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[HCG20-P03] NMR logging-based hydrological characterization of a fractured lava formation as a rapid groundwater flow path
Keywords:geological disposal of high-level radioactive nuclear waste, nuclear magnetic resonance well logging, rapid groundwater flow path, groundwater hydrology, porosity, permeability
1. Introduction
In geological disposal of high-level radioactive waste, it is important to obtain hydrogeological data such as porosity and permeability in-situ to understand regional groundwater flow. Geophysical logging based on proton nuclear magnetic resonance (NMR), which targets the 1H nuclei that make up water molecules, has the potential to acquire depth profiles of such hydrogeological data. In this study, we analyzed NMR logging data of a water-conducting formation (a formation with high permeability), which is particularly important for regional groundwater flow characteristics, and quantitatively evaluated the high permeability of a fractured lava layer [1].
2. NMR logging and results
NMR logging (Figs. 1-2) was conducted in a borehole drilled in the Suruga Bay coastal area [2]. Most of the strata are unconsolidated Quaternary fan deposits (sand and gravel with cobbles), but there is a basaltic lava layer (7 m thick). Figure 3 shows an example of a fracture in a basaltic lava core obtained from the borehole.
The raw NMR data are transient waveforms of the transverse magnetization of the 1H nucleus that decays exponentially with several time constants (T2). NMR enables to convert the T2 value into the pore size. As a result, based on Fig. 4, the original fracture aperture in Fig. 3 was calculated to be 13 mm [1]. Assuming a planar Poiseuille flow, the effective hydraulic conductivity of a system with a planar fracture with an aperture of 13 mm running through a 7 m thick lava layer with an impermeable matrix was calculated to be 0.22 m/s.
3. Discussion
A fan deposit with a thickness of approximately 140 m and a hydraulic conductivity of ≈10-5 m/s (by in-situ aquifer tests) is deposited above the 7-m-thick lava. The large contrast (0.22 m/s vs. 10-5 m/s) suggests that a fractured lava with a thickness of only 7 m, cannot be ignored when considering regional groundwater flow in the area.
However, the dimension of the sensitive area is only 2 cm in thickness (Fig. 2), and other validations are needed to discuss the regional groundwater flow on the km scale with an aperture calculated on this small scale. Fortunately, we obtained a lot of supportive data. For example, (i) active seafloor seepage was observed on the seafloor ≈3 km away from the borehole (≈140 m below sea level)[2], which can be roughly explained by groundwater flow calculation assuming a ≈1-cm-wide flat-plate fracture. (ii) Depth profiles of formation water temperature, electrical conductivity, dissolved chloride ion concentrations, and isotope ratio of 14C measured in the same borehole as the NMR logs show drastic changes in the depth interval of 100-200 m, which roughly coincides with the depth at which the lava exist (about 150 m) [2]. This suggests that the lava layer in Fig. 3, as a water channel, has a significant influence on the transport of geothermal energy and groundwater and its dissolved materials over a wide area along the Suruga Bay coast.
Thus, NMR logging, with its superior proton quantification capability, was able to quantify fracture aperture width, which has a decisive influence on the evaluation of hydraulic performance. In the current situation where there are few methods that to measure fracture aperture in situ, NMR logging is a promising method for detecting water-conducting features.
This study was conducted as part of a R&D supporting program entitled ‘The project for validating assessment methodology of geological disposal system in coastal region (2019FY-2023FY)’ under a contract with the Ministry of Economy, Trade and Industry (METI) (Grant Number: JPJ007597).
References.
[1] Nakashima, Y. and Ikawa, R. (2025) J. Nucl. Sci. Tech. https://doi.org/10.1080/00223131.2025.2451025
[2] Agency for Natural Resources and Energy (METI). https://www.enecho.meti.go.jp/category/electricity_and_gas/nuclear/rw/library/library06.html
In geological disposal of high-level radioactive waste, it is important to obtain hydrogeological data such as porosity and permeability in-situ to understand regional groundwater flow. Geophysical logging based on proton nuclear magnetic resonance (NMR), which targets the 1H nuclei that make up water molecules, has the potential to acquire depth profiles of such hydrogeological data. In this study, we analyzed NMR logging data of a water-conducting formation (a formation with high permeability), which is particularly important for regional groundwater flow characteristics, and quantitatively evaluated the high permeability of a fractured lava layer [1].
2. NMR logging and results
NMR logging (Figs. 1-2) was conducted in a borehole drilled in the Suruga Bay coastal area [2]. Most of the strata are unconsolidated Quaternary fan deposits (sand and gravel with cobbles), but there is a basaltic lava layer (7 m thick). Figure 3 shows an example of a fracture in a basaltic lava core obtained from the borehole.
The raw NMR data are transient waveforms of the transverse magnetization of the 1H nucleus that decays exponentially with several time constants (T2). NMR enables to convert the T2 value into the pore size. As a result, based on Fig. 4, the original fracture aperture in Fig. 3 was calculated to be 13 mm [1]. Assuming a planar Poiseuille flow, the effective hydraulic conductivity of a system with a planar fracture with an aperture of 13 mm running through a 7 m thick lava layer with an impermeable matrix was calculated to be 0.22 m/s.
3. Discussion
A fan deposit with a thickness of approximately 140 m and a hydraulic conductivity of ≈10-5 m/s (by in-situ aquifer tests) is deposited above the 7-m-thick lava. The large contrast (0.22 m/s vs. 10-5 m/s) suggests that a fractured lava with a thickness of only 7 m, cannot be ignored when considering regional groundwater flow in the area.
However, the dimension of the sensitive area is only 2 cm in thickness (Fig. 2), and other validations are needed to discuss the regional groundwater flow on the km scale with an aperture calculated on this small scale. Fortunately, we obtained a lot of supportive data. For example, (i) active seafloor seepage was observed on the seafloor ≈3 km away from the borehole (≈140 m below sea level)[2], which can be roughly explained by groundwater flow calculation assuming a ≈1-cm-wide flat-plate fracture. (ii) Depth profiles of formation water temperature, electrical conductivity, dissolved chloride ion concentrations, and isotope ratio of 14C measured in the same borehole as the NMR logs show drastic changes in the depth interval of 100-200 m, which roughly coincides with the depth at which the lava exist (about 150 m) [2]. This suggests that the lava layer in Fig. 3, as a water channel, has a significant influence on the transport of geothermal energy and groundwater and its dissolved materials over a wide area along the Suruga Bay coast.
Thus, NMR logging, with its superior proton quantification capability, was able to quantify fracture aperture width, which has a decisive influence on the evaluation of hydraulic performance. In the current situation where there are few methods that to measure fracture aperture in situ, NMR logging is a promising method for detecting water-conducting features.
This study was conducted as part of a R&D supporting program entitled ‘The project for validating assessment methodology of geological disposal system in coastal region (2019FY-2023FY)’ under a contract with the Ministry of Economy, Trade and Industry (METI) (Grant Number: JPJ007597).
References.
[1] Nakashima, Y. and Ikawa, R. (2025) J. Nucl. Sci. Tech. https://doi.org/10.1080/00223131.2025.2451025
[2] Agency for Natural Resources and Energy (METI). https://www.enecho.meti.go.jp/category/electricity_and_gas/nuclear/rw/library/library06.html