A Comprehensive Analysis of Swiss Alpine Glaciers Using Helicopter-Borne Ground-Penetrating-Radar
Abstract:Detailed information exists on the surface area of glaciers in Switzerland and long-term mass balance observations are available but because glacial thickness remains elusive and so only a rough estimate of the present ice volume is available. After the successful recording of approximately 1000 km of helicopter ground penetrating radar (GPR) profiles on Swiss glaciers during the last three years, the Swiss Competence Center for Energy Research (SCCER) and the Swiss Geophysical Commission (SGPK) began an initiative to obtain for the first time an accurate estimate of the total ice volume located in the Swiss Alps. Steps towards this goal include the delineation of 3D bedrock topography beneath glacerized regions. The final ice volume estimation will comprise an ice flux computation model constrained by a dense network of helicopter-borne GPR profiles.
Different systems that have been recently tested for acquiring helicopter GPR data in the Swiss Alps include towed systems (the HERA-G+ and the BGR-P30) and rigidly mounted systems of standard commercial GPR ground units (the GSSI and PulsEkko), all operating in the frequency range of 30 to 70 Mhz. Some measurements were ground-truthed using the same GPR antenna systems.
Analyses of these data sets revealed a wealth of useful information on the glacier bed topography and some internal structures. For instance, at depths between 30 and 60 m, we often observe zones of low backscattering followed by a more reflective zone. In the glacial accumulation areas these features are interpreted as firn layers, in which the water percolates down to its base.
The same test flights also provided useful technical information on the radar installation. For towed systems it is difficult to maintain a constant orientation of the antennas during the flight. In contrast, the rigidly mounted systems do not suffer from the orientation problem, but ringing effects are pronounced. We applied an SVD-based (singular value decomposition) multi-channel filter, which enabled this “system ringing” to be removed. Mostly, ground GPR surveys on coincident lines produce better quality GPR images of the glacier bed. However, it turned out that the orientation of the antennas relative to the glacier may be more important to retrieve good quality GPR data, than the surveying mode (airborne or ground).