Low temperature dependence of electrical resistivity: Implications for near surface geophysical monitoring

Electrical resistivity imaging surveys are used to monitor variations in pore fluid chemistry and saturation as well as timelapse changes. Temperature variations in the near surface can produce larger magnitude changes in electrical conductivity than changes due to slow moving solute plumes or spatial variations in chemistry and soilmoisture. Relationships between temperature and electrical conductivity based on previous studies conducted over 25-200 degrees C do not explain 0-25 degrees C laboratory data. A modification to the temperature dependence within a petrophysical model is proposed that may allow general application over this temperature range. An empirical linear approximation of 1.8 to 2.2 percent change in bulk electrical conductivity per degree C is consistent with low temperature electrical conductivity studies and the predictions of the petrophysical model used. This relationship can be used to account for the effect of temperature variations within individual images or time-lapse difference images.