Some fundamental considerations in induced polarization

Summary In a growing number of studies, researchers have presented induced polarization as a dielectric-related phenomenon arising from the complex nature of the permittivity in the constitutive law associated with the displacement current. Using Ampères’s law, they then write the conductivity as a complex number with an imaginary part related to the real part of the permittivity mixing Maxwell-Wagner-Sillars polarization and induced polarization phenomena. This presentation may be misleading to students and new researchers in the field. In terms of underlying physics, induced polarization phenomena have nothing to do with the displacement current density and Maxwell-Wagner-Sillars polarization should not be considered as an induced polarization mechanism. At the microscopic level, low-frequency polarization arises because the ionic fluxes responsible for the local electrical current are coupled not only through the long-range Coulombic effect but also through their physical interactions associated with diffusion phenomena driven by the random thermal motion of charge carriers. The thermodynamic force associated with ionic migration is not the electrical field alone but electrochemical potential gradients (induction effect can be accounted for, if needed, in the electrical field component of the Nernst-Planck equation governing the fate of ions in the pore space and along the surface of the mineral grains in their so-called electrical double layer). This has been known since the seminal papers of D.J. Marshall & T.R Madden in the 50s and Vinegar & Waxman in the 80s. This point seems, however, forgotten in recent studies leading to some misconceptions preventing a mechanistic understanding of the induced polarization processes and broad-band spectroscopic data.