Frequency Dependence In The Complex Resistivty Of Ottawa Sand Containing Water-Air And Water-Napl Mixtures

The interpretation of spectral induced polarization (IP) and electromagnetic geophysical<br>surveys requires an understanding of the frequency dependence of the electrical properties<br>of geological materials over as wide a frequency range as possible. The frequency<br>dependence observed in survey data acquired using these methods, at sites that are either<br>NAPL contaminated or partially saturated will depend on the pore-scale fluid distribution.<br>Complex resistivity measurements were performed on an Ottawa sand sample during<br>drainage-imbibition experiments to investigate the effects of water/NAPL and water/air<br>pore-scale fluid distributions on the electrical properties from 1 kHz to 1 MHz. The<br>resistivity spectra show a strong dispersion in the upper part of this frequency range. As<br>the water saturation decreases, the dispersion strengthens and shifts to lower frequencies.<br>The data were successfully modelled using an equivalent circuit model, which includes the<br>effects of low frequency electrolytic conduction, the high frequency dielectric polarization<br>limit and a Cole-Cole type relaxation phenomenon. Variations in the frequency<br>dependence of the complex resistivity with saturation and saturation history are interpreted<br>in terms of the pore-scale fluid distribution. The saturation history was found to have an<br>effect on the frequency response, particularly on the modelled chargeability.