Vertical mapping of hydrothermal fluids and alteration from bulk conductivity: Simple interpretation on the USDP-1 site, Unzen Volcano, SW Japan

Volcanic edifices often have hydrothermal systems in the shallow subsurface, and these hydrothermal systems may discharge volcanic gases. Electrical conductivity (resistivity) surveys have potential for the estimation of the fluxes of volcanic gases, because interstitial water conductivity steeply increases when salinity increases by the dissolution of volcanic gases. However, in the subsurface of volcanoes, there are essentially two factors increasing bulk conductivity: the salinity of hydrothermal fluids and conductive clays by hydrothermal alteration. Therefore, the estimation of the fluxes of volcanic gases by electromagnetic methods requires to separate bulk conductivity simply into pore water conductivity (σf) and matrix conductivity (σs), which are related to salinity and hydrothermal alteration, respectively. The objective of this study is to separate observed bulk conductivity simply into σf and σs by the use of drillcore samples based on Revilʹs model and to map the vertical variation of these two conductivity components for examination relation between bulk conductivity and hydrothermal fluids as the first step toward demonstrating the feasibility and practicality of using geophysical methods to constrain volcanic gas fluxes. As a case study, this study was conducted at the USDP-1 borehole site located on Unzen Volcano, SW Japan, and the bulk conductivity was successfully separated. The results consistently explained the vertical profiles of temperature and core permeability and suggested little hydrothermal alteration at this study site. The simple method of separating bulk conductivity may be applicable to vertical or lateral mapping of hydrothermal fluids and alteration at volcanoes, and separated values of σf and σs may be of great help for the estimation of the fluxes of volcanic gases and the intensity of hydrothermal alteration.