We have performed a multidisciplinary geophysical survey combined with geotechnical investigations over a degrading alpine rock glacier. A dense grid of helicopter-borne ground-penetrating radar data allowed the 3D shape of the bedrock topography and the gross transition from ice-rich to ice-poor parts of the rock glacier to be delineated. The bedrock topography served as a 2D structural constraint for tomographic inversions of seismic and geoelectric data acquired on coincident profiles parallel and perpendicular to the rock glacier flow direction. These profile data were complemented by a small 3D geoelectric tomography experiment. Only a combined interpretation of all the results allowed reliable and unambiguous interpretation of the tomograms. We could distinguish between the active layer, bedrock, ice-bearing rock glacier material, and degraded permafrost within the rock glacier. The latter could be further distinguished in areas where the ice must have melted only recently, and regions that had degraded some time ago. Additionally, high-resolution cross-hole radar tomography, performed in an area of opening crevices, allowed small-scale structures to be resolved, which were indicative of the dominant deformation mechanisms style of the rock glacier. The success of our study was primarily based on the availability of 3D data sets that allowed important structures to be traced over larger areas and the integrated interpretation of several data types. We have identified the internal structure of the rock glacier to be surprisingly heterogeneous with several small-scale features that were judged to be critical for assessing its stability. This underpinned the need for comprehensive 3D structural investigations to augment geotechnical measurements linearly with inclinometers or at points in boreholes.

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