Permafrost mapping using quasi-3D resistivity imaging, Murtel, Swiss Alps

To investigate recent permafrost characteristics in detail, quasi-3D resistivity imaging was applied to map the extent of frozen ground in an area of a glacier forefield in the Engadin/Eastern Swiss Alps. This approach combines a number of parallel and perpendicular electrical resistivity tomography (ERT) profiles into one data set and enables a subsequent 3D inversion and interpretation of geoelectric resistivity results. Spatial information about resistivity distribution within the heterogeneous subsurface of the investigated area is gained and allows a detailed analysis of permafrost occurrence, which is superior to two-dimensional resistivity surveying. Information about the consistency of the model results is obtained from the modification of the damping parameters during the inversion process. Data sets based on different combinations of ERT profiles and array types were compared to optimize the adequate number and orientation of profiles required for 3D inversion. Our findings show the importance of perpendicular tie lines in improving the image of the subsurface resistivity distribution. Inversion results from a Wenner-Schlumberger array are regarded as the best trade-off between accuracy and acquisition time in this periglacial environment. Spatial resistivity data facilitate a good delineation of a distinct permafrost body within the measurement grid. In addition, borehole temperatures are used as ground truth data. Our results show that quasi-3D resistivity imaging allows a spatial interpretation of ground conditions and thus constitutes an improvement compared to scattered 2D ERT profiles. In particular, single landforms of small- to medium-scale can be regarded as objectives and favour the application of detailed quasi-3D surveys.