Reflection seismic survey has not generally been considered as applicable geophysical method for geothermal explorations due to mainly the complex subsurface geology in the geothermal area. However, the high resolution seismic survey using vibroseis as seismic source carried out in 2000 in the Akinomiya geothermal area in northern Japan leads us to understand satisfactorily the regional subsurface geology, especially the assessment of characteristic of fractures. Two types of fracture system are revealed through seismic interpretation and logging analysis of adjacent wells; these are associated with high-angle dip fault and those of low-angle. The series of left-lateral strike-slip faults with their dip of approximately 80 degree, which originate from as deep as -3,000 masl to or near the surface, are interpreted as formed in the area under the active compressions of the Pacific Plate. Each strike-slip fault has been inducing to build up corresponding flower structures in either transpression domain or transtention domain in its shallow part, which formulate a zone of fractures. Six fracture zones are recognized in the seismic section with approximately 500 m apart. Each fracture is thin with its width of 25 m at most and can be picked in the seismic section by its much weaker seismic reflections than those of adjacent rocks. These weak seismic reflectivities are interpreted as associated with either hydrothermally altered rocks by steam and/or hot water, or intrusive rocks, or both. Generally, upper parts of these fractures seem to be of closed nature except those found in the eastern part of the area where fractures sometimes reach to or near the surface, which is coincident with the surface hydrothermal alterations anomalies, current geothermal activities etc. The continuous strong seismic reflections can be picked between 0 masl and -500 masl. Its dip is approximately 20 degree and its strike is of NW-SE orientation, which coincides with the results of FMI (Formation MicroImager). The similar feature of the seismic reflections can be recognized in deeper part, of which dip is similar but strike is of different orientation. These low-angle dip faults are interpreted as surface planes of either thrust or land slide, which occurred several times in the Pleiocene period.
Horizontal Ground Heat Exchangers (GHEs) for geothermal heat pump systems are not well distributed in Japan due to the requirement of large land space for installing the heat exchange pipes in the shallow ground. For reducing the land space to bury the GHEs, two-layered horizontal GHE systems, which install the loops at -1.0 m and -2.0 m, are proposed in this research. For evaluating the heat exchange performance of the GHEs, long-term cooling and heating tests and forced heat rejection tests were carried out in a field test site located in Itoshima City, Fukuoka, Japan. The results of field tests showed that the application of two-layered GHEs remarkably improve the heat exchange capacity of GHEs per unit area, which enable the significant reduction of the land space requirement. The field tests also showed that Slinky-coil horizontal GHEs have higher heat exchange capacity than conventional straight type horizontal GHEs in case the cost of pipes for the two types of GHEs is equalized.
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