Induced polarization applied to landslides. Part 1: imaging hydraulic barriers

SUMMARY Water content and pore fluid pressure increases have been recognized as important drivers of shallow landslides, especially through the role of strong rainfalls promoting gravitational instabilities. Less recognized is the role of vertical hydraulic barriers impeding the flow of ground water at the feet of areas prone to landslides. Induced polarization is a non-intrusive geophysical technique able to image hydraulic properties of the shallow subsurface. Recently developed petrophysical models bridging the gap between hydraulic and electrical properties of soft sediments, soils and rocks have been developed. Thanks to these relationships, this geophysical method can be used to image the water and clay contents of the formations and their permeability. Therefore, induced polarization can be used to image the occurrence of vertical permeability barriers. We focus our approach on a large landslide that occurred in March 1931 (reactivated in 1971–1972) above Le Châtelard village (Bauges, France). This landslide started inside a kilometer-scale syncline hosting clayey formations and moraines. We performed a 2.2 km profile crossing the syncline and the sliding area including resistivity, induced polarization and self-potential measurements. In addition, 22 samples were taken from the different formations outcropping at the field site including limestones, sandstones and clayey formations. The petrophysical investigations are combined with the field data to image the water content and cation exchange capacity as well as their permeability. The data set shows the existence of a vertical permeability barrier at the bottom of the landslide corresponding to the tight Urgonian limestone formation. We combine the permeability distribution, the resistivity and self-potential data by forward modelling the groundwater flow and electrokinetic response. We then invert the self-potential measurements to refine the image of the Darcy velocity distribution. The results show a strong upflow of the ground water just above the Manauds canyon where several gravitational instabilities occured in the past.