17:15 〜 19:15
[SEM15-P09] Three-dimensional resistivity structure in the western Owakudani region, Hakone Volcano, inferred from CSAMT data
キーワード：比抵抗、CSAMT、火山、箱根
At Hakone Volcano, a bell-shaped low-resistivity body has been imaged on a broad scale by Yoshimura et al. (2018, EPS) and directly beneath the Owakudani area by Seki et al. (2020, GRL), strongly suggesting the presence of a cap rock. However, how the cap rock structure imaged by Seki et al. extends westward from Owakudani remains unclear, and the overall structure of the cap rock from the Owakudani fumarolic area to the western region is not yet well understood.
To address this issue, a dense CSAMT survey was conducted in the western Owakudani region from 2021 to 2022. CSAMT observations provide a high signal-to-noise ratio and are capable of detecting even small temporal variations in resistivity (e.g., Mannen et al. 2019, EPS). A previous two-dimensional (2-D) analysis of the east-west CSAMT survey lines by Mannen et al. (2024, JpGU) revealed a distinct low-resistivity body consistent with a cap rock, suggesting that the cap rock imaged directly beneath Owakudani is part of a more extensive structure extending westward. The 2-D resistivity model also indicates the presence of localized high-resistivity zones within the conductive cap rock layer, potentially corresponding to vapor pockets.
In this study, we further investigate the detailed structure of the cap rock and the presence of vapor pockets through a three-dimensional (3-D) analysis of the CSAMT data. Our preliminary results confirm that a similar cap rock structure observed in 2-D also appears in the 3-D model (Minami et al., 2024, Fall Meeting of the Volcanological Society of Japan). However, our previous inversion used an error floor set at 10% of the impedance—significantly larger than the actual observational error. In this presentation, we present an updated 3-D resistivity structure using a smaller error floor, more representative of real CSAMT observational errors, and discuss the detailed strucuture of the cap rock and the presence of high-resistivity zones within the cap rock in the western Owakudani region.
To address this issue, a dense CSAMT survey was conducted in the western Owakudani region from 2021 to 2022. CSAMT observations provide a high signal-to-noise ratio and are capable of detecting even small temporal variations in resistivity (e.g., Mannen et al. 2019, EPS). A previous two-dimensional (2-D) analysis of the east-west CSAMT survey lines by Mannen et al. (2024, JpGU) revealed a distinct low-resistivity body consistent with a cap rock, suggesting that the cap rock imaged directly beneath Owakudani is part of a more extensive structure extending westward. The 2-D resistivity model also indicates the presence of localized high-resistivity zones within the conductive cap rock layer, potentially corresponding to vapor pockets.
In this study, we further investigate the detailed structure of the cap rock and the presence of vapor pockets through a three-dimensional (3-D) analysis of the CSAMT data. Our preliminary results confirm that a similar cap rock structure observed in 2-D also appears in the 3-D model (Minami et al., 2024, Fall Meeting of the Volcanological Society of Japan). However, our previous inversion used an error floor set at 10% of the impedance—significantly larger than the actual observational error. In this presentation, we present an updated 3-D resistivity structure using a smaller error floor, more representative of real CSAMT observational errors, and discuss the detailed strucuture of the cap rock and the presence of high-resistivity zones within the cap rock in the western Owakudani region.