The thermal activity of Iwodake volcano is characterized by the predominant volcanic gas ejection at the summit crater and the heat discharge from high ground temperature area which is distributed widely from the summit crater to hillside of the mountain. The volcanic gas at the summit crater is of magmatic origin and its temperature is observed to be 880°C in maximum. The total amount of the volcanic gas discharged from the summit area is estimated to be 400kg/s from the SO₂ flux measurement and the chemical composition of the volcanic gas.The heat discharge rate from high ground temperature area is estimated to be 46MW at the summit and 83MW at the hillside of the mountain, respectively.These thermal activities of Iwodake volcano is thought to be continued for more than 800 years. A plausible model to explain the long degassing of large amount of volcanic gas is magma circulation in a conduit which is extended from the deep-seated magma reservoir. The continuous active degassing probably causes the hydrothermal system within the volcano because the volcanic gas ascending the vent from the top of the magma to the surface is diffused to the surrounding formation. The development of such hydrothermal system is studied by using the numerical simulation. The result shows that the overall thermal activity of Iwodake volcano such as the volcanic gas ejection at the summit crater, widely distributed ground temperature anomaly at hillside and hot springs along the coast can be caused by the degassing activity. The important factors in order to induce the wide-ranging hydrothermal system are permeability of the volcanic edifice and the depth of the degassing. The simulation indicates that the permeability of 10^-13m² at the mountain edifice, the permeability of 6×10^-11m² at the vent, and the degassing above sea level are suitable conditions for the Iwodake thermal activity.

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Morphometric observations of the crater lake at Aso Volcano, Japan, was carried out and the change of its water level, surface area and water volume between Aug 2000 and Aug 2003 were estimated. The lake water was sampled three times during this period and the concentrations of chloride in the water were measured. Mass balance was evaluated by solving the water and chloride budgets simultaneously.Results showed that the lake water was mainly supplied by volcanic input (3500-6200 ton/day) which occupied at least 60% of the water influx.Meteoric water flux (420-2500 ton/day) was not dominant, which occupied less than 20% of the water influx between Aug 2000 and Apr 2003. Lake water was mainly dissipated by evaporation from the lake surface (3800-5900 ton/day). Seepage from the lake bottom (880-2200 ton/day) occupied about 13-37% of the water outflux. Heat balance indicated that thermal energy of volcanic input was about 150-200MW, which occupied more than 95% of energy influx.Most heat loss occurred at the lake surface through evaporation (100-150MW). The specific enthalpy of volcanic input was estimated to be about 2500-4200kJ/kg, which corresponds to the enthalpy of superheated steam at several hundred degrees C.It was suggested that mass and heat are input into the lake by the addition of superheated steam at several hundred degrees C.Water flux and enthalpy of volcanic input between Apr 2003 and Aug 2003 were estimated to be larger than those between Aug 2000 and Apr 2003.This is consistent with the observation that the water temperature became higher, the water level was decreased and small mud eruption was occurred.Our results indicate that the crater lake at Aso ranks among the peak-activity volcanic lakes in the world.

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Distributions of aquifers and groundwater yields are studied using a well database to evaluate the potential installation of the groundwater heat pump systems in the Nobi plain of Gifu prefecture. The distribution of two aquifers is deduced from that of well screens that are getting deep gradually toward the south.The distribution of the groundwater yields indicates that the groundwater heat pump systems using pre-existing wells have a potential installation in the whole of the study area. The distribution of the groundwater yields drawn from the aquifers shallower than 50m depth shows that the northern part of the study area is a suitable area for the installation of the groundwater heat pumps with a well drilling.The potential installation of the groundwater heat pump systems using the groundwater drawn from the total depth and shallower than 50m depth are 52.9% and 26.0% for the houses in the study area, respectively.These results suggest that the study area has a large potential for the installation of the groundwater heat pump systems.

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Three production wells of H-17, H-28, and H-29 discharged fluids from the reservoir formed along the Hatchobarufault. Pressure interferences among these wells were recognized by the change of discharge rate and geochemical monitoring data. The productivity of H-28 located between H-17 and H-29 declined due to the interferences, and the supply of steam to the power plant was often discontinued. Therefore a lumped parameter reservoir model for estimating hydraulic characteristics of reservoir and interferences from other production wells was developed and applied for analyzing Cl concentration and silica temperature monitored both in H-28 and 11-29.
Analysis results indicated that discharges of wells H-17 and H-29 affected that of H-28. Pressure interference among wells H-17 and H-28 with H-29 was negligible small. Moreover it is presumed that an abrupt change of reservoir pressure is apt to occur in H-28 because the reservoir volume related to H-28 is about half in magnitude that of H-29. Accordingly the pressure interference of H-17 and H-29 resulted in appreciable pressure drop in H-28 reservoir. Thus analysis using the developed lumped parameter model can be used for quantitative evaluation of pressure interference influences in geothermal reservoir management.

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