Delineation of subsurface variability in clay-rich landslides through spectral induced polarization imaging and electromagnetic methods

Although commonly triggered by heavy or long-lasting precipitation events, landslides in clay-rich formations are hardly predictable as their triggering mechanisms are still not fully understood. Hence, detailed information about the internal structure of landslides with high spatial resolution is fundamental for an improved understanding of triggering mechanisms and management. In this context, electrical resistivity tomography (ERT) is a well-established method used for the delineation of lithological interfaces in landslides and changes in water content. However, the quantitative interpretation of the ERT images can be challenging, particularly for clay-rich landslides, considering that the electrical signatures are dominated by the variations in the clay fraction rather than saturation. Moreover, large scale investigations also demand the development of faster surveying techniques. Hence, in this study we discuss the combined investigation of a landslide with low induction number electromagnetic (EMI) mapping and induced polarization (IP) imaging. We investigate the application of such methods to assess subsurface variability across different scales: near surface EMI mapping and their correlation with geomorphological data, whereas IP images are used to investigate the extension at depth from the shallow structures. Measurements are performed at a shallow clay-rich landslide in Lower Austria (Austria), characterized by lithological structures of the Flysch and Gresten Klippen Zone, with both formations known to be highly susceptible to landsliding. The interpretation of the imaging results for data collected along 15 profiles is performed using separately acquired hydrogeological and geotechnical data. We observe a relationship between the electrical properties and geotechnical parameters which permits to delineate areas associated to different weathering stages controlling the groundwater flow. Moreover, spectral induced polarization (SIP) data collected along one transect shows sensitivity to changes in textural composition, such as the fraction of clay minerals. The study shows the suitability of an integrated investigation with EMI mapping and IP imaging in combination with extensive geotechnical data for an improved characterization of subsurface variability and thus, understanding of clay-rich landslides.