9:00 AM - 9:15 AM
[SVC28-01] Subsurface Resistivity Structure in the Western Part of the Owakudani Steaming Area, Hakone, Japan
Keywords:Owakudani, Hakone volcano, Steaming area, CSAMT, vapor pocket, Steam-heated hot springs
The Owakudani steaming area of Hakone volcano can be divided into two parts: east and west parts by a catchment boundary running from north to south. The eastern part mainly forms a deep valley, and in its bottom, a steaming area develops. This area was the center of a hydrothermal eruption that occurred in June 2015, and many fumaroles were formed during the volcanic unrest, which are still active. On the other hand, in the western part, fumaroles and small mud pots are scattered on the slopes of Kakunagatake, where shrubs cover the landscape. In this area, Cl/SO4 ratio of a few spring waters increased after the eruption (Kikukawa, 2021), while no significant increase in surface activity was observed before and after the 2015 eruption.
In Hakone volcano, MT surveys have detected a bell-shaped, low-resistivity structure that extends over the entire edifice of the volcano with its top near Owakudani. The bell-shaped low-resistivity structure was interpreted as a caprock structure, which is a smectite-bearing low permeability zone that nests the hydrothermal system (Yoshimura et al., 2018). In the eastern part of Owakudani steaming area, the resistivity structure of the subsurface has been clarified by CSAMT and MT methods, and the cap layer, which can be recognized as a bell-shaped low-resistivity structure, occurs in shallow, and a high-resistivity zone that was interpreted as a vapor pocket was recognized within the cap layer (Seki et al., 2020; Mannen et al., 2019). On the other hand, the western part of the Owakudani steaming area was slightly out of the exploration area, and the detailed resistivity structure has been unknown.
We conducted a series of CSAMT survey in 2021 and 2022 to clarify the shallow resistivity structure of the entire Owakudani steaming area. The survey area extends approximately 1300 m (EW) × 1000 m (NS). Within the area, 53 observation points were distributed. The distance between the closest observation points were about 200 m. The transmitter composed of two groups of electrodes placed about 2,500 m apart in EW direction was located approximately 2 km south of the measurement area. From the transmitter, 23 frequencies (1 to 5120 Hz) of electrical current 5.9 A in 2022 and 6.3 A in 2023 were injected into the ground. To the obtained data, one-dimensional inverse analysis to investigate data quality and a rough resistivity structure was applied. Then, a two-dimensional inverse analysis was performed to obtain a resistivity structure along cross-sections in the east-west direction. The depth of exploration estimated by Bostick Depth was approximately 500 m.
In Hakone volcano, MT surveys have detected a bell-shaped, low-resistivity structure that extends over the entire edifice of the volcano with its top near Owakudani. The bell-shaped low-resistivity structure was interpreted as a caprock structure, which is a smectite-bearing low permeability zone that nests the hydrothermal system (Yoshimura et al., 2018). In the eastern part of Owakudani steaming area, the resistivity structure of the subsurface has been clarified by CSAMT and MT methods, and the cap layer, which can be recognized as a bell-shaped low-resistivity structure, occurs in shallow, and a high-resistivity zone that was interpreted as a vapor pocket was recognized within the cap layer (Seki et al., 2020; Mannen et al., 2019). On the other hand, the western part of the Owakudani steaming area was slightly out of the exploration area, and the detailed resistivity structure has been unknown.
We conducted a series of CSAMT survey in 2021 and 2022 to clarify the shallow resistivity structure of the entire Owakudani steaming area. The survey area extends approximately 1300 m (EW) × 1000 m (NS). Within the area, 53 observation points were distributed. The distance between the closest observation points were about 200 m. The transmitter composed of two groups of electrodes placed about 2,500 m apart in EW direction was located approximately 2 km south of the measurement area. From the transmitter, 23 frequencies (1 to 5120 Hz) of electrical current 5.9 A in 2022 and 6.3 A in 2023 were injected into the ground. To the obtained data, one-dimensional inverse analysis to investigate data quality and a rough resistivity structure was applied. Then, a two-dimensional inverse analysis was performed to obtain a resistivity structure along cross-sections in the east-west direction. The depth of exploration estimated by Bostick Depth was approximately 500 m.