5:15 PM - 6:45 PM
[SVC28-P03] Resistivity and permeability measurements of rock samples collected from geothermal and volcanic areas, Beppu and Aso, SW Japan
Keywords:Electrical resistivity, Permeability, Rock physics, Surface conduction
A hydrothermal system within a volcanic edifice is known to be one of the driving forces of volcanic and geothermal activities. The electrical resistivity structure has often been used to determine the fluid distribution and flow in hydrothermal systems. The spatial extent of the low resistivity region can be interpreted as the flow pathways of geothermal fluid, and the water content and permeability in the region can be estimated using resistivity. However, the estimated water content is strongly dependent on the choice of rock physics model, including empirical formulas and theoretical models. In addition, previous studies have constructed only a simple permeability structure, because the scheme for constructing subsurface permeability from resistivity has not been well established. The resistivity in geothermal areas may include the influence of surface conductivity due to hydrothermal alteration.
One way to investigate a rock physics model that describes the relationship between resistivity, permeability, and porosity is to measure the values of drill cores from volcanic and geothermal areas. In this study, we measured these properties of samples collected around Beppu and Aso, which are among the most active volcanic and geothermal areas in Japan, respectively. We also estimated surface conductivity on the basis of Revil’s model with impedance measurements using NaCl solutions with some initial salinities.
The experimental results showed that the electrical resistivity decreased with increasing porosity, where the effects of surface conduction were significant for both Aso and Beppu samples. The effects of surface conduction were related to the degree of alteration of the sample. The measured permeability of the lava and agglutinate samples exhibited a logarithmically linear relationship with changes in resistivity. However, different resistivity values were obtained for lava and sandstone samples with similar permeabilities. Our results indicate the difficulty in precisely interpreting subsurface resistivity in volcanic and geothermal areas using a classical rock physics model based on sandstone.
One way to investigate a rock physics model that describes the relationship between resistivity, permeability, and porosity is to measure the values of drill cores from volcanic and geothermal areas. In this study, we measured these properties of samples collected around Beppu and Aso, which are among the most active volcanic and geothermal areas in Japan, respectively. We also estimated surface conductivity on the basis of Revil’s model with impedance measurements using NaCl solutions with some initial salinities.
The experimental results showed that the electrical resistivity decreased with increasing porosity, where the effects of surface conduction were significant for both Aso and Beppu samples. The effects of surface conduction were related to the degree of alteration of the sample. The measured permeability of the lava and agglutinate samples exhibited a logarithmically linear relationship with changes in resistivity. However, different resistivity values were obtained for lava and sandstone samples with similar permeabilities. Our results indicate the difficulty in precisely interpreting subsurface resistivity in volcanic and geothermal areas using a classical rock physics model based on sandstone.