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 small-scale 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 ice-saturated 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 coarse- as 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.
Highlights
Permafrost in high-mountain areas occurs within different subsurface materials and textures
As the physical properties of the different materials will affect the thermal regime of the permafrost in different ways, detailed knowledge about the active layer material and its seasonal changes is essential to understand the long-term impact of climatic changes on mountain permafrost
During the last decades various geophysical methods have become a useful tool for characterizing the subsurface material in permafrost areas without disturbing it (e.g. Barsch, 1973; Fisch and Haeberli, 1977; Scott et al, 1990; Wagner, 1996; Vonder Mühll et al, 2001; Musil et al, 2002; Hauck et al, 2004; Maurer and Hauck, 2007; Kneisel et al, 2008)
Summary
Permafrost in high-mountain areas occurs within different subsurface materials and textures. During the last decades various geophysical methods have become a useful tool for characterizing the subsurface material in permafrost areas without disturbing it Barsch, 1973; Fisch and Haeberli, 1977; Scott et al, 1990; Wagner, 1996; Vonder Mühll et al, 2001; Musil et al, 2002; Hauck et al, 2004; Maurer and Hauck, 2007; Kneisel et al, 2008). Changes in temperature can cause different behaviour of resistivities and seismic velocities. A combination of both methods can reduce the ambiguity in the interpretation (e.g. a field-based comparison of both methods is given by Otto and Sass, 2006, and Hauck and Kneisel, 2008a)
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