Japan Geoscience Union Meeting 2023

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[E] Oral

S (Solid Earth Sciences ) » S-EM Earth's Electromagnetism

[S-EM14] Electric, magnetic and electromagnetic survey technologies and scientific achievements

Tue. May 23, 2023 1:45 PM - 3:00 PM 103 (International Conference Hall, Makuhari Messe)

convener:Kiyoshi Baba(Earthquake Research Institute, The University of Tokyo), Tada-nori Goto(Graduate School of Science, University of Hyogo), Yuguo Li(Ocean University of China), Wiebke Heise(GNS Science, PO Box 30368, Lower Hutt, New Zealand), Chairperson:Kiyoshi Baba(Earthquake Research Institute, The University of Tokyo), Takeshi Suzuki(Institute for Geothermal Sciences, Kyoto University)

2:15 PM - 2:30 PM

[SEM14-03] Electrical conductivity of rocks containing smectite computed by finite element method

*Kentaro Aoyama1, Takeshi Hashimoto1 (1.Hokkaido University)

Keywords:Electrical Conductivity, Smectite

The resistivity of the subsurface inferred from electromagnetic surveys reflects various physical properties and conditions, such as porosity, pore fluid salinity, and temperature. In particular, smectite, a clay mineral that is thermodynamically stable in a temperature range of about 50 to 230°C, sometimes plays a vital role in reducing the bulk resistivity in the shallow subsurface. Nevertheless, the quantitative relationship between smectite and bulk resistivity is not fully understood. A previous study (Levy et al., 2018) proposed an equivalent circuit model using core data from the Krafla volcano in Iceland to calculate the bulk conductivity of smectite-bearing rocks. However, the mass fraction of smectite in the cores used in that study was limited to a maximum of 50 wt%. In addition, it was unclear how the smectite particle arrangement pattern and temperature relate to bulk conductivity. Therefore, we investigated the effects of the parameters: volume fraction of smectite, temperature, NaCl concentration of pore water, and porosity, by numerical simulations using the finite element method. In our calculations, we randomly assigned the conductivity tensor either of smectite, quartz, or pore water to each element in the model space, based on the macroscopic physical properties (i.e., porosity and volume fraction of smectite in the quartz matrix). We introduced an anisotropy to the conductivity tensor of smectite, taking into account the conductivity of interlayer water in a parallel direction to the T-O-T layer and the conductivity of the SiO4 tetrahedron in the perpendicular direction (10-12 S/m in constant). Besides, we regarded the conductivities of the quartz and NaCl solution as isotropic. For porosities between 0.01 and 0.4, we found that the bulk conductivity highly depended on the spatial arrangement of mineral and liquid phases. In contrast, for porosities less than 0.01, the bulk conductivity was less affected by the spatial arrangement. Our study suggests the possibility of estimating the volume fraction of smectite, temperature, salinity, pore porosity, and spatial arrangement between phases from the bulk conductivity, given that some a priori constraints on these parameters are provided from field data such as drilling cores or geochemical analyses.