A geothermal system related to a caldera by geophysical surveys - An example of the Ontake volcano, Central Japan
Abstract
A geothermal system related to a caldera is often found in the world. Since a caldera structure is usually buried by new volcanic products and it is difficult to recognize only by surface geology. To investigate a caldera structure, geophysical surveys are useful, especially the gravity and the MT survey have proven successful. Here we present an example of MT survey to clarify buried old caldera in the Ontake volcano, Central Japan. We conducted the comprehensive MT survey and constructed 3D resistivity model in the southeast flanks of the volcano. The resistivity structure showed a columnar conductive structure in this area and it was expected that there will be a buried funnel type caldera that spewed lava and pyroclastic flow during 0.7 and 0.4 ma. The volcanic activity of the age could be a heat source for geothermal resources. A slightly old volcanic activity is suitable for geothermal system formation, but a caldera structure formed such old time was often covered by new volcanic products. Temperature measurement in the boreholes showed that temperature at 1000m depth were 60-80 degrees in this area. Though temperature at deep was not cleared, the bottom of the hypocentral distribution of earthquakes, suggesting 300-350 degrees, is about 4 km depth beside the caldera. This means the heat source has still remained at deep of thecaldera. A caldera is usually filled with soft deposits, meteor water is easy to penetrate deeply from the surface, and the temperature is low at the shallow depth. Hot water upflows from deep along a fracture zone and reach a certain depth in the caldera and forms a geothermal reservoir.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMNS41A0805K
- Keywords:
-
- 0920 Gravity methods;
- EXPLORATION GEOPHYSICSDE: 0925 Magnetic and electrical methods;
- EXPLORATION GEOPHYSICSDE: 0935 Seismic methods;
- EXPLORATION GEOPHYSICSDE: 1835 Hydrogeophysics;
- HYDROLOGY