A preliminary analysis of intrinsic fluid and rock resistivity in active hydrothermal systems


  • B. Moskowitz,

  • D. Norton


Electrical resistivity data are utilized in interpretations of subsurface envitonments and to explore for geothermal and mineral resources. Abnormally low resistivity data are alternatively interpreted to indicate the presence of high-temperature fluids or conductive minerals (metal sulfides) at depth, even though relative contributions of thermal, porosity, and fluid composition effects appear to be poorly known. An analysis of intrinsic rock resistivities, calculated electrical porosities, and two-dimensional heat and mass transfer computations indicates that the host rock resistivity distribution around igneous intrusives is directly related to the mode of dispersion of thermal energy away from the pluton. Comparisons between numerical results and field observations in geothermal areas indicate that resistivity values in the vicinity of thermal anomalies are a complex function of fluid circulation patterns, fluid composition, and the distribution of conductive minerals produced by the reaction between circulating fluids and rocks; therefore in many cases, low near-surface resistivity anomalies cannot be entirely accounted for by hot circulating saline fluids, and observations of high thermal gradients associated with low-resistivity anomalies are not unique indications of a high-energy geothermal resource at shallow crustal depths.