Seismic investigations of firn in Antarctica
Creators: name: Hollmann, Hannes
Publisher: name: University of Tasmania
Year: 2024

description: The firn layer represents the natural interface between glacier ice and the atmosphere. Firn is a highly heterogeneous medium, where the interplay of physical processes on the microscale determines its evolution on the macro-scale. Determining firn characteristics provides insight into weather and climate history, active glacier processes, and knowledge of firn depth and density is crucial for the accurate assessment of ice mass changes of major ice sheets. Direct constraints on firn structure, however, are limited to a few locations in Antarctica as the acquisition of firn cores or borehole-based measurements has a high logistic cost. Seismic surveys are one example of a geophysical method that can be used to determine some firn properties, at a lower resolution, with efficiency in field logistics. Such surveys have the potential to improve the spatial coverage of firn measurements, particularly for Antarctica.<br>The research documented in this thesis firstly investigates the firn layer of the Amery Ice Shelf, at the confluence of two ice streams, using a seismic refraction approach together with the Wiechert-Herglotz method. Field data from seismic surveys carried out in the along-flow and across-flow directions are examined revealing differences in travel times according to survey line orientation and the upstream origin of the ice stream flow. The firn acts as a means of recording glacier processes and shows evidence for seismic anisotropy. The findings are consistent with dynamic coupling between the firn and the underlying ice.<br>Subsequent studies documented in the thesis further investigate the use of seismic refraction methods applied to firn, and the firn-ice transition zone. Computation-based numerical experiments are carried out to understand the sensitivity of the inferred results to workflow and modelling parameter choices, and to inform the use of the WH method to characterise the depth and seismic structure of firn.<br>A large-scale study of firn layer density determination is then presented, informed by the results of the numerical experiments, which uses field datasets from multiple locations where both seismic refraction surveys and borehole?based measurements have been obtained in close proximity. The systematic modelling of seismic waves in firn, and comparison with real measurements, highlights the potential of seismic refraction, especially for dry locations. The use of WH ensembles is presented, tailoring the seismic data processing to the investigation of the firn layer, and is found to be informative in providing context to the inferred apparent firn depth. After constraining the depth, the seismic structure of the firn, and hence the density structure of the firn layer, can be found by using the WH method in an optimised way.<br>The research documented in this thesis will assist those conducting seismic surveys that target the firn layer to make informed choices when planning, executing and processing such surveys; and thus contribute to methods that will reduce uncertainty in future estimates of ice sheet evolution and mass change. Further, it is hoped that that the research will lead to the increased, and improved, use of seismic surveys for the investigation of firn in Antarctica, and thus expand the coverage of reliable constraints on firn depth and structure from currently undersampled regions.