Influence of the inner rock surface roughness on the SIP response – a numerical study

The electrical double layer (EDL) at the inner solid-water-interface controls the electrical polarization of saturated rocks without metallic constituents in the low frequency range. Consequently, spectral induced polarization (SIP) laboratory measurements show a strong correlation between polarization strength and inner surface area of rocks. So far published mechanistic SIP models consider this correlation based on grain or pore sizes but neglect the influence of the inner surface roughness. We study the influence of the inner surface roughness on the SIP response by simulating the frequency dependent complex conductivity of simple micro-scale rock models using Comsol Multiphysics®. Starting from smooth grain and pore models, we introduce surface roughness using a fractal approach and randomly generated surface structures.

We find that distinct surface roughness leads to additional polarization at higher frequencies compared to grains and pores of equal size with smooth surfaces. Additionally, the polarization peak of rough grains shifts to lower frequencies compared to smooth grains. These effects lead to an ambiguity in the interpretation of SIP spectra with respect to structural parameters (e.g., grain size or pore structure), e.g., a mixture of large and small grains could lead to the same SIP response as these large grains with rough surfaces. Overall, our simulation results show the same dependence of chargeability on inner surface area as laboratory measurements and thus the strong influence of inner surfaces roughness on the SIP response. Thus, quantitative interpretation of SIP measurements should account for the inner surface roughness of rocks.