5:15 PM - 6:45 PM
[SCG46-P08] A shallow subsurface electrical resistivity model beneath Zao volcano, and interpretation of the shallow conductive hydrothermal alteration layer
Keywords:electrical resistivity, hydrothermal alteration layer, permeability
1. Introduction
Studies of subsurface electrical resistivity structures beneath active phreatic volcanoes suggest whether a shallow impermeable hydrothermal alteration layer allows or prevents raising hydrothermal fluid is the key feature to result in a phreatic eruption or not. The objective of this study is 1) to reassess the resistivity structure down to 2 km depth below sea level by expanding the audio-frequency magnetotelluric (MT) observations at Zao volcano, and 2) to estimate the permeability variation of the alteration layer depending on temperature change based on the confidence interval of the resistivity.
2. Data and method
Two-component electric and three-component magnetic field variations in 1-10 kHz were observed at 22 stations around Okama from September 17 to 20, 2014, and at 40 stations in a 1 km × 1 km area centered on Goshikidake, the east side of Okama from September 28 to October 22, 2020, respectively. The MT frequency response functions and geomagnetic transfer functions were estimated using BIRRP method. The obtained functions were inverted into a three-dimensional resistivity model using WSINV3D-MT code (Siripunvaraporn & Egbert, 2009). The central area of the model, which was ±2 km from east to west, north to south, and from 2000 m above sea level to 2000 m below sea level, was discretized with 50 m meshes. The initial reference (prior) model was a subsurface 100 Ωm uniform resistivity model, and the reference model was updated three times into the optimal model in the previous inversion result. To estimate the resistivity confidence interval for the surface conductive zone in the final model, we chased the Jackknife sample changes for the data misfits normalized by the standard error of all data for each observation site with varying the resistivity value of the surface conductive zone. Because the Jackknife samples obey a Gauss distribution based on central limit theorem, we applied the F-test assessing significant Chi-squared average change to estimate the resistivity confidence interval.
3. Results and discussion
Ichiki et al. (2021 Volcanological Society of Japan Fall Meeting) reported that a conductive zone is expected to exist just below Goshikidake based on the spatial distribution of the MT phase tensor attitude and the in-phase Parkinson vectors. The resultant three-dimensional resistivity model certainly represents a conductive zone within ±1 km from east to west, north to south, centered on Goshikidake, down to 500 m depth above sea level. The conductive zone is interpreted as the hydrothermal alteration or smectite-chrolite mixed layer. The root mean squared misfit is 3.48. In the presentation, we will show the resistivity confidence interval for the conductive zone and a simulation of the permeability change against temperature variation based on the resistivity confidence interval.
Acknowledgments
This research was funded by integrated program for the next generation volcano research and human resource development, and earthquake and volcano hazards observation and research program, the ministry of education, culture, sports, science and technology of Japan. English abstract was described referring to translation by Deep L free version.
Studies of subsurface electrical resistivity structures beneath active phreatic volcanoes suggest whether a shallow impermeable hydrothermal alteration layer allows or prevents raising hydrothermal fluid is the key feature to result in a phreatic eruption or not. The objective of this study is 1) to reassess the resistivity structure down to 2 km depth below sea level by expanding the audio-frequency magnetotelluric (MT) observations at Zao volcano, and 2) to estimate the permeability variation of the alteration layer depending on temperature change based on the confidence interval of the resistivity.
2. Data and method
Two-component electric and three-component magnetic field variations in 1-10 kHz were observed at 22 stations around Okama from September 17 to 20, 2014, and at 40 stations in a 1 km × 1 km area centered on Goshikidake, the east side of Okama from September 28 to October 22, 2020, respectively. The MT frequency response functions and geomagnetic transfer functions were estimated using BIRRP method. The obtained functions were inverted into a three-dimensional resistivity model using WSINV3D-MT code (Siripunvaraporn & Egbert, 2009). The central area of the model, which was ±2 km from east to west, north to south, and from 2000 m above sea level to 2000 m below sea level, was discretized with 50 m meshes. The initial reference (prior) model was a subsurface 100 Ωm uniform resistivity model, and the reference model was updated three times into the optimal model in the previous inversion result. To estimate the resistivity confidence interval for the surface conductive zone in the final model, we chased the Jackknife sample changes for the data misfits normalized by the standard error of all data for each observation site with varying the resistivity value of the surface conductive zone. Because the Jackknife samples obey a Gauss distribution based on central limit theorem, we applied the F-test assessing significant Chi-squared average change to estimate the resistivity confidence interval.
3. Results and discussion
Ichiki et al. (2021 Volcanological Society of Japan Fall Meeting) reported that a conductive zone is expected to exist just below Goshikidake based on the spatial distribution of the MT phase tensor attitude and the in-phase Parkinson vectors. The resultant three-dimensional resistivity model certainly represents a conductive zone within ±1 km from east to west, north to south, centered on Goshikidake, down to 500 m depth above sea level. The conductive zone is interpreted as the hydrothermal alteration or smectite-chrolite mixed layer. The root mean squared misfit is 3.48. In the presentation, we will show the resistivity confidence interval for the conductive zone and a simulation of the permeability change against temperature variation based on the resistivity confidence interval.
Acknowledgments
This research was funded by integrated program for the next generation volcano research and human resource development, and earthquake and volcano hazards observation and research program, the ministry of education, culture, sports, science and technology of Japan. English abstract was described referring to translation by Deep L free version.