| Title | The Correlation Between Seismic and Logging Data and its Geological Interpretation in the Akinomiya/Wasabizawa Geothermal Area |
|---|---|
| Authors | Shigeki Mizutani |
| Year | 2011 |
| Conference | Asian Geothermal Symposium |
| Keywords | reflection seismic survey, strike-slip fault, fracture, flower structure |
| Abstract | Seismic surveys have not generally been applied for geothermal explorations due to mainly their complex subsurface geology. In the Akinomiya geothermal area in the Akita prefecture in northern Japan, the high resolution seismic survey using vibroseis as seimic source was acquired and processed in 2000, and more than 20 geothermal exploration and appraisal wells were drilled in the past 30 years in which the resistivity and pressure-temperature logs were acquired. The seismic interpretation and analysis of logging data was made in 2010. The geological interpretation with seismic-log correlation leads us to understand satisfactorily the regional subsurface geology, especially the assessment of characteristic and type of fractures; ones associated with high-angle dip fault and others of low-angle. The series of left-lateral strike-slip faults with their dip of about 80 degrees, which originate from deep situated consolidated magma 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 is 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, generally thin with its width of 25 m at most, can be picked in the seismic section by its anomalous weaker seismic reflections than those of adjacent rocks, and is correlated with the zones of porosity build-up by well log analysis. These weak seismic reflectivities are interpreted as associated with either thermally altered rocks by steam and/or hot water, or intrusive rocks, or both. Upper parts of these fractures seem often to be of closed nature, possibly due to swelling of clay minerals formed through thermal alteration. The continuous strong seismic reflections can be picked between 0 masl and -500 masl. Its dip is estimated as about 20 degrees and its strike of NW-SE orientation, which coincides with the results of FMI (Formation MicroImager). The similar feature of the seismic reflections can be also 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 land slide possibly associated with the caldera formation process, which occurred several times in the Tertiary period. |