Using active geophysical methods to characterise a temperate glacier’s hydrological system

Abstract

Worldwide, glaciers are receding as a consequence of climate change. Due to the global glacier recession, glaciers are experiencing enhanced melting, which results in an increase in meltwater availability. This increased meltwater has the ability to alter the glacier's hydraulic conditions and ultimately affects the dynamics of glaciers. Glaciers move through a combination of basal motion and internal ice deformation. The motion at the ice-bed interface comprises of both ice sliding along the bed and the deformation of subglacial sediments. Sliding at the bed is partially controlled by the water pressure at the ice-bed interface and therefore, knowledge of the subglacial hydraulic system is paramount in order to predict glacier movement and fundamentally the future evolution of glaciers. To date, our understanding of the glacier’s hydrological system is limited due insufficient field observations. As part of my thesis, I used surface-based geophysical methods to detect and characterise an alpine glacier's hydrological system. The aim is to improve our understanding of the drainage network of temperate alpine glaciers by using active seismic and ground-penetrating radar (GPR) to determine the properties in space and time of the hydrological system. To detect and characterise englacial flow paths, I performed active seismic and GPR on Rhonegletscher (Switzerland). Both geophysical methods detected a hydrological flow path within the glacier. With the use of amplitude-versus-offset analysis from the 2017 active seismic data, I confirmed that the englacial conduit was water-filled, thereby indicating that this technique is able to characterise hydrological features of glaciers. From both geophysical datasets, I estimated the conduit thickness to be between 0.5 and 4 m. With the acquisition of a 2D GPR grid in 2018, the spatial extent of the englacial conduit was estimated to be approximately 14,000 m$^2$. The water within the englacial conduit was sourced by surrounding surface meltwater and morainal streams. These streams enter the glacier subglacially before flowing into an efficient conduit system indicating that surrounding melt water streams directly impact the glacier's hydrological system. Upon successfully detecting an englacial conduit in 2017 on Rhonegletscher, I performed repeated GPR surveys in 2018 and 2019 to detect seasonal and annual changes to the englacial conduit. I extracted the GPR reflectivity using an impedance inversion workflow. The workflow allowed an interpretation of the englacial conduit's infill material. During the summer melt season, a flowing, water-filled sediment-transporting englacial conduit was detected having a sinusoidal shape with shallow inclination and being 0.4 $\pm{}$ 0.35 m thick. The repeated GPR measurements revealed that the shape of the conduit did not dramatically alter throughout the melt season. During the winter periods, the englacial conduit transported neither water nor sediment. It was either physically closed, or it became very thin (< 0.1 m). Such detailed interpretations were only possible with the use of the impedance inversion workflow and synthetic GPR waveform modelling. Even though temperate glaciers have a dynamic hydrological system, I was able confirm that the englacial conduit reactivated in an identical position from 2018 to 2019. The observations into Rhonegletscher's englacial conduit were conducted using 2D geophysical data. Such 2D geophysical reflection data represents the current state-of-the-art in glaciological applications; however, 2D reflection data falsely assume that all reflections originate from the vertical plane of acquisition. Complex englacial structures or basal geometries cause this assumption to be violated. Therefore, for accurate subsurface imaging, 3D GPR acquisition is required. Consequently, I acquired a 3D GPR survey that provided unprecedented high-resolution and unaliased 3D images of the Rhonegletscher's drainage network situated in the lower ablation zone. With this dataset, I was able to confirm a long-standing theory regarding melt water flow pathways around overdeepenings. With the development of unmanned aerial vehicles, future 3D GPR surveys are looking to be carried out in a fast and efficient manner. The research carried out and presented in this thesis demonstrates that both active seismic and GPR can be used to successfully identify, characterise and monitor a temperate glacier's hydrological network. Such studies will have a substantial impact on future investigations of the glacier hydrological system, and I conclude that future 3D GPR surveys conducted by unmanned aerial vehicles have the potential to revolutionise the way GPR data are acquired and processed in cryosphere applications.