Gravitationally induced caves and other discontinuities detected by 2D electrical resistivity tomography: Case studies from the Polish Flysch Carpathians

Widened discontinuities (among them crevice-type caves) are characteristic phenomena of landslides in anisotropic rock massifs forming the Flysch Carpathians. We tested the reliability of 2D electrical resistivity tomography (ERT) in the detection of well-mapped cave passages and then tried to explore new subsurface cavities in three sites of crevice-type caves in the Polish Flysch Carpathians: the Jaskinia Miecharska, Jaskinia Malinowska and Jaskinia Diabla Dziura caves; each comprising a different stage of mass-movement development and thus being a representative case study. The total of 14 ERT profiles was selected, while each of them was measured by several electrode arrays (dipole–dipole, Wenner and/or Wenner–Schlumberger). Our results show that the selection of an appropriate electrode array and spacing are crucial for the detection of subsurface cavities. Measurement using the dipole–dipole electrode array gave best results in the detection of known structures and its interpretation seems to be most reliable in the extrapolation of non-investigated areas. The application of ERT shows abundant occurrence of air-filled voids in gravitationally deformed flysch massifs and in several places its results enable comparison with open spaces detected by speleological exploration. ERT enables us to identify fractures in deformed slopes before the proper failure of these slopes affected by a landslide as well as in landslides themselves: fissure zones connected with trenches, less dense segments of colluvial ramparts and some lithological-tectonical or gravitationally induced elements of slope structures. The reliability of ERT detection of potential caves was verified by speleological exploration of a newly discovered cave at the Jaskinia Miecharska site following the ERT campaign.