Hydrological links between shallow and deep zones in a flysch landslide revealed by repeated FDEM surveys and 3D AMT imaging

The interplay between internal structure, deformation mechanisms, and subsurface hydrogeological processes controls the long-term stability of large landslides. A key unresolved issue is whether infiltrating groundwater is confined to the landslide body or can migrate into the underlying bedrock along deep-seated structural discontinuities. This problem is particularly relevant in areas underlain by steeply dipping flysch formations, where structural anisotropy may promote vertical groundwater connectivity and influence landslide reactivation. This study focuses on the Cisiec landslide in the Żywiec district of southern Poland, aiming to identify groundwater percolation pathways and their relationship to slope deformation. The landslide affects a ski slope located in a forest–meadow transition zone and moves predominantly east–northeast, with an elevation difference of approximately 100 m. Previous monitoring indicated complex kinematics but did not resolve the depth extent of groundwater infiltration or its coupling with deep geological structures.

We apply an integrated electromagnetic approach explicitly designed to resolve processes across complementary depth ranges. Shallow groundwater dynamics were monitored using time-lapse Frequency Domain Electromagnetics (FDEM), which is sensitive to depths of approximately 0–3 m and was repeated over a three-year interval. FDEM conductivity variations were used to map spatial and temporal patterns of near-surface water percolation within the landslide body. In addition, the in-phase component of the FDEM signal was exploited to detect positional changes of buried infrastructure on the ski slope. When combined with high-precision Differential GPS (DGPS) measurements, these data provided quantitative constraints on surface displacement and landslide activity. To resolve the intermediate-depth range and provide robust constraints for deep imaging, Electrical Resistivity Tomography (ERT) was conducted along five profiles across the landslide. The resulting resistivity sections, which image the subsurface to approximately 30 m depth, were incorporated as a priori resistivity constraints and starting models for the inversion of Audio-Magnetotelluric (AMT) data. This constrained inversion strategy significantly reduced ambiguity in the AMT results and ensured consistency between shallow, intermediate, and deep resistivity structures.

AMT imaging extended the investigation below 30 m depth and enabled the construction of a three-dimensional resistivity anomaly model of the landslide and its geological basement. The model reveals pronounced, near-vertical resistivity structures associated with the Carpathian flysch beneath the landslide, interpreted as preferential pathways for deep groundwater migration. The integrated interpretation of FDEM, ERT, and AMT data indicates that infiltrating groundwater is not restricted to the landslide mass but can penetrate into the bedrock along steeply oriented discontinuities. This hydrogeological connectivity between shallow infiltration zones and deep structural features provides a plausible mechanism for delayed landslide reactivation and long-term slope instability. The study highlights the importance of multi-scale, constraint-driven electromagnetic imaging for improving hazard-relevant conceptual models of complex landslide systems.