Abstract
Abstract. Interactions between different formative processes are reflected in the internal structure of rock glaciers. Therefore, the detection of subsurface conditions can help to enhance our understanding of landform development. For an assessment of subsurface conditions, we present an analysis of the spatial variability of active layer thickness, ground ice content and frost table topography for two different rock glaciers in the Eastern Swiss Alps by means of quasi-3-D electrical resistivity imaging (ERI). This approach enables an extensive mapping of subsurface structures and a spatial overlay between site-specific surface and subsurface characteristics. At Nair rock glacier, we discovered a gradual descent of the frost table in a downslope direction and a constant decrease of ice content which follows the observed surface topography. This is attributed to ice formation by refreezing meltwater from an embedded snow bank or from a subsurface ice patch which reshapes the permafrost layer. The heterogeneous ground ice distribution at Uertsch rock glacier indicates that multiple processes on different time domains were involved in the development. Resistivity values which represent frozen conditions vary within a wide range and indicate a successive formation which includes several advances, past glacial overrides and creep processes on the rock glacier surface. In combination with the observed topography, quasi-3-D ERI enables us to delimit areas of extensive and compressive flow in close proximity. Excellent data quality was provided by a good coupling of electrodes to the ground in the pebbly material of the investigated rock glaciers. Results show the value of the quasi-3-D ERI approach but advise the application of complementary geophysical methods for interpreting the results.
Highlights
In areas of sporadic and discontinuous permafrost, subsurface conditions 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)
A boundary which reflects the characteristic sharp increase in both resistivity and P -wave velocity at the transition from unfrozen to frozen subsurface conditions can be obtained through the complete profiles at Nair rock glacier (Fig. 3a, b)
The latter values are considerably lower than the maximum values of the electrical resistivity tomography (ERT) model, which are in the range of several hundred k m in an area that is not included in the seismic refraction tomography (SRT) profile
Summary
In areas of sporadic and discontinuous permafrost, subsurface conditions (e.g. 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. grain size composition, snow cover distribution), topographical attributes (e.g. aspect, slope) and characteristics of the contributing area (e.g. source of debris, extent of root zone) (see Monnier et al, 2013; Luetschg et al, 2004; Hanson and Hoelzle, 2004; Harris and Pedersen, 1998) which influence the local ground thermal regime during landform formation (Otto et al, 2012; Lambiel and Pieracci, 2008). A detailed knowledge of three-dimensional subsurface conditions can support the interpretation of results from such models
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