Bull. Volcanol. Soc. Japan
Vol. /0 (,*++) No. . · /, pp. +/-ῌ+0*

LetterControlled Source Audio-frequency Magnetotelluric (CSAMT) andTime Domain Electromagnetic (TDEM) Resistivity Measurements
at Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan
Yoshihiko GOTOῌ and Akira JOHMORIῌῌ
(Received February +*, ,*++ ; Accepted August 3, ,*++)
Controlled source audio-frequency magnetotelluric (CSAMT) and time domain electromagnetic (TDEM)
resistivity measurements were performed in April ,**2 at the Noboribetsu Geothermal Field, Kuttara Volcano,
Hokkaido, Japan. Both sets of measurements were carried out using a high-precision electromagnetic system,
Geo-SEM, controlled by GPS (Global Positioning System). The ,ῌ, km survey area covered the entire
geothermal ﬁeld and included 00 measurement sites. Interpretation of the CSAMT and TDEM data revealed thesubsurface resistivity structure shallower than +,.** m below sea level (b.s.l.). The most prominent feature of theresistivity structure is a region of low resistivity (῍+* W · m) beneath the geothermal ﬁeld. The low resistivity
varies in lateral extent and outline at di#erent depths. At ,** m above sea level, it comprises two domains withlong axes oriented NNWῌSSE. From * to ,** m b.s.l., the two resistivity lows combine, forming a large
semicircular low of +,/**ῌ+,/** m in lateral extent. From .** to 0** m b.s.l., the low resistivity has an irregularoutline and includes linear low-resistivity zones trending NNWῌSSE and ENEῌWSW. From 2** to +,.** m b.s.
l., a linear NNWῌSSE-trending region of low resistivity is apparent in an irregular overall pattern of resistivity.
In a NῌS vertical cross-section, the region of low resistivity extends vertically for more than +,.** m. We
attribute the low resistivity beneath the geothermal ﬁeld to the presence of conductive clay minerals produced byhydrothermal alteration, which was in turn induced by high-temperature geothermal ﬂuid ascending along
fractures.
Key words : resistivity structure, controlled source audio-frequency magnetotelluric (CSAMT) survey, time
domain electromagnetic (TDEM) survey, Noboribetsu Geothermal Field, Kuttara Volcano+. Introduction
Resistivity surveying provides valuable informationon underground geological structures at active volcanoes
and within geothermal ﬁelds (e.g., Risk et al., ,**- ;Aizawa et al., ,**2 ; Srigutomo et al., ,**2). Suchdata are particularly useful for understanding hydrothermal systems, as the resistivity of volcanic rocks
shows a marked change with the presence of alteration
minerals and thermal waters. Resistivity data are
therefore essential for scientiﬁc studies of volcanic activity and for geothermal exploration.
We performed controlled source audio-frequency magnetotelluric (CSAMT) and time domain electromagnetic (TDEM) resistivity measurements at Noboribetsu
Geothermal Field, Kuttara Volcano, Hokkaido, Japan.
The CSAMT method employed here used electric currents at frequencies from + to 2+3, Hz. The TDEM
῎College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology,
Mizumoto-cho ,1ῌ+, Muroran, Hokkaido */*ῌ2/2/, Japan.
῎῎Neo Science Co., Ltd., Tarui .ῌ,ῌ-*, Sennan, Osakamethod employed here was the long o#set transient
electromagnetic method (LOTEM ; Strack, +33,). The
Noboribetsu Geothermal Field is one of the major geothermal ﬁelds in Japan, and resistivity measurements area promising tool for studying subsurface geologic structures within the geothermal ﬁeld and for examining its
hydrothermal systems. Sixty-six receiver stations were
distributed in a ,ῌ, km area covering the whole geothermal ﬁeld, and processing of the CSAMT and
TDEM data revealed the subsurface resistivity structure
shallower than +,.** m below sea level (b.s.l.). This
paper describes the results of the resistivity measurements and discusses the nature of subsurface geologic
structures beneath the geothermal ﬁeld.
,. Noboribetsu Geothermal Field
The Noboribetsu Geothermal Field lies in the western/3*ῌ*/,+, Japan.
Corresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp

154

Yoshihiko GOTO and Akira JOHMORI

Fig. +. Location map of the Noboribetsu Geothermal Field at Kuttara Volcano, Hokkaido, Japan, showing thelocations of the transmitter (line AῌB) and receiver stations (red circles) for CSAMT and TDEM measurements. A and B are the ends of the dipole source. Topographic contour interval is +* m.

part of Kuttara Volcano (Fig. +), an andesitic to rhyolitic composite volcano (elevation, /.3 m above sea level)with a small caldera at its summit (Lake Kuttara).
The volcano evolved over the period 2*ῌ./ ka, involving
early silicic explosive activity and subsequent stratovolcano building associated with caldera collapse at .*ka (Katsui et al., +322 ; Yamagata, +33. ; Moriizumi,+332). The geothermal ﬁeld, which is inferred to haveformed after the collapse of the caldera (Katsui et al.,+322), is approximately + km wide (NEῌSW) and +./km long (NWῌSE), and occurs at an altitude of ,**ῌ
-1* m above sea level.
The Noboribetsu Geothermal Field is characterized
by a dacitic cryptodome (Hiyoriyama Cryptodome), a
volcanic lake (Oyunuma Lake), and a fumarolic valley(Jigokudani Valley) (Fig. ,A). The Hiyoriyama Cryptodome, in the northern part of the geothermal ﬁeld, iselliptical in plan view (oriented NWῌSE), is -/*ῌ//* min diameter, and rises +-* m above the surrounding area.
Its highest point is -11 m above sea level. The dome
has a small explosion crater with active fumaroles at thesummit. Oyunuma Lake (++/ῌ,+* m in area), which
is located in the central part of the geothermal ﬁeld, isthe largest lake in the ﬁeld and is ﬁlled with hot acidic
water. The Jigokudani Valley, in the southern part of
the geothermal ﬁeld, extends for /** m in an ENEῌWSW orientation and hosts active fumaroles. The geothermal manifestations of the geothermal ﬁeld (active

fumaroles, hot springs, and hydrothermal alteration
zones) are distributed in a zone extending NNEῌSSW
from the Hiyoriyama Cryptodome to the Jigokudani
Valley.
The geology of the Noboribetsu Geothermal Field
consists mainly of pyroclastic ﬂow deposits derived from
Kuttara Volcano (Moriizumi, +332). The deposits are
more than ,** m thick (NEDO, +33+) and are composed mainly of clasts of dacitic pumice (up to tens ofcentimeters across) in a cogenetic matrix. Faults in
the geothermal ﬁeld are oriented mainly NNWῌSSE and
ENEῌWSW (Saito et al., +3/-). There are few published drilling data on the geothermal ﬁeld (NEDO,+33+).
-. CSAMT and TDEM resistivity measurements-ῌ+ Electromagnetic system
CSAMT and TDEM resistivity measurements were
performed using a high-resolution electromagnetic system(Geo-SEM ; Neoscience Co., Ltd) consisting of a transmitter and receiver, both controlled by GPS (Figs. and .). The transmitter (Fig. .A) consists of a transformer, rectiﬁer, switching circuit, GPS clock, and adipole source (grounded electrical source) that is , kmlong and that has +** electrodes at each termination(Fig. -). The transmitter is powered by separate generators for CSAMT and for TDEM (see Table + for
speciﬁcations). The receiver (Fig. .B) consists of anControlled Source Audio-frequency Magnetotelluric (CSAMT) and Time Domain Electromagnetic (TDEM)
Resistivity Measurements at Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan

155

Fig. ,. (A) Topographic map showing the location of the Noboribetsu Geothermal Field. (B) Locations ofreceiver stations (red circles) for CSAMT and TDEM measurements over the Noboribetsu Geothermal Field.
Also shown is the location of the vertical cross-section (DῌDῑ) depicted in Fig. 2. Topographic contour
interval is +* m.

ampliﬁer, ﬁlter, data logger, GPS clock, and sensor.
The sensor consists of a pair of electrodes and coils,
powered by a +, V car battery. The transmitter andreceiver are synchronized by a high-precision quartz
clock system using GPS (accuracy, +῍+*ῌ0 seconds).
The speciﬁcations of the Geo-SEM system are listed inTable +, and details of the system can be found in
Johmori et al. (,*+*).
-ῌ, Measurements
The positions of the transmitter and receiver stations,used for both CSAMT and TDEM measurements, areshown in Figures + and ,. The transmitter was positioned / km northeast of the Noboribetsu Geothermal
Field with its ,-km-long dipole source directed N-*ῐW(Fig. +). The receiver stations were located .ῌ0 km
southwest of the transmitter (Fig. +). The 00 receiver
stations were distributed in a ,῍, km area covering thewhole geothermal ﬁeld (Fig. ,B).
The CSAMT/TDEM measurements were performed
from ,- April to ,/ May ,**2. During the CSAMT
measurements, the transmitter injected +ῌ2 A electric
currents into the ground at frequencies of +, ,, ., 2, +0,-,, 0., +,2, ,/0, /+,, +*,., ,*.2, .*30, and 2+3, Hz,
and another series at frequencies of ,*, .*, 2*, +0*, -,*,
0.*, +,2*, ,/0*, and /+,* Hz, in order to minimize thenoise related to the commercial frequencies of /* and 0*
Hz and their higher harmonics. The receiver recorded
the electric ﬁeld parallel to the dipole source, and themagnetic ﬁeld perpendicular to the source. During the
TDEM measurements, the transmitter injected +/ A
electric currents into the ground in an 2-s cycle consisting of ,-s periods of ‘on’ and ‘o#’, with alternating
polarity. The receiver recorded the vertical magnetic
response of the waveform at .*-ms intervals.
-ῌ- Data processing
The CSAMT data were processed using a band-passﬁlter, Fourier transform, and stacking, to remove noise.
The stacking was performed ῏.**,*** times at 2+3,
Hz or ῏-** times at + Hz. The apparent resistivity
and phase were then calculated from the electric ﬁeld
and magnetic ﬁeld. The TDEM data were stacked /**times at most, rejecting the /* Hz noise, and thensmoothed using a moving average of ,/ data points (.*ms῍,/ data points῎+ ms). A transient response wasthen calculated by time integration of the smoothed
data. The time integration was performed to obtainweightings for the low frequency data, which is suitable
for analyses of deep resistivity structure. An exampleof TDEM data processing (for location D.) is shown in
Figure /.
-ῌ. Resistivity structure
The resistivity structure was calculated by onedimensional (+D) joint inversion of the CSAMT and
TDEM data at each measuring location. The apparent
resistivity and phase angle of the CSAMT data, and thetransient response of the TDEM data, were inverted to

156

Yoshihiko GOTO and Akira JOHMORI

Fig. -. Electromagnetic system (Geo-SEM) used for
CSAMT and TDEM measurements. A pair ofelectrodes and a horizontal coil were used for
CSAMT, whereas a vertical coil was used for
TDEM.

Table +. Speciﬁcations of the electromagnetic system(Geo-SEM) used for CSAMT and TDEM measurements.

yield the resistivity structure, assuming a horizontal
layered structure. Analytical theory for the layered
structure was based on Ward and Hohmann (+321).
The inversion was carried out by comparing the ﬁeld
data with the calculated results, using the nonlinear
least-squares method (Fig. 0). The depth of penetration was determined by the sensitivity of the deepest
layer to resistivity. We tested the sensitivity of the
deepest layer to resistivity by doubling or halving theresistivity of the deepest layer and then checking thechange in the root mean square (RMS) values obtained
from the ﬁeld data and from the calculated result. If
the original RMS value showed no change, the deepest
layer was interpreted to be insensitive to resistivity andthe second-deepest layer was then examined.
The resistivity structure, obtained by the +D joint

Fig. .. Photographs of the transmitter (A) and receiver(B) of the electromagnetic system (Geo-SEM)
used in the CSAMT and TDEM measurements.
The white coil for CSAMT measurements (asshown in B) is 1, cm long.

inversion analysis, contained errors due to static shift
and topographic e#ects. To reduce these errors, ,D
analysis (cf., Sasaki, +320) was performed employing
the CSAMT data, using the ﬁnite element method andsetting the result of the +D analysis as the initial model.
,D resistivity structures were calculated along sevensections oriented NNWῌSSE (A+ῌA/, B+ῌB1, C+ῌC++,
D+ῌD++, E+ῌE++, F+ῌF1, and G+ῌG0 in Fig. ,). Finally, the -D resistivity structure was produced fromthe seven ,D sections.
.. Results
Processing of the CSAMT and TDEM data revealed

Controlled Source Audio-frequency Magnetotelluric (CSAMT) and Time Domain Electromagnetic (TDEM)
Resistivity Measurements at Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan

157

Fig. /. Example of TDEM-data processing (forlocation D.). (A) Raw TDEM data including
noise. (B) TDEM data after stacking .*, times
and smoothing. (C) Transient response producedby time integration.

a subsurface resistivity structure shallower than +,.** mb.s.l. (Figs. 1 and 2). Figure 1 shows horizontal sections of the resistivity structure at vertical intervals of,** m. The most prominent feature of the resistivity
structure is the presence of a region of low resistivity(῍+* W · m) beneath the geothermal ﬁeld. In planview, this region varies in lateral extent and outline at
di#erent depths. At the ground surface (Fig. 1A), twodomains of low resistivity are recognized : a northern
low at Oyunuma Lake (receiver stations D0 and D1)
and a southern low at Jigokudani Valley (D+*). Theselows occur at similar locations to geothermal features
such as hot springs, fumaroles, and hydrothermal alteration zones. At ,** m above sea level (Fig. 1B), thetwo lows combine to deﬁne a linear low-resistivity zonethat extends for +,/** m in a NNWῌSSE orientation,with a width of /** m. At sea level (Fig. 1C), the twolows are completely united, showing a semicircular or
quadrangular shape that is +,*** m across, centered at
Oyunuma Lake (receiver station D1). At ,** m below
sea level (Fig. 1D), the region of low resistivity isenlarged to an area of +,/**ῌ+,/** m. From .** to
0** m below sea level (Fig. 1E, F), the low shows amore irregular outline and appears to be an assemblageof linear low-resistivity zones trending NNWῌSSE and
ENEῌWSW. From 2** to +,.** m below sea level (Fig.
1GῌJ), a linear low-resistivity zone trending NNWῌSSE(receiver stations D+ῌD3) is observed among an irregular overall pattern of resistivity.
Figure 2 shows a vertical cross-section of the resistiv-

Fig. 0. Example of one-dimensional joint inversion
at location D.. (A) Comparison of CSAMT
ﬁeld data (green circles) and calculated results(black circles). (B) Comparison of TDEM
transient ﬁeld data (red line) and calculated
results (blue circles). (C) Calculated resistivity
structure.

ity structure along the line DῌD῎ (Fig. ,B). In thesection, a region of low resistivity occurs beneath thegeothermal ﬁeld, centered at Oyunuma Lake and extending to +,.** m depth below sea level.
/. Discussion
The resistivity of rocks and sediments is generally
lowered by the presence of conductive minerals (e.g.,smectite-series clays), thermal water in pores and fractures, and high ground temperatures (e.g., Risk et al.,,**-). In the Noboribetsu Geothermal Field, the regionof low resistivity is located beneath geothermal features
such as fumaroles, hot springs, and hydrothermal alteration zones, consistent with the presence of clay minerals produced by hydrothermal alteration related to upwelling high-temperature ﬂuids and/or the presence ofhigh-temperature, thermal water in porous or fractured
rocks. In explaining the low resistivity, we favor a
combination of the presence of clay minerals, thermal
water, and high ground temperatures.
The low resistivity at .**ῌ0** m below sea level (Fig.
1E and F) is irregular in outline and appears to be an

158

Yoshihiko GOTO and Akira JOHMORI

Fig. 1. Horizontal resistivity pseudosections of the Noboribetsu Geothermal Field at a vertical interval of ,** m.
The locations and numbers of receiver stations are shown in Fig. ,B.

Controlled Source Audio-frequency Magnetotelluric (CSAMT) and Time Domain Electromagnetic (TDEM)
Resistivity Measurements at Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan

Fig. 1.

Fig. 2. Vertical resistivity pseudosection of theNoboribetsu Geothermal Field along the line DῌDῌ(see Fig. ,B).

159

(continued)

assemblage of linear lows trending NNWῌSSE and ENEῌ
WSW, implying the presence of clay minerals and/or
high-temperature geothermal ﬂuid along fractures developed in these orientations. These fractures would
enable the upwelling of high-temperature ﬂuid, causinghydrothermal alteration. Faults in the Noboribetsu
Geothermal Field are developed mainly in NNWῌSSE
and ENEῌWSW orientations (Saito et al., +3/-), indicating the linear lows are related to the presence ofclay minerals and/or high-temperature geothermal ﬂuid
along faults at depth.
The linear NNWῌSSE-trending low at 2**ῌ+,.** mbelow sea level (D+ῌD+* in Fig. 1GῌJ) is inferred to berelated to deep fractures developed in this orientation.
The low at this depth is similar in location to the low
resistivity at the ground surface (D+ῌD+* in Fig. 1A,
where geothermal features are located), suggesting thatthe surface features have developed above the deep
fractures. The fractures at 2**ῌ+,.** m below sea

160

Yoshihiko GOTO and Akira JOHMORI

level are therefore inferred to be important pathways
for upwelling high-temperature ﬂuids.
Acknowledgments
This research was sponsored by the Ministry of Education, Culture, Sports, Science and Technology ofJapan (MEXT), and was supported ﬁnancially by the
Muroran Institute of Technology. We thank N.
Johmori, T. Kondou, and T. Takahashi (Neo Science
Co., Ltd.) for help in the ﬁeld. We are grateful to M.
Tamura (Geological Survey of Hokkaido) and an anonymous referee for reviewing the manuscript. T.
Hashimoto (Hokkaido University) is thanked for editing the manuscript.
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