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ABSTRACTS Bulletin of the Geological Survey of Japan Vol.55 No.11/12 2004
Background and summary of the basic study on the assessment of influence by
heat and hydrothermal water for geological disposal of nuclear waste
Shiro Tamanyu
The purpose of the research is to comprehend earth scientific characteristics and genetic
origins of high-temperature anomalous areas, and to examine the evaluation methods for the effects of
heat and hydrothermal water on surroundings of geological repository of high-level nuclear waste at
the stages of extraction of Preliminary Investigation Areas (PIAs) and of its following site-specific
surveys. This special issue is prepared to introduce the outline of the project and preliminary result of
each subdivided research subject. The conceptual model of subsurface thermal anomalies is tentatively
proposed. The research contents and results are described briefly as follows.
Consideration on the analytical method for heat anomalies based on distribution
of heat discharge by hot springs
Keiichi Sakaguchi Distribution of heat discharge by hot spring is a valuable feature for evaluating geologic repository site because it reflects both underground temperature and underground hydrologic conditions. Considering preparatory procedure of updating the manner of heat discharge calculation, several points have been noted. The following two points are particularly significant. For the time being naturally discharging hot springs occupy about 30 percent of the total amount of hot spring water in Japan. Therefore (1) we must evaluate natural discharge and pumped hot water separately. Recently developed hot springs, especially those by deep wells, are likely to be suffered from changes in their condition such as decrease in water temperature and/or production rate in a shot period after development. Therefore (2) hot spring data should be carefully treated for evaluating heat discharge by them.
Combination of geological and geophysical data by database
system
Shiro Tamanyu and Tatsuya Sato
In order to clarify the situation and origin of subsurface anomalous high temperature zone
in non-volcanic area, the midde eastern part of Tohoku is selected as the model field for this case
study. The distribution maps of topography, geology, microseismic hypocenter, electric resistivity,
Curie-point depth, and subsurface isotherms were collected and summarized as input data to the database
for 2-D and 3-D visualization. After digitizing, 6 kinds of maps are correlated each other. Following
three data sets resulted in good correlation sets: volcanic front and microseismic hypocenter,
resistivity and microseismic hypocenter, Curie-point depth and microseismic hypocenter.
Current status of regional hydrogeological
studies and numerical simulations on geological disposal
Shinsuke Nakao, Tsuneo Kikuchi and Tsuneo Ishido Current status of regional hydrogeological studies on geological disposal including hydrogeological modeling using numerical simulators is reviewed in this report. A regional scale and boundary conditions of numerical models are summarized mainly from the results of the RHS (regional hydrogeological study) project conducted by Japan Nuclear Cycle Development Institute (JNC) in the Tono area. We also refer to the current conceptual models of hydrology and numerical models of unsaturated zone flow at Yucca Mountain, Nevada, which is the arid site proposed for consideration as the United States’ first underground high-level radioactive waste repository. Understanding behavior of a freshwater-saltwater transition zone seems to play a key role in the hydrogeological modeling in a coastal region. Technical features of a numerical simulator as a tool for geothermal reservoir modeling is also briefly described.
Interpretation of deep resistivity structure by magnetotelluric data in the
Abukuma area, northeastern Japan
Toshihiro Uchida Magnetotelluric (MT) data obtained in the Abukuma area, eastern Japan, has been interpreted using a two-dimensional (2D) inversion scheme. The survey line, approximately 50 km long, spanned from Motomiya Town in central Fukushima Prefecture in the west to Namie Town near Pacific Ocean in the east. The survey area is mostly underlain by granitic plutons in Mesozoic time. The estimated 2D resistivity model exhibits highly resistive formations beneath the entire survey line, with resistivity of 1000 - 10000 ohm-m. There is a low-resistivity shallow layer at the eastern end of the survey line, which corresponds to Tertiary and Quaternary sedimentary formations. At depths greater than approximately 15 km, which correspond to the lower crust, there is a relatively low resistivity zone of several hundreds ohm-m. However, this resistivity is mush higher than the lowresistivity lower crust which has been found in other Outer Volcanic Front region in northeastern Honshu Island, such as northern Miyagi Prefecture. This indicates that the thermal or hydrothermal activity in the Abukuma area is weak in the lower crust as compared with other areas in northeastern Honshu Island.
Investigation on analysis method
for deep structure using hypocenter data - example in northern Honshu -
Kinichiro Kusunose, Hironori Kawakata and Jun-ichi Takeuchi Using the earthquake catalog data provided by the Meteorological Agency at the existing observation points, the correlation between the hypocenter distribution and deep-seated geothermal resources areas under plains was investigated in northern Honshu. Although the used data might not be reflecting the steady seismic activity of northern Honshu enough since the period after completion of the dense network of seismic stations is short, it turns out that seismic activity is not so active in deep-seated geothermal resources areas. Ito et al (1990) pointed out that the lower limits of depth distribution of the hypocenters are correlative with degree of heat flow. Therefore, we tried to extract some informations on the temperature distribution of the crust from hypocenter distribution data. The result indicates that the circumference of “Furukawa” deep-seated geothermal resource area is active in seismic activity in comparison with “Furukawa” geothermal resource area itself. Many researches such as identification of the positions of earthquake reflective sides using the seismic wave forms, have carried out. Moreover, the 2003 northern Miyagi earthquake is under investigation, then it is expected that the situation in the crust under “Furukawa” geothermal area will become more clear.
Analyses for long-term changes of geothermal systems using radiometric
dating
Keiko Mizugaki
In order to evaluate effects of geothermal anomalies at high-level radioactive wastes disposal
sites, information on long-term development and possible future activity of geothermal systems
are needed. The only method to measure the time in the geothermal history is radiometric dating
resetted by geothermal heat, such as electron spin resonance (ESR) and thermoluminescence (TL)
methods.
Geochemical features of groundwaters in granitoids
Munetake Sasaki Geochemistry of groundwaters in granitoids of Japan and those of Baltic and Canadian shields was reviewed in order to construct a basis which is available for future extraction and investigation of thermal anomaly and hydrothermal waters in granitic areas without volcanic activity. Groundwaters in granitoids derived from meteoric water are generally Ca•Na-HCO3 type, low in salinity (<1 g/l), slightly acidic to neutral and oxidized at shallow levels and Na-HCO3 type, low in salinity, slightly alkaline (pH<10) and reduced at deep levels with the exception of more varied geochemistry at areas where fossil or present seawater has infiltrated or fluids derived from magma or hot rocks have been left. Groundwaters in the shields are generally Ca(•Na)-Cl type and highly saline (>300 g/l) at deep levels. The following tasks to be solved are necessary for more detailed understanding of geochemistry of groundwaters in granitoids: i) quantitative evaluation of water-rock interaction process using a geochemical simulator and ii) identification of origin, genesis and mobilization mechanisms for saline groundwaters at deep levels.
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