Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 2, pp. 365-376

ArticleResistivity Structure of the Hiyoriyama Cryptodome
at Kuttara Volcano, Hokkaido, Japan
Yoshihiko GOTO＊ and Akira JOHMORI＊＊(Received March 1, 2012 ; Accepted March 15, 2013)
Direct current (DC) electrical and controlled source audio-frequency magnetotelluric (CSAMT) surveys were
performed over the Hiyoriyama Cryptodome in Kuttara Volcano, Hokkaido, Japan. Both surveys were performed on thesame survey line across the cryptodome in a NE‒SW orientation. Two-dimensional joint inversion of the DC electricaland CSAMT data revealed the underground resistivity structure at depths less than 400 m beneath the cryptodome. The
resistivity structure suggests that the cryptodome comprises a dacite intrusion of 150 m wide and 80 m thick (20-50 Ω･m), and overlying pyroclastic deposits that are 10-30 m thick (>100 Ω･m). The dacite intrusion is underlain by aconvex-shaped, low-resistivity layer (<5 Ω･m) that is interpreted to be smectite-rich, altered pyroclastic deposits thathave been subjected to low-temperature (<200℃) hydrothermal alteration. The low-resistivity layer is underlain by aslightly higher-resistivity layer (10-30 Ω･m) that is interpreted to be altered pyroclastic deposits that were subjected tohigher-temperature (> 200℃) hydrothermal alteration in a relatively deep, hot region near the conduit of thecryptodome.
Key words : resistivity structure, DC electrical method, CSAMT method, Hiyoriyama Cryptodome, Kuttara Volcano

1．Introduction
Resistivity surveying provides valuable information on
the underground geological structures of active volcanoes(e.g., Risk et al., 2003; Aizawa et al., 2008; Srigutomo etal., 2008). We performed direct current (DC) electrical (fordetails of the method, see Milsom, 2003) and controlled
source audio-frequency magnetotelluric (CSAMT) surveys(see Sandberg and Hohman, 1982) at the Hiyoriyama
Cryptodome in Kuttara Volcano, southwestern Hokkaido,
Japan, in order to investigate its internal structure. The
cryptodome is inferred to have formed by the uplift of preexisting pyroclastic deposits due to the intrusion of dacitic
magma (Katsui et al., 1988), but its internal structures arepoorly constrained. This paper presents the results of theresistivity surveys and discusses the nature of subsurface
geological structures beneath the cryptodome.
2．Hiyoriyama Cryptodome
The Hiyoriyama Cryptodome is located in the northernarea of the Noboribetsu Geothermal Field, in the westernpart of Kuttara Volcano (Fig. 1). The volcano consists
mainly of an andesitic stratovolcano (summit at 549 m
above sea level) with a small caldera at the summit (Lake
Kuttara). The volcano evolved over the period 80-45 ka,
involving early silicic explosive activity and subsequent＊

College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology, Mizumotocho 27-1, Muroran, Hokkaido 050-8585, Japan.
＊＊
Neo Science Co., Ltd., Tarui 4-2-30, Sennan, Osaka

stratovolcano building associated with caldera collapse at
40 ka (Katsui et al., 1988; Yamagata, 1994; Moriizumi,
1998; Moriya, 2003).
The Noboribetsu Geothermal Field, which is inferred tohave formed after the collapse of the caldera (Katsui et al.,1988), is approximately 1 km wide (NE‒SW) and 1.5 kmlong (NW‒SE) (Fig. 1). The geology of the Noboribetsu
Geothermal Field consists mainly of pyroclastic deposits
erupted from Kuttara Volcano (mainly the Kt-1 pyroclastic
flow deposit; Moriizumi, 1998). The deposits are >200 m
thick (NEDO, 1991) and consist of dacitic pumice clasts
up to tens of centimeters in diameter.
The Hiyoriyama Cryptodome (Fig. 2) is elliptical in planview, ranging in diameter from 350 m (NE‒SW) to 550 m(NW‒SE). In cross-section, it has a pyramidal form withsteeply sloping sides (Fig. 2A). It rises 130 m above thesurrounding area, with the highest point being 377 m above
sea level. The surface of the cryptodome is covered with
pyroclastic deposits (<15 m thick) that were uplifted by
growth of the dome (Katsui et al., 1988). The cryptodome
has an explosion crater (Hiyoriyama Summit Crater; Fig.
2B) at its summit. The crater is 40×95 m in area (elongate
along a NW‒SE axis) and 20 m deep, and contains active
fumaroles (Goto et al., 2011a, b) with a maximum temperature of 134℃ (measured on 6 July 2009). The slopes of the
590-0521, Japan.
Corresponding author: Yoshihiko Gotoe-mail: ygoto@mmm.muroran-it.ac.jp

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Yoshihiko GOTO and Akira JOHMORI

Fig. 1. Location map of the Hiyoriyama Cryptodome (yellow area) in the Noboribetsu Geothermal Field, lying in thewestern part of Kuttara Volcano, southwestern Hokkaido, Japan. Also shown is the location of the survey line for the DC
electrical and CSAMT surveys (red line). The points marked A and B are the ends of the grounded-wire (dipole source)of the transmitter for the CSAMT survey (blue line). Topographic contour interval is 10 m.

cryptodome are widely covered with dense vegetation,
apart from the southeastern slope that has been severely
eroded by past phreatic eruptions from Oyunuma Lake
(Fig. 2A). Fission-track dating indicates that the cryptodome
formed at ca. 15 ka (Goto and Danhara, 2011).
The dacite intrusion beneath the Hiyoriyama Cryptodome is exposed on the wall of the Hiyoriyama Summit
Crater, and is massive with columnar joints spaced atintervals of 1. 0-1. 5 m. The dacite is generally hydrothermally altered, although fresh dacite occurs sporadicallyat the crater rim. The fresh dacite is grey and porphyritic,
containing phenocrysts of plagioclase, quartz, hypersthene,
and trace amounts of augite, opaque minerals, and hornblende. The groundmass is granophyric, containing silica
minerals, feldspars, and opaque minerals.
Geomorphological studies (Goto et al., 2011a) suggestthat the growth of the Hiyoriyama Cryptodome caused
damming of a river and the deposition of sediments behindthe dam, resulting in the formation of a plain to thenortheast of the dome (Hiyoriyama Plain; Fig. 2B). The
Hiyoriyama Plain is semi-rectangular in plan view, being
300 m (NW‒SE)×800 m (NE‒SW) in size, and is coveredby short bamboo and grasses. Drilling of the Hiyoriyama
Plain (unpublished data) has revealed a 15-m-thick layer ofpeat underlain by fresh pyroclastic deposits (>6 m thick).

3．Resistivity surveys
The DC and CSAMT surveys were performed to obtaina continuous resistivity structure at depths of up to 400 m
beneath the Hiyoriyama Cryptodome. The two surveys
were performed on the same survey line, which crosses the
Hiyoriyama Cryptodome in a NE‒SW orientation (Figs.
2B and 3). Given that the cryptodome is elongated in a
NW‒SE orientation (Fig. 3), the feeder dyke that is themagma conduit beneath the dome is inferred to strike
NW‒SE (Goto et al., 2011a). The NW‒SE-trending feeder
dyke probably occurs below the summit of the cryptodome(location M6 in Fig. 3). The survey line was oriented insuch a way as to obtain a resistivity section perpendicular
to the inferred feeder dyke.
3-1 DC electrical survey
The DC electrical survey was performed using the‘high-density electrical prospecting’ method (Matsuoka,2005), which employs a number of electrodes that are laidout on the ground surface in an array. In this method, atransmitter injects electrical currents to a pair of current
electrodes, and the electric potential is measured using apair of potential electrodes. The distance between thecurrent electrodes and the potential electrodes dictates thedepth of measurement. The resistivity structure is usuallyobtained by two-dimensional (2D) inversion using theResistivity Structure of the Hiyoriyama Cryptodome at Kuttara Volcano, Hokkaido, Japan

Fig. 2. (A) Photograph of the Hiyoriyama Cryptodome
viewed from the south. (B) Oblique, three-dimensional, digital terrain model (DTM) of the Hiyoriyama
Cryptodome viewed from the south (after Goto et al.,2011a), showing the survey line for the DC electricaland CSAMT surveys (red line).

finite element method.
The DC electrical survey was carried out using a highresolution electrical exploration system consisting of atransmitter-logger unit (McOHM Model-2115; Oyo Corporation), a switching box, take-out cables, and electrodes(Fig. 4A). The transmitter‒logger unit comprises a transmitter with 400 Vp-p output voltage and a data logger withresolution of 20 µV, powered by a 12 V car battery. The
electrodes were connected to the transmitter‒logger unit
via the switching box and take-out cables. The specifications of the electrical survey system are listed in Table 1.
The survey line was 700 m long and oriented N35° E,
passing over the summit of the Hiyoriyama Cryptodome(Fig. 3). The electrode configurations used were the Wenner
and Eltran arrays (Fig. 4B), both of which are characterized by regular intervals of electrodes. We employed thesearrays because the Wenner array is sensitive to horizontal
structures, whereas the Eltran array is sensitive to verticalstructures. The electrodes were set on the survey line atregular intervals of 10 m in the horizontal, using a transit
and Global Positioning System (GPS). The number of367

Fig. 3. Topographic map of the Hiyoriyama Cryptodome,
showing the survey line for the DC electrical and
CSAMT surveys (red line). Receiver stations (bluecircles; M1‒M11) for the CSAMT survey are also
shown. The survey line is 700 m long and oriented
N35° E, passing over the summit of the cryptodome.
Receiver station M6 is located at the summit of thecryptodome.

electrodes was 71.
Measurements were carried out from 20 to 21 May 2011
(Fig. 5). During the measurements, two current electrodes(C1 and C2 in Fig. 4B) and two potential electrodes (P1and P2 in Fig. 4B) were selected from the 71 electrodes.
The transmitter‒logger unit injected 20-50 mA electricalcurrents into the current electrodes, while electric potential
was measured at the potential electrodes. A total of 1610
datasets were obtained by changing the sites and intervalsof measuring electrodes using the switching box (Fig. 5).
The distances between the measuring electrodes ranged
from 10 to 230 m.
3-2 CSAMT survey
The CSAMT survey was performed following the‘scalar CSAMT’ method (Matsuoka, 2005), whereby atransmitter injects electrical currents into the ground ataudio (and near-audio) frequencies via a grounded-wire(dipole source), while a receiver records the electric fieldparallel to the grounded-wire and records the magnetic
field perpendicular to the grounded-wire. The resistivity
structure is usually obtained by 2D inversion using thefinite element method, based on magnetotelluric (MT)theory.

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Yoshihiko GOTO and Akira JOHMORI

Fig. 5. Photograph of the DC electrical survey in the fieldusing a transmitter‒logger unit (McOHM) and a switching box, which is connected to 71 electrodes via takeout cables. The survey involved changing the sites andintervals of the electrodes using the switching box.

Table 1. Specifications of the exploration systems used
for the DC electrical (McOHM) and CSAMT surveys(Geo SEM).

Fig. 4. (A) Schematic of the electrical exploration systemused for the DC electrical survey. The system consistsof a transmitter‒logger unit (McOHM), a switchingbox, take-out cables (multiconductor cables), and 71
electrodes connected to the take-out cables. (B)Schematic of the electrode configuration (Wenner andEltran arrays) used for the DC electrical survey. C1
and C2 are current electrodes; P1 and P2 are potential
electrodes. V is the voltage difference between P1 andP2 due to a current (I) flowing between C1 and C2.

The CSAMT survey was carried out using a highresolution electromagnetic system (Geo-SEM; Neoscience
Co. Ltd) consisting of a transmitter and a receiver, both ofwhich are synchronized by GPS (Fig. 6). The transmitter(Fig. 7A) consists of a transformer, a rectifier, a switchingcircuit, a GPS clock, and a generator. A grounded-wire, 1.1
km long with 30 electrodes at each termination, wasconnected to the transmitter (Fig. 6). The receiver (Fig.
7B) consisted of an amplifier, a filter, a data logger, a GPS
clock, and sensors. The sensors consist of a pair ofelectrodes and a coil. The transmitter and receiver weresynchronized by a high-precision quartz clock systemusing GPS with an accuracy of 1×10‒6 sec. The specifications of the Geo-SEM system are listed in Table 1, andfurther details of the system can be found in Johmori et al.
(2010).
The transmitter was positioned 3. 5 km WNW of the
Hiyoriyama Cryptodome, with its 1.1-km-long groundedwire oriented N35° E (Fig. 1). The receiver stations were
distributed along the same survey line as that used for the
DC electrical survey (Fig. 1). The distance between thetransmitter and receiver stations was 3.5 km. The survey
line was 700 m long and was also oriented N35° E (i.e.,parallel to the grounded-wire), passing over the summit of
the Hiyoriyama Cryptodome (M1‒M11; Fig. 3). The 11
receiver stations (M1‒M11) were horizontally spaced atregular intervals of 60-70 m, using a transit and GPS.
The CSAMT survey was performed from 22 to 24 May
2011. The transmitter injected 1-8 A electrical currents

Resistivity Structure of the Hiyoriyama Cryptodome at Kuttara Volcano, Hokkaido, Japan

369

Fig. 6. Schematic of the electromagnetic system (GeoSEM) used for the CSAMT survey. The groundedwire for the transmitter is 1. 1 km long and has 30
electrodes at each termination. The receiver has
sensors consisting of a pair of electrodes and a coil.
The distance between the grounded-wire and receiver
stations is 3.5 km.

into the ground at frequencies of 1, 2, 4, 8, 16, 32, 64, 128,
256, 512, 1024, 2048, 4096, and 8192 Hz, and anotherseries at frequencies of 20, 40, 80, 160, 320, 640, 1280,
2560, and 5120 Hz, in order to minimize the noise related
to the commercially used frequency of 50 Hz and its higher
harmonics. The receiver recorded the electric field parallelto the grounded-wire, and the magnetic field perpendicular
to the grounded-wire. The measuring time at each receiver
station was 1 hour (2 minutes at 8192 Hz, 8 minutes at 1
Hz). The CSAMT data were processed using a band-passfilter and Fourier-transform, to remove noise. The numberof sampling waves was >400,000 at 8192 Hz or >300 at
1 Hz. The apparent resistivity and phase were thencalculated from the electric and magnetic fields. Measurement errors for the apparent resistivity and phasecannot be calculated as the CSAMT data was treated as asingle data to maximize the frequency resolution (seeJohmori et al., 2010).
Fig. 8 shows the apparent resistivity and phase angles at
all receiver stations. The CSAMT data are inferred to havebeen measured mainly in a far-field region, beyond thenear-field region, because (1) the phase angles range from
9° to 30° at 1 Hz (e.g., 20.4° at M5 station, 30° at M6, and9.3° at M7), with no angles of 0°; and (2) the apparent
resistivity does not show abrupt V-shaped decreasing andincreasing trends (i.e., a ‘notch’ or ‘transition zone’; Hishida
and Takasugi, 1998) at low frequencies. Below 10 Hz, theapparent resistivity shows a gentle increase with decreasing frequency, whereas the phase angle shows a gentle
decrease. This finding may indicate the presence of a highresistivity body beneath a low-resistivity body, and/or aminor influence on the CSAMT data of the near-field
effect. Therefore, it is possible that the CSAMT data areslightly influenced by the near-field effect. Even if this isFig. 7. Photographs of the transmitter (A) and receiver (B)of the electromagnetic system (Geo-SEM) used for the
CSAMT survey. The white coil (as shown in B) is 72
cm long.

the case, we infer that the near-field effect has no marked
influence on the modeling of resistivity structure at thedepths of interest (down to 400 m), given the relation
between this depth and the distance between the transmitter and receiver stations (3.5 km, which is 8.8 times
larger than the depth of 400 m) (cf. Sandberg and Hohmann,
1982).
4．Data analysis
Two-dimensional joint analysis of the DC and CSAMT
data was performed to obtain a continuous resistivity
structure at depths of up to 400 m beneath the Hiyoriyama
Cryptodome. The joint analysis was complicated by thefact that the DC electrical data and the CSAMT data have

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Yoshihiko GOTO and Akira JOHMORI

Fig. 8. Apparent resistivity and phase angles of CSAMT field data (blue lines). Calculated results of 2D inversion for theapparent resistivity (RMS=0.08) are also shown (red lines). Location numbers (M1-11) correspond to those shown in
Fig. 3.

Resistivity Structure of the Hiyoriyama Cryptodome at Kuttara Volcano, Hokkaido, Japan

Fig. 9. Flow chart of the procedure for 2D joint inversionof the DC electrical and CSAMT data.

different sensitivities to the three-dimensional structure. In
this study, the joint analysis was attempted using seesawlike calculations of repeated 2D inversions of these data
(Fig. 9).
The analytical methods applied to the DC electrical data
and the CSAMT data were taken from Sasaki (1981) andSasaki (1986), respectively. The algorithm of Sasaki(1981) is a 2D inversion using a finite element method for
DC electrical surveys, whereas the algorithm of Sasaki(1986) is a 2D inversion using a finite element method for
MT surveys. The mesh size of the finite element method
for DC data analysis was 5 m (element size, 5×5 m), whilethat for CSAMT data analysis was 10 m (element size, 10
× 10 m). Each inversion block (i. e., the unit used to
calculate resistivity by 2D inversion; Sasaki, 1986) consisted of four elements. In the joint analysis, the 5-m-mesh
DC resistivity data were interpolated and used as an initialmodel for the 10-m-mesh CSAMT data analysis; conversely, the 10-m-mesh CSAMT data were interpolated
and used as an initial model for the 5-m-mesh DC data371

analysis.
While the CSAMT data consisted of apparent resistivity
and phase angles, the 2D inversions were performed using
the apparent resistivity only (cf. Sasaki, 1986). Phase
angles were not used in the 2D inversions. Static shift for
the CSAMT data was reduced by the 2D inversions. Evenif a near-surface resistivity-anomaly was present, it was
detected by the DC electrical data and incorporated into
the CSAMT analytical model. Topographic effects for the
DC and CSAMT data were reduced by the 2D inversions.
High-resolution, aerial, laser-scanner mapping data (Gotoet al., 2011a) were used to produce topographic models forthe 2D inversions.
The calculation process involved the following five
steps (Fig. 9). (1) A resistivity structure was calculated by
2D inversion of the CSAMT data, setting a uniform
resistivity (3 Ω･m) as the initial model. The value of 3 Ω･m was based on the very low resistivity in the Noboribetsu
Geothermal Field (Goto and Johmori, 2011). (2) A
resistivity structure was calculated by 2D inversion of the
DC electrical data, setting the result of the 2D inversion of
the CSAMT data as the initial model. (3) A resistivity
structure was calculated by 2D inversion of the CSAMT
data, setting the result of the 2D inversion of the DC
electrical data as the initial model. (4) A resistivity
structure was calculated by 2D inversion of the DC
electrical data, setting the result of the 2D inversion of the
CSAMT data as the initial model. (5) Root mean square(RMS) values were calculated from the field data and fromthe results of steps (3) and (4). The RMS value (δ) is
defined as δ=[Σ{ln (ρaf) ‒ ln (ρac) }2/n]1/2, where ρaf isthe field measurement (apparent resistivity) and ρac is thecalculated result. An RMS value equal to zero means thatthe calculated result perfectly matches the field data. RMS
values were calculated separately for the CSAMT data andthe DC electrical data.
If the RMS values exceeded 0.2, steps (3) and (4) wererepeated. If the RMS values were lower than 0. 2, thecalculated result was accepted as the resistivity thatsatisfies both the CSAMT and DC electrical data. In thisstudy, steps (3) and (4) were repeated four times, yielding
RMS values of 0.08 (at step 3) and 0.19 (at step 4). Fig.
10A and B shows the calculated results of 2D joint
inversion when steps (3) and (4) were repeated three times(RMS=0.09 at step 3; RMS=0.26 at step 4), whereas Fig.
10C and D shows the calculated results when steps (3) and(4) were repeated four times (RMS=0.08 at step 3, RMS=0.19 at step 4). Fig. 8 compares the CSAMT field data(apparent resistivity) with the calculated result at step (3)(RMS=0.08), while Fig. 11 compares the DC electricalfield data (apparent resistivity) with the calculated result at
step (4) (RMS=0.19).
Before accepting the calculated result (Fig. 10D), we
tested an alternative 2D joint analysis, employing both
apparent resistivity and phase angles in the algorithm of372

Yoshihiko GOTO and Akira JOHMORI

Fig. 11. Comparison of the DC electrical field data (apparent
resistivity; A1: Wenner array; B1: Eltran array) and calculated results of the 2D inversion (A2: Wenner array;
B2: Eltran array) (RMS=0.19).

Fig. 10. Process of 2D joint inversion of the DC electricaland CSAMT data. (A) and (B) Calculated results of
2D joint inversion for three repetitions of steps (3) and(4) (see Fig. 9) (RMS=0.09 at step 3; RMS=0.26 atstep 4). (C) and (D) Calculated results of 2D joint
inversion for four repetitions of steps (3) and (4) (RMS
=0.08 at step 3, RMS=0.19 at step 4). Small dots in(A) - (D) indicate the distributions of the centers of
inversion blocks.

Resistivity Structure of the Hiyoriyama Cryptodome at Kuttara Volcano, Hokkaido, Japan

373

Fig. 12. Resistivity section beneath the Hiyoriyama Cryptodome (A) and its geological interpretation (B). Zone A isinterpreted to represent fresh (unaltered) pyroclastic deposits erupted from Kuttara Volcano. Zone A at the Hiyoriyama
Cryptodome (10-30 m thick; above zone B) is interpreted to be pyroclastic deposits uplifted by growth of thecryptodome. Zone B (150 m wide, 80 m thick) is interpreted as the dacite intrusion of the cryptodome. Zone C isinterpreted as altered pyroclastic deposits containing abundant conductive clay minerals, such as smectite, formed by
low-temperature (<200℃) hydrothermal alteration in the region between the cold ground surface and deeper, hotter
regions. Zone D is interpreted as altered pyroclastic deposits that have been subjected to higher-temperature (>200℃)
hydrothermal alteration in a deeper, hotter region near the conduit of the Hiyoriyama Cryptodome. The location of thefeeder dyke is estimated from the topographic features of the Hiyoriyama Cryptodome.

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Yoshihiko GOTO and Akira JOHMORI

Sasaki (1986). This calculation resulted in a reduced
degree of convergence compared with the 2D joint analysis using apparent resistivity only; steps (3) and (4) wererepeated four times, yielding RMS values of 0.19 (at step3) and 0.19 (at step 4). The reduced degree of convergence could be attributed to data dispersion in the phaseangles (see Fig. 8). The obtained resistivity structure wasalmost identical to that using apparent resistivity only, but
was slightly more irregular. In this study, we placed
greater emphasis on the degree of convergence rather thanphase angles, and adopted the resistivity structure obtainedusing apparent resistivity only (Fig. 10D) as the final model.
The depth of penetration was determined from the skindepth (Cagniard, 1953) and the Bostick depth (Murakami,1987). The skin depth is defined as the depth at which theamplitude of electromagnetic waves decrease to 1/e (e isthe base of the natural logarithm). The Bostick depth isdefined as 1/ 2 of the skin depth, and is considered torepresent a more accurate depth of penetration than theskin depth. The skin depth calculated from the frequencyof electric currents for the deepest layer (1 Hz) and from its
apparent resistivity (3-5 Ω･m) for the CSAMT data, yields
a penetration depth of 866-1120 m, whereas the Bostick
depth calculated from the same data yields a penetration
depth of 612-792 m. Given that the topography along thesurvey line varies in elevation from 225 to 350 m and has
relatively steep slopes, we conservatively estimate thedepth of penetration to be about 400 m below the groundsurface.
5．Results and discussion
Processing of the DC electrical and CSAMT data
revealed the subsurface resistivity structure at depths ofless than 400 m beneath the Hiyoriyama Cryptodome (Fig.
12A). The resistivity structure is divided into four zones
based on the resistivity values (A‒D in Fig. 12B). In
general, the resistivity of rocks and sediments is loweredby the presence of conductive minerals (e. g., smectiteseries clays), thermal water in pores and fractures, andhigh temperatures (e. g., Risk et al., 2003). Geologicalinterpretations of the four zones (A‒D in Fig. 12B) areprovided as follows.
Zone A (resistivity of > 100 Ω･m) extends subhorizontally along the ground surface and is 〜100 m thickbeneath the Hiyoriyama Plain, and 10-30 m thick at the
Hiyoriyama Cryptodome. Because the surface geology inthis area consists of pyroclastic deposits erupted from
Kuttara Volcano at ca. 40 ka (mainly the Kt-1 pyroclastic
flow deposit), zone A is interpreted to represent these
deposits. The high resistivity of this zone suggests that thedeposits are fresh and unaltered. Zone A at the Hiyoriyama
Cryptodome (10-30 m thick; above zone B) is interpreted
to be pyroclastic deposits uplifted by growth of thecryptodome.
Zone B (20-50 Ω･m) is located below the Hiyoriyama

Cryptodome and occupies a semi-rectangular domain thatis 150 m wide and 80 m thick in cross-section. Given thatzone B is located immediately beneath the cryptodome, weinterpret this zone as the dacite intrusion of thecryptodome. Considering the emplacement age of the
Hiyoriyama Cryptodome (ca. 15 ka; Goto and Danhara,2011), the dacite intrusion is probably solidified. Theintermediate resistivity (20-50 Ω･m) is explained byhydrothermal alteration of solidified dacite. The bulge at
the northeastern slope of the cryptodome (Fig. 12A)
reflects lateral displacement of the pyroclastic deposits that
resulted from emplacement of the dacite intrusion during
growth of the cryptodome (displaced deposits; Fig. 12B).
Zone C occurs 100-200 m below the cryptodome and ischaracterized by very low resistivity (<5 Ω･m). This zone
is 〜100 m thick, convex-shaped, and is conspicuous in theresistivity section. The convex shape is symmetrical withrespect to a vertical line passing through the summit of thecryptodome. The top of the convex body appears to fadeout, with the zone being discontinuous at the vertical line.
Possible interpretations of zone C include: (1) alteredpyroclastic deposits containing abundant conductive clay
minerals, such as smectite; (2) pyroclastic deposits
containing thermal water in pores and fractures; and (3)high-temperature pyroclastic deposits. We favorinterpretation (1) because interpretations (2) and (3) cannot
readily explain the convex geometry of the zone. We inferthat the convex-shaped geometry reflects the distributionof the stability field of smectite (<200℃; Hyndman et al.,
1997; Aizawa et al., 2009; Aizawa, 2010; Lee et al., 2010).
The fade-out at the top of the convex body could be
attributed to the instability of smectite (which occurs attemperatures of > 200℃) due to the ascent of hightemperature fluids along the feeder dyke beneath the dome.
Zone C is therefore interpreted as altered pyroclastic
deposits subjected to low-temperature (< 200℃)
hydrothermal alteration in the region between the cold
ground surface and deeper, hotter regions. Zone C is richin clay minerals and may form an impermeable ‘sealingzone’ (Aizawa et al., 2009; Aizawa, 2010) below fresh,
permeable pyroclastic deposits (zone A).
Zone D occurs beneath zone C (150-400 m below theground surface) and shows higher resistivity (10-30 Ω･m).
The oval shape of zone D remains uncertain, as theresistivity resolution at this depth is insufficient to reliably
constrain its outline. A possible interpretation of zone D isaltered pyroclastic deposits containing fewer conductive
clay minerals than in zone C (e.g., it may contain chlorite
rather than smectite). The higher resistivity can potentiallybe attributed to the higher temperature of the zone, whichmeans that smectite is unstable (> 200℃). A higher
temperature is plausible because zone D is deeper thanzone C, and located closer to the inferred feeder dyke ofthe dome. Zone D is therefore interpreted as alteredpyroclastic deposits that have been subjected to higher-

Resistivity Structure of the Hiyoriyama Cryptodome at Kuttara Volcano, Hokkaido, Japan

temperature (>200℃) hydrothermal alteration in a deeper,hotter region near the conduit of the Hiyoriyama Cryptodome.
6．Conclusions
DC electrical and CSAMT surveys have revealed the subsurface resistivity structure at depths shallower than 400 m
beneath the Hiyoriyama Cryptodome. The resistivity structure suggests that the cryptodome comprises a dacite intrusion (150 m wide, 80 m thick) and overlying pyroclastic
deposits (10-30 m thick). The dacite intrusion is underlain
by hydrothermally altered pyroclastic deposits (>300 m)that are divided into two zones based on the assemblage ofalteration minerals. The upper zone contains abundantconductive clay minerals such as smectite that formed by
low-temperature alteration, whereas the lower zone contains
fewer conductive clay minerals and formed by highertemperature alteration in a deeper, hotter region near the
conduit of the cryptodome.
Acknowledgments
This research was sponsored by the Ministry of Education, Culture, Sports, Science and Technology of Japan(MEXT), and was financially supported by the MuroranInstitute of Technology. We thank N. Johmori, T. Kondou
and T. Takahashi (Neo Science Co. Ltd) for help in thefield. S. Kameyama (Tanaka Consultant Co. Ltd) is thanked
for providing topographic data for the Hiyoriyama Cryptodome. Comments by K. Aizawa (University of Tokyo), an
anonymous referee and T. Watanabe (University of
Toyama) significantly improved the manuscript.
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北海道南西部クッタラ火山，日和山潜在ドームの比抵抗構造
後藤芳彦・城森明

北海道南西部クッタラ火山，日和山潜在ドームの比抵抗構造探査を行い，日和山潜在ドームの内部構造（深度<400 m）を解明した．探査は高密度電気探査法および CSAMT 探査法を用い，ドームを北東-南西方向に横断する測線上で行った．測線長は 700 m で，受信点は高密度電気探査 71 電極，CSAMT 探査 11 測点を等間隔に配置した．データ解析は高密度電気探査および CSAMT 探査データの 2D ジョイント解析で行った．
日和山潜在ドームは，幅 150 m 高さ 80 m のデイサイト貫入岩体 (20-50 Ω･m) と，貫入岩体を覆う厚さ 10-30m の火砕堆積物 (>100 Ω･m) からなる．デイサイト貫入岩体の直下には，ハの字型の低比抵抗領域 (<10 Ω･m) が存在し，スメクタイトに富む低温 (<200℃) の熱水変質帯であると考えられる．この低比抵抗領域の直下には中比抵抗領域 (10-30 Ω･m) が存在し，ドームの供給岩脈（火道）に沿って形成された高温 (>200℃)の熱水変質帯であると考えられる．