Induced polarization as a tool to characterize shallow landslides

Abstract

The development of shallow landslides is strongly connected to the changes in the water content of soils on hillslopes, their clay content and permeability distribution, which, in turn, are playing an important role regarding their hydro-mechanical properties. A non-intrusive geophysical method able to map these properties would be very helpful. The most common geoelectrical method, DC (Direct Current) resistivity, cannot be used as a stand-alone technique for this purpose since it depends on two contributions (bulk and surface conductivities), which depend on the water content and the cation exchange capacity (CEC) of the material. Induced polarization is a geophysical method that can be now used to complement DC resistivity in providing key material properties that can be used to diagnose potential risks for failure. We first recall the basic principles behind the induced polarization method from laboratory to field scales and key findings in the underlying petrophysics needed to jointly interpret electrical conductivity and normalized chargeability tomograms. Then, we apply these relationships to a field survey carried out over a shallow landslide at Claix (Isère, France), close to Grenoble. A 3D induced polarization survey was carried out and interpreted in terms of the clay content, water content, and permeability distributions. We demonstrate that the landslide is associated with a channel of high water content corresponding with the presence of travertine, a flow-path, and a permeability barrier downslope corresponding to the presence of plastic clays. This study demonstrates that induced polarization can be used to characterize the impacted volume and therefore might have been useful to map the area before the landslide to assess the possible risk of failure. This methodology could play a key role in mitigation planning.

A linked geomorphological and geophysical modelling methodology applied to an active landslide

“…With the exception of certain models in the ERI series, which are associated with poor raw data quality (particularly in 2018 when the number of TR measurements drops off significantly), the time-series analysis could be taken further with petrophysical relationships between resistivity, moisture content (Uhlemann et al 2017) and other critical parameters for assessing slope stability such as soil suction (Crawford and Bryson 2018). Alternatively, more involved workflows could conceivably couple hydrological and geoelectrical modelling through petrophysical relationships (Johnson et al 2017;Revil et al 2020), allowing for robust assessments on the slope hydrogeology through time.…”

“…Numerous studies have shown that electrical resistivity imaging (ERI), also known as electrical resistivity tomography (ERT), can be an invaluable aid in interpreting changes in near surface hydrologic conditions (Binley et al 2015;Brunet et al 2010;Chambers et al 2014;Johnson et al 2017;McLachlan et al 2020;Perrone et al 2014;Revil et al 2020;Uhlemann et al 2017;Uhlemann et al 2016b). In the absence of any changes to geological structure, changes in electrical resistivity should be due to changes in temperature and the pore fluid (saturation/salinity) in the subsurface (Waxman and Smits 1968).…”

“…Crawford and Bryson (2018) presented a novel study directly relating electrical conductivity (the inverse of resistivity) to soil suction which is then used to compute an unsaturated shear strength. Recently, Revil et al (2020) demonstrated the use of the time domain induced polarisation (IP) method for use in clay rich, landslide prone, materials, transforming their geoelectrical models into both soil moisture content estimates and cation exchange capacities through petrophysical calibration. Once these parameters have been estimated, a volumetric approximation of permeability can be attempted (Soueid , hence this method may have future implications for coupled geoelectrical and hydrological modelling.…”

Advancing hydrological process understanding from long‐term resistivity monitoring systems

“…IP measures the degree to which the subsurface is polarized (somewhat like a capacitor) on application of an electric field; this polarization foremost exists due to the restricted movement of ionic charges in the EDL lining pore throats (or surrounding mineral grains). In soils/rocks devoid of electronically conducting minerals, IP measurements are directly proportional to the surface conductivity term (Equation (4)), meaning that IP measurements can reduce the inherent ambiguity of resistivity interpretation (Revil et al, 2020; Slater & Lesmes, 2002; Zarif, Kessouri, & Slater, 2017). The presence of electronically conducting (or semiconducting) minerals results in a much stronger polarization signal due to larger restricted movement of ionic charges surrounding the mineral in response to the polarization of charges inside the electron conductor.…”

Induced Polarization of Carbonates

“…a mechanistally based and testable model to understand induced polarization (Revil et al, 2017a), which is simple enough to be applicable in field conditions (Revil et al, 2020).…”