Abstract
Akita-Yakeyama Volcano is located at the northwestern edge of the Sengan Geothermal Area, northeast Japan, where many geothermal features such as fumaroles and hot springs are observed as well as many Quaternary volcanoes. In this area, geothermal explorations had been actively conducted since mid-1970s and four geothermal power plants are being operated now.
Magnetic analyses had been conducted in and around Akita-Yakeyama Volcano to reveal the regional and local subsurface structures of the area. For instance, we had conducted a magnetic forward modeling, with known structural and magnetic parameters to reveal a detail subsurface structure of Akita-Yakeyama Volcano. The magnetic model was composed of five polygons: three of them correspond to granitic intrusions below the northern flank, while rests of them correspond to the buried Old-Tamagawa Welded Tuffs below the southern flank. These analyses were very useful to better estimate the subsurface structure related to the geothermal activity of the study area but it took much time to conduct the modelling by trial and error.
Recently an aeromagnetic 3D subsurface imaging method has been developed and takes an important role in 3D visualization of subsurface structures especially in active volcanic regions. Therefore we have applied the method to interpret the magnetic anomalies of Akita-Yakeyama Volcano. The resultant magnetization intensities were superimposed on the same cross-sections of the previous forward modeling and compared with each other. In the N-S cross-section traversing the summit of the volcano, a magnetization high is well imaged below the northern flank, corresponding to the granitic intrusions. Whereas, negative magnetizations lie below the southern flank and are associated with the reversely magnetized volcanic rocks such as the Old-Tamagawa Welded Tuffs and/or the Pre-Yakeyama Andesite Pyroclastic Rocks. In the E-W cross-section of the northern flank, a magnetization high is centered at around the geothermal exploration well SN-5 below the western part. Whereas, no magnetic high is imaged at around the geothermal exploration well SN-7D below the eastern part, suggesting its higher subsurface temperature than that below the western part.
More new information was obtained by the 3D imaging. For instance, obvious negative magnetization intensities lie below Mt. Kurasawa south of Akita-Yakeyama Volcano and correspond to the distribution of the reversely magnetized Tamagawa Welded Tuffs and Old-Tamagawa Welded Tuffs. A magnetization high was imaged at the south of the Onuma Geothermal Power Plant, implying the existence of an intrusion associated with the heat source of the power plant.
Since the aeromagnetic data used for the 3D imaging was observed in 1970's, its resolution is not as good as that of them by recent high-resolution surveys. Nevertheless, it can estimate 3D subsurface structures with a fast calculation on ordinary modern PCs. New high-resolution aeromagnetic data will be expected to be obtained in domestic geothermal areas by JOGMEC in the near future. When it comes true, the 3D imaging method could be applied to the data and contribute to indispensable estimations of thermal structures of geothermal areas in Japan.
