5:15 PM - 6:45 PM
[STT34-P01] A subsurface magnetizaion model and an estimation of the magnetic field variation around Oana crater, Azuma volcano, NE Japan, using aeromagnetic surveys
Keywords:Aeromagnetic survey, Magnetization, Phreatic eruption, Demagnetization, Hydrothermal alteration
We conducted an aeromagnetic survey around Oana Crater, Azuma Volcano in 2021 and present a subsurface magnetization model. The 2021 aeromagnetic data were acquired using an unmanned aerial vehicle over a rectangular area of 2 km in the NNE-SSW and 1 km in the WNW-ESE direction, centered on Oana Crater. The 18 flight lines were aligned at 100 m intervals parallel to the short side of the rectangular area.
The raw magnetic field anomaly map was estimated on a reduction surface (Nakatsuka and Okuma, 2006a). The apparent mean magnetization was estimated using Grauch (1987), and the topographic effect caused by the apparent mean magnetization was corrected from the raw magnetic field anomaly map. The obtained magnetic field anomaly data were inverted into a subsurface magnetization model using the method of Nakatsuka and Okuma (2014). The model parameter corresponds to the magnetization perturbation against mean magnetization and was constrained to be over the negative mean magnetization to avoid a reverse magnetization.
The apparent mean magnetization was calculated to be 2.42 A/m. The obtained magnetization model shows 1) a highly magnetized body extending to about 180 m below the surface at about 300 m SE of Mt. Issaikyo and 2) a low magnetized body extending to about 180 m below the surface at about 400 m W of Oana Crater. The root mean squared misfit was 4.28 nT. The locations of the demagnetizing dipoles (Nihara et al. 2022) are the same depth or about 100 m deeper than that of the low magnetized body but are horizontally closer to Oana Crater. Himematsu and Ozawa (2023) estimated the point, ellipsoidal, and rectangular crack inflation source models from the ground deformation data recorded by both InSAR and GNSS in the 2015 and 2018 volcanic unrest. All inflation sources are located in deeper depths than those of the low magnetization and demagnetizating area but the low magnetization and demagnetizating area are located in the direction of the long axes of the ellipsoidal inflation source and of the rectangular crack inflation source. On the other hand, the point inflation sources are located at the center of the ellipsoidal and rectangular crack inflation sources, and the locations of the point inflation sources of 2015 and 2018 data almost coincide with each other. Thus, they interpret that hydrothermal fluids should pass through this point source location during the 2015 and 2018 volcanic unrest. These results suggest that there exists the hydrothermal fluid pathway from the point inflation source to the low magnetization and the demagnetization area. Although the low magnetization area is not sure to be a hydrothermal reservoir, the low magnetization is likely to be caused by hydrothermal alteration. The demagnetizing area is likely in the process of hydrothermal fluid infiltration.
The location of the low magnetized area in the model does not coincide with those of the subsurface demagnetizing dipoles (Nihara et al., 2022). The demagnetizing dipoles are estimated from the magnetic field observations at limited numbers of stationary sites on the ground. The discrepancy is possibly due to the sparse distribution of stationary sites and the locations of demagnetizing dipoles may be coincide with that of the magnetized area. Hence, we attempted to estimate the static magnetic field variation using the Ministry of Land, Infrastructure, Transport and Tourism (MLIT)'s aeromagnetic surveys in 2013. However, the result is intrinsically inconsistent with the data of the stationary sites on the ground, and we concluded the aeromagnetic data had poor quality to discuss the static magnetic field variation.
Acknowledgments
Fukushima Office of Rivers and National Highways, Tohoku Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism, and Nippon Engineering Consultants, Inc. provided the 2013 aeromagnetic data. Geospatial Information Authority of Japan provided the high-precision 1-meter DEM data.
The raw magnetic field anomaly map was estimated on a reduction surface (Nakatsuka and Okuma, 2006a). The apparent mean magnetization was estimated using Grauch (1987), and the topographic effect caused by the apparent mean magnetization was corrected from the raw magnetic field anomaly map. The obtained magnetic field anomaly data were inverted into a subsurface magnetization model using the method of Nakatsuka and Okuma (2014). The model parameter corresponds to the magnetization perturbation against mean magnetization and was constrained to be over the negative mean magnetization to avoid a reverse magnetization.
The apparent mean magnetization was calculated to be 2.42 A/m. The obtained magnetization model shows 1) a highly magnetized body extending to about 180 m below the surface at about 300 m SE of Mt. Issaikyo and 2) a low magnetized body extending to about 180 m below the surface at about 400 m W of Oana Crater. The root mean squared misfit was 4.28 nT. The locations of the demagnetizing dipoles (Nihara et al. 2022) are the same depth or about 100 m deeper than that of the low magnetized body but are horizontally closer to Oana Crater. Himematsu and Ozawa (2023) estimated the point, ellipsoidal, and rectangular crack inflation source models from the ground deformation data recorded by both InSAR and GNSS in the 2015 and 2018 volcanic unrest. All inflation sources are located in deeper depths than those of the low magnetization and demagnetizating area but the low magnetization and demagnetizating area are located in the direction of the long axes of the ellipsoidal inflation source and of the rectangular crack inflation source. On the other hand, the point inflation sources are located at the center of the ellipsoidal and rectangular crack inflation sources, and the locations of the point inflation sources of 2015 and 2018 data almost coincide with each other. Thus, they interpret that hydrothermal fluids should pass through this point source location during the 2015 and 2018 volcanic unrest. These results suggest that there exists the hydrothermal fluid pathway from the point inflation source to the low magnetization and the demagnetization area. Although the low magnetization area is not sure to be a hydrothermal reservoir, the low magnetization is likely to be caused by hydrothermal alteration. The demagnetizing area is likely in the process of hydrothermal fluid infiltration.
The location of the low magnetized area in the model does not coincide with those of the subsurface demagnetizing dipoles (Nihara et al., 2022). The demagnetizing dipoles are estimated from the magnetic field observations at limited numbers of stationary sites on the ground. The discrepancy is possibly due to the sparse distribution of stationary sites and the locations of demagnetizing dipoles may be coincide with that of the magnetized area. Hence, we attempted to estimate the static magnetic field variation using the Ministry of Land, Infrastructure, Transport and Tourism (MLIT)'s aeromagnetic surveys in 2013. However, the result is intrinsically inconsistent with the data of the stationary sites on the ground, and we concluded the aeromagnetic data had poor quality to discuss the static magnetic field variation.
Acknowledgments
Fukushima Office of Rivers and National Highways, Tohoku Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism, and Nippon Engineering Consultants, Inc. provided the 2013 aeromagnetic data. Geospatial Information Authority of Japan provided the high-precision 1-meter DEM data.