Geomorphology 145–146 (2012) 56–69

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Geomorphology

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Combined geophysical methods for detecting soil thickness distribution on aweathered granitic hillslope

Yosuke Yamakawa ⁎, Ken'ichirou Kosugi, Naoya Masaoka, Jun Sumida, Makoto Tani, Takahisa MizuyamaLab. of Erosion Control, Department of Forest Science, Graduate School of Agriculture, Kyoto Univ, Kyoto 606-8502, Japan

⁎ Corresponding author. Tel.: +81 75 753 6493; fax:E-mail address: yosuke82@kais.kyoto-u.ac.jp (Y. Yam

0169-555X/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.geomorph.2011.12.035

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 January 2011Received in revised form 1 November 2011Accepted 20 December 2011Available online 27 December 2011

Keywords:Electrical resistivity imagingSoil thicknessSeismic surveyShallow landslideWeathered granite

The usefulness of electrical resistivity imaging (ERI) as a highly accurate method for determining the soilthickness distribution on hillslopes was validated by combining intensive measurements using invasivemethods, i.e., cone penetration testing and boreholes, with ERI in three granitic watersheds. Areas of highelectrical resistivity (ρ) contrast reflecting soil–bedrock interfaces were found in all three study watersheds.However, ρ values of soil and weathered granite just below the soil mantle varied over a relatively wide rangeat each site, as well as considerably from site to site. The patterns of low–high contrast in ρ profiles, reflectingthe soil–bedrock interface, also differed from site to site despite similarly dry conditions. Differences in thewater retention characteristics of soil and weathered granitic bedrock, as found by a previous study of bed-rock hydrological properties, may have been a major factor in the observed subsurface ρ variations. TheERI method, with electrode spacing of 0.5 to 2.0 m, was successful in determining soil thickness distributionsranging from about 0.5 to 3 m depth based on its ability to detect high contrast in ρ in the subsurface zone.Closer electrode spacings are expected to more sensitively reveal the distribution of ground material proper-ties and thus more accurately replicate the soil–bedrock interface. ERI failed to clearly identify the soil–bed-rock interface at some points along our measurement lines because of local intermediate materials withdifferent properties such as unconsolidated soil and clayey intermediation just below the soil–bedrock inter-face. Two types of seismic survey (SS) techniques were also used, combining seismic refraction (SR) and thesurface wave method (SWM) with the ERI method in a granitic watershed to compare ERI with other geo-physical methods. The profile of S-wave velocity (Vs) by SWM also reasonably duplicated the soil–bedrockinterface; the Vs profile showed larger variation in lateral direction and corresponded to the soil thicknessdistribution better than the P-wave velocity (Vp) profile by SR. The combined use of ERI and SWM may bemore effective in detecting the soil–bedrock interface because each method compensates for the deficienciesof the other method.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

On steep mountainous hillslopes in humid temperate regions likeJapan, shallow landslides induced by heavy rainfall often occur, espe-cially in granitic regions when rainfall destabilizes the soil mass. Theoccurrence of shallow landslides has been ascribed to soil hydrologi-cal properties such as the relationship between volumetric watercontent and pore water pressure, as well as saturated hydraulic con-ductivity and its physical properties, including the unit weight, cohe-sion, and internal friction angle of soil (e.g., Sammori et al., 1991;Suzuki, 1991; Sidle and Ochiai, 2006). Additionally, the uppermostbedrock layer could act as a considerable hydrological interface, con-tributing to the generation of positive pore pressures that destabilizethe surface soil layer located just above the interface (Montgomery et

+81 75 753 6088.akawa).

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al., 2002; Kosugi et al., 2006). Accordingly, the assessment of soilthickness (i.e., depth of soil–bedrock interface) and its distributionover a slope, which contributes directly to slope stability due to itsweight and volume as well as control of groundwater movement, isessential for the prediction of shallow landslides. However, such as-sessments are difficult due to the irregular distribution of soil thick-ness. Indeed, previous studies failed to identify a clear relationshipbetween soil (regolith) depth and topographical factors such asslope gradient, curvature, and hillslope position, especially in steepmountainous regions (e.g., Okimura, 1989; De Rose et al., 1991;Ohsaka et al., 1992). These studies pointed out that soil thickness atany point on a hillslope was dependent not only on slope topography,but also many other interacting factors such as vegetation factors in-cluding soil overturned by wind-thrown trees and the types and in-tensity of erosion processes; even though De Rose et al. (1991)demonstrated that at least mean soil depths for swale profiles wereroughly inversely related to mean slope gradient. Furthermore, thetype and intensity of erosion processes are considered to be affected

57Y. Yamakawa et al. / Geomorphology 145–146 (2012) 56–69

by structural conditions including the nature of the base material andthe existence of faults, as well as hydrological properties such as rain-fall characteristics and the frequency of saturation.

Although invasive methods such as a penetration test using a conepenetrometer yield more reliable soil depth measurements (Ohsaka etal., 1992; Katsura et al., 2005; Kosugi et al., 2006), they are laboriousand time consuming. Thus, large-scale (>10m) two-dimensional (2-D) or three-dimensional (3-D) soil-depth distributions with sufficient-ly high resolution are difficult to obtain using a cone penetrometer.

Over the last decade geophysical techniques to image subsurfacestructures have become popular in many geomorphological studies(Otto and Sass, 2006; Schrott and Sass, 2008). They include electricalresistivity imaging (ERI), seismic survey (SS), and ground penetratingradar (GPR) techniques, which detect the spatial distribution of elec-trical resistivities (ERs), seismic velocities, and dielectric constants,respectively. The advantages of current geophysical field methodsare the greatly increased power of modern computers and the avail-ability of light-weight equipment, which permit relatively user-friendly, efficient, and non-destructive data gathering. Improved so-phistication in current data analytical methods may allow us toimage more complicated and heterogeneous subsurface structures.Geophysical methods, therefore, could offer an important advantagein improved rapid detection of soil thickness distributions on moun-tain hillslopes compared with analog measurement.

Indeed, combinations of ERI, SS, and GPR techniques have beenapplied to mountain slopes for detecting the thickness of overburden(Godio et al., 2006), unstable landslide material (Bichler et al., 2004;Gökhan et al., 2008), and talus deposits (Mauritsch et al., 2000; Ottoand Sass, 2006). These successful applications efficiently developed2-D or 3-D interpretations of the internal structure of each studysite based on the combined data, with geological verification derivedfrom boreholes. However, both Mauritsch et al. (2000) and Otto andSass (2006) demonstrated that some disagreements may existamong ERI, SS, and GPR estimates of bedrock surfaces below talus de-posits. That is, there are problems related to the sensitivity and spatialresolution of indirect measuring methods. This is mainly because theelectrical resistivity, seismic velocity, and dielectric constant ofground materials observed using such noninvasive methods are allproxies for several mechanical and physical properties. Another com-mon disadvantage of the SS technique is the ‘hidden layer problem,’which may lead to incorrect interpretations of subsurface structures(Foti et al., 2003). The ‘hidden layer’ is a low-velocity layer sand-wiched between higher velocity units of refracted first P-waves. Sim-ilarly, GPR performance has been found to deteriorate in electricallyconductive media such as clayey soils because of attenuation ofground waves, but to be optimal in soil with a coarse texture(Hubbard et al., 1997; Michot et al., 2003).

Neither the practical application nor the limitations of these geo-physical methods have been studied thoroughly because of the highcost of drilling a sufficiently dense network of stratigraphic and geo-morphologic observations. Consequently, questions remain aboutthe reliability of these geophysical methods, especially in very shal-low zones (b5 m depth) on mountain hillslopes, which are somewhatshallower than the regions (approximately b50 m depth) of interestin most previous studies (Mauritsch et al., 2000; Bichler et al., 2004;Godio et al., 2006; Otto and Sass, 2006; Gökhan et al., 2008). To im-prove the methodology for quantifying soil thickness distributionsover large areas, it is necessary to directly validate these techniquesand to understand in greater detail their advantages and limitations.

In this study, we validated the capability of ERI in detecting soilthickness distributions on hillslopes by comparing cone penetrationtests (CPT) along a total of four lines in three weathered granitic wa-tersheds. Along one line, we used the ERI method with three differentelectrode spacings. For comparison with the ERI method, we also con-ducted two types of SS along this line, namely seismic refraction (SR,for P-wave velocity profile) and the surface wave method (SWM, for

S-wave velocity profile). Additionally, we evaluated the ER profilesreflecting the subsurface structure, including the soil mantle and shal-low bedrock, based on hydrological properties derived from previousresearch using soil and bedrock core samples collected at each studysite.

2. Methods

2.1. Study sites

The experiments applying ERI, using three different electrode spac-ings (0.5, 1.0, and 2.0 m), two types of SS (SR and SWM), and the pen-etration tests, were conducted along a line in the Nishi'otafuku-YamaExperimental Catchment (line A-1 at site A, Fig. 1a). The comparisonsbetween ERI and the penetration test were conducted along an addi-tional line in the same catchment (line A-2 at site A, Fig. 1a) and alsoalong lines in the Fudoji Experimental Catchment (line B at site B,Fig. 1b) and the Kiryu Experimental Catchment (line C at site C,Fig. 1c). All of the watersheds are underlain by granitic bedrock. Thegeophysical techniques and the acquisition parameters applied toeach line are summarized in Table 1. All measurements at each sitewere conducted under precipitation-free conditions. Antecedent pre-cipitation prior to each geophysical survey at sites A to C that mighthelp the interpretation of the geophysical survey profiles is summa-rized in Table 2. Boreholes were drilled for geotechnical investigationsand groundwater level observations at sites A–C (Fig. 1).

2.1.1. Site A: Nishi'otafuku-Yama Experimental WatershedThe Nishi'otafuku-Yama Experimental Watershed (site A, Fig. 1a)

is in the Rokko Mountains (34°46'N, 135°16'E), Hyogo Prefecture,Japan. The bedrock underlying the watershed is equigranular Creta-ceous granite, characterized by light pink feldspar, cross-cut withsome late Cretaceous to Paleogene quartz porphyry dykes. This gran-ite is called the ‘Rokko granite’ and contains the primary mineralsquartz, plagioclase feldspars, alkali feldspars, and biotite (Kasama,1968).

The two measurement lines (line A-1 and line A-2) were both55 m long horizontally and almost parallel to the contour lines. Thetwo boreholes, R1 and R2, were located along the main hollow ofthe observation slope, and R3 was located around the middle of lineA-2 (Fig. 1a). The total depths of the boreholes R1, R2, and R3 were70, 38, and 50 m, respectively. The depth to the water table in bore-holes R1, R2, and R3 is summarized in Table 2.

2.1.2. Site B: Fudoji Experimental WatershedThe Fudoji Experimental Watershed (site B, Fig. 1b) is in the Tana-

kami Mountains (35°54'N, 135°59'E), Shiga Prefecture, Japan. TheCretaceous granite bedrock underlying the watershed is called the‘Tanakami granite’ (Hashimoto et al., 2000). As shown in Fig. 1b, theERI survey was conducted along the hollow of an unchanneled head-water catchment (line B in Fig. 1b). The array line was 55 m long. Aspring was found in the downstream end of the hollow where bothsoil and bedrock layers were considered to be wholly saturated.

A borehole (F1) located in the middle of the slope about 100 mnortheast of line B (Fig. 1b) was selected as a reference for evaluatingthe weathering profile around line B. The selection was based on hy-drological observations conducted around line B (Woldie et al., 2009)and F1 (Fujimoto et al., 2008), which indicated similar hydrologicalprocesses, i.e., abundant rainwater infiltrating the weathered bedrockin the middle- and up-slope regions and exfiltrating in foot-slope re-gions. The total depth of the borehole was 15.0 m. On the day of ERImeasurement, the depth of the groundwater table was 3.1 m (Table 2).

2.1.3. Site C: Kiryu Experimental WatershedThe Kiryu Experimental Watershed (site C, Fig. 1c) is also in the

Tanakami Mountains (35°54'N, 135°59'E), Shiga Prefecture, Japan.

Fig. 1. Locations and topography of the Nishi'otafuku-Yama (a; contour interval=5 m), Fudoji (b; 2 m) and Kiryu (c; 1 m) Experimental Watersheds. Square symbols show thelocations of observation borehole wells ( R1, R2, R3, F1, K1, K2, and K3).

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The Cretaceous granite bedrock underlying the watershed is Tana-kami granite (Hashimoto et al., 2000). As shown in Fig. 1c, the arrayline (69 m long in horizontal distance) was made across a ridge(line C in Fig. 1c).

The total depths of the boreholes at points K1, K2, and K3 alongline C (Fig. 1c), were 12, 20, and 15 m, respectively; all were drilledby Katsura et al. (2009) for geotechnical investigations andgroundwater-level observations.

During the ERI measurement, the water table depths in boreholesK1, K2, and K3 were 6.3, 13.3, and 6.4 m, respectively (Table 2). At siteC, no precipitation was observed during the preceding 1 week(Table 2). The soil mantle and shallow bedrock along line C were ex-tremely dry.

2.2. Electrical resistivity imaging

ERI surveys measure the electrical potential between a pair ofelectrodes caused by direct current injection from a second pair ofelectrodes. The electrical resistivity (ER) of a ground material is con-sidered to be a proxy for the spatial and temporal variability of the

Table 1Geophysical techniques and acquisition parameters for each experimental site.

Site Method Electrode/geophone spacing (m)

A (Nishi'otafuku-Yama), line A-1 ERI 0.5, 1.0, 2.0A (Nishi'otafuku-Yama), line A-1 SR, SWM 0.5A (Nishi'otafuku-Yama), line A-2 ERI 1.0B (Fudoji), line B ERI 1.0C (Kiryu), line C ERI 0.75

other properties (e.g., structure, water content, or fluid composition;Samouëlian et al., 2005). ER data are commonly analyzed usingArchie's equation (Archie, 1942), which relates the resistivity of soilor rock (ρ) to the resistivity of pore water (ρw), effective porosity(ϕ), and degree of saturation (S) of a medium:

ρ ¼ aϕ−mS−nρw ð1Þ

where a, m, and n are empirical constants with positive values, all ofwhich depend on soil and rock characteristics. Properties that can af-fect the values of the constants a, m, and n in Archie's law include dif-ferences in porosity, pore configuration, clay content, clay type,mineralogy, mineral coatings, and weathering of bedrock lithologies(Watson et al., 2005). ER distributions obtained on hillslopes couldreflect the soil mantles and underlying bedrock.

ER data were acquired using the E60CNMulti-electrode ResistivitySystem (Geopen Shanghai, Ltd., Shanghai, China) with 0.5–2.0 melectrode spacing along each measurement line (Table 1). As elec-trode spacing decreases, the resolution of the profile generally in-creases; therefore, an ER profile representing the soil-thicknessspatial distribution may be affected by changes in electrode spacing.Commonly used electrode geometry includes Wenner, Schlumberger,pole–dipole, and dipole–dipole arrangements. We chose the dipole–dipole arrangement for this study, as it provides the greatest spatialresolution and most sensitivity to vertical resistivity boundaries(Zhou et al., 2000). The data were processed to generate 2-D resistiv-ity models of the subsurface area using E-Tomo ver. 4 software for in-version analyses, developed by DIA Consultants Company, Tokyo,Japan.

Table 2Antecedent precipitation before geophysical surveys and groundwater levels in boreholes at sites A–C on each measurement day.

Site Geophysicaltechnique

Date Antecedent precipitation (mm) Depth ofgroundwater tablea

(m)For 30 days For 1 week For 3 days For 1 day

[R1] [R2] [R3]A (Nishi'otafuku-Yama), line A-1 ERI 7-Aug-2009 (end of rainy season) 315.6 (215.6)b 113.8 6.8 0 50.3 18.1 36.8A (Nishi'otafuku-Yama), line A-1 SR, SWM 26-Nov-2009 72.0 (77.0)b 14.4 8.8 0 55.0 22.2 41.2A (Nishi'otafuku-Yama), line A-2 ERI 12-Nov-2008 79.0 (112.4)b 24.4 0 0 58.1 25.2 44.4

[F1]B (Fudoji), line B ERI 23-Jul-2009 (middle of rainy season) 384.0 (326.6)b 238.5 95.5 0 3.1

[K1] [K2] [K3]C (Kiryu), line C ERI 26-Aug-2009 165.1 (102.0)b 0 0 0 6.3 13.3 6.4

a Depth from the ground surface.b The average value of the same period over the past 5 years.

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2.3. Seismic refraction (P-wave velocity profile)

As the seismic velocity of P-waves is dependent on the elasticmodulus and density of the material through which the seismicwaves propagate (Schrott and Sass, 2008), P-wave velocity distribu-tion could reflect soil thickness distribution. The SR method is basedon the measurement of the first arrivals of critically refracted P-waves, which propagate through different subsurface materials,such as sand, gravel, and bedrock, at different velocities. At the inter-face between different layers, the wave energy is refracted into thedeeper layer, and part of the energy is reflected back toward the sur-face. Increase in P-wave velocity with depth is a necessary precondi-tion for the SR method to measure the subsurface velocitydistribution, but it prevents the detection of a hidden layer. The reli-ability of P-wave and S-wave velocity profiles may be increased bygathering a larger data set using multiple shots and receiver positions.We used 4.5-Hz vertical geophones with a spacing of 0.5 m and a12.5-kg sledgehammer on a plate for seismic source at an interval of2 m. We carried out the acquisition using a 48-channel McSEIS-SX48 (OYO Corporation, Tokyo, Japan) seismograph. The tomographicinversion of P-wave velocities (Gökhan et al., 2008) was conductedusing SeisImager/SW software (OYO Corporation).

2.4. Surface wave method (S-wave velocity profile)

SWM has been recently receiving increased attention (Glangeaudet al., 1999), although SR is a more conventional method. S-wave(shear-wave) velocity could yield important information for deter-mining soil thickness, as this parameter is directly related to the stiff-ness of the material through which the seismic waves propagate (Fotiet al., 2003). S-wave velocity profiles are provided by means of sur-face wave analysis, utilizing the dispersive nature of surface wavesin heterogeneous media (Glangeaud et al., 1999; Park et al., 1999).Surface-wave records, usually dominated by Rayleigh waves, are pro-cessed to recover the dispersion curve, i.e., the relationship betweenthe frequency and the phase velocity of Rayleigh waves. This charac-teristic depends on the S-wave velocity of sub-surface layers and canbe used in an inversion procedure to estimate the 1-D S-wave velocityprofile. Surface-wave data were acquired using a 48-channel McSEIS-SX48 (OYO Corporation) seismograph and 4.5-Hz vertical geophoneswith a spacing of 0.5 m. A 12.5-kg sledgehammer on a plate was usedfor seismic source at intervals of 2 m. The processing to estimate 1-DS-wave velocity profiles based on the frequency–phase velocity (f–c)approach (Park et al., 1999; Cercato, 2009) and the 2-D tomographicinterpretation were carried out using SeisImager/SW software (OYOCorporation). Because SWM overcomes some of the inherent limita-tions of the refraction technique caused by refraction equivalence,such as the hidden layer problem and gradual variations in velocity

(Foti et al., 2003), it could provide more reliable results for complexsubsurface distributions of physical parameters than the SR method.

2.5. Definition of the soil–bedrock interface using the cone penetrationtest

The cone penetrometer test (CPT), which provides vertical profilesof penetration resistance within a soil mantle, is generally conductedto investigate the depths of boundaries between the soil mantle andbedrock in mountainous forested watersheds (e.g., Katsura et al.,2005; Kosugi et al., 2006). The penetration resistance is usually indi-cated as the number of blows with a weight required for 10-cm pen-etration and is expressed by the variable N with a subscript for eachdifferent gauge.

On the basis of previous investigations in granite mountains (e.g.,Okunishi and Iida, 1978; Okimura and Tanaka, 1980; Kosugi et al.,2006), in which a cone penetrometer with a 60° bit, a cone diameterof 30 mm, a weight of 5 kg, and a fall distance of 50 cm was used, apenetration resistance (Nc) of 50 provides a rough indicator of theboundary between soil and bedrock. Okimura and Tanaka (1980)conducted CPT at and around slopes where landslides occurred fol-lowing heavy storm events in granitic watersheds. They demonstrat-ed that most of the slip surfaces occurred in a layer with Ncb13, andthat the cone was hardly able to penetrate the layer with Nc>50.This suggests that interfaces where Nc≥50 have adequate strengthto serve as a stationary underlayer (i.e., bedrock) in the case of shal-low landslides on granitic slopes.

However, there are few studies demonstrating the weathering de-gree of the Nc=50 layer based on certain classification systems ofweathered granite. Fig. 2 shows the lithological logs of shallowzones in boreholes K3 and K2 (line C at site C, Fig. 1c), drilled byKatsura et al. (2009), together with the results of CPT conducted atthe corresponding points by Kosugi et al. (2006). These boreholelogs were described by Katsura et al. (2009) based on the system ofclassifying granite cores by weathering degree (Japanese Society ofSoil Mechanics and Foundation Engineering, 1985). Table 3 summa-rizes the characteristics of each weathering class (A to D). Weatheringclasses C and D are both divided into three subcategories, with sub-scripts H, M, and L. Additionally, to better illustrate the geologicalcondition around the soil–bedrock interface defined in this study,we established stratigraphic interpretations for these borehole logs(Fig. 2) based on the six-stage weathering profile classification sys-tem (i.e., I through VI) summarized by Dearman (1974), shownhere as Table 4. The weathering profile of granite is characterized bya mixture of solid rock, i.e., core stones, and residual soil. In principle,the ratio of solid rock to residual soil gradually increases with increas-ing depth and the weathering degree of rock decreases (e.g., Ruxtonand Berry, 1957; Dearman, 1974). At K3, Nc exceeded 50 at the

Fig. 2.Weathering-class profiles observed at boreholes K3 (a) and K2 (b) along line C atsite C (Fig. 1c), as determined by Katsura et al. (2009). The profiles include stratigraph-ic weathering profiles, estimated based on borehole logs, and profiles of penetration re-sistance (Nc), measured at the corresponding points by Kosugi et al. (2006).

Table 4Types of weathering profile characteristics (Dearman, 1974).

Weatheringsection

Character

I. Fresh Parent rock showing no discoloration, loss of strength or anyother weathering effects.

II. Slightlyweathered

Rock may be slightly discolored, particularly adjacent todiscontinuities, which may be open and will have slightlydiscolored surfaces; the intact rock is not noticeably weaker thanthe fresh rock.

III. Moderatelyweathered

Rock is discolored; discontinuities may be open and will havediscolored surfaces with alteration starting to penetrate inwards;intact rock is noticeably weaker, as determined in the field, thanthe fresh rock.

IV. Highlyweathered

Rock is discolored; discontinuities may be open and will havediscolored surfaces, and the original fabric of the rock near to thediscontinuities may be altered; alternation penetrates deeplyinwards, but core stones are still present.

V. Completelyweathered

Rock is discolored and changed to a soil but original fabric ismainly preserved. There may be occasional core stones. Theproperties of the soil depend in part on the nature of parent rock.

VI. Residualsoil

Rock is discolored and completely changed to a soil in which theoriginal rock fabric is completely destroyed. There is a largechange in volume.

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depth in the section classified as the surface soil layer or zone V(Fig. 2a), probably reflecting a core stone embedded within the sur-face soil layer. At K2, Nc exceeded 50 at the depth nearly correspond-ing to the interface between the surface soil and DL-class bedrock, orzones V and IV (Fig. 2b). Consequently, the layer with Nc≥50 couldaccount for the top of the layer primarily composed of DL-class andless-weathered bedrock and roughly corresponding to the interface

Table 3Classification of weathering class (Japanese Society of Soil Mechanics and FoundationEngineering, 1985).

Weatheringclass

Degree of weathering Color Hardness

A Fresh Bluish gray tomilk-whitishgray

Very hard, emits aclank whenhammered

B Generally fresh Milk-whitishgray to (light)brownish gray

Hard, emits alightclank whenhammered

CH Weathered along fractures,plagioclase and biotite arepartly altered

Brownish grayto (light)grayish brown

Moderately hard,emits a dull clankwhen hammered

CM Weathered except inside rockbody, plagioclase and biotiteare altered further

Grayish brownto lightyellowishbrown

Slightly soft tohard, easily splitwhen hammered

CL Weathered entirely whileretaining rock fabric andstructure, plagioclase isgenerally altered, quartz is leftunweathered

Light yellowishbrown toyellowishbrown

Soft, partlycrushed by fingerpressure

DH Entirely weathered,plagioclase is altered into clay

Yellowishbrown

Soft, crushed byfinger pressure

DM Entirely weathered, most offeldspar is generally altered

Yellowishbrown

Very soft, crushedby finger pressure

DL Entirely weathered, most offeldspar is generally alteredinto clay, quartz isdecomposed into fine grains

Yellowishbrown

Very soft,powdered byfinger pressure

between zones V and IV. However, it should be noted that CPT resultsare significantly affected by the presence of core stones.

Moreover, Katsura et al. (2009) demonstrated that the saturatedhydraulic conductivity, Ks, of the DL-class bedrock (10−4 cm s−1)was dramatically lower than that of the soil (10−1 cm s−1). Thesefindings were based on core samples obtained from boreholes drilledon line C (Fig. 1c) and they suggest that the boundary between sur-face soil and DL-class bedrock (i.e., zones V and IV) acts as a consider-able hydrological interface, even though DL-class bedrock isextremely highly weathered and is almost soil. The hydrological in-terface is considered to hinder rainwater infiltration into the deeperzone, as Kosugi et al. (2006) demonstrated in field observations, inwhich a heavy rainstorm generated positive pore pressures in thesoil layer shallower than the interface, with the potential to destabi-lize the soil mass.

In this study, we conducted CPT using a Hasegawa-type penetrom-eter (Nishimura et al., 1987) with a 60° bit, a cone diameter of 20 mm,and a weight of 2 kg, and expressed the penetration resistance usingNh. Simply based on specific energy per drop, Es, is proportional to M/A, where M is the falling mass and A is the cross-sectional area of acone tip (Sanglerat, 1972; Clayton et al., 1995), and Es for M=5 kgand A=7.07E-04 m2 divided by Es for M=2 kg and A=3.14E-04 m2=1.11. Accordingly, Nc=50 is considered equivalent toNh=55.55. However, the cone diameter of the Hasegawa-type pene-trometer (20 mm) is smaller than that used by Okimura and Tanaka(1980) (30 mm). Accordingly, the force of impact of the Hasegawa-type penetrometer may be attenuated to a higher degree than for thepenetrometer used by Okimura and Tanaka (1980) because of skin fric-tion between soil profiles and the penetrometer rods, the diameters ofwhich are both 16 mm. For this reason, the ground penetration resis-tance of Nc=50 could correspond to a value of Nh>55. Meanwhile,the cone was hardly able to penetrate the layer with Nh>100. There-fore, for this study we defined the soil–bedrock interface as the layerwhere Nh≥55–100. Dynamic resistance, RD, can be expressed usingthe following formula (Sanglerat, 1972; Clayton et al., 1995):

RD ¼ MghNAz

ð2Þ

where g is the gravitational acceleration (9.8 m s−2), h is the fallingheight (0.5 m), N is the number of drops, and z is the depth intervalconsidered for counting drops (0.1 m). RD with M=5 kg, N=50 and

61Y. Yamakawa et al. / Geomorphology 145–146 (2012) 56–69

A=7.07E-04 m2; that withM=2 kg, N=55 and A=3.14E-04 m2; andthat with M=2 kg, N=100 and A=3.14E-04 m2 are 17.3, 17.2, and31.2 Mpa, respectively. Accordingly, the layers with Nc≥50 andNh≥55–100, defined as the soil–bedrock interface in this study, areconsidered to correspond to the layer with RD≥17.2–31.2 Mpa, assummarized in Table 5.

CPT was conducted at 36 points along line A-1 (Fig. 1a), five pointsalong line A-2 (Fig. 1a), and 36 points along line B (Fig. 1b). On line A-1, Nh profiles from the ground surface to the soil–bedrock interfaceswere continuously measured at nine of the 36 points, while only layerdepthswithNh values≥100were confirmedwithoutmeasuring the con-tinuousNh profiles at the other 27 points. On line C (Fig. 1c), we observedsoil thickness distribution with invasive methods for only the northwest-ern slope to validate ERI results. That is, along the northwestern half ofline C, we circumstantially measured soil thickness at 48 points in the di-rection of the slope using the iron-pole method, which was validated bycomparing the results with those of penetration tests in which layerswith Nc values≥50 were detected (Kosugi et al., 2006).

3. Results and discussion

3.1. Comparison of ERI and SS with CPT on line A-1

3.1.1. Direct measurement of soil thickness using the cone penetrometerFig. 3 shows the depths at which Nh=100, invasively detected as

soil–bedrock interfaces by CPT performed at nearly 1 m intervals,along line A-1 at site A (Fig. 1a). These depths ranged from 38 to303 cm. Most of the interfaces with Nh values=100 detected at the36 points were considered reasonable soil–bedrock interfaces, asthe depths of the soil–bedrock interface were relatively similaramong adjacent points despite their high spatial variation. However,the possibility of the CPT incorrectly detecting a core stone anoma-lously embedded within a residual soil layer as a soil–bedrock inter-face cannot be ruled out.

As shown by the Nh profiles measured at the nine points (P1through P9 in Figs. 3 and 4), Nh values suddenly increased andexceeded 100 at a certain depth at each point. The depth of the dee-pest layer where Nh=55 was almost the same as the depth of thelayer where Nh=100 at the nine points. The depth differences be-tween those layers ranged from 1.4 cm (at point P6) to 14.9 cm(P5), and the average value of the nine points was 7.0 cm. The layerswhere Nh≥55–100 probably correspond to the interface between theresidual soil layer and the layer primarily composed of DL-class andless-weathered bedrock, and thus correspond to the interface be-tween zones V and IV. While the soil layers at P2, P5, P6, and P8exhibited relatively small penetration resistances (Nhb20) up to thedepth corresponding to the soil–bedrock interface, anomalous Nh

peaks at points P1, P3, P4, P7, and P9 were detected, the values ofwhich occasionally exceeded 55, likely corresponding to core stones.

3.1.2. Detecting the soil thickness using ERIThe three electrical resistivity (ρ) profiles along line A-1 at site A

surveyed by ERI with electrode spacings of 0.5, 1.0, and 2.0 m arenearly identical except around the 6-m point (Fig. 3), demonstrating

Table 5Geophysical and mechanical properties of subsurface layers at sites A–C.

Site Electrical resistivity: r (ohm-m) Seismic velocity: Vp

Soil layer Surface layer in bedrocklayer

Soil layer

A (Nishi'otafuku-Yama) 500–5000 5000–90000 b350–500, b140–22B (Fudoji) 500–1800 2000–10000 –

C (Kiryu) 1000–3800 200–700 –

the high reproducibility of the ERI method. The inconsistency aroundthe 6-m point is probably attributable to high grounding resistance inthat area.

These ρ profiles (Fig. 3) all imply the existence of three layers. Thethickness of the shallowest thin layer (layer 1), which had relativelylow resistivity values of about 5000 ohm-m or less, varied fromabout 0.5 to 3 m. Layer 2 had relatively high resistivity values ofabout 5000 ohm-m or more. Consecutive and spotty anomalies withextremely high ρ>10,000 ohm-m were observed over line A-1.Layer 3 showed a relatively low ρ of 500–4000 ohm-m. As shown inFig. 3, the positions of the layer where Nh=100 (i.e., the soil–bedrockinterface) were well correlated with the boundaries between layers 1and 2 or the outlines of the anomalies of the extremely high resistiv-ity, ρ> 10,000 ohm-m. The ρ profiles at points P1 to P9 using ERI withan electrode spacing of 0.5 m (Fig. 4) (which were extracted from the2-D resistivity profile; Fig. 3a) indicated that ρ of the depths of soil–bedrock interfaces detected by CPT at points P1 through P9 fell withinthe range of 5000–10,000 ohm-m. That is, the low-high contrast of ρprofiles might reflect the interface between the residual soil layer andthe soil layer primarily composed of DL-class and less-weathered bed-rock, and thus correspond to the interface between zones V and IV.Accordingly, we concluded that the ERI method with electrode spac-ing of 0.5 to 2.0 m was successful at detecting soil thickness distribu-tions ranging from about 50–300 cm depth based on the high contrastof ρ in the subsurface zone.

However, from comparison between the ρ profiles and the soilthickness distribution, the region of ρ> 5000 ohm-m was absentaround point P7 on the ER profile, most notably using a 0.5-m elec-trode spacing (Fig. 3a), which prevented detection of the soil–bed-rock interface. The bedrock sample collected using a motor auger at15–30 cm depth below the soil–bedrock interface at point P4 (identi-fied in Figs. 3 and 4) was gravelly (Fig. 5a) and, despite the highly dis-turbed condition, could be estimated as DM- to DH-class heavilyweathered granite. In contrast, the sample collected at 80–90 cmdepth below the soil–bedrock interface at point P7 (localized inFigs. 3 and 4) was clayey (Fig. 5b) and disintegrated readily underflowing water (Fig. 5c). The gravelly bedrock sample from point P4was composed of clasts>2 mm in diameter, whereas the percentcontents of fine gravel, sand, silt, and clay for the clayey bedrockfrom point P7 were 3.8%, 20.6%, 31.5%, and 44.1%, respectively(Fig. 6). That is, the clayey material from point P7 was categorizedas clay based on the USDA soil-classification system, although it wasconsidered to be bedrock by CPT. The difference in ground materialbetween points P4 and P7, which was not detected by CPT, was alsothought to be reflected on the ρ profiles (Fig. 3). The irregularly dis-tributed low resistivity zone under the surface soil layer aroundpoint P7 as well as point P1 could reflect the existence of a clay-richgouge attributed to fault fracturing (Anderson et al., 1983; Noharaet al., 2006). This conclusion is supported by the intrusion patternof the anomaly, with low resistivity on the ERI profile as demonstrat-ed by Marescot et al. (2008). In addition, we considered the effect ofelectrode spacing on the detection of the soil–bedrock interfacethrough a precise comparison between the ρ profiles and the soilthickness distribution detected by CPT as follows.

, Vs (m s−1) Penetration resistance: Nc, Nh (drop 10-cm−1), Rd(MPa)

Surface layer in bedrocklayer

Soil layer Surface layer in bedrocklayer

0 >350–500, >140–220– b10–13, b20–55, b3–17 >50, >55–100, >17–31–

Fig. 3. Electrical resistivity (ρ) profiles along line A-1 at the Nishi'otafuku-Yama Experimental Catchment (site A, Fig. 1a) surveyed by ERI with electrode spacing of (a) 0.5, (b) 1.0,and (c) 2.0 m. Black ‘x’ symbols show the positions where Nh=100 (i.e., the soil–bedrock interface) as determined by CPT. At points P1 through P9, penetration resistance (Nh)profiles were measured by CPT as shown in Fig. 4. Square symbols at points P4 and P7 show the locations of core sample collection using a motor auger.

Fig. 4. Vertical profiles of penetration resistance (Nh) at points P1 through P9 along line A-1 at the Nishi'otafuku-Yama Experimental Catchment (site A, Fig. 1a) measured with CPT,and those of electrical resistivity (ρ), P-wave velocity (Vp), and S-wave velocity (Vs) for the corresponding points extracted from the two-dimensional resistivity profile surveyed byERI with electrode spacing of 0.5 m (Fig. 3a), P-wave velocity profile by SR (Fig. 7a), and S-wave velocity profile by SWM (Fig. 7b), respectively.

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Fig. 5. Bedrock samples collected using a motor auger (a) at 15–30 cm depth below thesoil–bedrock interface at point P4 and (b) at 80–90 cm depth below the soil–bedrockinterface at point P7 along line A-1 at site A (Fig. 1a). See Figs. 3 and 4 for sample loca-tions. (c) The same sample as shown in Fig. 5b but disintegrated under flowing water.The minimum scale value of the ruler is 1 mm.

Fig. 6. Grain-size distributions of the bedrock samples collected at 15–30 cm depthbelow the soil–bedrock interface at point P4 and at 80–90 cm depth below the soil–bedrock interface at point P7 along line A-1 at site A (Fig. 1a). See Figs. 3 and 4 for sam-ple locations.

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Comparison of the three ρ profiles revealed that they were verysimilar, although the ρ profile with 0.5-m electrode spacing was themost sensitive, and the resolution of the profiles appeared to decreasewith electrode spacing. Indeed, on the spotty, extremely high ρanomaly at the 18-m point on line A-1, detected by ERI with 0.5-melectrode spacing (Fig. 3a), the upper outline corresponded exactlyto the shallow soil depth, locally measured using CPT. This signalwas dampened by the increasing electrode spacing (Fig. 3b,c). More-over, the intermediation of lower resistivities around point P7, whichrepresented the collected clayey material, became smoothed with in-creased electrode spacing (Fig. 3). Thus, ERI measurements madeusing close electrode spacing depict the distribution of ground mate-rial properties more sensitively than those with wider electrode

spacing and, as a consequence, define the soil–bedrock interfacemore accurately.

Additionally, within the regions of the 36-m through 39-m points,where the ERI profiles demonstrated low reproducibility and theweathering profiles are considered to be more ambiguous comparedto other points, the inconsistencies between ρ profiles and the soil–bedrock interface detected by CPT at the 37-m and 39-m (corre-sponding to P6) points on line A-1 could be attributed to a corestone anomalously embedded within the residual soil layer.

3.1.3. Detecting the soil thickness using SSFig. 7a,b shows P-wave (Vp) and S-wave (Vs) velocity profiles for

line A-1 surveyed by both SR and SWM methods using a geophonespacing of 0.5 m. The position of the layer with Nh=100 (i.e., thesoil–bedrock interface measured by CPT) is also shown for compari-son. Both Vp and Vs increased with depth due to the compaction ofground materials and less-weathered conditions. The contour linesof Vp appear parallel to the ground, and the intervals between thelines were almost the same; that is, Vp showed a gradual increase inthe vertical direction and little variations laterally (Fig. 7a). By con-trast, the Vs profile showed larger lateral variation than the Vp profile.In addition, velocity inversion layers, where shallower layers havegreater velocity than deeper layers, were detected, for example, atthe 17- and 35-m points on line A-1 in the Vs profile.

The soil mantle detected by CPT corresponded to the Vp layer atb350–500 m s−1 and the Vs layer at b140–220 m s−1 (Figs. 4, 7a,b),as summarized in Table 5. In particular, the Vs contour line at180 m s−1 almost exactly corresponded to the soil–bedrock interface(i.e., the interface between the residual soil layer and the layer pri-marily composed of DL-class and less-weathered bedrock) showinglaterally high spatial variation. However, the soil–bedrock interfacedetected with CPT at the 24- to 27-m points was approximately 1 to1.5 m shallower than the position of the Vs contour line of180 m s−1. This could be attributed to differences in the patterns ofweathering profiles around and below the region Nh=100 betweenthe 24- to 27-m points and the other points along line A-1. That is,a likely explanation is that the top region of bedrock detected byCPT at the 24- to 27-m points was either composed of more-weathered bedrock or had a higher percentage of residual soil thanintact rock, resulting in Vs values lower than those of the other points.In contrast, around the middle position of line A-1, including the 24-to 27-m points, the soil–bedrock interface corresponded very well tothe outlines of the anomalies of the extremely high ρ>10,000 ohm-mdetected by ERI with electrode spacing of 0.5, 1.0, and 2.0 m (Fig. 3).

The velocities corresponding to the surface bedrock just below thesoil–bedrock interface on the Vp and Vs profiles did not show large var-iation in lateral directions implying the local existence of some anom-alies, while the ρ profiles locally showed relatively low resistivities

Fig. 7. Profiles of seismic wave velocity. The P-wave velocity (Vp) (a) and S-wave velocity (Vs) (b) profiles along line A-1 at Nishi'otafuku-Yama Experimental Catchment (site A,Fig. 1a) surveyed by SR and SWM, respectively, with geophone spacing of 0.5 m. Black ‘x’ symbols show the positions where Nh=100 (i.e., the soil–bedrock interface) as deter-mined by CPT. At points P1 through P9, penetration resistance (Nh) profiles were measured by CPT. Square symbols at points P4 and P7 show the collection locations of core samplesusing a motor auger.

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corresponding to the clayey material below the soil–bedrock interfaceat point P7 on line A-1 (Fig. 3). That is, the Vp and Vs profiles did not in-dicate the existence of the clayey material, but instead successfullyreflected the soil–bedrock interface detected by CPT around point P7.

Moreover, the heterogeneous spatial distribution of Vs may reflectthe spatial distribution of core stones or certain soil properties espe-cially in shallow regions of the soil mantle, as Vs profiles at pointsP3 and P5 showed a slight increase around the depth where anoma-lous Nh peaks were detected (i.e., 55 cm deep at P3 and 30 cm deepat P5), which were detected in neither Vp nor ρ profiles (Fig. 4). How-ever, Vs profiles did not exactly correspond to such anomalous Nh

peaks; for example, the Vs profile at point P1 monotonically in-creased, whereas the Nh profile showed numerous anomalous peaks.

Inconsistencies in identifying the depth of a soil–bedrock interfaceand the characteristics of a soil layer are practically inevitable whenusing the different methods (CPT, SR, SWM, and ERI) because thesemethods involve dissimilar responses in their identification of the in-terfaces among different weathering zones. Accordingly, the compar-ison of the results of ERI, SR, and SWM with those of CPT indicatedthat SWM might be more useful than SR for surveying the distribu-tion of soil thickness on mountain hillslopes, and that the combineduse of ERI and SWM could be more effective in detecting the soil–bed-rock interface because each method would compensate for disadvan-tages in the other method; ρ profiles sometimes detected a soil–bedrock interface that Vs profiles could not detect, and vice versa.

3.2. Detecting soil thickness using ERI on weathered granitic hillslopes

The preceding section demonstrates that ERI is effective fordetecting soil thickness distribution on a granitic hillslopes. To exam-ine the reliability of ERI, we evaluated the results of the methodologyon three additional lines, another line on the same slope at site A andtwo lines in different watersheds (i.e., sites B and C) underlain withgranitic bedrock. Moreover, on the basis of the results, we examinedthe relationship between the ρ profiles and the weathering profilesof the observed slopes.

3.2.1. Site A: Nishi'otafuku-Yama Experimental WatershedFig. 8a shows the ρ profile of line A-2 in the Nishi'otafuku-Yama Ex-

perimental Watershed (site A, Fig. 1a) surveyed by ERI with an elec-trode spacing of 1.0 m, as well as the positions of the layer whereNh=100 (i.e., the soil–bedrock interface measured by CPT). Like line

A-1, the ρ distribution of line A-2 also indicated the existence of threelayers. The shallowest thin layer (layer 1) had low ρ of about5000 ohm-m or less, with depth varying from about 0.5 to 3 m. The sec-ond layer (layer 2) had relatively high ρ of about 5000 ohm-m or more,which included spotty anomalies with extremely high ρ>10,000 ohm-m with relatively low resistivities of 4000–5000 ohm-m filling gapsamong the anomalies. The third layer (layer 3) had ρ of500–4000 ohm-m. At site A, ρ of both the soil layer and surface bedrockindicated little change between lines A-1 and A-2 in spite of the largedifference in antecedent precipitation occurring over the 30 days be-tween ERI surveys at the lines (Table 2).

As shown in Fig. 8a, on line A-2, soil thickness ranged from 160 to267 cm. Fig. 8a includes the vertical distribution of the weatheringclasses of rock samples collected from borehole R3 at the 24-mpoint on line A-2 at site A (Fig. 1a). Fig. 8b,c similarly indicates theweathering class profiles for boreholes R2 and R1, respectively.These profiles include a stratigraphic weathering profile estimatedfrom each borehole log. At R3, we found heavily weathered granite(DH to DM classes) at depths of 0.2–12.7 m, estimated as zone IV,below the surface soil mantle of 0.2 m depth and a layer of less-weathered granite (CL to CM classes), estimated as zone III, belowthe heavily weathered granite (Fig. 8a). On the basis of the boreholelithological logs shown in Fig. 8, layers below the surface soil mantleswere considered to be composed of about 10 m of thick, heavilyweathered granite (D class), estimated as zone IV, underlain byweathered granite (CL class) estimated as zone III.

The soil–bedrock interface appeared well correlated with theboundary between layers 1 and 2 also along line A-2, although soilthickness was determined by penetration tests at only five points,fewer than for line A-1. The soil layers between the 0- and 15-m pointshad higher ρ values than those found for line A-2, where the ρ profiledid not clearly show the position of the soil–bedrock interface. Thiswas probably attributable to high grounding resistance because of un-consolidated talus deposits in the hollow of the catchment correspond-ing to the northwest end of line A-2. The distinctive contrast in the ρprofiles of layers 1 and 2 probably reflects the layered structure com-posed of a surface soil layer (zones VI to V) and D-class highly weath-ered granite (zone IV) along both lines A-1 and A-2. The intermediationof low resistivities between spotty, extremely high resistivities(>10,000 ohm-m), which prevent detection of the soil–bedrock inter-face with ERI, may reflect the existence of clayey materials on line A-2similar to line A-1. However, D-class heavily weathered granite was

Fig. 8. Profile of electrical resistivity (ρ) along line A-2 at the Nishi'otafuku-Yama Experimental Catchment (site A, Fig. 1a) surveyed by ERI with electrode spacing of 1.0 m, includingthe weathering-class profile observed in borehole R3 located on lines A-2 (a), R2 (b), and R1 (c) at site A (Fig. 1a). Black ‘x’ symbols show the positions where Nh=100 (i.e., thesoil–bedrock interface) as determined by CPT.

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distributed continuously in the vertical direction, which did not corre-spond to the boundaries between layers 2 and 3. Therefore, the con-trasting ρ values between layers 2 and 3 could not be interpretedbased solely on the granite weathering classification or profile.

Fig. 9. Profile of electrical resistivity (ρ) along line B at the Fudoji Experimental Catchmenweathering-class profiles observed at borehole F1 (Fig. 1b) (b). Black ‘x’ symbols show thetip of inverted triangle in Fig. 9b indicates the position of the groundwater table in the bor

3.2.2. Site B: Fudoji Experimental WatershedFig. 9a shows ERI results from the Fudoji Experimental Watershed

(site B, Fig. 1b). Similar to Nishi'otafuku-Yama (site A), the ρ distribu-tion at site B indicated the existence of three soil layers. Resistivity

t (site B, Fig. 1b) surveyed by ERI with electrode spacing of 1.0 m (a), including thepositions where Nh=100 (i.e., the soil–bedrock interface) as determined by CPT. Theehole. GWL, groundwater level.

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values of the shallowest, thin layer (layer 1) were relatively low,ranging from 500 to 1800 ohm-m, and the thickness varied fromabout 0.3 to 3 m. Layer 2 had relatively high ρ values of about2000 ohm-m or more. Extremely high resistivities>2500 ohm-mwere observed in a spotty anomaly, with relatively low resistivitiesof 1000–2000 ohm-m. Layer 3 had relatively low ρ ranging from1000 to 1800 ohm-m.

As shown in Fig. 9a, soil thickness (the depth to the layer whereNh=100) ranged from 15 to 207 cm. The surface-soil interface corre-lated well with the boundary between layers 1 and 2. Consequently,the first layers of low resistivity (b1800 ohm-m) were thought to cor-respond to the surface soil layer. However, at some points, ERI did notclearly mark the position of the soil–bedrock interface. For example,the boundary between layers 1 and 2 was unclear at points around31 and 49 m. This is probably due to local intermediate materialswith different properties, similar to the area around point P7 on lineA-1.

Fig. 9b presents the vertical distribution of the weathering classesof rock samples collected from borehole F1, about 100 m northeast ofthe ERI line at site B (Fig. 1b). Fig. 9b includes a stratigraphic weath-ering profile estimated from the log. Core samples collected fromthe borehole showed that the stratum at site B was similar to thatat site A: a 2.4-m-thick surface soil layer estimated as zones VI andV underlain by a 2.7-m-thick heavily weathered granite layer (Dclass) estimated to be zone IV. Below the heavily weathered layer(b5.1 m) was a less-weathered layer (CL class) estimated as zone III.Although borehole F1 was about 100 m from the ERI line, the layerbelow the surface soil mantle on line B was considered to be com-posed of approximately 3-m-thick heavily weathered granite (DL toDH classes, i.e., zone IV) underlain by weathered granite (CL class,i.e., zone III). Similar to site A, the contrast in the ρ profiles of layers1 and 2 probably reflected a layered structure composed of a surfacesoil layer (zones VI to V) and D-class highly weathered granite (zoneIV) along line B. Nonetheless, it was difficult to interpret the entire ρprofile including layer 3 based solely on the granite weathering clas-sification or profile, as at site A.

3.2.3. Site C: Kiryu Experimental WatershedFig. 10 shows ERI results from the Kiryu Experimental Watershed

(site C, Fig. 1c). The ρ distribution indicated the existence of threelayers along almost the entire length of line C. The shallowest thinlayer (layer 1) had relatively high resistivity values of about1000 ohm-m or greater and varied in thickness from about 1 to 2 m.Layer 2 had relatively low resistivity values of about 700 ohm-m orless. The boundary between layers 1 and 2 was clearer than those atNishi'otafuku-Yama (site A) and Fudoji (site B). Layer 3 had ρ valuesof about 700 ohm-m or more.

Fig. 10. Profile of electrical resistivity (ρ) along line C at the Kiryu Experimental Catchmweathering-class profiles observed in boreholes K1, K2, and K3 located on line C. Black ‘x’mined by CPT. The tips of inverted triangles indicate the positions of the groundwater tabl

As shown in Fig. 10, the soil thickness (i.e., the depth to the layerwhere Nh=100) ranged from 20 to 126 cm, and the soil–bedrock in-terface was well correlated with the boundary between layers 1 and2. Consequently, the first layer of high resistivity (>1000 ohm-m)was thought to correspond to the surface-soil layer. In particular,note that ρ of the shallow bedrock located just below soil mantlewas lower than that of the soil mantle on line C, unlike on lines A-1,A-2, and B (Figs. 3, 8, and 9). In the following discussion (inSection 3.2.4), we will consider the characteristics of the ER profiles.

Core samples collected from the drilling borehole K3 at the 21-mpoint on the ERI line at site C (Fig. 10) revealed a heavily weathered,4.55-m-thick, D-class granite layer below the soil mantle. At K2 andK1, the D-class layers were 12.55 and 1.77 m thick below soil mantlesof 0.3 and 1.23 m thicknesses, respectively. Similar to both site A andsite B, the layers below the surface soil mantles are considered to becomposed of thick heavily weathered granite (D class), estimated aszone IV, underlain by weathered granite (CL class), estimated aszone III. However, the weathering profile of line C was more orderedthan at sites A and B, with the degree of granite weathering decreas-ing with downward depth.

Consequently, the contrast in the ρ profiles of layers 1 and 2 prob-ably reflects a layered structure composed of a surface soil layer(zones VI to V) and D-class highly weathered granite (zone IV)along line C, even though the low–high contrasts in ρ profiles at siteC are the opposite of those at sites A and B. Again, however, it was dif-ficult to interpret the entire ρ profile including layer 3 based solely onthe granite weathering classification or profile.

3.2.4. Evaluation of ER profiles based on borehole evidenceAs shown in Table 5, ρ of both soil and shallow bedrock just below

soil–bedrock interfaces varied in a relatively wide range in each site,as well as from site to site. Larger variations were especially foundin the surface bedrock layer, although there were significant differ-ences between the ρ values of the soil mantle and shallow bedrockin each site (i.e., sites A, B, and C, Fig. 1). This result is consistentwith the review by Giao et al. (2008), who found that the resistivityof granitic rock varies across a relatively wide range and considerablyfrom site to site (1.3–160×106 ohm-m). Previous studies have dem-onstrated that ρ values of several types of soil and rock are primarilyaffected by pore-water electric conductivity, effective porosity, de-gree of water saturation, and clay content (Archie, 1942; Patnodeand Wyllie, 1950; Chiba and Kumada, 1994; Nishimaki et al., 1999).These factors are also considered to complexly contribute, to varyingdegrees, to the wide variations in ρ values that we observed at sites A,B, and C.

Notably, ρ of the granitic bedrock in sites A and B were muchhigher than ρ of site C, even though shallow bedrock layers down to

ent (site C, Fig. 1c) surveyed by ERI with electrode spacing of 0.75 m, including thesymbols show the positions where Nh=100 (i.e., the soil–bedrock interface) as deter-es in the boreholes. GWL=groundwater level.

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about 5 mwere considered to be highly dried at both sites A and C be-cause of the low groundwater levels (Figs. 8 and 10) and dry anteced-ent conditions (Table 2). Accordingly, the ρ value of the soil mantlewas much lower than that of the shallow bedrock located justbelow the soil mantle at sites A and B, while the ρ value of the soilmantle was much higher than that of the shallow bedrock at site C;the pattern of ρ distribution reflecting soil and shallow bedrock layerswas opposite among the sites.

We consider the contrast in the ρ profiles of the surface soil andthe shallow bedrock located just below the soil mantle at site A(Fig. 1a) and site C (Fig. 1c), as well as the low-high contrast in ρ pro-files, to reflect differences in the soil–bedrock interface between thetwo sites, despite similar dry conditions. We base this suggestion onthe water retention characteristics of the soil and bedrock at eachsite, as explained below.

Fig. 11a presents the water retention curves (WRCs), i.e., the rela-tionship between water pressure head (ψ) and volumetric water con-tent (θ), for the soil and weathered granitic bedrock for some coresamples collected in boreholes R1 and R2 at site A. Fig. 11b showsthose corresponding to the soil and bedrock collected in boreholesK1, K2, and K3 at site C. All retention curves shown in Fig. 11 weremeasured by Katsura et al. (2009). WRCs represent the number of mi-cropores and macropores in soil (Kosugi, 1994) and bedrock (Katsuraet al., 2009). The WRCs of the D- and C-class bedrock at sites A and Cexhibited little change in θ especially in the range of ψb−200 cm H2-

O, which is less than 0.3 for whole range of ψ. However, the θ values ofbedrock in the range of ψb−200 cm H2O tended to increase as afunction of weathering progression, reflecting an increase in thenumber of micropores (Katsura et al., 2009). In contrast, the WRCsof the soil at sites A and C were characterized by extremely largechanges in θ especially in the range of ψ>−200 cm H2O, comparedto weathered bedrock, reflecting a larger number of soil macropores.However, the soil θ of site A in the range of ψb−200 cm H2O wasmuch larger than that of site C which was comparable to values ofthe other bedrock samples of sites A and C. This result suggests thatthe soil of site A contains more micropores due to the higher percent-age of clayey material, unlike the soil of site C from which some of theclayey material might have been leached out by rain water. Accordingto Salem (2001), clay minerals have the property of significantly de-creased resistivity of ground material, as explained in terms of elec-trochemical principles. Thus, WRCs provide information on twoaspects of ground material properties, both closely related to the ρvalue: (i) the variation in θ in response to ψ and (ii) the number ofmicropores, which indicates the percent content of clayey materialwithin ground materials.

At site A, the much lower ρ of the soil mantle (b5000 ohm-m)compared to the ρ of shallow bedrock (>5000 to 10,000 ohm-m)

Fig. 11. Mean water retention curve for soil and weathered granitic bedrock of D and CL class(a) R1 and R2 in Nishi'otafuku-Yama (site A, Fig. 1a) and (b) K1, K2, and K3 in Kiryu (site C

can be explained by Archie's equation (Eq. (1)), i.e., the effect of thehigher water content, and the electrochemical effect of clay conduc-tivity. At site C, θ of DL-class bedrock is higher than that of the soilin unsaturated conditions except for the considerably low suctionrange (0 to −55 cm H2O; Fig. 11) at the core scale. It is highly possi-ble that this difference in θ explains the difference between the ρ pro-files of shallow bedrock regions (ρb700 ohm-m) and those of the soilmantle (ρ>1000 ohm-m), following Eq. (1). This is supported by thefact that regions just below the soil–bedrock interface are consideredto be homogeneously composed of DL-class weathered granite, basedon the borehole logs of K1, K2, and K3 (Fig. 10) and the extremely dryconditions resulting from the small antecedent precipitation at site C(Table 2). Thus, there is the possibility that the focus of this study,variations in ρ clearly reflecting interfaces between soil and weath-ered granite just below the soil mantle, can be explained by thewater retention characteristics of the ground materials.

However, there is still a limit to the interpretation of ρ profiles, in-cluding those of deep bedrock regions (layer 3), based on water re-tention characteristics at the core scale. Indeed, the ρ value of layer3 was significantly lower than that of layer 2 at sites A (Figs. 3 and8) and B (Fig. 9), although a deeper lying and less-weathered bedrocklayer with fewer micropores would be expected to have a ρ valuehigher than that of shallower zones. At site C, the top of layer 3, cor-responding to the region probably composed of DM-class bedrock,had a lower ρ value than layer 1, although the opposite result wasexpected. These inconsistent and irregular ρ profiles clearly indicatethat parameters such as clay content and water content do not inde-pendently affect the ρ value; rather, the structure and composition ofthe ground materials and the amount of fluid seem to complexly con-tribute to determining the ρ value. Another possible explanation forthe lower ρ of layer 3 than of layer 2 at sites A and B is that local dis-tributions of clay-rich gouges or fluid-filled cracks are indistinctlyreflected in the ρ profiles; such localized anomalies may explain thedecreased ρ value of the entire region corresponding to layer 3. Thisassumption is supported by resolution decreases in the ρ profilewith increasing depth, which is in turn related to the measuring prin-ciple of ERI (Singha and Gorelick, 2006; Schrott and Sass, 2008).

4. Conclusions

We validated the capability of electrical resistivity imaging (ERI)in detecting soil thickness distributions on hillslopes in granite water-sheds by comparison with cone penetration tests (CPT) along fourlines in three weathered granitic watersheds: lines A-1 and A-2 atsite A (Nishi'otafuku-Yama), line B at site B (Fudoji), and line C atsite C (Kiryu). Along line A-1, the ERI method was evaluated usingthree different electrode spacings; also two types of seismic surveys

es derived from the hydraulic properties test using core samples collected in boreholes, Fig. 1c), which were compiled from data of Katsura et al. (2009).

68 Y. Yamakawa et al. / Geomorphology 145–146 (2012) 56–69

(SS), seismic refraction (SR, for the Vp profile) and the surface wavemethod (SWM, for the Vs profile), were conducted for comparisonwith ERI. The low–high contrasts in electrical resistivity (ρ) by ERIsuccessfully reflected the soil–bedrock interfaces along all of the ob-served lines, which were detected by CPT and estimated as the inter-face between the residual soil layer and the layer primarily composedof DL-class (highly weathered) and less-weathered bedrock (corre-sponding to the interface between zones V and IV). However, the ρvalues of both soil and shallow bedrock just below the soil–bedrockinterfaces varied over a relatively wide range within and betweensites. Furthermore, the patterns of low-high contrast in ρ profiles dif-fered between sites A and C, despite the similarly dry conditions oftheir watersheds, especially involving the soil and shallow bedrocklayers. The differences between the two sites could be explained bydifferences in water retention characteristics, as determined usingwater retention curves (WRCs) that indicate the water contentunder dry conditions and the percent content of clayey material insoil, measured at a core scale in a previous study (Katsura et al.,2009). Nonetheless, we were unable to reliably interpret the entireρ profile, including deeper bedrock regions, based on WRCs at thecore scale.

Although the ERI method with electrode spacing of 0.5 to 2.0 mcould appropriately detect soil thickness distributions ranging fromabout 50 to 300 cm, smaller electrode spacing are expected to moresensitively depict the distribution of ground material properties and,consequently, to more accurately replicate the soil–bedrock interface.ERI failed to clearly identify the soil–bedrock interface at some pointsalong our measurement lines because of local intermediate materialswith different properties, such as clayey intermediation probably dueto fault fracturing just below the soil–bedrock interface and unconsol-idated soil causing high grounding resistance.

The Vs profile determined by SWM, with larger variation in lateraldirections, reflected the soil–bedrock interface more appropriatelythan the Vp profile obtained by SR and successfully detected the inter-face in regions where ERI failed to do so because of clayey intermedi-ations in shallow bedrock regions. However, SWM also failed toaccurately estimate soil thickness, with an error of approximately 1to 1.5 m in some locations along line A-1, whereas the ERI profile suc-cessfully reflected the soil–bedrock interface.

On the basis of the present results, we conclude that ERI is a highlyeffective geophysical technique for investigating the soil thicknessdistribution in granitic hillslopes. Furthermore, the combined use ofERI and SWM, by compensating for the respective deficiencies ineach method, could be more effective than either method alone indetecting the soil–bedrock interface.

Acknowledgments

This work was partly supported by grants from the Foundation ofMonbukagakusho for Scientific Research (22248018) and Foundationof River & Watershed Environment Management Foundation of Mit-sui & Co., Ltd. for Environmental Research.

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