Stick-slip imaging through the GPR phase: Turning temperate ice 'noise' into signal

Ground Penetrating Radar (GPR) is a major tool to investigate, map and monitor polar ice sheets and alpine glaciers. Alpine glaciers are often composed of temperate ice, which has significantly different backscatter properties from cold ice. Radar attenuation is much stronger in temperate ice than in cold ice, because the radar signal encounters strong scattering in temperate ice. 
A major candidate for this scattering is the presence of liquid water inclusions, which are much smaller than the radar wavelength. The large contrast between water and ice dielectric permittivity would explain the diffuse radar scattering in temperate ice. Indeed, recent numerical modelling of the radar signal in temperate ice confirmed the contribution of liquid water inclusions on the scattering of the radar signal (Ogier, 2023). 
Here, we investigate if the strong scattering caused by liquid water inclusions, which is usually treated as noise, can be in fact exploited to unravel dynamic processes inside the glacier. This strong scattering results in large radar phase variations in space, which remain constant over short timescales (hours - days), during which the glacial water content remains constant. During that timescale, however, the mountain glacier might experience sudden internal deformation due to intermittent sliding at the glacier base, also called glacier stick-slip.  This deformation might be resolved using difference imaging and the spatio-temporal properties of the radar phase.
We numerically model radar wave propagation throughout temperate ice (i.e. with the presence of liquid water inclusions) before and after an idealized glacier deformation and show, that through phase difference imaging the internal movement of the sub-wavelength scatterers can be mapped. 
Finally, we discuss how this novel type of monitoring could be applied in the field, which is planned for spring 2024.

 

Ogier, Christophe, Dirk-Jan van Manen, Hansruedi Maurer, Ludovic Räss, Marian Hertrich, Andreas Bauder, and Daniel Farinotti. 2023. “Ground Penetrating Radar in Temperate Ice: Englacial Water Inclusions as Limiting Factor for Data Interpretation.” Journal of Glaciology, September, 1–12. https://doi.org/10.1017/jog.2023.68.