Permafrost mapping using quasi‐3D resistivity imaging, Murtèl, Swiss Alps

Abstract

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.

Internal structure of two alpine rock glaciers investigated by quasi-3-D electrical resistivity imaging

“…The separation factor of X lines was adjusted due to site-specific reasons, like deep snow fields or topographical obstacles and ranges between a factor of 3 and 6 of the along-line electrode spacing. However, this is still assumed to be sufficient as the application of perpendicular grid lines is not a mandatory requirement for 3-D data acquisition but useful for the delimitation of small-scale structures orthogonal to the survey line direction (Loke et al, 2013;Rödder and Kneisel, 2012;Chambers et al, 2002). Despite the known limitations of the DipDip electrode array under rough surface conditions (rather low signal strength), its application as a basic electrode array in this study provided very good results.…”

“…These differences are quantified as a mean absolute misfit error value (absolute error). The inversion continues until acceptable convergence between the calculated and the measured data is reached (see Rödder and Kneisel, 2012, for more details on inversion settings). To investigate model reliability, a resolution matrix approach (Hilbich et al, 2009;Wilkinson et al, 2006;Stummer et al, 2004) was performed on all data sets.…”

“…However, these approaches do not take into account the actual three-dimensionality of the subsurface resistivity distribution and the so-called "3-D effects", which result from small-scale heterogeneities (Loke et al, 2013). This is achieved by running a 3-D inversion on a data set consisting of multiple intersecting 2-D data sets ("quasi-3-D" approach), which enables a horizontal mapping of resistivity variations, an assessment of the geometry of the detected structures and consequently a more realistic characterisation of three-dimensional subsurface conditions (Kneisel et al, 2014;Rödder and Kneisel, 2012).…”

Geoelectrical monitoring of frozen ground and permafrost in alpine areas: field studies and considerations towards an improved measuring technology

“…(), Vanhala et al . (), Laxton and Coates (), Oldenborger (), Rodder and Kneisel () and Lewkowicz et al . ().…”

A spatial and temporal analysis of different periglacial materials by using geoelectrical, seismic and borehole temperature data at Murtèl–Corvatsch, Upper Engadin, Swiss Alps

“…• C at 5 m depth; Roedder and Kneisel, 2012). All three investigations suggest the occurrence of discontinuous permafrost which is highly sensitive to increasing air temperatures (Maurer and Hauck, 2007).…”