Journal of Volcanology and Geothermal Research 264 (2013) 164–171

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Short communication

Effect of the hydrothermal alteration on the surface conductivity ofrock matrix: Comparative study between relatively-high and lowtemperature hydrothermal systems

Shogo Komori a,⁎, Tsuneomi Kagiyama b, Shinichi Takakura c, Shinji Ohsawa d, Mamoru Mimura e, Toru Mogi f

a Institute of Earth Sciences, Academia Sinica, 128 Academia Road, Section 2, Nankang, Taipei 115, Taiwan, ROCb Aso Volcanological Laboratory, Kyoto University, Minamiaso, Kumamoto 869-1404, Japanc Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japand Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Noguchibaru, Beppu, Oita 874-0903, Japane Department of Urban Management, Graduate School of Engineering, Kyoto University, C1, Kyodai-Katsura, Kyoto 615-8540, Japanf Institute of Seismology and Volcanology, Hokkaido University, N10W8, Kita-ku, Sapporo 060-0810, Japan

⁎ Corresponding author at: 128, Academia Road, SeTaiwan, ROC. Tel.: +886 2 2783 9910.

E-mail addresses: komori@earth.sinica.edu.tw (S. Komkagiyama@aso.vgs.kyoto-u.ac.jp (T. Kagiyama), takakura-ohsawa@bep.vgs.kyoto-u.ac.jp (S. Ohsawa), mimura@geo(M. Mimura), mogitisv@mail.sci.hokudai.ac.jp (T. Mogi).

0377-0273/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.jvolgeores.2013.08.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 June 2013Accepted 16 August 2013Available online 27 August 2013

Keywords:Beppu geothermal areaPore water conductivitySurface conductivity of the rock matrixGeneration and stability of smectiteHydrothermal alterationExposure temperature

In volcanic areas, hydrothermal systems are characterized by high bulk electrical conductivity, which is a combi-nation of high-salinity/temperature pore water and hydrothermally-altered rock matrix. The present study fo-cuses on better understanding the influence of the hydrothermal alteration on the surface conductivityassociated with generation and stability of smectite. As a case study, the GSB borehole site at Beppu geothermalarea in southwest Japan was selected; at this site, hydrothermal fluids with a relatively high temperature(150 °C) flow at a shallow depth.The physical properties related to electrical conductivity (formation factor, surface conductivity, and porosity)were estimated on the basis of Revil model using conductivitymeasurements of the drillcores. Strong hydrother-mal alteration at the site GSB was shown by high surface conductivity (10−2–10−1 S/m) and high cementationexponent (2.5–4.5). From the comparison between the vertical profiles of the bulk conductivity, pore waterconductivity, and surface conductivity, it was shown that the rock matrix makes a non-negligible contributionto the bulk conductivity at the site. This contribution to bulk conductivity is quite different from that oflow-temperature hydrothermal systems, where the contribution from the pore water dominates because thereis little or no hydrothermal alteration. Furthermore, comparison between the findings of this study and low-temperature hydrothermal systems showed that the surface conductivity could simply reflect temperature towhich the rock has been exposed. The surface conductivity maintains the small value at the low temperaturessuch as b40 °C, and significantly increases at the relatively high temperatures (100–150 °C). At the higher ex-posed temperatures N150 °C, its value decreases relative to that at the temperatures of 100–150 °C. This relation-ship is consistent with the generation and stability of smectite at active hydrothermal systems, and places strongconstraints on the quantitative interpretation of the electrical conductivity structure of a volcano.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

In volcanic areas, volcanic fluids released frommagma are dissipatedwithin the volcanic edifice by the groundwater flow, driven by the rain-fall recharge. Volcanic fluids usually have high salinity and high temper-ature, resulting in hydrothermal activity and a region of high electricalconductivity in the shallow part of the edifice (Keller and Rapolla,1974; Wannamaker et al., 2004; Kanda et al., 2008; Komori et al.,

ction 2, Nankang, Taipei 115,

ori),s@aist.go.jp (S. Takakura),tech.dpri.kyoto-u.ac.jp

ghts reserved.

2013). The spatial distribution of temperature and salinity in the aquifermay be controlled by the influx of mass and heat from degassingmagma(Hurwitz et al., 2003; Matsushima, 2011); in return, the spatial extent ofthe high electrical conductivity regionmay also reflect theflux of the vol-canic fluids (Komori et al., 2012). It is therefore crucial to understand therelation between temperature, salinity and bulk electrical conductivity.

Generally, bulk electrical conductivity, obtained from electromag-netic surveys at volcanic areas, contains two conductivity components:the pore water conductivity and the surface conductivity of the rockmatrix. Pore water conductivity can be relatively easily represented asa function of the salinity and temperature, using known equations(Arps, 1953; Revil et al., 1998; Atkins et al., 2009). On the other hand,surface conductivity depends predominantly on the smectite contentof the rock matrix, because smectite has a large surface area and de-velops an electrical double layer on its surface (Revil et al., 1998,

165S. Komori et al. / Journal of Volcanology and Geothermal Research 264 (2013) 164–171

2002). A certain level of high temperature is usually necessary to gener-ate smectite by hydrothermal alteration; however, smectite is unstableunder higher temperature conditions, changing into other, less conduc-tive clay minerals such as illite (Pytte and Reynolds, 1989; Andersonet al., 2000). To date, the effect of temperature on the surface conductiv-ity associated with generation and stability of smectite has been poorlyinvestigated quantitatively, although a pioneering study was done byMogi (1992) with respect to the effect of hydrothermal alteration onthe electrical properties of the rock matrix.

One way to investigate this problem is to examine the surfaceconductivity of drillcores taken from volcanic and geothermal areas,because these drillcores are exposed to a variety of temperatures,depending on the strength of subsurface hydrothermal activity. Ideally,this would provide quantitative relationships between surface conduc-tivity and the temperatures of alteration, with the eventual goal of eval-uating the mass flux of volcanic fluids using the spatial distributions ofpore water and surface conductivities in the volcanic aquifer. This ispossible because these conductivity components can be related withthe temperature and salinity distributed by the influx of the volcanicfluids released from magma. Additionally, drillcores may be useful forinferring the contributions to the observed bulk conductivity frompore water and the rock matrix beneath the drilling site; which couldinform an understanding of the flow of hydrothermal fluids and thestructural features controlling them, from the viewpoint of electricalconductivity (Komori et al., 2010).

To date, Komori et al. (2010) have estimated the surface conductiv-ity using drillcores from borehole USDP-1, a borehole at Unzen volcano(SW Japan) that penetrated the shallow low-temperature hydrother-mal system. Those investigators found that the rock matrix demon-strates low surface conductivity, due to little/no alteration by the low-temperature fluid. The present study focuses on drillcore samplesfrom a geothermal well at Beppu, in SW Japan, where relatively high-temperature fluids flow in the shallow part of the well. There is also alarge number of geophysical and geochemical data from well testingand the analysis of rock and water samples. The drillcore sampleswere used to make measurements of bulk conductivity by changingthe porewater conductivity, after which surface conductivities were es-timated using the Revil model. The vertical profile of estimated surfaceconductivity was used for comparison with the profiles of bulk andpore water conductivities, to investigate their relationship under rela-tively high-temperature conditions. Furthermore, this study examinedthe effect of exposure temperature on surface conductivity by compar-ing the present results with the case of low temperature fluids at theUnzen USDP-1 site.

2. Site description— Beppu geothermal area

The Beppu geothermal area is one of the most active geothermalfields in Japan. It is located in the pull-apart-like depression developedin the northern part of Kyushu Island, SW Japan (Takemura et al.,1994; Takemura, 2004). The andesitic volcaniclastic rocks that outcropin Beppu area have been supplied by the Tsurumi and Garan volcanoeslocated to thewest; these are the source rocks of the alluvial fan that ex-tends towards the coast of Beppu Bay (New Energy DevelopmentOrganization, 1990). Hot springs and fumaroles, which originate fromthe NaCl-type deep hydrothermal fluid with a temperature of 250–300 °C, have been developed extensively at the surface, and heat dis-charge in the area is estimated to be a few hundreds of MW (Allis andYusa, 1989). In this study area, drilling was conducted to investigatethe flow paths of the hot spring waters with different origins (Ohsawaet al., 1994; Yusa et al., 1994).

Fig. 1(b)–(e) shows the vertical profiles of temperature, permeabil-ity by in-situ aquifer testing, and authigenic minerals (Gianelli et al.,1992; Yusa et al., 1994), together with the profile of bulk electrical con-ductivity obtained from CSMT (Controlled Source Magnetotellurics)surveys (Mogi et al., 1995). Note that the in-situ permeability is

normalized by its uppermost value (~10−12 m2) for comparison withthe core permeability at the Unzen USDP-1 site, as discussed later. InCSMT, NS-oriented 11 receivers were used for acquisition of the electro-magnetic waves with frequencies of 4.2–8700 Hz, which were generat-ed by the transmitter located at about 4 km west of the receivers[Fig. 1(a) (Mogi et al., 1995)]. The bulk conductivity increases withdepth, and has maximum values of 3 × 10−1–100 S/m at depths be-tween 150 and 250 m. The low permeability layer composed of alarge amount of smectite has developed at depths between 150 and200 m, below which hydrothermal fluids at a temperature of 150 °Care flowing. The high bulk electrical conductivity region correspondsto both the low permeability layer and the fluid-bearing layer.

This study used the drillcores sampled from the low permeabilitylayer (174 m and 201 m depths), the fluid-bearing layer (228 mdepth), and the temperature-decreasing section (298 m depth). Thesesamples, which have been retained in the Institute for Geothermal Sci-ences, Kyoto University, were used for porosity and bulk electrical con-ductivity measurements under controlled conditions of pore waterconductivity.

3. Electrical conductivity measurements

3.1. Principle of the estimation of the surface conductivity

In general, the bulk electrical conductivity is represented as the func-tion of the pore water and the surface conductivities (Revil et al., 1998,2002):

σ ¼σ fF 1−t þð Þð Þþ F σs

σ fþ1

2 t þð Þ−σsσ f

� �1− 1

t þð Þσsσ f

þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1− 1

t þð Þσsσ f

� �2

þ 4 Ft þð Þ

σsσ f

r� �� �

σ sþ1−t þð Þ

F σ f

for σ f ≧σst þð Þ

for σ f ≦σst þð Þ

8>><>>:

ð1Þ

where: σ is the bulk electrical conductivity (S/m), σf is the pore waterconductivity (S/m), σs is the surface conductivity of rock matrix (S/m),and t(+) is theHittorf transport number of a cation in the free electrolyte(Revil et al., 1998). F is the formation factor, defined as:

F ¼ ϕ−m ð2Þ

where: ϕ is the porosity, andm is the cementation exponent (m N 0). Inthis study, there are two unknown parameters: the formation factor Fand the surface conductivity σs. These unknowns can be determined ifthere are at least twomeasured pairs of bulk and porewater conductiv-ities (Komori et al., 2010).

3.2. Core measurements

The drillcores are column-shaped bodies, composed mainly of an-desitic volcaniclastic deposits (Gianelli et al., 1992). Table 1 showstheir cross-sectional areas, and lengths. They were initially saturatedwith pure water in a vacuum using the desiccator and the vacuumpump, to remove the soluble materials on the grains. After drying at100 °C for 48–96 hours, theywere saturatedwith NaCl solutions at var-ious controlled salinities, in a vacuum as mentioned previously. Afterthe saturation for 48 hours, the electrical conductivity of the water in-side the desiccator was measured, and its value was defined as thepore water electrical conductivity for each. Bulk electrical conductivityof the samples was measured by the four electrodes method, asshown in Komori et al. (2010). During the measurements, the roomtemperature was maintained at 25 °C.

Almost all the measurements were conducted by the OyoCorporation's “miniOHM”; which applies the square wave currentwith a frequency of 8 Hz to the samples, and measures the currentand the electric potential between the electrodes with a precision oftwo significant digits. Some measurements were done by the Agilent's

Fig. 1. Location of the Beppu geothermal area, borehole GSB, and the vertical profiles of the temperature, electrical conductivity, permeability, and the distribution of the authigenicminerals. Temperature and permeability logs are quoted from Yusa et al. (1994). Note that the permeability is normalized by its uppermost value. Electrical conductivity is obtainedfrom CSMT surveys done by Mogi et al. (1995). The distribution of the authigenic minerals is obtained from Powder X-ray diffraction analysis done by Gianelli et al. (1992).

166 S. Komori et al. / Journal of Volcanology and Geothermal Research 264 (2013) 164–171

4263A LCRmeter under the condition of the high porewater conductiv-ity, because themeasured electric potentials were below the detectablerange of the miniOHM, due to a high bulk electrical conductivity. Be-cause the fluid-bearing rock has a dielectric constant larger than thoseof its individual component materials (either the pore fluid or therock-forming minerals), the electrical conductivity of the rock usuallyhas a dependency on the frequency of the applied current waves(Yokoyama, 1980; Olhoeft, 1985; Leloy et al., 2008). This situation arisesas a result of the inhomogeneity of the rock (pore and particle geome-tries and their connectivity), and is known as the “Maxwell–Wagner”effect (Dukhin, 1971; Hasted, 1973). The Agilent's 4263A LCR meterhas the applicable frequencies of 100 Hz, 1 kHz, 10 kHz, and 100 kHz.In this study, no significant differences in the measured electrical con-ductivity were found between the different frequencies when usingthis LCR meter. It is considered that the conduction current dominatesthe whole conduction system with respect to the displacement currentat the above frequency ranges (Guéguen and Palciauskas, 1992); which

Table 1Drillcores used in this study.

Depth Cross-sectional area Length Weigh

(m) (mm2) (mm) (g)

174 3.06 ± 0.38 × 103 3.20 ± 0.27 × 101 194201 a 1.85 ± 0.13 × 103 4.78 ± 0.31 × 101 189

b 1.83 ± 0.17 × 103 5.00 ± 0.04 × 101 187228 a 9.77 ± 0.50 × 102 4.00 ± 0.06 × 101 103

b 1.13 ± 0.07 × 103 3.97 ± 0.00 × 101 114c 1.21 ± 0.03 × 103 4.01 ± 0.03 × 101 127d 1.23 ± 0.06 × 103 4.20 ± 0.21 × 101 132

298 a 2.33 ± 0.04 × 103 2.78 ± 0.03 × 101 149b 2.31 ± 0.14 × 103 2.77 ± 0.06 × 101 146c 2.21 ± 0.32 × 103 2.76 ± 0.24 × 101 152d 2.26 ± 0.08 × 103 2.98 ± 0.04 × 101 157

is due to the high pore water conductivity (e.g., a few tens S/m) used inthis study. Therefore, it is reasonable to use the data from the two differ-ent instruments together under the conditions of the high pore waterconductivity.

The bulk conductivity measurements were repeated for NaCl solu-tions with the initial salinities of 0.0005, 0.005, 0.05, 0.2, 1.2, and2 mol/l; thesemeasurements were made in order of increasing salinity.Table 1 shows theporewater conductivity used to saturate each sample.Themeasured bulk conductivities were plotted as a function of the porewater conductivity. In addition, sample porosities were measured bycomparing the density of a sample saturatedwithwaterwith the densi-ty after drying at 100 °C for 48–96 hours.

4. Results

Fig. 2 shows the results of the measurements. From the figure, it canbe seen that bulk electrical conductivity is positively correlated with

t after saturation Dry weight Salinity of NaCl aq. used for saturation

(g) (mol/l)

131 0.005, 0.05, 0.2140 0.005, 0.2, 1.2133 0.0005, 0.0577.2 0.000589.1 0.005, 1.290.8 0.05, 2

105 0.2139 0.005, 1.2, 2134 0.05140 0.2145 0.0005, 1.2

(a)

(c)

(b)

(d)

Fig. 2. Results of electrical conductivity measurements on drillcore samples and data fitted with Revil's model. Solid circles and open diamonds represent measurement points and the“isoconductivity point”, respectively.

167S. Komori et al. / Journal of Volcanology and Geothermal Research 264 (2013) 164–171

pore water conductivity. The figure also shows the relation betweenbulk conductivity and pore water conductivity calculated from theRevil's model for an assumed formation factor F and each surface con-ductivity σs in the range of 0–1 S/m. Diamonds in Fig. 2 represent the“isoconductivity point”, which is defined by:

σ f ¼ σ s=t þð Þ: ð3Þ

The isoconductivity point is that point forwhich surface conductivitydue to cations is equal to the fraction of electrical conductivity due to thecations in the bulk porewater (Revil et al., 1998).When porewater con-ductivity is much above the isoconductivity point, the bulk conductivityis linearly related to the pore water conductivity. On the other hand, aspore water conductivity approaches the isoconductivity point, a non-linear trend emerges due to the predominance of conduction resultingfrom themigration of cationsweakly absorbed at the porewater–mineralinterfaces (Revil et al., 1998).

On the basis of these features, the best-fit models of surface conduc-tivity and the formation factor were estimated, using the least-square

Table 2Estimated parameters.

Depth Porosity Formation factor Surface conductivity

(m) (S/m)

174 0.64 ± 0.10 5.3 ± 1.2 5.1 ± 1.6 × 10−2

201 0.57 ± 0.04 6.6 ± 0.8 6.2 ± 0.5 × 10−2

228 0.60 ± 0.02 10 ± 1 6.6 ± 0.0 × 10−2

298 0.17 ± 0.00 120 ± 0 1.8 ± 0.0 ± 10−2

method. Solid lines in Fig. 2 show the best-fitmodels for the relationshipbetween σf and σ. Table 2 shows the estimated porosity, formation fac-tor F, cementation exponentm and surface conductivityσs. Each samplehas a surface conductivity of the order of 10−2 S/m, and relativelyhigher surface conductivity was estimated for the samples at the depthsof 174, 201 and 228 m. The Formation factor shows a greater variationthan the surface conductivity, and its value at the depth of 298 m is larg-er than those of the other samples. Porosities are estimated at around60%, with the exception of the porosity at a depth of 298 m, which hasa low value of 17%. The estimated parameters at the depth of 174 mhave relatively large errors; this is due to the distorted shape of thecore at this depth, which results in large errors for the cross sectionalarea and height of the drillcore.

5. Discussions

5.1. Rock alteration inferred from formation factor and porosity

Previous studies of the drillcores, as well as visual inspections duringthe course of the present study, have shown that the rock matrix isstrongly altered. In this section, rock alteration is examined from theviewpoint of the electrical properties of rock, based on the relationbetween formation factor and porosity.

At depths of 174, 201, 228 m, estimated porosities are around 60%.These drillcores have unconsolidated rock matrices, and contain puresmectite. In general, large clay contents lead to high porosity of therockmatrix (Revil and Cathles, 1999). Especially, smectite makes great-er contribution to an increase in porosity than other clayminerals, alongwith a commensurately large amount of water absorbed within its

168 S. Komori et al. / Journal of Volcanology and Geothermal Research 264 (2013) 164–171

layers (Takakura, 2000). These are considered to be themain reasons forthe large porosities estimated at depth. On the other hand, a relativelysmall porosity of 17% at the depth of 298 m is considered to be due toa consolidated matrix, and to the relatively small content of smectiteas shown in Fig. 1(e).

Fig. 3 shows the relationship between the formation factor, porosity,and cementation exponent, together with the previous studies fromJouniaux et al. (2000), Revil et al. (2002), Komori et al. (2010), andHermitte (unpublished). In this study, drillcore samples with large po-rosity tend to have a small formation factor, as is to be expected underEq. (2). Revil et al. (2002) showed that the cementation exponent “m” in-creaseswith increasing rock alteration, due to its complex texture. In thisstudy, the estimated cementation exponents are in the range of 2.5 and4.5, and are substantially larger than those of unaltered rocks (1.5–2),implying that the drillcores sampled in this study are strongly altered.

5.2. Estimated surface conductivity and its relationship to rock alteration

The cementation exponents “m” observed in this study confirmedthat the drillcores are strongly altered. In this section, features of the es-timated surface conductivity are examined and comparedwith themin-eral phases obtained from Gianelli et al. (1992).

In the present study, the estimated surface conductivities are in therange of 1.8–6.6 × 10−2 S/m. These values are 1–2 orders of magnitudegreater than those of clay-free rocks [3 × 10−4–1 × 10−3 S/m (Komoriet al., 2010); 3 × 10−3 S/m (Revil et al., 2002)], and comparable withthose of smectite-rich rocks [3 × 10−2–1 × 10−1 S/m (Revil et al.,2002)], suggesting that the drillcores sampled from this study area arerich in conductive clay minerals.

Fig. 1(e) shows the distribution of authigenic minerals withdepth, identified by Powder X-ray diffraction analysis (Gianelliet al., 1992). Cores with pure smectite are sampled at depths of170–250 m; there is good correspondence to the high surface con-ductivity of ~6 × 10−2 S/m estimated at the depths of 174, 201,and 228 m. Below a depth of 250 m, the distribution of clay mineralschanges from pure smectite to illite/smectite-mixed. Correspondingto its vertical change, the surface conductivity at a depth of 298 m isdecreased to 1/3 to 1/4 of those of the shallower depths.

Fig. 3. Plots of formation factor versus porosity for the drillcore samples from this studyand previous studies.

In addition, the vertical profile of stilbite, a kind of zeolite, does notcorrespond to the expected vertical profile of surface conductivity, inspite of the fact that the high cation exchange capacity (CEC) of zeolitessuggests the potential for high surface conductivity (Hlvay et al., 1983).This result is consistent with a study by Revil et al. (2002), in which theinvestigators found that zeolite contributes little to surface conductivity.

5.3. Surface conductivities at low-permeability and fluid-bearing sections

In general, smectite makes contributions not only to the increase inthe surface conductivity, but also to the effective decrease in the perme-ability (Revil and Cathles, 1999). This section examines the influence ofclay minerals on the permeabilities at low-permeability and fluid-bearing sections, and their relation to the surface conductivities.

It has been known that pure smectite has permeability about two or-ders of magnitude smaller than pure illite (Revil and Cathles, 1999). Asshown in Fig. 1(e) and (d), pure smectite is abundant at depths of170–200 m; that leads to low permeability and high surface conductiv-ity at those depths. The fluid-bearing section at depths of 200–250 m ispresent at the transition from pure smectite to illite/smectite-mixed,and has relatively lesser smectite content compared to the upper sec-tion. This could cause the permeability about one order of magnitudelarger than the upper section. On the other hand, the fluid-bearing sec-tion has no significant decrease of the surface conductivity correspond-ing to decrease in smectite content. According to Revil et al. (2002), thesurface conductivity of rock matrix increases to the order of 10−1 S/meven when it has only 10% of smectite content. This fact suggests thatthe fluid-bearing section contains still enough smectite to maintain itshigh surface conductivity. The coexistence of electrically-conductivesmectite and hydrothermal fluids at the fluid-bearing section is ingood agreement with the interpretation of the high bulk electrical con-ductivity region provided by Aizawa et al. (2009) and Revil et al. (2011).

5.4. Comparison with the low-temperature hydrothermal activity:northeastern part of Unzen volcano

In this case study at Beppu geothermal area, strong hydrothermal al-teration of the rockmatrixwas indicated by elevated cementation expo-nents and surface conductivity. This section examines the influence ofthe strength of the hydrothermal alteration on the physical propertiesof the aquifer in terms of the electrical conductivity and permeability,by comparing the properties of relatively high- and low-temperaturehydrothermal systems.

Fig. 4(a) shows the vertical profiles of bulk, pore water, and surfaceconductivities, temperature, and normalized permeability at the studysite. Note that the surface conductivity is adjusted to the temperatureof the borehole GSB at each depth, using the following equation (Revilet al., 2002):

σ s Tð Þ ¼ σ s T0ð Þ 1þ θs T−T0ð Þ½ � ð4Þ

where: T0 is the reference temperature (°C), T is the temperature duringmeasurement (°C), σs(T0) is the surface conductivity at the referencetemperature (S/m), σs(T) is the surface conductivity at the temperatureT (S/m), and θs is the temperature dependence (°C−1). The value of θs isabout 0.04 for the cations expected to be contained within hydrother-mal fluids in geothermal and volcanic areas (Revil et al., 1998). Addi-tionally note that the profile of pore water conductivity was obtainedduring the drilling (Meidai Industry, 1988). Fig. 4(b) shows the verticalprofiles of bulk, pore water, and surface conductivities, and temperatureat borehole USDP-1, Unzen volcano, together with the profile of core per-meability normalized by its uppermost value (~10−14 m2) (Komori et al.,2010). Unzenvolcano is located about 100 kmwest of theBeppugeother-mal area, as shown in Fig. 1(a), and its northeastern flank bears a low-temperature hydrothermal fluid of 37 °C at the depth of 40 m.

(a) (b)

Fig. 4. Vertical profiles of temperature, bulk conductivity, porewater conductivity and estimated surface conductivity of thematrix after temperature correction. The profile of porewaterconductivity was obtained during drilling (Meidai Industry, 1988).

169S. Komori et al. / Journal of Volcanology and Geothermal Research 264 (2013) 164–171

As described in the following sentences, the conductivity features atthe Beppu GSB are quite different from those observed at the UnzenUSDP-1 borehole. At Beppu GSB, the low permeability section(150–200 m depth) develops above the hydrothermal-fluid-bearingsection (200–250 m depth), and has a large surface conductivity of2 × 10−1–3 × 10−1 S/m. The vertical profile of the bulk conductivityseems to be similar to the profiles of both pore water conductivity andsurface conductivity. Calculated isoconductivity points are 0.53 for174 m depth, 0.82 for 201 m depth, 1.0 for 228 m depth, and 0.18 for298 m depth, using Eq. (3) and the estimated surface conductivitiesand assuming t(+)0.38 for NaCl solution. The pore water conductivitiessampled at each depth are almost the same or smaller than theisoconductivity points. This means that the surface conductivity of therock matrix makes a non-negligible contribution to the bulk conductiv-ity at the strongly-altered hydrothermal field. On the other hand, thesection with low permeability and high surface conductivity does notdevelop at Unzen USDP-1; this is due to no/little hydrothermal alter-ation under conditions of very low temperature. At Unzen USDP-1, thevertical profile of the bulk conductivity is difficult to explain from theprofile of surface conductivity of the rock matrix alone, and essentiallyreflects the distribution of pore water conductivity. Comparison be-tween these two different hydrothermal systems quantitatively showsthat hydrothermal alteration could significantly change the contribu-tion to bulk conductivity from the rock matrix/pore water.

5.5. Quantitative relation between the exposed temperature and the surfaceconductivity

In general, the type anddegree of rock alteration dependon the tem-perature to which the rock has been exposed. This section examines the

relationship between exposure temperature and the surface conductiv-ity of the matrix.

The authors used the surface conductivity of the drillcore estimatedin this study and in Komori et al. (2010) and the temperature of theborehole at each depth. Prior to the above examination, it is necessaryto consider whether or not the exposure temperature corresponds tothe temperature at the present stage. In the case of Beppu GSB, Gianelliet al. (1992) found titanite in the drillcores below the depth of 250 m,as shown in Fig. 1(e), and suggested that it could be stabilized at a highertemperature (150–200 °C) than that of the present stage. This sugges-tion is also consistent with the phase transition from smectite to illite/smectite at the depths below 250 m, as also shown in Fig. 1(e). As men-tioned in Introduction, smectite is unstable under high temperature con-ditions, irreversibly transforming into other, less-conductive minerals,such as illite. Therefore, the surface conductivity at a depth of 298 mcould be predominantly influenced by previous rock alteration takingplace under temperatures between 150 and 200 °C. The above discus-sion also implies that the rocks at depths shallower than 270 m havenot previously been exposed to such high temperature conditions.Therefore, it would be a good approximation to assume that the exposedtemperatures of the drillcores at depths of 174, 201, 228 m are repre-sented by those observed at the present stage and the indicated depths.In the case of Unzen USDP-1, visual inspection found that the drillcoresdemonstrate predominantly fresh matrices (Komori et al., 2010). Con-sidering their small surface conductivities, it is likely they have little/noalteration, and have been exposed only to low temperature conditions.Therefore, at Unzen USDP-1, it would be a good approximation to as-sume that the exposed temperatures of the drillcores are the same asthose of the present stage at the indicated depths.

Fig. 5 shows the surface conductivity of the matrix as a functionof the exposure temperature, modeled on the basis of the above

170 S. Komori et al. / Journal of Volcanology and Geothermal Research 264 (2013) 164–171

discussions. Note that the surface conductivity at the reference temper-ature of 25 °C is used for plotting. Under low temperature conditions,the rock matrix maintains a small surface conductivity in the range of10−4–10−3 S/m, as a result of the relative lack of hydrothermal alter-ation. With an increase in the exposure temperature, the rock matrixbecomes more susceptible to hydrothermal alteration, and developsan elevated surface conductivity on the order of 10−1 S/m at tempera-tures of 100–150 °C, due to an increase in smectite content. When tem-peratures rise to greater than 150 °C, the surface conductivity is one-half to one order of magnitude smaller than those at 100–150 °C. Highsurface conductivities at temperatures of 100–150 °C suggest thatsmectite is stable at relatively higher temperatures compared to its sta-ble condition at sedimentary basin (Morrison and Parry, 1986; Hillieret al., 2006). The stability of smectite at relatively high temperatureshas been reported at many active hydrothermal systems (Ji et al.,1997; Inoue et al., 2004); some of its possible reasons are discussed byVidal et al. (2012). In this study, it is assumed that surface conductivitydecreases at temperatures in the range of 150–200 °C. These tempera-tures are consistent with the findings of Anderson et al. (2000); inthat study of the relationship between the phase of the clay mineralsand temperature, the investigators reported that 70% of smectite ischanged into a mixed phase of illite and smectite at a temperature of180 °C.

It has been suggested that surface conductivity can be represent-ed as a simple function of exposure temperature. This representa-tion has been suggested by many previous investigators, as aresult of comparisons between bulk conductivity from borehole log-ging and the emergence of smectite in recovered drillcores (Stanleyet al., 1976; Wright and Nielson, 1986; Galanopoulos et al., 1991).However, the presence/absence of smectite should ideally be com-pared with the surface conductivity of the rock matrix, becausethat quantity is directly correlated with smectite content. In thatsense, this study is the first to show quantitatively the influence ofhydrothermal alteration on the rock matrix from the viewpoint ofelectrical conductivity. Further studies will provide a better under-standing of the relationship between surface conductivity and theexposure temperature of the in-situ rock. Eventually, this relation-ship, together with the pore water conductivity as a function of tem-perature and salinity, could be combined with the bulk conductivitydistribution and the distributions of temperature and salinity associat-ed with the influx of volcanic fluids to allow an evaluation of volcanicfluid flux from observed electrical conductivity structure.

Fig. 5. Plots of surface conductivity versus exposure temperature from this study and Komoriet al. (2010). Note that the value of surface conductivity is adjusted to that at 25 °C.

6. Conclusions

In this study, the physical properties related to electrical conductivity(porosity, formation factor, and surface conductivity) of volcaniclastic de-posits were estimated, using property measurements on drillcores thathave been strongly altered under relatively high temperature conditions.By comparison with the properties of unaltered drillcores from the lowtemperature portion of the system, it was shown that the pattern of thecontributions to the bulk conductivity from pore water and the rock ma-trix is quite different from each other. Under low temperature conditions,bulk conductivity is dominated by the contribution from pore water. Onthe other hand, the rock matrix has a non-negligible contribution tobulk conductivity under high temperature conditions. It was also showedthat the surface conductivity of the rock matrix could simply reflect theexposure temperature; which is consistent with conditions necessaryfor the generation and stability of smectite at the active hydrothermalsystems. In the future, further conductivity measurements will betterclarify the relation between the surface conductivity and the exposuretemperature, using drillcores of the volcaniclastic materials. These dataare expected to give strong constraints on the quantitative interpretationof the electrical conductivity structure of a volcano, especially in terms ofthe evaluation of volcanic fluid fluxes.

Acknowledgments

We thank H. Shimizu and T. Inui for providing the laboratory and in-struments, and J.P. Fairley for improving the manuscript. We also thankK. Takemura, T. Goto, N. Oshiman, and R. Yoshimura (Kyoto Univ.) fortheir valuable discussions.

We would like to appreciate two anonymous reviewers for criticalreview and constructive comments, and A. Aiuppa for editorial support.This work was supported by the Grant-in-Aid for Scientific Research(No. 21403003 and No. 23310120, T. Kagiyama) from the Ministry ofEducation, Culture, Sports, Science and Technology, Japan.

Appendix A. Supplementary data

Supplementary data associated with this article can be found in theonline version, at http://dx.doi.org/10.1016/j.jvolgeores.2013.08.009.These data include Google map of the most important areas describedin this article.

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