In the Uenotai geothermal power station, hydrogen and oxygen isotopic ratios of geothermal water from production wells have been observed since the start of operation in 1994. The Uenotai geothermal water has hydrogen isotopic ratio (δD, SMOW) in a range of -70∼-73‰, and oxygen isotopic ratio (δ¹⁸O, SMOW) in a range of -10.2∼-10.8‰. Although the δD is about 10‰ lower than the local meteoric water, the relationship of δD and δ¹⁸O suggests meteoric water origin of the geothermal water. The Uenotai geothermal water is classified into three kinds different in isotopic character. Two of them are minor, and one of those two has little higher δ¹⁸O, while the other little higher δD than the major one. The occurrence of three different geothermal waters may be explained by a possible case that they are individually ascending through separate paths, or by an alternative case that a single geothermal water is isotopically modified through reaction with rock, mixing with other water, boiling and other processes little before flowing into the production wells. The former case seems more likely, because the latter processes of isotopic modification may be fairly difficult to happen for rather short period before flowing of the geothermal water into the wells. If the former case is a fact, the original meteoric waters are expected to circulate in fairly large and deep part of underground, and infiltration area and time(age) of the original meteoric water require to explain. Residual hot water separated from steam and also cooling tower drainage are reinjected to depth almost the same as those of production wells. A considerable portion of the reinjected water comes out quickly from production wells together with the original geothermal water. During flowing of several hundreds meters from the reinjection wells to the production wells, the reinjected water is extremely heated by surrounding rock, and the amount of heat received is estimated to be as much as 2000 kJ/kg.

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Ungaran Volcano is located in the province of Central Java, Indonesia, and is a Quaternary volcano that consists of older and younger volcanoes. The older Ungaran volcano formed over 500,000 years ago, and the younger volcano was active until 300,000 years ago. The younger volcano seems to have been constructed inside a caldera formed by the older Ungaran activity. In this study, gravity data was used in an attempt to determine the exact location of the younger and older Ungaran volcanoes, and to investigate the relationship between fault structure and geothermal manifestations.
A positive Bouguer anomaly was observed over the volcanic body. From detailed analysis of the gravity data, high anomalies were located over the northern part of the summit that correlate with the older Ungaran volcano. Various interpretation methods, such as horizontal gradient analysis, spectral analysis, and 2-D forward modeling, were applied to the gravity data. The younger Quaternary volcanic rocks, which consist of hornblende-augite-andesite (andesite lava) of the Gajahmungkur volcanics, have an average density of 2,390 ± 120 kg/m³, while the older Quaternary volcanic rocks, consisting of augite-olivine basalt flows (basaltic lava) from the Kaligesik formation, have an average density of 2,640 ± 100 kg/m³. The structural setting of the Ungaran volcano has characterized by circular structure where most geothermal manifestations are located. The result of gravity analysis shows that Ungaran volcano seems to have occurred in tectonic depression, and prominent caldera depression has not formed within Ungaran volcano without surficial caldera rim. The horizontal gradient analysis indicates that geothermal features at Ungaran volcano are structurally controlled and are located within the younger volcano.

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We describe direct observation techniques for inaccessible and extremely acidic crater lakes surrounded by steep crater walls. Observation equipment and an attached weight suspended beneath a pulley are lowered to the lake surface or lake bottom under gravity along a rope stretched between opposite sides of the crater rim. The weight is tied to the observation equipment using paper string that dissolves in the lake water. This enables the rapid descent of the system from the top of the crater rim to the lake, the automatic deployment of the weight from the observation equipment once the system reaches the lake, and the simple manual retrieval of the system from the lake to the crater rim following the completion of observations.
Using the above technique, we deployed two temperature telemetry buoys, including wireless thermometers, upon the crater lake of Aso volcano, Japan, to monitor water temperature. We manually ran a 400 m polypropylene rope (8 mm diameter) between opposite sides of the crater rim. The strength of the rope was sufficient to lower 3-5 kg of equipment to the lake surface. The protective buoy housing the wireless thermometer is made of expanded polystyrene (EPS), enabling it to float on the lake surface. The buoy has a 1-mm-thick coating of polyurethane resin to prevent UV damage. The buoys were fixed at the lake bottom using anchors and 18 m fluororesin ropes initially tied to the system using paper string, enabling the system to readily descend to the lake surface along the stretching rope. The paper strings dissolved several minutes after the equipment had landed on the lake surface, at which point the anchor descended to the lake bottom.
To obtain samples of lake sediments, we developed a mud sampler that consists of a stainless steel frame covered by a polyester mesh net. A doughnut-shaped fluororesin block, designed to prevent chemical contamination of the dredged sample, was used as a weight inside the net. The mud sampler and an attached weight suspended beneath two pulleys were lowered to the lake bottom under gravity along the polypropylene rope. The attached weight was tied to the sampler using paper string. We also obtained a sample of lake water using a polypropylene bottle with a weight tied beneath the bottle using paper string.
These techniques are expected to contribute to the monitoring of active crater lakes and the underlying hydrothermal system.

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