Lawrence Berkeley National Laboratory

Abstract Among the subsurface geophysical methods used in the critical zone investigations, induced polarization (IP) shows great vitality thanks to its unique ability to assess porosity via bulk conduction and estimate permeability through surface conduction and/or polarization. However, such an advantageous separation between bulk and surface is mostly implemented by multi‐salinity experiments in the laboratory, which is incredibly difficult to realize in the field. One promising approach to address such an obstinate issue is to gauge the surface conductivity ( σ s ) from the quadrature conductivity ( σ ″) or normalized chargeability ( M n ) with the ratios between ( l  =  σ ″/ σ s , l mn  =  M n / σ s ). While these ratios are known not to be universal, the underlying principles are not fully understood and relevant theoretical studies are rare, which makes quantitative IP applications difficult. Here we scrutinize the conduction and polarization mechanisms of geomaterials and pinpoint that the two ratios are inherently functions of salinities and frequencies rather than only determined by the properties of the electrical double layer (EDL), hence representative samples from the investigated field must be calibrated in the laboratory and a characteristic frequency should be chosen for their usage. Besides the macro‐scale ratios l and l mn , we define two micro‐scale ratios χ and χ mn directly from the EDL, such that the new ratios exclude the effect of salinity and frequency and offer the opportunity to characterize and monitor changes of the EDL. Our study demonstrates that the existing macro‐scale ratios converge toward the values of novel micro‐scale ratios at high water salinity. Plain Language Summary True formation factor F is key to the porosity and permeability assessment when electrical and electromagnetic methods are applied to the subsurface investigations. F is easily obtained from the bulk conductivity with the well‐known Archie's law for “clean” geomaterials, but becomes difficult to acquire for “dirty” ones, such as clay and carbonate, because additional surface conduction occurs besides the pore water. Although in the laboratory F can be model‐fitted by measuring bulk conductivities under different water conductivities to exclude the surface conductivity, this means cannot be realized in the field. One promising approach is to use IP to gauge the surface conductivity from the polarization ability of the geomaterial, as pore water is not polarizable at the IP frequencies. Unfortunately, during the past decades IP practitioners have found quite distinct ratios between the polarization magnitude and surface conductivity but the underlying principle is not clear. To fill this gap, we delve into the geomaterial's conduction and polarization mechanisms and find that these ratios are not parameters exclusively related to mineralogy but mixed ones with both water and interface properties. Accordingly, the specific laboratory calibration of the representative samples is required for their quantitative applications in the field. Key Points Ratios between the polarization magnitude and surface conductivity are ratios including both water and electrical double layer properties Macro‐scale salinity dependency of the ratios can be described by an effective medium model The existing macro‐scale ratios converge toward the values of the newly defined micro‐scale ratios at high water salinity