11:00 AM - 11:15 AM
[SEM14-08] Wideband-MT surveys and 3D resistivity modeling by combined use of data from land, sea, and lake areas at Toya Caldera region, Hokkaido, Japan.
Keywords:MT, resistivity, combined analysis of land, sea, and lake areas, Toya Caldera
The present study shows an outline for three-dimensional magnetotelluric (MT) study at the Toya Caldera region, Hokkaido, Japan. The investigated area has a caldera lake called Lake Toya, which has a water depth of 140-180m and a diameter of 10 (N-S) to 11 (E-W) km. The caldera-forming eruption has occurred at c.a. 110 Ka, and its post-caldera activities, including Usu volcano, are present around the area. The aim of this survey is to reveal the regional structural characteristics beneath the Toya Caldera.
The MT surveys were conducted in 2018 to 2019, by deploying the observation points not only on land surface, but also on ocean and lake floor. In general, a precise resistivity imaging based on MT requires spatially balanced observations to avoid biased model sensitivity. In this case study, the investigated area has the centrally located caldera lake with the wide and deep geometry, and is surrounded by the Pacific Ocean in the southern part (only 10 km apart from the caldera). Therefore, this study has attempted to obtain MT data at the interval of 5-6 km, resulting in the deployment of 92 survey points on land, 16 points on ocean floor, and 9 points on lake floor. The following two observation systems were used in accordance with the deployment situation: Phoenix Geophysics MTU-5A system for land areas, and shallow marine MT measurement system (Ueda et al., 2014) for sea and lake areas. It is notable that a measurement device for electrical field in the shallow marine MT system were adjusted to measurement in lake water, which has a resistivity 300 times higher than sea water. Consequently, time-series data were successfully acquired, and the processed impedance tensors were inverted by WSINV3DMT code (Siripurvaraporn et al.,2009), to obtain three-dimensional resistivity model beneath the Toya caldera.
[Acknowledgments]
This study was conducted under the collaborative research project with Hanshin Consultants Co.,Ltd., entitled “a research on subsurface structure beneath Toya caldera,” which is a part of an impact evaluation of volcanic activity on Tomari power plant conducted by Hokkaido Electric Power Co., Inc. We thank Hokkaido Electric Power Co., Inc., Hokuden Sogo Sekkei Corporation, Nittetsu Mining Consultants Co., Ltd., Geothermal Energy Research & Development Co., Ltd., local governments, local fishery association, and Dr. Seunghee Lee from SEKOGEO for kind supports for the MT surveys.
The MT surveys were conducted in 2018 to 2019, by deploying the observation points not only on land surface, but also on ocean and lake floor. In general, a precise resistivity imaging based on MT requires spatially balanced observations to avoid biased model sensitivity. In this case study, the investigated area has the centrally located caldera lake with the wide and deep geometry, and is surrounded by the Pacific Ocean in the southern part (only 10 km apart from the caldera). Therefore, this study has attempted to obtain MT data at the interval of 5-6 km, resulting in the deployment of 92 survey points on land, 16 points on ocean floor, and 9 points on lake floor. The following two observation systems were used in accordance with the deployment situation: Phoenix Geophysics MTU-5A system for land areas, and shallow marine MT measurement system (Ueda et al., 2014) for sea and lake areas. It is notable that a measurement device for electrical field in the shallow marine MT system were adjusted to measurement in lake water, which has a resistivity 300 times higher than sea water. Consequently, time-series data were successfully acquired, and the processed impedance tensors were inverted by WSINV3DMT code (Siripurvaraporn et al.,2009), to obtain three-dimensional resistivity model beneath the Toya caldera.
[Acknowledgments]
This study was conducted under the collaborative research project with Hanshin Consultants Co.,Ltd., entitled “a research on subsurface structure beneath Toya caldera,” which is a part of an impact evaluation of volcanic activity on Tomari power plant conducted by Hokkaido Electric Power Co., Inc. We thank Hokkaido Electric Power Co., Inc., Hokuden Sogo Sekkei Corporation, Nittetsu Mining Consultants Co., Ltd., Geothermal Energy Research & Development Co., Ltd., local governments, local fishery association, and Dr. Seunghee Lee from SEKOGEO for kind supports for the MT surveys.