Relationship between spectral induced polarization and hydraulic properties of saturated and unsaturated sandstone

There is growing interest in the use of geophysical methods for hydrological model parameterization. Empirical induced polarization (IP)-hydraulic conductivity (K) relationships have been developed, but these are only applicable to sediments in which the IP response shows limited variation with electrical current frequency. Here we examine the spectral IP response of samples taken from a UK sandstone aquifer and compare measured parameters with physical and hydraulic properties. We demonstrate the limited value of existing IP-K models due to the inherent IP frequency dependence of these samples. Our results show how the mean relaxation time, tau, is a more appropriate measure of IP response for these sediments. A significant inverse correlation between the surface area to pore volume ratio and tau is observed, suggesting that tau is a measure of a characteristic hydraulic length scale. This is supported by a measured strong positive correlation between log tau and log K. Our measurements also reveal evidence of a relationship between tau and a dominant pore throat size, which leads to postulations about the parallelism between the spectral IP behavior and unsaturated hydraulic characteristics. Additional experiments show how the relaxation time is affected by degree of fluid saturation, indicating that saturation levels must be accounted for if our empirical relationships are applied to vadose zone studies. Our results show clear evidence of the potential value of frequency-based IP measurements for parameterization of groundwater flow models.