Investigating Landslide Behaviour Under Varying Environmental Pressure: A Multi‑Method Geophysical Approach

The long-term stability and reactivation potential of large landslides are primarily controlled by their deep internal structure, the geometry and connectivity of shear planes, fault systems, and subsurface hydraulic pathways. Despite their importance, these features remain difficult to investigate, and geophysical methodologies capable of resolving those at sufficient resolution and depth are still not fully established. This study addresses this challenge through a comprehensive, multi-method geophysical investigation of the Cisiec landslide, located in the Żywiec district of southern Poland.

The study area comprises a forested clearing surrounded by meadow terrain, with the landslide moving predominantly east–northeast and exhibiting an elevation difference of approximately 100 m between its crown and toe. Annual monitoring campaigns, including drone-based photogrammetry and laser scanning, as well as geophysical measurements like seismic or resistivity tomography, conducted between 2018 and 2022, provided valuable insights into the general geometry and kinematics of the landslide. Still, process-understanding of the complex and non-linear landslide behaviour could not be fully obtained. These datasets allowed the construction of a preliminary structural models and indicated temporal variability in displacement patterns; however, the nature of movement along individual slip surfaces remained unresolved. In particular, it was unclear whether deformation occurred as a coherent, uniform displacement or as a progressive, sequential sliding process involving multiple layers or discrete blocks. Furthermore, the role of groundwater circulation within the landslide body and its influence on mechanical stability could not be conclusively determined.

To overcome these limitations, we conducted advanced geophysical surveys during dedicated field campaigns in October 2024 and April 2025. The investigation integrated high-resolution seismic imaging based on Distributed Acoustic Sensing (DAS) with Spectral Ground Penetrating Radar (SGPR) and a suite of electrical and electromagnetic methods, including Electrical Resistivity Tomography (ERT), Frequency Domain Electromagnetics (FDEM), Time Domain Electromagnetic (TDEM) soundings, and Spectral Induced Polarization (SIP). Each technique was carefully selected and optimized to resolve complementary aspects of the landslide architecture, ranging from shallow deformation features to deeper structural controls and subsurface hydraulic pathways.

The combined dataset provided a detailed, multi-scale image of the landslide, revealing significant spatial heterogeneity in both mechanical and hydrogeological properties. Seismic imaging resolved fine-scale structural and geomechanical variations, while electrical and electromagnetic methods highlighted zones of enhanced moisture content and groundwater flow. The results confirm a division of the landslide into three distinct kinematic zones and reveal previously unresolved shallow slip surfaces and groundwater-related effects. Notable observations include focused groundwater discharge at the lower slope and an anomalous signal attenuation zone near the crown, interpreted as evidence of microseismic activity and the development of a potential new failure zone. These findings demonstrate the value of integrated, high-resolution geophysical approaches for improving conceptual models of complex landslide systems and for supporting long-term hazard assessment.