Modelling of spectral induced-polarization measurements on cm-sized metallic spheres in sand–water mixtures

SUMMARY Geologic materials containing metallic particles exhibit a high electrical chargeability due to polarization processes taking place at the microscale. Mechanistic models describe these processes theoretically, while laboratory experiments are a suitable tool to further characterize the polarization behaviour. However, since no comprehensive model exists and most models are highly parametrized, matching the results of theoretical and experimental studies is challenging. To enable a direct comparison of model and experiment, we consider a simplest possible system of one cm-sized metal sphere in a sand-electrolyte mixture. We then use a numerical model to simulate a corresponding complex-conductivity spectrum by adapting the model to be as close to the measurement as possible. We model the influence of the measurement setup on the polarization response, finding that for commonly used setups no significant deviations occur. We also model the effect of dissolved and reactive metal ions since the presence of these ions significantly influences the predicted polarization signal, especially if the dissolved ions have a higher valence. We then compare the simulated complex-conductivity spectrum to a measured spectrum, with both spectra showing a similar shape. However, the theoretically predicted polarization peak occurs at higher frequencies than in the measured data, indicating that the theoretical model does not account for all effects associated with the polarization of metallic particles and/or the influence of the granular embedding medium. Our results show how the direct comparison of model and experiment can highlight gaps in our current understanding of the processes involved.