Subsurface resistivity and sulfide filling microfractures: a rock physical analysis at Mt. Ontake volcano's southeast flank

SUMMARY The interpretation of subsurface resistivity structures in volcanic areas remains challenging and requires the selection of the most plausible configuration from various geological features that affect the resistivity measurements. A comprehensive physical study of the rocks in the target area is essential for accurate interpretation. A magnetotelluric (MT) survey conducted in the southeast flank of Mt. Ontake volcano detected several kilometres of low resistivity below 30 Ωm. However, the interpretation of resistivity anomalies remains to be verified; borehole data have been used to resolve this problem. In this study, geological, fracture, temperature and resistivity structures obtained from borehole investigations were analysed using rock physical methods, and core samples were subjected to physical property measurements and microscopic observations. The geology observed in the borehole was sedimentary rock with a porosity of less than a few per cent, except for the surface volcanic breccia and some intrusive granites. The borehole wall was poorly fractured. The groundwater temperature in the borehole was aligned with the standard geothermal gradient, 40 °C at a depth of 800 m. These results indicate the absence of a hydrothermal reservoir at the site. In contrast, the core samples contained pyrite-filled microfractures. All the samples had poor porosity and contained a small amount of clay minerals but showed uniformly lower resistivity values than expected for similar porosity. Some samples exhibited robust induced polarization effects. The pyrite contents of the samples were low. A pseudo-high-frequency conductivity model test using electrostatic field analysis on a numerical model of microfractured rock properties reproduced the low resistivity observed in the borehole investigation and MT survey. The specifically low-resistivity values observed were appropriately reproduced. The findings of this study indicate that pyrite filling the microfractures can be a major contributor to the low resistivity below 1 Ωm in the study area and suggest that it is necessary to consider the network of microscale veins formed by conductive sulfide minerals, mainly pyrite, for the interpretation of subsurface resistivity in geothermal areas.