Complex Conductivity Measurements of Sand-Clay Mixtures with Varying Grain Size, Clay Content and Solution Chemistry

We have measured the electrical-impedance response (10-3 Hz to 103 Hz) of sand-clay mixtures, with varying grain size, clay content, and pore solution chemistry in an attempt to reproduce and expand upon the classic experiments of Klein and Sill (Geophyscis, 1982). Klien and Sill used a series form of the empirical Cole-Cole model to invert their data for a central relaxation time and a chargeability parameter. A major result of their study was that the relaxation time was generally observed to increase with the size of the sand grains (or glass beads): where, and the relaxation time was also observed to be dependent upon the solution conductivity and the amount of clay in the mixture. These results, however, were somewhat inconsistent with the model of Madden and Marshall (1959) and subsequent models that are based on diffusive polarization mechanisms which theoretically predict a value of a=2. Similar to Klein and Sill we found the relaxation time to increase with the size of the sand grains and to be a function of the solution conductivity and clay content. Using the original form of the Cole-Cole model, however, we obtained a larger value for the power law exponent:, which is more consistent with the predictions of the theoretical models. Lastly, in trying to reproduce Klein and Sill's experiments we found that the method of sample preparation significantly affects the complex conductivity response of the sand-clay mixture. In particular, the distinct relaxation peak observed by Klein and Sill is not reproduced unless the sand-clay mixtures are oven dried and then vacuum saturated. Samples made from "wet" sand-clay slurries do not have distinctive relaxation peaks - the measured phase of the sample does not approach zero degrees even at 10-3 Hz. NMR measurements of the samples indicate that the "wet" sand-clay mixtures have a much broader effective pore size distribution than the oven-dried and vacuum saturated sand-clay mixtures.