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

Abstract

Abstract. Different geophysical investigations, such as electrical resistivity tomography (ERT) and refraction seismic tomography (RST), allow for an improved characterization of subsurface conditions in mountain permafrost areas. The knowledge of the permafrost internal composition constitutes a major prerequisite for climate-related modelling studies, for detailed hazard or local infrastructure assessments. To detect the smallscale variations of permafrost characteristics and its varying sensitivity to climate influencing factors, two ERT and RST monitoring profiles were installed in 2009 at two different sites called Chastelets and Murtèl forefield located in the Murtèl-Corvatsch area, Upper Engadin, eastern Swiss Alps. The geophysical profiles extend over four existing boreholes and are characterized by strong small-scale variations of surface as well as subsurface structures such as bedrock, fine material or coarse debris. Here we present ERT measurements carried out in a bimonthly interval during the years of 2009 to 2012 and RST measurements which were performed once a year, normally in August, during the same period. Based on these data sets the so-called four-phase model, based on petrophysical relationships, was applied to determine the volumetric fractions of ice, water and air within the heterogeneous ground, resulting in a relatively precise description of the subsurface material around the existing boreholes.The observations revealed a permafrost occurrence at the Chastelets rock glacier with an estimated icesaturated layer of at least 10 m thickness and the detection of a thawed layer with increased water content in the lower frontal part of the rock glacier within an area of fine material. In the area of the Murtèl forefield the analysis revealed strongly weathered bedrock, which is in the upper part covered by a pronounced layer of coarse debris establishing a thermal regime which is able to sustain permafrost beneath. In addition, the high temporal ERT measurements revealed a seasonal formation of ice during wintertime within the coarseas well as the fine-grained active layer zones. It can be concluded that the combination of existing borehole temperature measurements, the ERT/RST measurements and the application of the four-phase model resulted in an in-depth view of the investigated area, which is a major prerequisite for future modelling studies allowing for a better treatment of the present small-scale spatial ground variabilities.

Resolution capacity of geophysical monitoring regarding permafrost degradation induced by hydrological processes

“…As electrical resistivities are well suited to differentiating between ice and water, and seismic velocities are sensitive to transitions between air and ice or rock, the combination of these methods is in principle suitable for estimating the lateral heterogeneity and quantifying the volumetric contents of ice-rich bodies in a rock glacier. The model has already been successfully applied to alpine permafrost bodies Schneider et al, 2013;Pellet et al, 2016) and uses the following equations to determine the volumetric ice (f i ), water (f w ) and air content (f a ) for a given porosity model (x, z) ( = 1 − f r ; f r being the rock content):…”

Ground temperature variations in a talus slope influenced by permafrost: a comparison of field observations and model simulations

“…Initial conditions and spatio-temporal variations in the composition of the substrate (Arenson et al, 2002;Kneisel et al, 2008), especially of the ground ice and water content (Hanson and Hoelzle, 2005;Hilbich et al, 2011;Schneider et al, 2013), are important for the response of ground temperatures to warmer atmospheric conditions (Engelhardt et al, 2010;Scherler et al, 2013). Ice-rich landforms with a thick, coarse-blocky active layer showed a slower GT increase during a simulation with warming surface conditions , whereas sites with small amounts of ground ice and water may react faster to ground-heating events in terms of GT increase Luetschg et al, 2008;Zenklusen Mutter and Phillips, 2012).…”

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

“…thickness of the active layer, frost table topography, ice content) can be highly heterogeneous within small distances and can vary within single landform units (Schneider et al, 2013;Langston et al, 2011;Scapozza et al, 2011;Kneisel, 2010a). This is due to complex interactions between small-scale surface conditions (e.g.…”

“…For rock glaciers and similar periglacial landforms, geophysical investigations enable an assessment of the subsurface material composition and to distinguish between frozen and unfrozen areas (e.g. Schneider et al, 2013;Musil et al, 2006). This information can be used to investigate inferences with creep velocities Kneisel and Kääb, 2007) or to reveal glacier-permafrost interactions during the development of rock glaciers (see Dusik et al, 2015;Krainer et al, 2012;Ribolini et al, 2010).…”