The induced polarization geophysical method applied to permafrost at various scales and for various frozen environments

The Dynamic Stern Layer (DSL) model is a reliable petrophysical model to comprehend induced polarization data at various scales from the representative elementary volume of a porous rock to the interpretation of field data at the cm to 100 m scales. We first review the DSL model in presence of ice and discuss the role of ice as an interfacial protonic dirty semi-conductor in the complex conductivity spectra of rocks and sediments. The electrical current polarizes the surface of the ice crystals and generates a very high chargeability that can reach one depending on the value of the volumetric content of ice. We apply the petrophysical model to a new set of complex conductivity spectra obtained in the frequency range 10 mHz-45 kHz using a collection of 25 rock samples including metamorphic and sedimentary rocks in the temperature range +15/+20°C to -10/-15°C. We observe that the model explains very well the observed data. We also investigate the role of porosity, cation exchange capacity, and freezing curve parameters on the complex conductivity spectra of crystalline and non-crystalline rocks during freezing. Laboratory experiments demonstrate that in most field conditions including permafrost conditions, surface conductivity associated with conduction on the surface of clay minerals (and alumino-silicates in general) is expected to dominate the overall conductivity response. Therefore Archie’s law cannot be used as a conductivity equation in this context because of the contribution of surface conductivity and has been strongly abused in the context of the applications of geoelectrical methods in the realm of the cryosphere. Time-domain induced polarization data obtained in field conditions are interpreted thanks to this updated DSL model. We selected three different test sites in order to apply the DSL model to very different conditions of low and high ice contents. A first survey is performed along a cross-section of a ridge in the Kangerlussuaq mountains of Greenland. We also performed a field survey close to Col des Vés (2846 m a.s.l., Tignes, French Alps, Site II). This site corresponds to a complex ground ice body overlying a substratum made of a low-porosity marble, both having high resistivity values. The front of this body is characterized by a small amount of residual ice while the roots are ice-rich. Therefore the porosity at this site is high and the ice content highly variable. This case study showcases the role of ice in the induced polarization data in terms of high chargeability values (close to 1 as predicted by the theory) at the roots of the complex ground ice body. A third site (Site III) corresponds to a profile crossing the Aiguille du Midi (3842 m a.s.l., Chamonix), also in the French Alps in a low porosity granitic environment. We end up with an application to a rock glacier (Site IV) to show how we can image the ice content.