Effect of the hydrothermal alteration on the surface conductivity of rock matrix: Comparative study between relatively-high and low temperature hydrothermal systems

Abstract

In volcanic areas, hydrothermal systems are characterized by high bulk electrical conductivity, which is a combination of high-salinity/temperature pore water and hydrothermally-altered rock matrix. The present study focuses on better understanding the influence of the hydrothermal alteration on the surface conductivity associated with generation and stability of smectite. As a case study, the GSB borehole site at Beppu geothermal area in southwest Japan was selected; at this site, hydrothermal fluids with a relatively high temperature (150°C) flow at a shallow depth.The physical properties related to electrical conductivity (formation factor, surface conductivity, and porosity) were estimated on the basis of Revil model using conductivity measurements of the drillcores. Strong hydrothermal alteration at the site GSB was shown by high surface conductivity (10−2–10−1S/m) and high cementation exponent (2.5–4.5). From the comparison between the vertical profiles of the bulk conductivity, pore water conductivity, and surface conductivity, it was shown that the rock matrix makes a non-negligible contribution to the bulk conductivity at the site. This contribution to bulk conductivity is quite different from that of low-temperature hydrothermal systems, where the contribution from the pore water dominates because there is little or no hydrothermal alteration. Furthermore, comparison between the findings of this study and low-temperature hydrothermal systems showed that the surface conductivity could simply reflect temperature to which the rock has been exposed. The surface conductivity maintains the small value at the low temperatures such as <40°C, and significantly increases at the relatively high temperatures (100–150°C). At the higher exposed temperatures >150°C, its value decreases relative to that at the temperatures of 100–150°C. This relationship is consistent with the generation and stability of smectite at active hydrothermal systems, and places strong constraints on the quantitative interpretation of the electrical conductivity structure of a volcano.