Abstract
Several regions in Japan, including the Kakkonda geothermal field in Iwate Prefecture, have been identified where magma-derived fluids rise to shallow depths (several kilometers deep), forming supercritical geothermal reservoirs (non-conventional geothermal reservoir composed of magma-derived geothermal fluids). This makes it a promising candidate for supercritical geothermal development. However, despite the high likelihood of the existence of supercritical geothermal reservoirs based on geological and geochemical considerations, direct evidence from geophysical exploration other than MT surveys has not been found. In this study, attempts were made to image the supercritical geothermal reservoir based on microseismic observations and seismic reflection surveys using natural earthquakes and vibrator sources in the Kakkonda geothermal field. In addition, numerical calculations (ray tracing) based on models that simulate the actual seismic sources, receivers, and obtained reflection structures were conducted. In the numerical calculations, synthetic reflection records were calculated, and the effects of artifacts potentially caused by P-wave multiple reflections, P-S converted waves, etc. were evaluated to assess the reliability of the obtained reflection structures. As a result, it is likely that, for the first time in Japan, successful detection of reflectors corresponding to the upper boundary of the supercritical geothermal reservoir was achieved through seismic reflection imaging.
