Investigating firn structure and density in the accumulation area of Aletsch Glacier using Ground Penetrating Radar

Abstract. The role of firn structure and density in geodetic glacier mass balance estimation has been constrained, with studies in alpine conditions primarily relying on models. Our research focuses on understanding firn structures and firn density-depth profiles in the Aletsch Glacier's accumulation area using field methods, Ground-Penetrating Radar (GPR) as a geophysical tool, glaciological methods, and firn compaction models. We aim to characterize the firn structure and determine the spatial firn density-depth profiles by estimating electromagnetic wave velocities. Identifying reflection hyperbolae via semblance analysis from common midpoint (CMP) data sets are used for this purpose. Three density-depth profiles, up to 35 m depth, were obtained at various locations within the accumulation area. The Ligtenberg (LIG) and Kuipers Munnekee (KM) firn compaction models were selected from the community firn models (CFM) to evaluate how well the model results matched the observations. These models were adjusted to fit the estimated 1-D firn density profiles from CMP gathered by optimising model coefficients based on regional climatic conditions.

We developed a method to estimate accumulation history by chronologically identifying GPR-derived internal reflection horizons (IRHs) as annual firn layers, validated against estimated snow water equivalent (SWE) from long-term stake measurements. Our findings emphasize the importance of direct measurements, such as snow cores, firn cores, and isotope samples, in identifying the previous summer horizon. We investigated the spatial firn density distribution and the glacier's accumulation history over the past 12 years using a 1.8 km GPR transect, supported by CMP-derived density-depth profiles. Our study underscores the potential of integrating GPR, direct measurements, and firn compaction models in monitoring firn structures and density, ultimately enhancing glacier mass balance estimation in future research.