Geophysical imaging of alpine rock glaciers

Abstract

ABSTRACT. Slope instabilities caused by the disappearance of ice within alpine rock glaciers are an issue of increasing concern. Design of suitable counter-measures requires detailed knowledge of the internal structures of rock glaciers, which can be obtained using geophysical methods. We examine benefits and limitations of diffusive electromagnetics, geoelectrics, seismics and ground-penetrating radar (georadar) for determining the depth and lateral variability of the active layer, the distributions of ice and water, the occurrence of shear horizons and the bedrock topography. In particular, we highlight new developments in data acquisition and data analysis that allow 2-D or even 3-D structures within rock glaciers to be imaged. After describing peculiarities associated with acquiring appropriate geophysical datasets across rock glaciers and emphasizing the importance of state-of-the-art tomographic inversion algorithms, we demonstrate the applicability of 2-D imaging techniques using two case studies of rock glaciers in the eastern Swiss Alps. We present joint interpretations of geoelectric, seismic and georadar data, appropriately constrained by information extracted from boreholes. A key conclusion of our study is that the different geophysical images are largely complementary, with each image resolving a different suite of subsurface features. Based on our results, we propose a general template for the cost-effective and reliable geophysical characterization of mountain permafrost.

A new model for estimating subsurface ice content based on combined electrical and seismic data sets

“…A delineation of regions with enhanced ice-, water-and air content can not be determined directly from the two inversion models, but requires additional information such as borehole data or additional geophysical data sets (cf. Maurer and Hauck, 2007). Figure 7 shows the calculated ice-, water-and air contents for a constant porosity model of 50 % over the same model domain as in Fig.…”

“…Results for both rock glaciers and both model approaches are shown using the inversion results described in Maurer and Hauck (2007). We will discuss the results for Muragl rock glacier in more detail than Murtèl, because (1) the differences between porosity dependent and general model results are similar for both rock glaciers and (2) Muragl rock glacier shows much more spatial heterogeneity than Murtèl rock glacier.…”

“…As we focus on the development of the 4-phase model in this paper, we will not repeat specifics regarding measurement accuracy, inversion parameters and validation of resistivity and P-wave velocity results. We refer the reader to Maurer and Hauck (2007) for the details on data acquisition and inversion of both seismic and electrical data sets.…”

“…Both approaches were applied to geophysical data sets from two well studied rock glaciers in the Upper Engadine, Swiss Alps, namely the rock glaciers Muragl and Murtèl described in more detail in Maurer and Hauck (2007). At both field sites borehole data and additional geophysical data are available for model validation (Musil et al, 2002;Maurer et al, 2003;Arenson and Springman, 2005;Hilbich et al, 2009).…”

“…The tomographic geoelectric and refraction seismic data sets for the case studies presented here were obtained during several field campaigns and include an almost 300 m cross profile near the tongue of Muragl rock glacier and a shorter 145 m longitudinal profile at Murtèl rock glacier (Fig. 5, see also Maurer and Hauck, 2007).…”