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Thermal structure beneath Kuju volcano, central Kyushu, Japan
Journal of Volcanology and Geothermal Research, 54 ( 1992 ) 107-115
107
Elsevier Science Publishers B.V., Amsterdam
Thermal structure beneath Kuju volcano, central Kyushu, Japan Sachio Ehara Geothermal Laboratory, Faculty of Engineering, Kyushu University, Fukuoka, 812, Japan (Received May 15, 1991; revised and accepted May 20, 1992 )
ABSTRACT Ehara, S., 1992. Thermal structure beneath Kuju volcano, central Kyushu, Japan. J. VoIcanol. Geotherm. Res., 54: 107115. Thermal conditions beneath Kuju volcano, central Kyushu, Japan are simulated in two manners. Simulation of the regional high heat flow around Kuju volcano shows that the underlying magma reservoir is not too!ten but maintains high temperatures of 400-700°C. Simulation of the thermal processes beneath the solfatara field in the central part of Kuju volcano suggests that the geothermal reservoir is a two-phase reservoir heated by magmatic steam.
Introduction
Utilization of volcano energy is an important subject in future geothermal energy development. Although extracting energy directly from crustal magma sources (Chu et al., 1990) is an interesting subject, it may be more feasible to extract energy from shallow but hot fluid reservoirs which exist between surficial intense fumarolic activities and crustal magma sources in active volcanic geothermal systems. In order to develop such volcano energy, it is very important to clarify thermal processes beneath active fumarolic areas. However, numerical models for active volcanic geothermal systems have not been well developed so far. though geological and geochemical models have been presented (e.g., Gianelli et al., 1988; Traineau et al., 1989). In this paper a numerical model of a volcanic geothermal system at Kuju volcano, Japan is presented. Correspondence to: S. Ehara, Geothermal Laboratory, Faculty of Engineering, Kyushu University, Fukuoka, 812 Japan.
The Kuju volcano group, composed of many lava domes, is situated in the Beppu-Shimabara volcanic graben in central Kyushu. Japan (Matsumoto, 1979; Fig. 1 ). It is a typical andesitic island-arc volcano. The main rock type is hornblende andesite (VSJ and IAVCEI, 1972).The volcanic activity started about 0.3 Ma and the youngest radiometric age is several tens of thousands of years (Watanabe et al., 1987; Hayashi, 1988). In historic times, only phreatic explosions have occurred several times (JMA, 1975). An active solfatara field in an explosive crater of Mt. Hosshoyama of the group shows the most intense geothermal activity in central Kyushu. The natural heat discharge is estimated at about 100 MW, most of it from steaming ground and fumaroles (Ehara et al., 1981 ). Temperatures of fumaroles generally exceed 200°C and the maximum observed temperature is 508 °C (Mizutani et al., 1986). The thermal state beneath Kuju volcano is numerically simulated by computer modelling in two different manners. The regional high heat flow around Kuju volcano is
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simulated in terms of conductive cooling of the magma reservoir beneath Kuju volcano. The thermal processes beneath the active solfatara field in the central part of Kuju volcano are simulated in terms of convective heat transfer in the geothermal reservoir beneath the solfatara field.
Regional thermal structure around Kuju volcano
Fig. 2. Distribution of heat flow and Curie point depth in and around Kyushu. Solid and broken lines show heat flow and Curie point depth, respectively. Solid circles show heat flow stations. 7 km and 8 km depths to Curie point are shown in central and south Kyushu, respectively. Solid triangles show active volcanoes and open triangles show other Quaternary volcanoes.
incide very well. This means that the upper crust of central and south Kyushu is at high temperatures at the present time.
(1) Heat flow and Curie isotherm (2) Conductive thermal model Heat flow in and around Kyushu island increases gradually from the Pacific side to the Japan Sea side (Ehara, 1989). A revised heat flow map is shown in Figure 2. Heat flows higher than 100 m W / m 2 are observed in central and south Kyushu. The heat flow ranges from 100 to 250 m W / m 2 around Kuju volcanoes. High heat flow is also observed around Sakurajima and Kirishima volcanoes in south Kyushu, where heat flow ranges from 100 to 160 m W / m 2. Okubo et at. (1985) determined Curie point depths for t~he Island of Kyashu and surrounding areas. They showed very shallow Curie point depths in volcanic areas of central and south Kyushu (Fig. 2 ). The distributions of heat flow and Curie point depth co-
In this region of high heat flow around Kuju volcano, there are several geothermal areas, where hot water circulates in the upper crust. The underground temperatures beneath such geothermal areas are very high in the shallow part of the crust but such hot regions are limited to relatively narrow zones as shown in Figure 3 (Fujita and Abe, 1988; NEDO, 1988). Thus the effects of local convective systems are superimposed on the regional conductive structure. A two-dimensional heat conduction analysis along line B-B' in Figure 3 was done using a two-dimensional finite-element code developed by Lee et al. (1980). The region for calculation is 50 km long in the north-south
THERMAL STRUCTURE BENEATH KUJU VOLCANO, CENTRAL KYUSHU, JAPAN
HEAT FLOW
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Fig. 3. Heat flow distribution in and around Kuju volcano and local high temperature anomalies (A and B) due to convective systems. The underground temperatures at sites A and B are those at 300 m and 500 m below sea level, respectively. Solid circles show locations of wells where heat flow values were determined. Solid triangles show some peaks of the Kuju volcano group.
direction and about 10 km deep (from the surface to 10 km below sea level). The crustal rocks are divided into four layers, that is, Holocene to upper Pleistocene Kuju volcanic rocks, middle to lower Pleistocene Hohi volcanic rocks, Pliocene volcanic rocks and Cretaceous plutonic and metamorphic rocks (basement rocks) as shown in Figure 4
Fig. 4. Simplified two-dimensional crustal model for heat conduction simulation. Layer l = H o l o c e n e to upper Pleistocene Kuju volcanic rocks; //--=Middle to lower Pleistocene Hohi volcanic rocks; 1II= Pliocene volcanic rocks; IV= Cretaceous plutonic and metamorphic rocks.
(NEDO, 1988). Physical properties such as thermal conductivity, thermal diffusivity and density are assigned to each layer based on measured values, as shown in Table 1 (NEDO, 1988). A representative value for granite is used for the value of heat generation (Buntebarth, 1984). Both steady-state and transient simulations were performed.
TABLE 1 Physical properties of each layer used for simulation
Layer Layer Layer Layer
I II III IV
Thermal diffusivity I (10-6 m2/s)
Thermal conductivity 2 (W/InK)
Density 2 (kg/m ~)
Specific heat 3 (J/kg K)
Radioactive heat production 3 (10 -6 W / m 3)
0.7 0.8 0.9 0.9
1.3 1.3 1.5 1.9
2350 2140 2120 2640
800 800 800 800
3.0 3.0 3.0 3.0
~calculated parameter ( = Thermal conductivity / (Density. Specific heat ) ). 2measured values (NEDO, 1988). 3assumed values (Buntebarth, 1984).
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A good-fit model is shown in Figure 5. However, this model shows that the temperature at 5 km depth reaches 1000°C. The calculated temperatures beneath Kuju volcano exceed the melting point of granitic rocks. However, seismic shear waves can pass through the inferred region of high temperatures (Ehara and Mogi, 1989 ). Therefore, the calculated steadystate temperature distribution beneath Kuju volcano is too high.
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Radiometric ages of volcanic rocks in the Kuju volcano group are about 0.3 Ma to several tens thousands years ago (Watanabe et al., 1987; Hayashi, 1988 ). In the transient simulations, it is assumed that the magma reservoir beneath Kuju volcano has cooled by conduction since 0.05 Ma. Thus, the observed heat flows are not in a steady state but in a transient one. In the transient simulation, an initial hightemperature distribution which represents the magma reservoir beneath Kuju volcano (Fig. 6 ) is allowed to cool for 0.05 Ma. The lateral extent of the initial magma reservoir is about 25 km wide, comparable to the lateral extent of the Kuju volcano group. The calculated
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Fig. 5. Upper: Calculated steady-state temperature distribution beneath Kuju volcano. Unit: °C. Lower: The solid line shows the calculated heat flow profile. The broken linesshow ranges of observed heat flows. Solid circlesshow observed heat flows.
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(3) Steady-state simulation In the steady-state simulations constant temperatures were specified at the upper and lower boundaries. The temperatures at the upper surface were fixed but depend on altitude at a lapse rate of - 0 . 5 °C/100m. The temperatures at the lower boundary are changed to simulate observed heat flow variations. The observed heat flow values are projected vertically on the line B-B' except the value at AS-2, which is projected on the intersection of the contour line of 100 m W / m 2 and the line B-B'.
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Fig. 6. Initial temperature distribution beneath Kuju volcano for transient simulations. The IO00°C isotherm shows the shape of the magma reservoir beneath Kuju volcano.
T H E R M A L S T R U C T U R E BENEATH K U J U VOLCANO, C E N T R A L K Y U S H U , JAPAN
1
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Kuju volcano group. Kuju-iwoyama is an explosive crater of Mr. Hosshoyama and shows the most intense solfataric activity. The solfatara field (ca. 0.2 km 2) is at the northeastern flank of Mt. Hosshoyama. The natural heat discharge is estimated at about 100 MW and most of it (more than 95%) is from fumaroles and steaming ground (Ehara et al., 1981 ). The fumarole temperatures generally exceed 200 °C and the observed maximum temperature is 508°C (Mizutani et al., 1986). Volcanic gas emissions are estimated at 168 tons/day (CO2), 53 tons/day (H2S), 26 tons/day (SO2), 3.1 tons/day (S), 3.1 tons/day (HC1) and 0.07 tons/day (HF) (Ehara et al., 1981 ).
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Fig. 7. Upper: Present temperature distribution beneath Kuju volcano based on 0.05 Ma of cooling. Lower: The solid line shows the calculated heat flow profile. The broken lines show ranges of observed heat flows. Solid circles show observed heat flows.
present temperature distribution is shown in Figure 7. The temperature beneath Kuju volcano is about 400-700°C at 5 km depth. This value is reasonable for seismic shear wave transmission. Kuju-iwoyama, which shows the most active solfataric activity, exists above the hottest region at depth as shown in Figure 7. Thermal process beneath the central part of Kuju volcano
(1) Solfatara field Mt.Hosshoyama is in the central part of the
Microseismic observations were conducted around the solfatara field (Ehara et al., 1990 ). An active seismic zone was detected just beneath the solfatara field down to about 1.5 km depth. The origin of such anomalous microearthquake activity may be high pore pressure due to rising hot fluid. An anomalously low P-wave velocity structure was also detected just beneath the solfatara field (Ehara et al., 1990), suggesting that the region beneath the solfatara field is fractured and, therefore, highly permeable.
(3) Low-resistivity zone ELF and ULF magnetotelluric soundings were made in and around the solfatara field (Mogi et al., 1988 ). An extremely low resistivity value of 1 ohm-m was detected just beneath the solfatara field down to deeper than 1 km depth. The localized low-resistivity zone is 900 m long in the E-W direction and 500 m long in the N-S direction. Such low resistivity beneath the solfatara field is considered to be due to hot saline geothermal fluids. In this field both salinity and heat are important factors causing such an extremely low resistivity, be-
112
~.EHAR~
cause fumarolic condensates contain very high CI content, up to 8000 ppm (Mizutani et al., 1986) and the reservoir temperature amounts to 340°C at 2 km depth as shown later.
(4) Conceptual model A conceptual thermal model is based on thermal, isotopical and structual data. Drilling (NEDO, 1988) and gravity (Komazawa and Kamata, 1985 ) data show the depth of basement is 1.5 to 2.0 km in and around the solfatara field. The low-resistivity zone extends to the depth of basement. Microearthquakes occur at a shallower depth above the basement. Isotopic study Of fumaroles, hot springs and groundwater shows mixing of meteoric water with magmatic water (Matsubaya et al., 1975; Mizutani et al., 1986 ). The conceptual model presented in Figure 8 summarizes the abovementioned data. Magmatic steam and downgoing meteoric water mix at about 2 km depth. Then a geothermal reservoir is formed and hot fluids move upward through the low-resistivity zone. Microearthquakes occur only in the upper part of the reservoir. Finally, hot water and steam discharge at the ground surface.
(5) Numerical model Based on the magmatic steam and meteoric water mixing model, the thermal processes beneath the solfatara field were simulated by computer modelling (Pruess and Schroder, 1980; O'Sullivan, 1985), assuming a body of radial symmetry. The diameter and depth of the area concerned are 5 km and 2 km, respectively, considering the area of recharge (Kawamura, 1984) as shown in Figure 9. The permeable zone extends from the earth's surface to 2 km depth (A1 to A8 in Fig. 9). The diameter of the permeable zone is 0.5 kin. Magmatic steam is supplied from the bottom surface of the permeable zone. Boundary conditions at the side are adiabatic and impermeable. The surface temperature ( 10 °C) and pressure (1 bar) are constant. Physical properties such as density, porosity, permeability, thermal conductivity and specific heat are assigned to each block as shown in Table 2. The following parameters provide a good fit model as shown in Figure 9 : 3 0 kg/s of magmatic !AI
--B 1
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3OKgl$ 3500 KJ I Kg 105 MW
0.29MW
1.6MW
Fig. 9. Quantitative thermal model beneath Kuju volcano. Upper: Block layout for simulation. Lower: Some parameters for a good fit model.
113
THERMAL STRUCTURE BENEATH KUJU VOLCANO, CENTRAL KYUSHU, JAPAN
TABLE 2 Physical properties of each block used for simulation
Block A Blocks B and C
Thermal conductivity1 (W/mK)
Density 1 (kg/m3)
Porosity ~ (%)
Specific heat] (J/kg K)
Permeability: (m darcy)
1.65 1.80
2450 2500
17.5 10.0
775 775
50 1
~assumed based on the data of NEDO (1988) and Buntebarth (1984). 2estimated through preliminary simulations. But they are kept constant in later simulations.
steam is supplied from below mixes with about 10 kg/s of the downgoing meteoric water and finally steam (35.8 kg/s) and hot water(4.3 kg/s) discharge at the surface. The calculated heat discharge is 104.4 MW. The enthalpy of the magmatic steam is 3500 kJ/kg and, therefore,the temperature is higher than 580 ° C, because the pressure of the magmatic steam is higher than that at the bottom of the reservoir about 15 MPa. Temperature, pressure and vapor saturation in the reservoir are shown in Figure 10. The Corey's expressions are applied to the relative permeability functions (Corey, 1954). The temperature of the first layer (0250 m ) at the central discharge part (A 1 in Fig. 9) is about 210°C, which agrees well with the observed fumarolic temperatures. The observed maximum temperature is much higher
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TEMPERATURE(°C) 20O i
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than the estimated temperatures of the twophase reservoir. But most of observed fumarolic temperatures are about 200 °C. Then it is considered that the reservoir is two-phase as a whole and some magmatic steam passes through the reservoir without equilibrating with liquid water. The temperature in the bottom layer (1750-2000 m) at the central discharge part (A8 in Fig. 9) exceeds 340°C. The central high-permeable zone is in two-phase conditions at any depth. The pressure in the central discharge part [P(A ) in Fig. 10 ] is a little (to 1 MPa) higher than hydrostatic in the upper part of the reservoir. The higher pressure in the high-permeable zone may be the origin of high seismic activity in this part, since elevated pore pressure lowers the strength of rocks.
30O ,
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0 --
VAPOR SATURATION 0.2 0.4 0.~, 0B ii 1 • __/
10
/
I00C
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; PRESSURE( MPa 1') o
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Fig. 10. Vertical distribution of temperature,pressure and vapor saturation in the reservoir. T(A) and T(C) show temperatures in the central permeable zone and outside the central zone, respectively. P(A)and P(C) show pressures in the central permeable zone and outside the central zone, respectively.
114
Conclusions
Heat flow values higher than 100 mW/m 2 exist around Kuju volcano, central Kyushu, Japan. The high heat flows are interpreted in terms of conductive cooling of the magma reservoir beneath Kuju volcano. The magma began to cool about 0.05 Ma. The magma reservoir is not molten at present but maintains temperatures of 400-700 °C at 5 km depth. An active solfatara field exists just above the hottest part of the magma reservoir. Thermal processes beneath the solfatara field were simulated by computer modelling. The following parameters provide a good fit model; 30 kg/s of magmatic steam (the enthalpy is 3500 kJ/ kg and the temperature is higher than 580°C) is supplied from below, mixes with the downgoing meteoric water (10 kg/s) and steam (35.8 kg/s) and hot water (4.3 kg/s) discharge at the surface. The calculated heat discharge is 104.4 MW. The central, high-permeability zone contains two-phase fluids to 2 km depth where the temperature exceeds 340 ° C.
References Buntebarth, G., 1984. Geothermics. Springer-Verlag, Berlin, 144 pp. Chu, T.Y., Dunn, J.C., Finger, J.T., Rundle, J.B. and Westrich, H.R., 1990.The magma energy program. Geotherm. Resour. Council Bull. 19 ( 1 ): 42-56. Corey, A.T., 1954. The interrelation between gas and oil relative permeabilities. Prod. Mon., 19: 38-41. Ehara, S., 1989. Thermal structure and seismic activity in central Kyushu, Japan. Tectonophysics, 159: 269-278. Ehara, S. and Mogi, T., 1989. Deep structure beneath volcanic high temperature geothermal systems. In: Working Group for Continental Scientific Drilling of Japan, Sites for Continental Scientific Drilling, (III): 1-22 (in Japanese). Ehara, S., Yuhara, K. and Noda, T., 1981. Hydrothermal system and the origin of the volcanic gas of Kuju-iwoyama volcano, Japan, deduced from heat discharge, water discharge and volcanic gas emission data. Bull. Volcanol. Soc. Jpn., 26:35-56 (in Japanese with Eng-
~ I/tIAR-,~ lish abstract ). Ehara, S., Bito, T., Ohi, T. and Kasai, H., 1990. Microseismic activity beneath active fumarolic areas-- A case study at Kuju-iwoyama in central Kyushu, Japan, J. Geotherm. Res. Soc. Jpn., 12:263-281 (in Japanese with English abstract). Fujita, T. and Abe, M., 1988. Survey of Oguni geothermal field, Kyushu, Japan. J. Jpn., Geotherm. Energy Assoc., 25:287-314 (in Japanese with English abstract). Gianelli, G., Puxeddu, M., Batini, F., Bertini, G., Dini, I., Pandeli, E. and Nicolich, R., 1988. Geological model of a young volcano-plutonic system: The geothermal region of Monte Amiata (Tuscany, Italy). Geothermics, 17: 719-734. Hayashi, M., 1988. Quaternary volcanism as the heat source for some geothermal fields in Kyushu, Japan. Proc. Int. Symp. on Geothermal Energy, 1988. Kumamoto and Beppu, Japan, p. 446. Japan Meteorological Agency, 1975. National Catalogue of the Active Volcanoes in Japan, ll9 pp. (in Japanese ). Kawamura, M., 1984. Thermal condition of the geothermal field around Mt. Waita in central Kyushu, Japan. ( 1 ) Distributions of subsurface temperature and heat discharge. J. Geotherm. Res. Soc. Jpn., 6:217-241. In Japanese with English abstract). Komazawa, M. and Kamata, H., 1985. The basement structure of the Hohi geothermal area obtained by gravimetric analysis in central-north Kyushu, Japan. Rep. Geol. Surv. Jpn., 264:303-333 (in Japanese with English abstract). Lee, T.C., Rudman, A.J. and Sjoreen, A., 1980. Application of finite-element analysis to terrestrial heat flow. Indiana. Geol. Surv. Occasional Paper 29, 53 pp. Matsubaya, O., Ueda, A., Matsuhisa, U., SakaL H. and Sasaki, A., 1975. Isotopic study of hot springs and volcanoes in Kyushu, Japan. Bull. Geol. Surv. Jpn., 26: 375-392 (in Japanese with English abstract). Matsumoto, Y., 1979. Some problems on volcanic activities and depression structures in Kyushu, Japan. Mem. Geol. Soc. Jpn., 16:127-139 (in Japanese with English abstract ). Mizutani, Y., Hayashi, S. and Sugiura, T., 1986. Chemical and isotopic compositions of fumarolic gases from Kuju-Iwoyama, Kyushu, Japan. Geochem. J., 20: 273285. Mogi, T., Nakama, K. and Shimoizumi, M., 1988. Magnetotelluric study of the geothermal system in Kuju Volcano. Proc. Int. Symp. on Geothermal Energy, 1988, Kumamoto and Beppu, Japan, pp. 539-542. New Energy Development Organization (NEDO), 1988. An interim report, Kuju area, pp. 65-120 (in Japanese). Okubo, Y., Graf, R.J., Hansen, R.O., Ogawa, K. and Tsu, H., 1985. Curie point depths of the Kyushu and surrounding areas, Japan. Geophysics, 53:481-494.
THERMAI_ STRUCTURE BENEATH KUJU VOLCANO, CENTRAL KYUSHU, JAPAN
O'Sullivan, M.J.. 1985. Geothermal reservoir simulation. Energy Res., 9: 319-332. Pruess, K. and Schroder, R.C., 1980. SHAFT79. User's Manual, LBE-10861, 47 pp. Volcanological Society of Japan and IAVCEI, 1972. List of World Active Volcanoes. 160 pp. Traineau. H., Westercamp, D. and Benderritter, Y., 1989.
1 15
Case study of a volcanic geothermal system. Mount Pelee, Martinique. J. Volcanol. Geotherm. Res.. 38: 4966. Watanabe, K., Hayashi, M. and Fujino, T., 1987. Fission track age of volcanoes in the Kuju volcanic region in relation to geothermal activity. J. Geotherm. Res. Soc. Jpn., 9:207-217.
107
Elsevier Science Publishers B.V., Amsterdam
Thermal structure beneath Kuju volcano, central Kyushu, Japan Sachio Ehara Geothermal Laboratory, Faculty of Engineering, Kyushu University, Fukuoka, 812, Japan (Received May 15, 1991; revised and accepted May 20, 1992 )
ABSTRACT Ehara, S., 1992. Thermal structure beneath Kuju volcano, central Kyushu, Japan. J. VoIcanol. Geotherm. Res., 54: 107115. Thermal conditions beneath Kuju volcano, central Kyushu, Japan are simulated in two manners. Simulation of the regional high heat flow around Kuju volcano shows that the underlying magma reservoir is not too!ten but maintains high temperatures of 400-700°C. Simulation of the thermal processes beneath the solfatara field in the central part of Kuju volcano suggests that the geothermal reservoir is a two-phase reservoir heated by magmatic steam.
Introduction
Utilization of volcano energy is an important subject in future geothermal energy development. Although extracting energy directly from crustal magma sources (Chu et al., 1990) is an interesting subject, it may be more feasible to extract energy from shallow but hot fluid reservoirs which exist between surficial intense fumarolic activities and crustal magma sources in active volcanic geothermal systems. In order to develop such volcano energy, it is very important to clarify thermal processes beneath active fumarolic areas. However, numerical models for active volcanic geothermal systems have not been well developed so far. though geological and geochemical models have been presented (e.g., Gianelli et al., 1988; Traineau et al., 1989). In this paper a numerical model of a volcanic geothermal system at Kuju volcano, Japan is presented. Correspondence to: S. Ehara, Geothermal Laboratory, Faculty of Engineering, Kyushu University, Fukuoka, 812 Japan.
The Kuju volcano group, composed of many lava domes, is situated in the Beppu-Shimabara volcanic graben in central Kyushu. Japan (Matsumoto, 1979; Fig. 1 ). It is a typical andesitic island-arc volcano. The main rock type is hornblende andesite (VSJ and IAVCEI, 1972).The volcanic activity started about 0.3 Ma and the youngest radiometric age is several tens of thousands of years (Watanabe et al., 1987; Hayashi, 1988). In historic times, only phreatic explosions have occurred several times (JMA, 1975). An active solfatara field in an explosive crater of Mt. Hosshoyama of the group shows the most intense geothermal activity in central Kyushu. The natural heat discharge is estimated at about 100 MW, most of it from steaming ground and fumaroles (Ehara et al., 1981 ). Temperatures of fumaroles generally exceed 200°C and the maximum observed temperature is 508 °C (Mizutani et al., 1986). The thermal state beneath Kuju volcano is numerically simulated by computer modelling in two different manners. The regional high heat flow around Kuju volcano is
0377-0273/93/$05.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
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108
35°N
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135
Fig. 1. Index map of Kuju volcano. Solid triangles show active volcanoes.
simulated in terms of conductive cooling of the magma reservoir beneath Kuju volcano. The thermal processes beneath the active solfatara field in the central part of Kuju volcano are simulated in terms of convective heat transfer in the geothermal reservoir beneath the solfatara field.
Regional thermal structure around Kuju volcano
Fig. 2. Distribution of heat flow and Curie point depth in and around Kyushu. Solid and broken lines show heat flow and Curie point depth, respectively. Solid circles show heat flow stations. 7 km and 8 km depths to Curie point are shown in central and south Kyushu, respectively. Solid triangles show active volcanoes and open triangles show other Quaternary volcanoes.
incide very well. This means that the upper crust of central and south Kyushu is at high temperatures at the present time.
(1) Heat flow and Curie isotherm (2) Conductive thermal model Heat flow in and around Kyushu island increases gradually from the Pacific side to the Japan Sea side (Ehara, 1989). A revised heat flow map is shown in Figure 2. Heat flows higher than 100 m W / m 2 are observed in central and south Kyushu. The heat flow ranges from 100 to 250 m W / m 2 around Kuju volcanoes. High heat flow is also observed around Sakurajima and Kirishima volcanoes in south Kyushu, where heat flow ranges from 100 to 160 m W / m 2. Okubo et at. (1985) determined Curie point depths for t~he Island of Kyashu and surrounding areas. They showed very shallow Curie point depths in volcanic areas of central and south Kyushu (Fig. 2 ). The distributions of heat flow and Curie point depth co-
In this region of high heat flow around Kuju volcano, there are several geothermal areas, where hot water circulates in the upper crust. The underground temperatures beneath such geothermal areas are very high in the shallow part of the crust but such hot regions are limited to relatively narrow zones as shown in Figure 3 (Fujita and Abe, 1988; NEDO, 1988). Thus the effects of local convective systems are superimposed on the regional conductive structure. A two-dimensional heat conduction analysis along line B-B' in Figure 3 was done using a two-dimensional finite-element code developed by Lee et al. (1980). The region for calculation is 50 km long in the north-south
THERMAL STRUCTURE BENEATH KUJU VOLCANO, CENTRAL KYUSHU, JAPAN
HEAT FLOW
l=
N Krn
tOOmW/m~
/
[ 09
S
.2~ooo3~~~'3:.,L~~~ v
//
IV
I I t l l l l l ' ~ l I T I I l " T I l ' l l l lO lO 30 ~0
ASO
Kin 0
VOLCANO
Fig. 3. Heat flow distribution in and around Kuju volcano and local high temperature anomalies (A and B) due to convective systems. The underground temperatures at sites A and B are those at 300 m and 500 m below sea level, respectively. Solid circles show locations of wells where heat flow values were determined. Solid triangles show some peaks of the Kuju volcano group.
direction and about 10 km deep (from the surface to 10 km below sea level). The crustal rocks are divided into four layers, that is, Holocene to upper Pleistocene Kuju volcanic rocks, middle to lower Pleistocene Hohi volcanic rocks, Pliocene volcanic rocks and Cretaceous plutonic and metamorphic rocks (basement rocks) as shown in Figure 4
Fig. 4. Simplified two-dimensional crustal model for heat conduction simulation. Layer l = H o l o c e n e to upper Pleistocene Kuju volcanic rocks; //--=Middle to lower Pleistocene Hohi volcanic rocks; 1II= Pliocene volcanic rocks; IV= Cretaceous plutonic and metamorphic rocks.
(NEDO, 1988). Physical properties such as thermal conductivity, thermal diffusivity and density are assigned to each layer based on measured values, as shown in Table 1 (NEDO, 1988). A representative value for granite is used for the value of heat generation (Buntebarth, 1984). Both steady-state and transient simulations were performed.
TABLE 1 Physical properties of each layer used for simulation
Layer Layer Layer Layer
I II III IV
Thermal diffusivity I (10-6 m2/s)
Thermal conductivity 2 (W/InK)
Density 2 (kg/m ~)
Specific heat 3 (J/kg K)
Radioactive heat production 3 (10 -6 W / m 3)
0.7 0.8 0.9 0.9
1.3 1.3 1.5 1.9
2350 2140 2120 2640
800 800 800 800
3.0 3.0 3.0 3.0
~calculated parameter ( = Thermal conductivity / (Density. Specific heat ) ). 2measured values (NEDO, 1988). 3assumed values (Buntebarth, 1984).
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. . . .
i
. . . .
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. . . .
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A good-fit model is shown in Figure 5. However, this model shows that the temperature at 5 km depth reaches 1000°C. The calculated temperatures beneath Kuju volcano exceed the melting point of granitic rocks. However, seismic shear waves can pass through the inferred region of high temperatures (Ehara and Mogi, 1989 ). Therefore, the calculated steadystate temperature distribution beneath Kuju volcano is too high.
S
i
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(4) Transient simulation I
30
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HORIZONTAL
....
3 0 0
+ ....
i ....
Radiometric ages of volcanic rocks in the Kuju volcano group are about 0.3 Ma to several tens thousands years ago (Watanabe et al., 1987; Hayashi, 1988 ). In the transient simulations, it is assumed that the magma reservoir beneath Kuju volcano has cooled by conduction since 0.05 Ma. Thus, the observed heat flows are not in a steady state but in a transient one. In the transient simulation, an initial hightemperature distribution which represents the magma reservoir beneath Kuju volcano (Fig. 6 ) is allowed to cool for 0.05 Ma. The lateral extent of the initial magma reservoir is about 25 km wide, comparable to the lateral extent of the Kuju volcano group. The calculated
< o
DISTANCE
(Km)
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-
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100
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.
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.
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.
.
i
3
.
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DISTANCE
.
.
.
I
.
¢ D
.
.
.
o
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(Km)
Fig. 5. Upper: Calculated steady-state temperature distribution beneath Kuju volcano. Unit: °C. Lower: The solid line shows the calculated heat flow profile. The broken linesshow ranges of observed heat flows. Solid circlesshow observed heat flows.
I
2
I
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i
. . . .
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(3) Steady-state simulation In the steady-state simulations constant temperatures were specified at the upper and lower boundaries. The temperatures at the upper surface were fixed but depend on altitude at a lapse rate of - 0 . 5 °C/100m. The temperatures at the lower boundary are changed to simulate observed heat flow variations. The observed heat flow values are projected vertically on the line B-B' except the value at AS-2, which is projected on the intersection of the contour line of 100 m W / m 2 and the line B-B'.
E-' E,-3 <:
' 2
i
m
J I
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t
. . . . 0
H O R I Z O N T A L
t 20
.
.
,
,
I
,
30
D I S T A N C E
,
I
. . . . .
40
( k r n )
Fig. 6. Initial temperature distribution beneath Kuju volcano for transient simulations. The IO00°C isotherm shows the shape of the magma reservoir beneath Kuju volcano.
T H E R M A L S T R U C T U R E BENEATH K U J U VOLCANO, C E N T R A L K Y U S H U , JAPAN
1
N
Kuju volcano group. Kuju-iwoyama is an explosive crater of Mr. Hosshoyama and shows the most intense solfataric activity. The solfatara field (ca. 0.2 km 2) is at the northeastern flank of Mt. Hosshoyama. The natural heat discharge is estimated at about 100 MW and most of it (more than 95%) is from fumaroles and steaming ground (Ehara et al., 1981 ). The fumarole temperatures generally exceed 200 °C and the observed maximum temperature is 508°C (Mizutani et al., 1986). Volcanic gas emissions are estimated at 168 tons/day (CO2), 53 tons/day (H2S), 26 tons/day (SO2), 3.1 tons/day (S), 3.1 tons/day (HC1) and 0.07 tons/day (HF) (Ehara et al., 1981 ).
I
E
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(2) Active seismic zone
KJ-5
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DISTANCE(km)
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HORIZONTAL DISTANCE
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(Km)
Fig. 7. Upper: Present temperature distribution beneath Kuju volcano based on 0.05 Ma of cooling. Lower: The solid line shows the calculated heat flow profile. The broken lines show ranges of observed heat flows. Solid circles show observed heat flows.
present temperature distribution is shown in Figure 7. The temperature beneath Kuju volcano is about 400-700°C at 5 km depth. This value is reasonable for seismic shear wave transmission. Kuju-iwoyama, which shows the most active solfataric activity, exists above the hottest region at depth as shown in Figure 7. Thermal process beneath the central part of Kuju volcano
(1) Solfatara field Mt.Hosshoyama is in the central part of the
Microseismic observations were conducted around the solfatara field (Ehara et al., 1990 ). An active seismic zone was detected just beneath the solfatara field down to about 1.5 km depth. The origin of such anomalous microearthquake activity may be high pore pressure due to rising hot fluid. An anomalously low P-wave velocity structure was also detected just beneath the solfatara field (Ehara et al., 1990), suggesting that the region beneath the solfatara field is fractured and, therefore, highly permeable.
(3) Low-resistivity zone ELF and ULF magnetotelluric soundings were made in and around the solfatara field (Mogi et al., 1988 ). An extremely low resistivity value of 1 ohm-m was detected just beneath the solfatara field down to deeper than 1 km depth. The localized low-resistivity zone is 900 m long in the E-W direction and 500 m long in the N-S direction. Such low resistivity beneath the solfatara field is considered to be due to hot saline geothermal fluids. In this field both salinity and heat are important factors causing such an extremely low resistivity, be-
112
~.EHAR~
cause fumarolic condensates contain very high CI content, up to 8000 ppm (Mizutani et al., 1986) and the reservoir temperature amounts to 340°C at 2 km depth as shown later.
(4) Conceptual model A conceptual thermal model is based on thermal, isotopical and structual data. Drilling (NEDO, 1988) and gravity (Komazawa and Kamata, 1985 ) data show the depth of basement is 1.5 to 2.0 km in and around the solfatara field. The low-resistivity zone extends to the depth of basement. Microearthquakes occur at a shallower depth above the basement. Isotopic study Of fumaroles, hot springs and groundwater shows mixing of meteoric water with magmatic water (Matsubaya et al., 1975; Mizutani et al., 1986 ). The conceptual model presented in Figure 8 summarizes the abovementioned data. Magmatic steam and downgoing meteoric water mix at about 2 km depth. Then a geothermal reservoir is formed and hot fluids move upward through the low-resistivity zone. Microearthquakes occur only in the upper part of the reservoir. Finally, hot water and steam discharge at the ground surface.
(5) Numerical model Based on the magmatic steam and meteoric water mixing model, the thermal processes beneath the solfatara field were simulated by computer modelling (Pruess and Schroder, 1980; O'Sullivan, 1985), assuming a body of radial symmetry. The diameter and depth of the area concerned are 5 km and 2 km, respectively, considering the area of recharge (Kawamura, 1984) as shown in Figure 9. The permeable zone extends from the earth's surface to 2 km depth (A1 to A8 in Fig. 9). The diameter of the permeable zone is 0.5 kin. Magmatic steam is supplied from the bottom surface of the permeable zone. Boundary conditions at the side are adiabatic and impermeable. The surface temperature ( 10 °C) and pressure (1 bar) are constant. Physical properties such as density, porosity, permeability, thermal conductivity and specific heat are assigned to each block as shown in Table 2. The following parameters provide a good fit model as shown in Figure 9 : 3 0 kg/s of magmatic !AI
--B 1
A2
8
!A3
B 3
!A~
!A6
C1
2
c
B4
C L.
B5
C5
B 6
I
,e
field
5
r
C6
A71 B7 iAS ~ 8 Solfatara
2
G3
c7 C8
Km
Meteoriwater c o o o o o o o o o o o o
. . . . . . . . . . . . . .
eoo ooo
104.4 MW 35.B 4.3
~ p
: : : :
!
. . . . . . .
,
t
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~
°* ~cr°earthquakea~:: : :~ll i ooo
÷
÷
~ •
go
8.7v~ts
1.3
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-b
~,~
~
'''l'SKm ~si "','8.~1 <----
STE,eavl
<
WATE]~
.--3.s
I
CONDUCTION Magcnat ic s t e a m
Fig. 8. Conceptual thermal model beneath the solfatara field of Kuju volcano.
3OKgl$ 3500 KJ I Kg 105 MW
0.29MW
1.6MW
Fig. 9. Quantitative thermal model beneath Kuju volcano. Upper: Block layout for simulation. Lower: Some parameters for a good fit model.
113
THERMAL STRUCTURE BENEATH KUJU VOLCANO, CENTRAL KYUSHU, JAPAN
TABLE 2 Physical properties of each block used for simulation
Block A Blocks B and C
Thermal conductivity1 (W/mK)
Density 1 (kg/m3)
Porosity ~ (%)
Specific heat] (J/kg K)
Permeability: (m darcy)
1.65 1.80
2450 2500
17.5 10.0
775 775
50 1
~assumed based on the data of NEDO (1988) and Buntebarth (1984). 2estimated through preliminary simulations. But they are kept constant in later simulations.
steam is supplied from below mixes with about 10 kg/s of the downgoing meteoric water and finally steam (35.8 kg/s) and hot water(4.3 kg/s) discharge at the surface. The calculated heat discharge is 104.4 MW. The enthalpy of the magmatic steam is 3500 kJ/kg and, therefore,the temperature is higher than 580 ° C, because the pressure of the magmatic steam is higher than that at the bottom of the reservoir about 15 MPa. Temperature, pressure and vapor saturation in the reservoir are shown in Figure 10. The Corey's expressions are applied to the relative permeability functions (Corey, 1954). The temperature of the first layer (0250 m ) at the central discharge part (A 1 in Fig. 9) is about 210°C, which agrees well with the observed fumarolic temperatures. The observed maximum temperature is much higher
........
~ "
~
100 ~
TEMPERATURE(°C) 20O i
p(c~---":'.-..P(A)
"L": .........
than the estimated temperatures of the twophase reservoir. But most of observed fumarolic temperatures are about 200 °C. Then it is considered that the reservoir is two-phase as a whole and some magmatic steam passes through the reservoir without equilibrating with liquid water. The temperature in the bottom layer (1750-2000 m) at the central discharge part (A8 in Fig. 9) exceeds 340°C. The central high-permeable zone is in two-phase conditions at any depth. The pressure in the central discharge part [P(A ) in Fig. 10 ] is a little (to 1 MPa) higher than hydrostatic in the upper part of the reservoir. The higher pressure in the high-permeable zone may be the origin of high seismic activity in this part, since elevated pore pressure lowers the strength of rocks.
30O ,
T A)
0 --
VAPOR SATURATION 0.2 0.4 0.~, 0B ii 1 • __/
10
/
I00C
3
200C
; PRESSURE( MPa 1') o
1;
Fig. 10. Vertical distribution of temperature,pressure and vapor saturation in the reservoir. T(A) and T(C) show temperatures in the central permeable zone and outside the central zone, respectively. P(A)and P(C) show pressures in the central permeable zone and outside the central zone, respectively.
114
Conclusions
Heat flow values higher than 100 mW/m 2 exist around Kuju volcano, central Kyushu, Japan. The high heat flows are interpreted in terms of conductive cooling of the magma reservoir beneath Kuju volcano. The magma began to cool about 0.05 Ma. The magma reservoir is not molten at present but maintains temperatures of 400-700 °C at 5 km depth. An active solfatara field exists just above the hottest part of the magma reservoir. Thermal processes beneath the solfatara field were simulated by computer modelling. The following parameters provide a good fit model; 30 kg/s of magmatic steam (the enthalpy is 3500 kJ/ kg and the temperature is higher than 580°C) is supplied from below, mixes with the downgoing meteoric water (10 kg/s) and steam (35.8 kg/s) and hot water (4.3 kg/s) discharge at the surface. The calculated heat discharge is 104.4 MW. The central, high-permeability zone contains two-phase fluids to 2 km depth where the temperature exceeds 340 ° C.
References Buntebarth, G., 1984. Geothermics. Springer-Verlag, Berlin, 144 pp. Chu, T.Y., Dunn, J.C., Finger, J.T., Rundle, J.B. and Westrich, H.R., 1990.The magma energy program. Geotherm. Resour. Council Bull. 19 ( 1 ): 42-56. Corey, A.T., 1954. The interrelation between gas and oil relative permeabilities. Prod. Mon., 19: 38-41. Ehara, S., 1989. Thermal structure and seismic activity in central Kyushu, Japan. Tectonophysics, 159: 269-278. Ehara, S. and Mogi, T., 1989. Deep structure beneath volcanic high temperature geothermal systems. In: Working Group for Continental Scientific Drilling of Japan, Sites for Continental Scientific Drilling, (III): 1-22 (in Japanese). Ehara, S., Yuhara, K. and Noda, T., 1981. Hydrothermal system and the origin of the volcanic gas of Kuju-iwoyama volcano, Japan, deduced from heat discharge, water discharge and volcanic gas emission data. Bull. Volcanol. Soc. Jpn., 26:35-56 (in Japanese with Eng-
~ I/tIAR-,~ lish abstract ). Ehara, S., Bito, T., Ohi, T. and Kasai, H., 1990. Microseismic activity beneath active fumarolic areas-- A case study at Kuju-iwoyama in central Kyushu, Japan, J. Geotherm. Res. Soc. Jpn., 12:263-281 (in Japanese with English abstract). Fujita, T. and Abe, M., 1988. Survey of Oguni geothermal field, Kyushu, Japan. J. Jpn., Geotherm. Energy Assoc., 25:287-314 (in Japanese with English abstract). Gianelli, G., Puxeddu, M., Batini, F., Bertini, G., Dini, I., Pandeli, E. and Nicolich, R., 1988. Geological model of a young volcano-plutonic system: The geothermal region of Monte Amiata (Tuscany, Italy). Geothermics, 17: 719-734. Hayashi, M., 1988. Quaternary volcanism as the heat source for some geothermal fields in Kyushu, Japan. Proc. Int. Symp. on Geothermal Energy, 1988. Kumamoto and Beppu, Japan, p. 446. Japan Meteorological Agency, 1975. National Catalogue of the Active Volcanoes in Japan, ll9 pp. (in Japanese ). Kawamura, M., 1984. Thermal condition of the geothermal field around Mt. Waita in central Kyushu, Japan. ( 1 ) Distributions of subsurface temperature and heat discharge. J. Geotherm. Res. Soc. Jpn., 6:217-241. In Japanese with English abstract). Komazawa, M. and Kamata, H., 1985. The basement structure of the Hohi geothermal area obtained by gravimetric analysis in central-north Kyushu, Japan. Rep. Geol. Surv. Jpn., 264:303-333 (in Japanese with English abstract). Lee, T.C., Rudman, A.J. and Sjoreen, A., 1980. Application of finite-element analysis to terrestrial heat flow. Indiana. Geol. Surv. Occasional Paper 29, 53 pp. Matsubaya, O., Ueda, A., Matsuhisa, U., SakaL H. and Sasaki, A., 1975. Isotopic study of hot springs and volcanoes in Kyushu, Japan. Bull. Geol. Surv. Jpn., 26: 375-392 (in Japanese with English abstract). Matsumoto, Y., 1979. Some problems on volcanic activities and depression structures in Kyushu, Japan. Mem. Geol. Soc. Jpn., 16:127-139 (in Japanese with English abstract ). Mizutani, Y., Hayashi, S. and Sugiura, T., 1986. Chemical and isotopic compositions of fumarolic gases from Kuju-Iwoyama, Kyushu, Japan. Geochem. J., 20: 273285. Mogi, T., Nakama, K. and Shimoizumi, M., 1988. Magnetotelluric study of the geothermal system in Kuju Volcano. Proc. Int. Symp. on Geothermal Energy, 1988, Kumamoto and Beppu, Japan, pp. 539-542. New Energy Development Organization (NEDO), 1988. An interim report, Kuju area, pp. 65-120 (in Japanese). Okubo, Y., Graf, R.J., Hansen, R.O., Ogawa, K. and Tsu, H., 1985. Curie point depths of the Kyushu and surrounding areas, Japan. Geophysics, 53:481-494.
THERMAI_ STRUCTURE BENEATH KUJU VOLCANO, CENTRAL KYUSHU, JAPAN
O'Sullivan, M.J.. 1985. Geothermal reservoir simulation. Energy Res., 9: 319-332. Pruess, K. and Schroder, R.C., 1980. SHAFT79. User's Manual, LBE-10861, 47 pp. Volcanological Society of Japan and IAVCEI, 1972. List of World Active Volcanoes. 160 pp. Traineau. H., Westercamp, D. and Benderritter, Y., 1989.
1 15
Case study of a volcanic geothermal system. Mount Pelee, Martinique. J. Volcanol. Geotherm. Res.. 38: 4966. Watanabe, K., Hayashi, M. and Fujino, T., 1987. Fission track age of volcanoes in the Kuju volcanic region in relation to geothermal activity. J. Geotherm. Res. Soc. Jpn., 9:207-217.