materials, which are the control factors of the permeability. Therefore we first made clear the criteria to select geophysical methods which constitute the integrated geophysical technique for the levee safety assessment as follows: (1) the method should be a non-destructive technique, which does not cause any damage to levee systems; (2) the method should identify the physical properties that are helpful in evaluating the safety of levee systems; (3) the method should be a near-surface survey technique that can image shallow depths up to 20 m; (4) the method should have enough resolution to identify an anomaly as small as 10 meters; (5) the method should provide a continuous profile along levees at an affordable cost, generating high quality field work; and, (6) the method should be technically transparent and open to be widely applied to levee surveys (Inazaki, 2007).

Then we demonstrated the technique at actual levees and showed it was able to identify anomaly structures in and beneath the levee systems (Inazaki, ditto). Next, we conducted direct and detailed field measurements at an open-cut of an earthen levee where integrated geophysical survey had been applied, and clarified clear relationship between geophysical properties and geotechnical characteristics (Inazaki, et al., 2008; Hayashi, et al., 2009). Both geophysical and geotechnical properties are not static but varies with time. Monitoring of change in these properties is essential to reveal the deterioration process of levee systems. Time-lapse DC resistivity measurement was carried out on a model levee to monitor a surface failure, and successfully detected water infiltration into the levee body (Inazaki, et al., 2010). Here we review the safety assessment process in the integrated geophysical surveying and related issues on the field measurements.

Safety Assessment of Levee Systems by means of Integrated Geophysical Surveying

Areal Resistivity Mapping As described above, the safety assessment of levee systems starts with the stage of preliminary

areal survey. Aerial photo analysis, compilation of previous flood disaster, and supplemental check drilling in spots has been adopted as the preliminary survey in Japan. Geophysical survey has been rarely utilized in this stage whereas it can provide areal data effectively. So we demonstrated Slingram EM survey to map near-surface structure around the levee on which we conducted the integrated geophysical survey (Inazaki, et al., 2008). Figure 1 shows the distribution map of electric conductivity

Preprint

SAGEEP 2011 Charleston, South Carolina USA http://www.eegs.org for an induction frequency at 5425 Hz. A total of 6,000 point data were measured over a 1 km by 3 km area around a levee using GEM2 in a dry season in 2009. Paleo-channels interpreted from aerial photos are superposed on the map with seepage points along the levee. It is characteristic that river side of the levee shows relatively high resistivity (low conductivity). In contrast, low resistivity zones are concordantly delineated along the interpreted paleo-channels at the land side. Accordingly, the map demonstrates that it is helpful to map 2D near-surface structure that affects ground water flow across a levee system.

Monitoring of Seasonal Change Figure 2 compares resistivity profiles along the same levee

in Fig. 1. The data were obtained in Nov. 17, 2006 using OhmMapper (Fig. 2 (a); Inazaki, et al., 2008), in Aug. 03 (Fig.2 (b)) and Dec. 06 (Fig.2 (c)) in 2009 by means of GEM2 (Miura & Inazaki,2010). Because the line was set on the top, the part down to 6 m in the profile corresponds to the levee body. Each profile shows the similar trend that the levee body has relatively high resistivity compared with the underlying strata. Note that resistivity at substrata drastically decreased in a rainy season, which indicates

Figure 1. Map showing electric conductivityaround a levee.

Figure 2. Seasonal change in resistivity profiles along the same levee. (a): OhmMapper profile for a dry season (Inazaki, et al., 2008). GEM2 profiles obtained in a rainy season (b) and in another dry season (c) (Miura & Inazaki, 2010).

Preprint

SAGEEP 2011 Charleston, South Carolina USA http://www.eegs.org ground water flowed through the levee perhaps from the land side to the river side. The profiles also indicate that GEM2 measurements are sensitive to artificial noises.

Review on the Characterization Process Resistivity is a function of many physical properties such as porosity, pore fluid conductivity,

water saturation condition, and grain size (complement to pore size) characteristics. For the

Figure 3. Schematic idea for vulnerability assessment using general relationship between geophysical properties and soil properties (left) and classification to 4 quadrants on crossplot of S-wave velocity and resistivity data (right) extracted from actual dataset shown in Fig. 4.

Figure 4. Survey profiles along a 1-km long line set on Iino Levee Segment of Kurobe River. (a): resistivity profile reconstructed from OhmMapper data; (b): S-wave profile delineated from HRSW data; (c): interpreted vulnerability profile derived from the crossplot analysis drawn in Fig. 3

Preprint

SAGEEP 2011 Charleston, South Carolina USA http://www.eegs.org unconsolidated porous sediments, resistivity is expressed as a function of porosity when assuming the fluid conductivity and water saturation are constant using Archie’s Equation. 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. So we can summarize the relationship among resistivity, S-wave velocity and geotechnical properties as illustrated in Fig.3 (left). In general, the soils characterized as coarse grained and loose may permeable. In contrast, fine grained and dense soils are usually treated as impervious. We therefore interpret the upper left quadrant on crossplot of resistivity and S-wave velocity as vulnerable or high permeable.

Figure 3 (right) demonstrates the classification of actual levee data as shown in Fig. 4 (a) and (b), on the crossplot diagram. The data which keep locality information are classified into 4 quadrants at Vs=250 m/s and 5000 Ω-m in resistivity. Making use of this classification, we can construct a vulnerability classification profile as shown in Fig. 4 (c). We can easily discriminate the vulnerable zones by means of the crossplot technique.

Quantitative Evaluation One of the goals of the integrated geophysical

surveying is to evaluate quantitatively the geotechnical properties in and beneath the levee systems. As mentioned above, we can derive geotechnical properties and their spatial distribution from the geophysical profiles. Figure 5 shows the close relationship between S-wave velocity and N-value (blow counts) measured by Standard Penetration Test (Inazaki, 2006). The collected data consisted of PS suspension logging data and the dataset of SPT measured at the same intervals in the same drill holes in which the PS suspension logging was carried out. We can derive an empirical equation which estimates N-values from the S-wave velocities. Using

Figure 6. (a): S-wave velocity profile calculated from HRSW dataset along the same levee as shown in Fig. 2 (Inazaki, et al., 2008); (b): An N-value profile converted from the profile (a) using the equation derived from Fig. 5.

Figure 5. Relationship between S-wave velocity and N-value (Inazaki, 2006)

Preprint

SAGEEP 2011 Charleston, South Carolina USA http://www.eegs.org

this equation, we can make an N-value profile from the S-wave profile as shown in Fig. 6. N-values are familiar for the geotechnical engineers, and we can interpret the profile based on the large volume of previous works on N-values. For instance, we could distinguish very low N-value zones at the base of the levee body. These zones were characterized as soft or loose soils having high risks of dislocation or failure when attacked by a large earthquake.

Figure 7 shows relationship between resistivity and geotechnical properties for levee systems. We collected the geotechnical data for the levees where integrated geophysical surveys were applied. As can be seen, the data on log-log scale crosspolots show linear trends between resistivity and each geotechnical property. For instance, resistivity increases with an increase in effective grain size (Fig.7 (a)). In contrast, it decreases with an increase in Fc or fine-grained content (Fig.7 (b)). Notice should be paid to Fig. 7 (c), in which linear relationship between resistivity and permeability is drawn. Utilizing this relationship, we can calculate permeability values from resistivity data. Figure 8 depicts permeability profile along the Iino Levee Segment of Kurobe River, resistivity profile of which is presented as Fig. 4 (a). We can estimate the permeability in the vulnerable zones at a maximum in 10-1 cm/s order.

Figure 7. Crossplots of resistivity and geotechnical properties. (a): Correlation between resistivity and effective grain size; (b): Correlation between resistivity and fine-grained content (Fc); (c): relationship between resistivity and permeability.

Figure 8. Permeability profile calculated from resistivity profile data shown in Fig. 4 (a) using empirical relationship between resistivity and permeability represented in the above Fig. 7 (c). (Inazaki, et al., 2010)

Preprint

SAGEEP 2011 Charleston, South Carolina USA http://www.eegs.org Detailed Geophysical Surveying at an Anomaly Part

As noted above, detailed geophysical surveying should be applied at an anomaly part delineated by a preceding integrated geophysical survey. Direct measurement or probing would play an important role in this phase. To do this, we have developed a slimhole type resistivity logging probe and applied it to a levee survey (Inoue & Inazaki, 2008). The flexible probe, 2.5cm in diameter, incorporates a total of 56 button-type electrodes at 2cm intervals. The probe is inserted by hand into a slimhole dug beforehand by another sounding tool such as a cone penetrometer.

Figure 10 demonstrates resistivity log profiles obtained by the slimhole type resistivity logging probe. A total of 15 holes were dug beforehand by means of a percussion type cone penetrometer down to 5 m along a toe part of a levee. The purpose of the survey was to clarify the extent of a small anomaly delineated by means of OhmMapper survey. Pole-pole array was adopted and apparent resistivity was measured for a total of 210 electrode array set. The apparent resistivity profiles illustrated in Fig. 10 corresponds to the electrode array at 4 cm intervals. The resistivity profiles are similar to each other. A thin high resistivity layer is recognizable at about 1.5 m in depth. High resistivity at the surface part is mainly due to the decoupling of electrodes to the wall of the hole. The apparent resistivity values become higher at shallow depths in B_70 and B_60, where high resistivity anomaly was reconstructed, however, still remains much difference with the OhmMapper profile. This may be due to the sparseness of the data in vertical direction for OhmMapper profile.

Figure 9. A photo showing the slimhole type resistivity probe just inserted into a hole dug on a slope of a levee

Figure 10. Resistivity log profiles along slope toe of a levee where a high resistivity anomaly zone was delineated by means of OhmMapper survey.

Preprint

SAGEEP 2011 Charleston, South Carolina USA http://www.eegs.org

Conclusions

Integrated geophysical surveys were conducted on levee systems to first evaluate the geophysical methods that can be used for levee safety assessment. Accordingly, we chose to utilize high-resolution surface wave (HRSW) method using Land Streamer, capacitively-coupled resistivity (CR) survey using OhmMapper, and supplemental Slingram electromagnetic (EM) survey method because of their effective field performance and ability to provide useful geophysical properties, S-wave velocity and resistivity. We tested the above methods at 20 sites and exemplified that they were able to identify anomaly structures in and beneath levee systems. We also proposed a crossplot analysis of the measured data for the qualitative vulnerability assessment of levee systems. Quantitative evaluation of the vulnerability was the next goal for the safety assessment. We then collected and compiled not only the geophysical data but also geotechnical data on permeability and N-value, Clear relationship was identified between them and was helpful to the quantitative evaluation. Using this relationship we were able to map the permeability distribution as well as N-value, conventionally provided as point or line data. In addition, we conducted direct and detailed field measurements to reveal the internal structure and geotechnical properties of anomaly or vulnerable zones. The systematic application of geophysical methods appropriate to the investigation phase is the characteristic feature of the integrated geophysical technique.

Geophysical properties determined by integrated geophysical methods would vary with survey parameters, period, temperature, and so on. We checked it by time-lapse monitoring. However it still remained ambiguity in the varying order and mechanism. It might be possible to evaluate non-linear response of the soils in and beneath levee systems from such the time-lapse change. That is our next target for precise and effective assessment of the safety of levee systems.

Acknowledgements

A part of this study was conducted in cooperation with Mr. Inoue, and Mr. Miura. The authors acknowledge them who took charge of field measurements and tool operations. We also wish to thank the Chikuma River Office, Kurobe River Office, and Arakawa Joryu River Office for the many conveniences afforded to the field works and the useful data provided to this research.

References

Geometrics, 2001, OhmMapper TR1 operation manual, Geometrics Inc., 147p. Hayashi, K. and Suzuki, H., 2004, CMP cross-correlation analysis of multi-channel surface-wave data,

Exploration Geophysics, 35, p7-13. Hayashi, K., Inazaki, T., and SEGJ Levee Consortium, 2009, Integrated geophysical investigation for

safety assessment of levee systems (Part 1): methodology, process and criterion for the safety assessment, Proceedings of the 9th SEGJ International Symposium, CD-ROM, 4p.

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), 207-216.

Preprint

SAGEEP 2011 Charleston, South Carolina USA http://www.eegs.org Inazaki, T., 2006, Relationship between S-wave velocities and geotechnical properties of alluvial

sediments, Proceedings of the 19th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP2006), CD-ROM, 1296-1303.

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 (SAGEEP2007), 8p.

Inazaki, T., Hayashi, K., Watanabe, F., Matsuo, K., Tokumaru, T., and Imamura S., 2008, Ground truth verification of an integrated geophysical investigation for the assessment of an earthen levee, Proceedings of the 21st Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP2008), CD-ROM, 731-738.

Inazaki, T., Hayashi, K., and SEGJ Levee Consortium, 2009, Integrated geophysical investigation for safety assessment of levee systems (Part 2): acquisition and utilization of ground truth data, Proceedings of the 9th SEGJ International Symposium, CD-ROM, 4p.

Inazaki, T., Inoue, M., Saito, Y., Arakane, S., and Yoshida, N., 2010, Time-lapse monitoring of the slope failure process of a model levee, Proceedings of the 23rd Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP2010), CD-ROM, 8p.

Inoue, M., and Inazaki, T., 2008, Development of precise measuring method to comprehend internal characteristics of levees using electrical log and measurement of core resistivity, Proceedings of the 118th SEGJ Conference, 225-228. (in Japanese with English abstract) .

Miura, G., and Inazaki, T., 2010, An example of resistivity mapping around a river embankment by the broadband electromagnetic sensor GEM-2, Proceedings of the 122nd SEGJ Conference, 293-295. (in Japanese with English abstract) .

Preprint