Temperature Dependency of Electrical Resistivity of Water Saturated Rocks and the Possibility of Underground Temperature Estimation

Abstract

In order to get more correct informations from the data obtained by electrical methods such as surface electrical survey, well logging or other applications ingeothermal fields, it is desired to know the relationship between the electricalresistivity of rocks and temperature under the condition of water saturation. Inthis paper, the results of laboratory measurements conducted on tipical igneous rocks, sedimentary rocks and altered rocks are described together with those about rocks cored from a geothermal well. The rock samples with rod shape were cut out from original rocks and saturated with water, and then set in a teflon cylindrical electrode cell which was designed so that the electrical leakage between electrodes of sample ends might not occur along outer solution. A teflon vessel was used as a container of the electrode cell, and was put into an autoclave equipped with outer lead terminals. The electrical resistance of rock samples was measured under water saturated condition (vapour free) as a function of temperature by Kohlrausch bridge method in the range of temperature from 10°C to 180°C. The dependency of electrical resistivity on temperature was approximately similar to that of aqueous solution, when the logarithm of the resistivity was plotted versus a reciprocal of Kelvin temperature. Thus the temperature dependency of electrical resistivity of water saturated rocks is understood by introducing Stokes' formula for ionic conduction of aqueous solution, Andrade's formula for temperature dependency of ionic mobility (viscosity) and Archie's formula expressing rock resistivity in terms of pore fluid. According to the results mentioned above, the resistivity formation factors of rocks and chemical characteristics of underground water in geothermal fields arevery important for predicting subsurface temperature profile from electrical methods, and since the ionic concentration of pore fluid strongly depends on the kind of rock minerals, an accumulation of statistical data of resistivity formation factor for rocks in geothermal field is desired for further development of underground iexploration.