Self-potential (SP) survey was carried out at 420 measurement points around the Waita volcano-Takenoyu area, Kyushu, Japan. This survey region covers the summit of the Waita volcano and the Takenoyu hot spring area of which elevation differs by 750 m. SP tends to decrease according to the elevation but high negative anomaly up to six-hundred millivolts can be observed at the middle height of the Waita volcano. Negative peak anomaly of an SP profile has been generally interpreted as an indicator of a recharge zone. However, a numerical modeling of the subsurface fluid flow and its corresponding SP shows that a negative peak anomaly at the middle height of a mountain can not be explained by a model with a higher permeability in the middle height nor by a homogeneous permeability model. Further simulation suggests the existence of a higher permeability column underneath the summit of the Waita volcano. The hot water flows up beneath the volcano and flows horizontaly toward the geothermal system which has been developed at the foot of the volcano.

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In order to monitor the mass variations in the reservoir that might occur due to the pressure changes that would be induced by the changes in the production and reinjection rates when the output doubled from 55 MW to 110 MW, in the Hatchobaru geothermal field, gravity changes have been monitored at 46 benchmarks since May 1990. According to the gravity and reservoir pressure changes monitored using capillary tubes set into several observation wells, a close correlation between these two changes was recognized. Therefore, the gravity changes are considered to indicate changes in reservoir mass behavior. Moreover, when the Hatchobaru No. 1 unit temporarily suspended operation for 47 days, from March 16 to May 1, 1996, because of a lightning accident, gravity changes were observed about two months after the forced outage. On the basis of a two-dimensional analysis, the gravity changes in, the Hatchobaru geothermal field are considered to reflect the changes in reservoir mass caused by variations in the production and reinjection rates.

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Mud-pulse is widely used in MWD systems to transmit bottom hole information to the ground. However, distortion of pulse, which is caused by pulse reflection and damping in drill pipe, limits the transmission rate to several bits per second. This report proposes the following signal processing method to measure the mud-pulse intervals accurately. The mud-pulse transfer function of the drill pipe is measured using reference mud-pulse, which is generated by driving the mud-pulse valve with the reference signal. The distortion of the mud-pulse is compensated by multiplying the inverse transfer function of the drill pipe in frequency domain. Verification experiment on this method is carried out by using a mud-pulse transmitted in the copper pipe of 60 m in total length. As a result, the pulse series of pulse rate over 8 pulses/s is well reproduced with thee pulse interval error less than 3 ms.

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We performed fluid inclusion study of an investigation well (WD-la; 3, 729 m in total depth) in the Kakkonda geothermal field, Japan. WD-la was drilled in the Quaternary Kakkonda granite from 2, 860 m through 3, 729 m in depth which is the heat source of the Kakkonda geothermal system. The boundary between hydrothermal convection and heat conduction zones was found around 3, 100 m by temperature loggings. We discussed thermal structure in the Kakkonda granite and permeation of meteoric water into the Kakkonda granite. Temperature of the Kakkonda granite shallower than 3, 100 m can be evaluated roughly from homogenization temperature of the liquid-rich inclusion which has a minimum salinity in every depth. Since the Kakkonda granite deeper than 3, 100 m is less permeable at greater depths, meteoric water permeates less into the Kakkonda granite. The evidence is as follows: (a) Minimum salinities of the liquid-rich inclusions shallower than 3, 250 m are close to 0 wt. %, although those deeper than 3, 300 m increase. (b) Homogenization temperature of the liquid-rich inclusion, which has minumum salinity, increases just slightly between 2, 750 m and 3, 250 m, then they increase steeply deeper than 3, 300 m. (c) According to the result of gas analysis for fluid inclusions by laser Raman microprobe spectroscopy, CO₂ and H₂S were not detected shallower than 3, 150 m, although they were detected deeper than 3, 350 m. We also built a geothermal model of the Kakkonda field based on the fluid inclusion study, showing thermal structure and fluid flow in the Kakkonda geothermal system.

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