Internal structure of an alpine rock glacier based on crosshole georadar traveltimes and amplitudes

Rapid melting of permafrost in many alpine areas has increased the probability of catastrophic rock slides. In an attempt to provide critical structural information needed for the design and implementation of suitable mitigation procedures, we have acquired low frequency (22 MHz) cross-hole radar data from within a fast-moving rock glacier, an important form of alpine permafrost. Since the ice, rock and pockets of water and air found in the underground of high alpine areas have very different dielectric permittivities and electrical conductivities, the radar method was well-suited for investigating the structure and state of the rock glacier. Our interpretation of the radar velocities and attenuations was constrained by geomorphological observations, borehole lithological logs and the results of a surface seismic survey. The radar data revealed the existence of a discontinuous 7-11 m thick ice-rich zone distinguished by high velocities (0.14-0.17 m/ns) and low attenuations (0.04-0.09 m(-1)) and a thin underlying ice-free zone characterized by moderate velocities (0.11-0.12 m/ns) and low attenuations (0.04-0.09 m(-1)). Beneath these two zones, we observed a prominent band of high velocities (0.14-0.17 m/ns) and moderately high attenuations (0.10-0.20 m(-1)) associated with unconsolidated glacial sediments and numerous large air-filled voids, which in the past were probably filled with ice. At greater depths, the variably dry to water-saturated sediments were represented by generally lower velocities (0.08-0.10 m/ns) and higher attenuations (0.16-0.24 m(-1)). The bedrock surface was represented by an abrupt similar to 0.03 m/ns velocity increase. We speculate that the disappearance of ice, both laterally and with depth, occurred during the past one to two decades.