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 ?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.