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[HCG23-P02] Three-dimensional resistivity distribution by magnetotelluric survey to evaluate the distribution of fossil seawater
Keywords:Fossil seawater, Magnetotelluric survey, Neogene sedimentary rock, Geological disposal
Introduction
The hydrological regime with “a slow groundwater movement that minimizes the transport of radioactive materials” is considered one of the most suitable geological conditions in the site selection of the geological disposal of high-level radioactive waste [1]. Fossil seawater is groundwater whose composition has been altered during the burial diagenetic process but has not been affected by the infiltration of meteoric fresh water [2]. Its presence indicates that the groundwater movement of the area is sufficiently slow. Thus, developing a workflow in the geological disposal project for investigating the distribution of fossil seawater is essential to determine where groundwater movement is sufficiently slow.
This study reports the preliminary results of the magnetotelluric (MT) survey aiming to evaluate the three-dimensional distribution of fossil seawater in the Horonobe area that is composed of the Neogene sedimentary rocks. Moreover, fossil seawater is characterized by high salinity. Consequently, the low resistive domain obtained by the MT survey can provide information about its distribution [3]. The audio-magnetotelluric (AMT) survey, where the depth of exploration is shallow, was applied in most of the previous electromagnetic surveys conducted in the Horonobe area. This study applied the MT survey, which can explore the deeper layers with high accuracy, to improve the resolution of fossil seawater distribution.
Materials and methods
The survey was conducted in a 9-km2 area adjacent to the Horonobe Underground Research Laboratory of Japan Atomic Energy Agency. Moreover, 60 points were arranged in a grid pattern within the area to effectively estimate the three-dimensional distribution of fossil seawater. Consequently, the MT data were obtained at each point. In the three-dimensional inversion, 99 points including 39 existing points were used to estimate the resistivity distribution.
Results and future perspective
The overall distribution of resistivity was generally low, consistent with the geology of the Horonobe area, which consists mainly of mudstone and shale. However, resistivity appeared to vary following the geological structure. The resistivity was relatively high (>10 Ω-m) until 50 m in depth, while it was relatively lower (1–5 Ω-m) at a depth of 50–1,000 m. The area adjacent to the Omagari Fault showed higher resistivity compared to the surrounding area, suggesting the infiltration of meteoric fresh water preferentially along the Omagari Fault similar to the previous AMT survey [4]. The Yuchi Formation, an upper stratum distributed to the southwest part of the survey area, also showed high resistivity, which may be due to the groundwater recharge.
Examining a more optimal resistivity distribution considering the geological structure is necessary for future work. Furthermore, the integration of the MT survey with borehole investigations is a critical issue to estimate the three-dimensional distribution of fossil seawater.
This study was conducted as a part of the project on “FY2020 Development and Improvement on groundwater flow evaluation technic in rock” funded by the Japanese Ministry of Economy, Trade and Industry.
[1] Nuclear Waste Management Organization of Japan, 2011, NUMO-TR-13-05.
[2] Muramatsue et al., 2016, J. Hot Spring Sci., 65, 216-233.
[3] Mizuno et al., 2017, 58, 178-187.
[4] Ishii et al., 2006, Jour. Geol. Soc. Japan, 112, 301-314.
The hydrological regime with “a slow groundwater movement that minimizes the transport of radioactive materials” is considered one of the most suitable geological conditions in the site selection of the geological disposal of high-level radioactive waste [1]. Fossil seawater is groundwater whose composition has been altered during the burial diagenetic process but has not been affected by the infiltration of meteoric fresh water [2]. Its presence indicates that the groundwater movement of the area is sufficiently slow. Thus, developing a workflow in the geological disposal project for investigating the distribution of fossil seawater is essential to determine where groundwater movement is sufficiently slow.
This study reports the preliminary results of the magnetotelluric (MT) survey aiming to evaluate the three-dimensional distribution of fossil seawater in the Horonobe area that is composed of the Neogene sedimentary rocks. Moreover, fossil seawater is characterized by high salinity. Consequently, the low resistive domain obtained by the MT survey can provide information about its distribution [3]. The audio-magnetotelluric (AMT) survey, where the depth of exploration is shallow, was applied in most of the previous electromagnetic surveys conducted in the Horonobe area. This study applied the MT survey, which can explore the deeper layers with high accuracy, to improve the resolution of fossil seawater distribution.
Materials and methods
The survey was conducted in a 9-km2 area adjacent to the Horonobe Underground Research Laboratory of Japan Atomic Energy Agency. Moreover, 60 points were arranged in a grid pattern within the area to effectively estimate the three-dimensional distribution of fossil seawater. Consequently, the MT data were obtained at each point. In the three-dimensional inversion, 99 points including 39 existing points were used to estimate the resistivity distribution.
Results and future perspective
The overall distribution of resistivity was generally low, consistent with the geology of the Horonobe area, which consists mainly of mudstone and shale. However, resistivity appeared to vary following the geological structure. The resistivity was relatively high (>10 Ω-m) until 50 m in depth, while it was relatively lower (1–5 Ω-m) at a depth of 50–1,000 m. The area adjacent to the Omagari Fault showed higher resistivity compared to the surrounding area, suggesting the infiltration of meteoric fresh water preferentially along the Omagari Fault similar to the previous AMT survey [4]. The Yuchi Formation, an upper stratum distributed to the southwest part of the survey area, also showed high resistivity, which may be due to the groundwater recharge.
Examining a more optimal resistivity distribution considering the geological structure is necessary for future work. Furthermore, the integration of the MT survey with borehole investigations is a critical issue to estimate the three-dimensional distribution of fossil seawater.
This study was conducted as a part of the project on “FY2020 Development and Improvement on groundwater flow evaluation technic in rock” funded by the Japanese Ministry of Economy, Trade and Industry.
[1] Nuclear Waste Management Organization of Japan, 2011, NUMO-TR-13-05.
[2] Muramatsue et al., 2016, J. Hot Spring Sci., 65, 216-233.
[3] Mizuno et al., 2017, 58, 178-187.
[4] Ishii et al., 2006, Jour. Geol. Soc. Japan, 112, 301-314.