geophysical surveying for imaging near-surface structure ... · schematic diagram assessing seepage...

2011 Korea-Japan Joint Symposium November 8th, 2011, Busan

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, 2011. 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.


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.


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

Figure 1 (b). Layout of a capacitively-coupled resistivity tool (OhmMapper).

Figure 1 (a). Schematic illustration of the high-resolution surface wave survey tool.


geophysical properties of soils and sediments is clarified, we can directly estimate the

vulnerability condition from the geophysical properties.

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).

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).


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.

2.2. 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 14C dating to the


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).


Two seismic lines, 1100-m long GS_AK_SLS1 and 900-m long GS_AK_SLS2, for S-

wave 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).


accumulation on the active faulting.

We therefore can calculate average

deformation rate of the Kakuda-

Yahiko fault in combination with

depositional age estimated from 14C

dating data labeled as figures beside

the column in Fig. 5. Figure 6 plots

the relative displacement and AMS 14C 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

suggests possible underestimation of

deformation rate of an active fault only

based on the evidence at major

faulted zone. In addition, high-

resolution seismic reflection surveying

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 surface-

wave data, Exploration Geophysics, 35, 7-13.

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).

Figure 6. A cumulative deformation curve regarding the activities of the major fault (MF) and a frontal fault FF2. The average deformation rate was estimated 0.5 m/kyr for FF2 and 1.0 m/kyr for MF (Inazaki, et al., 2011).


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), p207-216.

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, 101-108.

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).

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