Bulletin of the Geological Survery of Japan, vol. 53 (2/3), p. 253-263, 2002

Interpretation of DC resistivity data in the Bajawa geothermal field, central Flores, Indonesia

Toshihiro UCHIDA 1, Achmad ANDAN2 and ASHARI~1

Toshihiro UCHIDA, Achmad ANDAN and ASHAHI (2002) Interpretation of DC resistivity data in

the Bajawa g巴othermalfield, c巴ntralFlor巴s,Indon巴sia.Bull. Gθol. Surv. Japan, vol. 53 (2/3), p.

253 263, 8 figs.

Abstract: We have performed two-dimensional (2-D) interpretation of the Schlumberger sound-

ing and mapping data that were obtained by the Directorate of Mineral Resources Inventory,

Indonesia in the Bajawa g巴othermalfield, central Flores Island, eastern Indonesia. There ar巴

three major surface geothermal manifestations, Mataloko, Nage and Bobo, in the study area.

Th巴 aim of th巴 Schlumb巴rg巴rsurvey was to investigate th巴 d日日presistivity structure for explo

ration of g巴othermalr巴servoirsin these areas. The 2 D r巴日istivitymodels wer巴 obtained for ten,

two and four survey lines in Mataloko, Nage and Bobo, resp巴ctively.Resistivity mod巴lsof most

lines have a surface high-resistivity layer that corresponds to less-altered young volcanic rocks,

and a low-resistivity thick second layer that corresponds to alteration zones where conductive

clay minerals might be abundant. A high-resistivity deep layer (resistivity basement) was recog-

nized at a few zon巴sincluding th巴 Matalokomanifestation zon巴.The r巴sistivitystructure at the

Mataloko manifestation, which is characteriz巴dby a very low-resistivity lay巴rabove the high-

r巴日istivity bas巴m巴nt,sugg巴st巴d an exist巴nc巴 of a potential g巴othermal r巴日巴rvoir system.

However, the inv巴stigationdepth of the Schlumberger measurement, with a maximum electrode

spacing AB/2 of 1,000 m or 2,000 m, was not sufficient to detect another d巴巴phigh-resistivity

layer that indicates an existence of geothermal reservoir.

1. Introduction

Electric and electromagnetic methods are inten

sively applied in geothermal exploration to delineate

the resistivity structure of geothermal reservoirs.

The reason is that the resistivity structure is one of

the most important factors for understanding the

structure of geothermal reservoirs. Schlumberger

sounding is the most popular method among DC

resistivity methods that have been used for this

purpose in the past few decades, mainly because of

its large investigation depth.

Th巴 DCr巴sistivitysurv巴yswer巴 carriedout by

th巴 Directorate of Min巴ral Resourc巴s Inventory,

Indonesia (DMRI) in the Bajawa geothermal field,

central Flores, eastern Indonesia, in 1997 (Andan et al., 1998) and 1998. The surveys consisted of

Schlumb巴rgersounding, Schlumberg巴rmapping and

h日adon r巴sistivityprofiling. W巴 hav巴 conducteda

1 Institute for Geo-Resources and Envi1 onment, GSJ 2 Di1ectorate of Mineral Resources Inventory, Jl. Soekarno-Hatta No.444, Bandung-, 40254 Indonesia

九 Di1ectorate of Volcanology and Geological Hazard 孔1itig・ation,Jl. Diponeg・oro No.57, Bandung, 40122 Indonesia

-253-

two dimensional (2 D) inversion of the Schlumberger

data and compared the resistivity models with the

results of magnetotelluric (MT) surveys (Uchida et

al., 2002) and other geoscientific studies for the in

terpretation of geothermal reservoirs in the area.

2. Data

The Baiawa geothermal field is located in the cen

tral part of the Flores Island, Nusa Tenggara Timur (NTT) Province, eastern Indonesia (Fig. 1).

The survey area is on a high land whose average

elevation is approximately 1,000 m. The area is un

derlain mostly by young volcanic formations

(WestJec and MRC, 2000; Muraoka et al., 2000). An

active volcano, Mt. Inerie, is located west of the sur-

vey area. There are num巴roussmall volcanic cones

that are younger than 0.5 Ma. Three major surface

g巴othermal manifestations, Mataloko, Bobo and

Nage, ar巴 locatedin th巴 studyarea. Th巴 Schlum-

berger surv巴yswere conduct巴d to inv巴stigatethe

structure of these manifestation areas.

Figure 1 shows the Schlumberger sites that were

Keywords: DC resistivity method, Schlumberger array, 2-D interpretation, resistivity, geothermal reservoir, Ba.iawa geothermBl field, Flores Island, Indonesia

Bulletin of the Geological Survey of Japan, vol. 53 (2/3), 2002

used for the 2 D interpretation. Among the ten sur

vey lines in the Mataloko area, Lines B, C and D

are approximately in the east west direction, and

the rest are oriented approximately in the north

south direction. There are 19 Schlumberger sound

ing stations. The current electrode spacing, AB/2, for the sounding varies from 1.6 m to 2,000 m.

There are 81 mapping sites including the sounding

stations. The interval of the mapping sites is 500 m

along the survey lines. The electrode spacing AB/2 for mapping are 250, 500, 750 and 1,000 meters. The

巴xpansionof the巴lectrod巴 arrayat each station is

E車内

along the survey line for both sounding and map

ping. The subsequent magnetotelluric survey in 1999

was more concentrated in the southern half of the

Schlumberger survey area (Fig. 1; Uchida et a.l.,

2002). In the Nage area, two survey lines were inter

preted. There are two sounding stations whose elec

trode spacing varies from 1.6 m to 1,000 m. There are eight mapping sites including the sounding sta-

tions. The electrode spacing for mapping is the

same as the Mataloko area. In the Bobo area, four surv巴ylines w巴r巴 inter

・1・21"匝隠4『t

a

w

a

a

B S回目

融 Id皿mberg町•. s岨岨昭 oMT Q 'MAlpp

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言；zsmo -I ~ 民雄ケ 】 － ~oニ也 、可b 司自宅込内 11 11岨開

言 ..._4i)可~ Q曹司 11il団凪

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Fig. 1 Location of the Schlumberger mapping and sounding sites in the Bajawa geo-thermal field. Top figure shows the Flores Island and neighboring islands, eastern Indonesia. In the bottom figure, solid black circles are Schlumberger sounding sites, open black circles ar巴 Schlumbergermapping・ sites, and blue circles are MT stations The background is topography contours, white squares are surface manifestations, and black lines are major local roads.

-254-

Interpretation of DC resistivity data in Bajawa, Indonesia (UCHTD.fl. 告tal.)

preted. There are ten sounding stations, whose elec

trode spacing varies from 1.6 m to 1,000 or 2,000 m.

The number of mapping sites is 37 including the

sounding stations. The electrode spacing for map

ping is the same as the Mataloko area. The field instruments used for the data acquisi

tion are a DMRI designed transmitter for sending

current into the earth and an analog chart recorder

for measuring the potential difference on the sur-

face.

3. 2-D Inversion Scheme

We have applied 2 D inversion to ten survey lines

(B, C, D, F, H, J, K, L, M and N) in Mataloko, two

lines (AA and CC) in Nage, and four lines (Bl, B2,

B3 and B5) in Bobo (Fig. 1). All the sounding data and mapping data along these survey lines were

used for the inversion.

In the Schlumberger sounding measurement, we

usually have several segments of apparent resistivity

curves for one sounding, which correspond to differ

en t potential electrode spacings，λ！［N/2.羽Teshifted those segments along the apparent resistivity axis

of the sounding curve toward the larger MN/2 seg

ments to produce a single continuous sounding

curve.

The 2 D inversion method used here is based on

Uchida (1991, 1993a, 1993b). It utilizes the conven tional linearized least squares scheme and applies a

smoothness regularization for stabilizing the ill

posedness of the problem. The minimization of data

misfit and model roughness are simultaneously

achieved by introducing the Bayesian likelihood

(Uchida, 1993a, 1993b). The forward modeling utilizes the finite-element

method. Therefore, topography can be incorporated

in the modeling. The elevation data of the map

ping/sounding sites were provided by the DMRI's

geodesy survey. Starting with a homogeneous earth

of 10,000 ohm m with the topography as an initial model, the iterative inversion procedures almost con

verge after several iterations.

4. 2-D Interpretation

4.1 Mataloko Area Figure 2 shows 2-D models of the Lines B, C, D

and F. Figure 3 shows observed and modeled sound-

ing curv巴sof Lin巴S B, C and D. Note that th巴 reli-

ability of r巴sistivity distribution b巴neath the

mapping sit巴smay b巴 poorin both shallow (less

than 150 m depth) and deep portions (greater than 600 -700 m depth), because those sites only have

four different electrod巴 spacings.Resisti vi ties of

these portions were determined in conjunction with

th巴 dataof th巴 n巴ighboring sounding sites. Th巴

-255-

quality of the data with large electrode spacings is

sometimes poor because of rough terrain and low

signal levels. However, as we can recognize in Fig.

3, the fitting between the observed and calculated

apparent resistivities is very fine for both mapping and sounding sites.

Resistiviy structure of these lines can be summa

rized as follows.

• Almost all sites are underlain by a surficial resis

tive layer of approximately 50 300 ohm m. The

thickness of this resistive layer gradually changes from site to site, from less than 100 m to ap

proximately 500 m. The surficial resistive layer is

very thin beneath Sites C 51 and C 55 on Line C,

where the surface manifestation is located.

• Below this resistive layer is a conductive layer

that is distributed along all the lines. Resistivity of this conductive layer is mostly below 10 ohm-

m. Very conductive anomalies are seen beneath

sites in the eastern part of Line C and western

part of Line B.

• There is a high resistivity basement beneath the

eastern half of Line C. The sounding curves at Sites C 40 and C 51 clearly support this model

(Fig. 3). Other lines do not indicate the existence

of the high resistivity basement.

Figure 4 shows 2 D models of six north south

lines at the Mataloko area. The data of Lines H and N were simultaneously inverted. Resistivity struc

ture of these lines are similar to the west east lines

in Fig. 2. Most sites have a high resistivity surface

layer and low resistivity second layer. Curves at the

sounding sites K-30, K-40 and N-30, indicate the ex-

istence of a high-resistivity basement. Other sound-ings do not show such resistivity curves. A small

alteration zone was found between K-25 and K-30

by a surface geological survey (WestJec and MRC,

2000). Smectite was identified by X ray analysis of

a rock sample from the surface. The very low

resistivity anomaly may be associated with hydro thermal activity that created the alteration zone.

Figure 5 shows depth slice sections of a three

dimensional (3 D) inversion model of MT data at

Mataloko (Uchida et al., 2002). Figure 6 compares

Schlumberger 2 D models of Lines B, C and D with

the corresponding cross sections extracted from the MT 3-D model in Fig. 5. Line C crosses the

Mataloko manifestation zone, while Lines B and D

are locat巴dnorth and south of the manif巴station,resp巴ctiv巴ly.

The surficial high-resistivity layer is distribut巴d

along these three lines, except at the location of the manifestation. These features are easily detected by

both Schlumberger and MT models. However, this

resistive layer is generally thicker in the Schlum

berger mod巴lsthan th巴 MTmodels. Two possi bl巴

Bulletin of the Geological Survey of Japan, vol. 53 (2/3), 2002

reasons are; 1) the MT signal, with the highest fre

quency of 120 Hz (Uchida et alリ 2002),is less sensi

tive to the surface high resistivity layer and hence

the layer was not properly interpreted in the inver

sion, and 2) both Schlumberger and MT apparent

resistivities are contaminated by static shifts or

other biases and they were not satisfactorily ex

plained by the 2-D or 3-D models.

In the MT model, the low-resistivity second layer

is distributed beneath all three lines, and it is un-

derlain by a high-resistivity basement layer. The

low r巴sistivitylayer is thin in th巴 巴ast巴rnpart and

gradually incr巴asesto the W巴st. How巴ver,in the

【叫

1

刷附

｛同l Une手ヨ 掴副 al ~ rl 占

品目

m..111 ~ .. 車且j ll>

Schlumberger model the high resistivity basement

layer is seen only beneath Line C. Resistivity and

shape of the low resistivity layer of Line C is simi

lar between the Schlumberger and MT models. As

we can recognize in the MT models (Fig. 5), the

high resistivity basement is the shallowest, approxi

mately a depth of 600 m, at Line C near the mani

festation, if we define a layer of several tens of

ohm-m is resistive. Therefore, the maximum elec-

trode spacing AB/2 of 2,000 m was enough to detect

the high-resistivity basement at Sites C-40 and C-51.

On the other hand, the high resistivity bas巴m巴nt

can not be obs巴rvedin the Schlumb巴rg巴rmod巴lsfor

itbl

[ 司Mt戸1ヰ

岡...

Fig. 2 The 2 D models of the Schlumberger data in Mataloko for (a) east west lines: B, C and D, and (b) NW-SE line (Line F). Black site numbers indicate sounding sites, while blue numbers are mapping sites. No vertical exag・g・eration is applied.

Interpretation of DC resistivity data in Bajawa, Indonesia (UCHTD.fl. 告tal.)

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Fig. 3 Schlumberger sounding curves of Lines B, C and D with observed data (open circles) and theoretical values obtained from the models in Fig. 2a (Solid lines).

Lin巴S B and D. The low resistivity second layer is

very thick in th巴町 models. The d巴pthof the bas巴

m巴ntin the MT model is approximat巴ly800 m in

th巴巴ast巴rnpart of Lin巴日 B and D. Although the

basement is deeper only by a few hundred meters

than Line C, the Schlumberger investigation depth

is not sufficient to d巴tectthe basement beneath

Lines B and D. If w巴 havethe data with a larger

巴！日ctrodespacing on th巴setwo !in巴s,w巴 mayhave

-257-

obtained similar models as Line C, by havmg a

more conductive thinner low r巴sistivity layer.

4.2 Nage Area

Figure 7 shows 2-D models of Lines AA and CC

m the Nage area, together with the MT model of

the Bobo-Nage line (Uchida et al., 2002). Since the

maximum current electrode spacing AB/2 is 1,000

m, we only have a shallow inv巴stigationd日pth.

Bulletin of the Geological Survey of Japan, vol. 53 (2/3), 2002

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Fig. 4 The 2 D model日 ofthe survey lines in the N S direction in Mat>iloko: Lines J, K, L, M, H and N.

The data of Lines H and N were inverted simult>meously. Black site numb巴rsindicate soundin呂、日ites,

while blue numbers are mapping sites.

-258-

Interpretation ol DC resistivity data in Bajawa, Indonesia (UCHTD.fl. 告tal.)

View

6

4

4

4

司

tha

－－h

宅4

10舶

l官！開

HI

。司I 2 3, 誕(k・

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Fig. 5 Depth slic巴 sectionsof the MT 3-D model at four depth levels (Uchidaθt al., 2002). Black dotR ar日 MTstations, and thick g-ray lines are the Schlumberger lines. In the upper-left figure, the

white line indicates the zone of surface alteration, and purple dashed rectangfos indicate the blocks for th巴 sectiondisplay of the 3-D mod巴lin Fig. 6.

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’＇it~ ::c 1[ki司 叫同自 X柑m,

Fig. 6 Comparison of Schlumberger 2 D models of Lines B, C and D in Mataloko with the corresponding cross sect10ns of the MT 3-D model. Location of the sections are shown in Fig. 5. Not巴 thatthe direction of th巴 Schlumberg・erlines is not exactly east-west.

Bulletin of the Geological Survey of Japan, vol. 53 (2/3), 2002

(a) MFU,

~ : Si ; II:

E曲::-- c＝~ ー ード i

ill ii!Ai u 3,l':i .c.ciι!I lilll 図面回白骨

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Fig. 7 (a) The 2 D models of the Schlumberger lines in Nage, (b) the 2 D model of th巴 MTdata on the Bobo Nag日 line(Uchidaθt且1.,2002). Line AA is in the NS direction Line CC is in the WSW ENE direction and the MT line is in the NW SE direction. In the Schlumberger models, black site numbers indicate sounding sites, while blue numbers are mapping sites.

The resistivity of Line AA is generally low. The

minimum resistivity is found at a 200 300 m

depth, and the resistivity increases below that

d巴pth.However, th巴 minimumresistivity is approxi

mately 10 ohm m, which is not as low as Line C in

Mataloko. Line CC is mostly underlain by a resis-

tive layer except at Site CC-20.

Line AA crosses the MT line b巴tweenSites 208

and 209, and Line CC crosses at around Site 212

(Fig. 1). In the MT model, the shallow low-

resistivity anomaly is v巴rysmall at th巴 Nagemani-

festation zone, and resistivity below sea level is high

in the eastern part of the MT line. These results

correspond well to the models of Lines AA and CC.

The shallow low resistivity anomaly at Site 212 in

th巴 MT model is similar to Sit巴 CC25 in th巴

Schlumberger model.

Although the investigation depth of the Schlum

berger data is limited, the Schlumberger models are

very consistent with the MT 2 D mod巴land repre

senting the shallow resistivity features of th日 Nage

area very well. It is recognized from both Schlum-

berger and MT models that there are only small

low-resistivity anomalies beneath the Nag巴 area.

4.3 Bobo Area

Figur巴 8shows four 2-D sections in th巴 Bobo

area; three survey lines are in the SW-NE direction

and one line (Line B5) in th巴 E-Wdirection. Almost

all the sit巴shave a resistive surface layer and con-

ductive second layer. One exception is that a con

ductive anomaly is seen from th巴 surfaceat Site 30

Interpretation of DC resistivity data in Ba)awa, Indonesia (UCHTD/1 告tal.)

4叫 Llne-B11

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~ 141 u 晶且 &.!! Q IU 11..1;1 a喧

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EaE－－M

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(b)

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Fig. 8 The 2 D models of the Schlumberger data in Bobo for (a) SW NE lines: Bl, B2 and B3, and (b) east-west line (Line B5). Black site numbers indicate sounding・ sites, while blue numbers are mapping sites.

of Line B2, wh巴r巴 th巴 Bobomanifestation is located.

This anomaly is also recognized in the MT model in

Fig. 7. Central portions of the Bobo lines are on young

volcanic cones. The resistive surface layer is associ-

ated with non altered volcanic formations near the

surface. Th巴 lowresistivity second layer is possibly

indicating alteration zon巴s,which is the same as th巴

resistivity models in the Mataloko area. However,

the resistive bas巴mentwas not recognized in the

Bobo area because the investigation depth of the

Schlumb巴rgersoundings was not suffici巴nt.

5. Preliminary Interpretation

and Conclusions

When we apply electric and electromagnetic sur-

veys in geothermal exploration, the exist巴nceof a

g巴othermalr巴servoiris typically characteriz巴dby a

S巴qu巴neeof a very conductiv巴 shallow layer that

corresponds to clay cap and a resistive basement

layer that corresponds to the reservoir zone.

The Schlumberger surveys in the Bajawa area r巴－

vealed a low-resistivity second lay巴rbeneath all sur-

vey lin田 in Matsloko and Bobo. Howev巴r, most

-261-

Bulletin of the Geological Survey of Japan, vol. 53 (2/3), 2002

sounding sites did not have sufficient investigation

depth to detect the resistive basement beneath the

low resistivity clay cap, although the maximum

electrode spacing AB/2 was 1,000 m or 2,000 m.

An obvious exception is the eastern part of Line C in the Mataloko area. A resistivity structure with

a low resistivity second layer and high resistivity

third layer were found. A pilot drilling that was

conducted in 2000 to a depth of approximately 200

m indicated the existence of smectite in a shallow

zone of the area (WestJec and MRC, 2001). The low-resistivity layer corresponds to the clay rich zone.

Also, smaller but similar resistivity patterns are

recognized on Line K and Line N in the Mataloko

area. A very low resistivity layer is interpreted be

neath Line K, where smectite was found by an

analysis of a rock sample from the surface altera tion zone (WestJec and MRC, 2000). This may be an

indication of another hydrothermal system other

than that in the central Mataloko beneath Line C.

The pilot drilling in the Mataloko manifestation

area successfully produced steam from a shallow

zone in 2001 (WestJec and MRC, 2001). The inter-pretation of a low-resistivity clay-cap and high-

resistivity reservoir zone is applicable to the eval

uation of the geothermal resources in this area.

Therefore, the Schlumberger models have taken an

important role in the integrated interpretation of

the reservoir system in the Mataloko area.

Acknowledgments: The authors are grateful to

DMRI for giving them permission to use the

Schlumberger field data in the Baiawa area in this

work.

References

Andan A. Suhanto E. Sukirman A. Ashari

and Usmawardi (1997) Rθ'POrt on integrated geophysical investigation of乱1atalokogeo

-262-

thθI刀wlarea, Ngada 2nd Rθ•gency, Nusa Tengg・ara Timur Province (in Indonesian,

translated to English). Volcanological

Survey of Indonesia.

Muraoka, H., Nasution, A., Urai, M., Takahashi, M. and Takashima, I. (2000)

Regional geothermal geology of the Ngada

District, central Flores, Indonesia. Proc.

World Geothermal Congress 2000, 1473-1478.

Uchida, T. (1991) Two-dimensional resistivity in-

version for Schlumberger sounding. Geophys. Explor. (Butsuri Tansa), 44, 1 17.

Uchida, T. (1993a) Smoothness constrained 2D

inversion for DC resistivity data by ABIC

minimization method (in Japanese with English abstract). Geophys. Explor. (Butsuri

Tansa), 46, 105 119. Uchida, T. (1993b) Smooth 2 D inversion for

magnetotelluric data based on statistical

criterion ABIC. J. Geomag. Geoelectr., 45,

841-858.

Uchida, T., Lee, T. J., Honda, M., Ashari and

Andan, A. (2002) 2-D and 3-D interpretation of magnetotelluric data in the Baiawa geo-

thermal field central Flores Indonesia. Bulletin of Geological Survey of Japan, 53,

265 283.

WestJec and MRC (2000) FY 1999 Report on ＇’

Joint Research on Exploration of Small Scale Geothermal Reservoirs in Remote Islands in Eastern Indonesia”（in Japanese),

277p.

WestJec and MRC (2001) FY 2000 Report o刀

”'Joint Research on Exploration of Small-Scale Geothermal Reservoirs in Remote Islands in Eastern Indonesia”（in Japanese), 182p.

Received October 12 2001

Accepted February 21, 2002

Interpretation of DC resistivity data in Bajawa, Indonesia (UCHTD.fl. 告tal.)

インドネシア東部フローレス島パジャワ地熱地域における比抵抗法データの解析

内田利弘・ AchmadANDAN • ASHARI

要 ヒエ，目

インドネシア東部に位置するフローレス島パジャワ地熱地域においてインドネ シア鉱物資源調査局が

取得したシュ ランベルジャ法電気探金データについて 2次元解析を実施し，地熱貯留層に闘する考察を

行った．当該地域にはマ タロコ，ナゲ，ボボといった比較的規模の大きい地表徴候地がある．電気探金

はそれらの場所における貯留層構造の解明を目的として行われた.2次元解析はマタロ コ地灰 10測線，

ナゲ地区 2損rj線，ボボ地区4測線に士、lして行った．大部分の測線では，l也下浅部に高比抵抗層が分布 し，

これは変質をあまり受けていない新期火山噴出物層に対応するものと解釈される．その下部には厚い低

比抵抗層が広がっており，これは低比抵抗の粘土鉱物に富む層であると判断される マタ ロコ徴候地を

含むいくつかの箇所では，深部に高比祇抗基盤を捕らえた．マタロコ徴候i也では，低比抵抗層の比抵抗

は非常に小さく ，高比抵抗基盤が存在するこ とを考慮すると，下部に有望な地熱貯留層が存在するもの

と推測される しかし，本シュラ ンベルジャ法電気探査の最大電極間隔 AB/2は1000m あるいは2000

m であり， 探査深度は十分と はいえず他の場所で地熱貯留層の存在を示唆する高比抵抗基盤の存在を

確認することはできなかった

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