Subsurface Exploration of the Great Aletsch Glacier Using SAR Tomography

Kurzfassung

Synthetic Aperture Radar (SAR) and microwave technologies are powerful tools for investigating glacier internal structures due to their ability to penetrate snow, firn, and ice and their sensitivity to dielectric changes. These capabilities make them invaluable for improving glacier monitoring and refining mass balance models. This study investigates the subsurface characteristics of the accumulation zone of the Great Aletsch Glacier in Switzerland, where annual snowfall surpasses melt, leading to the gradual transformation of snow into firn and eventually into ice. The research uses multi-frequency, fully polarimetric SAR data from the SWESAR24 airborne campaign, acquired with the F-SAR sensor from the German Aerospace Center (DLR) in March 2024, focusing on a relatively flat, minimally crevassed study area. Complementary ground-based measurements, including ground-penetrating radar (GPR) surveys and in situ snow depth and density measurements, were conducted to enable a comprehensive comparison of subsurface layers detected by these techniques. The primary method used to investigate the glacier's internal structure is SAR tomography, which utilizes multiple SAR images captured at different incidence angles to generate three-dimensional representations of the subsurface. Multi-frequency SAR tomography, including X-, C- and L-band, allows us to capture both shallower and deeper subsurface features. In this study, the focus is on data acquired at L-band, which penetrates up to approximately 50 meters below the surface with a vertical resolution of 2.9 meters. Preliminary results of the tomographic analysis reveal distinct subsurface layers, with the upper layers possibly corresponding to the summer layers from previous years. These layers are formed by residual snow that undergoes melt-freeze processes, creating a hard, icy crust before the onset of new snowfall. However, interpreting deeper layers proves challenging, particularly when differentiating firn from ice and identifying the bedrock in the lower subsurface. This differentiation is crucial, as accurately identifying the transition from firn to ice and to bedrock is essential for understanding glacier dynamics and the internal structure. To address these challenges, we incorporate our complementary data, where GPR provides higher vertical resolution compared to SAR tomography and reveals numerous distinct layers within the subsurface. As part of this study, we perform a qualitative comparison between the layers visible in the tomographic representation and the GPR data. To improve the accuracy of this comparison, a 3D geocoding approach is applied to align the datasets to the same surface height and simulating a nadirlooking perspective for the tomographic analysis. Moreover, in-situ measurements provide density profiles of the upper layers, which directly influence the dielectric properties of snow, firn, and ice. Higher densities result in increased permittivity due to reduced air content and greater material compactness, with firn typically ranging from 400 to 830 kg/m³ and transitioning to ice above this range. Microwaves, being sensitive to dielectric changes, can detect density variations, and therefore enabling the distinction between firn, ice, and bedrock. Therefore, this study also aims to link GPR detected layers with those identified through SAR tomography and correlating them with dielectric properties and physical characteristics. These insights would improve the interpretation of glaciers through SAR tomography, advancing both cryospheric research and methods for monitoring glacier dynamics under frozen conditions.