5:15 PM - 6:45 PM
[STT34-P03] Shallow magnetization structure beneath Mt. Tokachidake based on UAV aeromagnetic survey
Keywords:Aeromagnetic survey, Drone, Mt. Tokachidake
Mt. Tokachidake is an active volcano, located at the SW end of the Daisetsu-Tokachi volcanic chain in the central Hokkaido, Japan. Major historical magmatic eruptions of Mt. Tokachidake occurred in 1857, 1887, 1926, 1962 and 1988-89. Now, there is a general concern about a future eruption as 35 years have already passed since the latest magmatic activity.
Hashimoto et al. (2010) has conducted ground geomagnetic repeat surveys since 2008, and estimated a spherical demagnetization at a depth of 150 m (1,600 m a.s.l.) beneath the 62-II crater based on the 2008-09 data. The recent surveys has revealed that the demagnetization trend continues to the present day, suggesting that a high temperature zone is expanding at a very shallow depth. In addition, an ongoing thermal anomaly around 1,600 m a.s.l. of the Mae-Tokachi, WNW side of the 62-II crater, suggests shallow migration of hydrothermal water along the bedding planes within the Tairagatake lava (Hashimoto et al., VSJ 2023). In light of this situation, we conducted a drone-borne aeromagnetic survey in August 2023 to clarify the subsurface thermal activity around the 62-II crater.
In this study, we collected geomagnetic total intensity data with a GEM GSMP-25U Potassium Magnetometer (rented from Tierra Technica, Ltd.) mounted on a DJI Matrice 300 RTK. The drone flew at a speed of 5 m/s measuring the magnetic field at a sampling interval of 0.05 s. The survey area performed in a rectangular area of 1.5 km by 1.3 km in the NW-SE and NE-SW directions, including the 62-II crater, the Furikozawa fumarolic zone and the geothermal anomaly on Mae-Tokachi, with a line spacing of ~100 m. We made a reduction of temporal changes of the extra-terrestrial origins using the simultaneous record measured with a GEM GSM-19 Overhauser magnetometer at the reference point, Shirogane Mohan Farm in Biei, ~8km NNW of the survey area. The drone was operated automatically using the mapping mission on the DJI Pilot 2 App. The flight altitude from the ground surface was set to be constant at 149 m with respect to the topographic data around the 62-II crater using the terrain follow mission on the DJI Pilot 2. The topographic data was generated using the DEM of 5 m resolution provided by Japan’s Geospatial Information Authority (GSI).
Based on the geomagnetic field data collected during the autonomous drone surveys, we inverted the distributions of three-dimensional magnetization intensity beneath the geothermal area using the code developed by Koyama et al. (2021). The horizontal extent of the model space for the inversion was 2 km by 2 km, whereas the vertical extent was 2 km with four layers of 200, 400, 600, and 800 m thick from the surface to the depths. The estimated average magnetization intensity was ~2 A/m. The hyperparameter balancing the data misfit term and the model roughness term in the objective function of inversion were determined by the ABIC minimization concept (Akaike, 1980). Our 3D model showed the low magnetization region beneath the 62-II to Taisho craters. The distribution of the low magnetization area was consistent with the location of the major fumarolic areas at Mt. Tokachidake. In addition, another weakly magnetized region was found at an elevation of ~1,550 m a.s.l. on the NW side of the 62-II crater, showing a good correspondence with the surface thermal anomaly on the NW slope.
This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research Program.
Hashimoto et al. (2010) has conducted ground geomagnetic repeat surveys since 2008, and estimated a spherical demagnetization at a depth of 150 m (1,600 m a.s.l.) beneath the 62-II crater based on the 2008-09 data. The recent surveys has revealed that the demagnetization trend continues to the present day, suggesting that a high temperature zone is expanding at a very shallow depth. In addition, an ongoing thermal anomaly around 1,600 m a.s.l. of the Mae-Tokachi, WNW side of the 62-II crater, suggests shallow migration of hydrothermal water along the bedding planes within the Tairagatake lava (Hashimoto et al., VSJ 2023). In light of this situation, we conducted a drone-borne aeromagnetic survey in August 2023 to clarify the subsurface thermal activity around the 62-II crater.
In this study, we collected geomagnetic total intensity data with a GEM GSMP-25U Potassium Magnetometer (rented from Tierra Technica, Ltd.) mounted on a DJI Matrice 300 RTK. The drone flew at a speed of 5 m/s measuring the magnetic field at a sampling interval of 0.05 s. The survey area performed in a rectangular area of 1.5 km by 1.3 km in the NW-SE and NE-SW directions, including the 62-II crater, the Furikozawa fumarolic zone and the geothermal anomaly on Mae-Tokachi, with a line spacing of ~100 m. We made a reduction of temporal changes of the extra-terrestrial origins using the simultaneous record measured with a GEM GSM-19 Overhauser magnetometer at the reference point, Shirogane Mohan Farm in Biei, ~8km NNW of the survey area. The drone was operated automatically using the mapping mission on the DJI Pilot 2 App. The flight altitude from the ground surface was set to be constant at 149 m with respect to the topographic data around the 62-II crater using the terrain follow mission on the DJI Pilot 2. The topographic data was generated using the DEM of 5 m resolution provided by Japan’s Geospatial Information Authority (GSI).
Based on the geomagnetic field data collected during the autonomous drone surveys, we inverted the distributions of three-dimensional magnetization intensity beneath the geothermal area using the code developed by Koyama et al. (2021). The horizontal extent of the model space for the inversion was 2 km by 2 km, whereas the vertical extent was 2 km with four layers of 200, 400, 600, and 800 m thick from the surface to the depths. The estimated average magnetization intensity was ~2 A/m. The hyperparameter balancing the data misfit term and the model roughness term in the objective function of inversion were determined by the ABIC minimization concept (Akaike, 1980). Our 3D model showed the low magnetization region beneath the 62-II to Taisho craters. The distribution of the low magnetization area was consistent with the location of the major fumarolic areas at Mt. Tokachidake. In addition, another weakly magnetized region was found at an elevation of ~1,550 m a.s.l. on the NW side of the 62-II crater, showing a good correspondence with the surface thermal anomaly on the NW slope.
This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research Program.