Electrical Conductivity of Clayey Rocks and Soils: A Non‐Linear Model

• Published in: Geophysical research letters 2022-05, Vol.49 (10), p.n/a
• Main Authors: Qi, Youzheng, Wu, Yuxin
• Format: Article
• Language: English
• Subjects: Archie's law; Clay; Clay minerals; Clay soils; Conduction; Conductivity; Data interpretation; Effective medium theory; Electrical conductivity; electrical properties; Electrical resistivity; electrical resistivity tomography (ERT); Electrochemistry; Geophysical data; hydrogeophysics; Lithology; Mathematical models; Minerals; Modelling; Parameter estimation; petrophysics; Pore water; Porosity; Rock; Rocks; Soil; Soil porosity; Soils; Subsurface investigations
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL097408

It is well‐accepted that Archie's law is only applicable to “clean” rocks and soils but fails in “dirty” ones where clay minerals possess an additional component of surface conductivity. Although several models, for example, Waxman‐Smits model, were presented to account for this phenomenon, surface conductivity is always inappropriately treated as constant, which actually only holds at high salinities. The essential non‐linear characteristic differing over fluid salinities has not been physically or mathematically explained well in those models. We scrutinize the conduction mechanism of clayey rocks and soils and ascribe this non‐linear feature to (a) variation of the electrical double layer and (b) the intrinsic clay‐and‐water conduction pattern. With effective medium theory, we develop an easy‐to‐use non‐linear model that both reflects electrochemical theories and explains the measurement data well. Our model can be used to produce more accurate results for laboratory‐ and field‐scale petrophysical parameter evaluations than the previous models. Plain Language Summary Electrical and electromagnetic methods are commonly used hydrogeophysical methods for large‐scale non‐invasive subsurface investigations, where different conductivities between Earth materials lay their foundations. Unfortunately, for rocks and soils with clay minerals, their conductivities behave non‐linearly with the pore water. Inappropriately using Archie's law or other linear models could lead to erroneous rock/soil parameter estimation or field data interpretation. Here we develop an easy‐to‐use non‐linear model which would produce more quantitative petrophysical and geological results from measured geophysical data, for example, more accurate porosity assessment and lithological discrimination. Key Points Conductivity of clayey rocks and soils manifests an obvious non‐linear behavior with respect to changes of the pore water salinity Non‐linear conductivity is resulting from (i) variation of electrical double layer and (ii) the intrinsic clay‐and‐water conduction path Our non‐linear model reflects electrochemical theories, explains measured data well, and is very easy‐to‐use