無人ヘリを利用した樽前山における繰り返し磁気測量

概要

Repeated aeromagnetic surveys using an unmanned autonomous helicopter was conducted at Mt. Tarumae Volcano. In October 2020, we performed the fourth survey following the previous ones in 2011, 2012 and 2013. In this study, we analyzed the aeromagnetic data in 2020 and compared to the previous data in 2011 and 2013 to obtain the temporal changes of the magnetic anomaly between them. The accuracy of the repeated flight was satisfactory as gaps between the actual flight trajectories were mostly within 5 m. After the reduction of the geomagnetic disturbances of extra-terrestrial origins, we obtained the temporal changes in the total magnetic field by taking the simple differences of the selected nearest points between 2013 and 2020. We also calculated the temporal changes for the pair between 2011 and 2013 through the same procedure. Both the pairs showed positive and negative changes in the south and north, respectively, around the summit lave dome, suggesting the cooling re-magnetization. We explained them as a single magnetic dipole source beneath the lave dome with magnetic moment changes of 7.9 × 106 Am2 and 8.0 × 106 Am2, respectively for the former and the latter pairs. The comparable amount of the magnetic moment for the two periods suggested the possibility of slowdown in the magnetization rate after 2013. This result was consistent with the ground-based monitoring that was annually performed by the Japan Meteorological Agency and Hokkaido University (Hashimoto et al.,2018). Such a cooling trend in the 2010s was also supported by other monitoring data such as the ground eformation and fumarolic temperatures.

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