Spectral Induced Polarization measurements on New Zealand sands - dependence on fluid conductivity

Spectral induced polarization (SIP) measurements explore the variation of the complex conductivity (sigma*) of a material with frequency. Much of this variation results from polarization effects associated with the electric double layer on the surfaces of pore spaces. The consequent dependence of the SIP signature on pore structure thus has the potential to provide a link to the hydraulic properties of the material, which have a similar dependence. We report here on the variation of the SIP signature of unconsolidated sands, typical of those found in coastal aquifers in New Zealand, with the conductivity of the pore fluid. The SIP parameters of the measurements are modelled in terms of a Cole-Cole model and demonstrate the independence of relaxation time on fluid conductivity. The contribution of surface conductivity to the overall conductivity is calculated and the variation of the imaginary part of the surface conductivity with fluid conductivity is tested against two models for the origin of surface conductivity. The measured hydraulic conductivity is also compared with estimates provided by three proposed equations relating hydraulic properties to structural and electric properties.