ABSTRACT

The concept of specific polarizability cp, being the ratio between imaginary conductivity and specific surface area, can be used to represent the polarization of the mineral-fluid interface per unit pore-volume-normalized surface area Spor and to account for the control of the fluid chemistry and/or mineralogy on induced polarization (IP) measurements. We used a database of IP measurements on sands and sand-clay mixtures to investigate the variation in cp as a function of clay content and/or mineralogy. We found an apparent variation in cp between sands and sand-clay mixtures when Spor was calculated using the nitrogen adsorption (Brunauer-Emmett-Teller — BET) method, with clays having an apparently higher cp than sands. However, this variation was considerably reduced when Spor was calculated using a wet-state methylene blue (MB) method that also sensed the surface area associated with internal layers of clay minerals inaccessible with the dry-state BET method. Furthermore, the imaginary conductivity was significantly better correlated with Spor determined from the MB method relative to Spor determined from the BET method. We found no evidence for a strong difference in the specific polarizability of quartz and clay minerals. This finding contradicted predictions from recent mechanistic formulations of the IP response of the Stern layer. Our findings have significant implications for improving and simplifying the interpretation of IP measurements in near-surface materials.

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