The mass balance of glaciers is influenced by their bed elevation below sea level, surface melt, and ocean-induced ice melt at calving fronts. It is essential to know the glacier thickness, bed elevation and fjord bathymetry to interpret the glacier evolution in the ongoing warm climate. Traditional methods for mapping ice thickness from radar sounding fail in the terminal valleys occupied by glaciers and near glacier calving fronts because of challenging conditions: side returns, rough surface, warm ice, water inclusions. We had to explore new ways to infer ice thickness. The advent of modern airborne gravimeters capable of sub milligal precision made it possible to explore the usage of gravity. Such instrument had been used widely for oil and mineral surveys and we applied it to glacier ice.
In this dissertation, we use airborne gravity data collected in August 2012 with a 0.5 mGal precision at a spatial resolution 750 m, combined with measurements of the fjord bathymetry and mass conservation reconstruction solution to obtain novel mapping of bed topography for several glaciers in Greenland. We use both 2D and 3D modeling to interpret the gravity data in these areas. The models are heavily constrained by geological information as much as we got, such as ocean bathymetry data, rock density information from Geological Survey of Denmark and Greenland with the selection of more optimized initial solutions. We use the gravity misfit to quantify the uncertainty of the inversion. The inversion significantly reduces the gravity misfit from the initial bed, as expected. The gravity misfit ranges from -3 to +3 mGal, the nominal precision of our bed mapping is about 60 m. Our study demonstrated the practical use of high-resolution airborne gravity to fill critical gaps in bed elevation in Greenland, especially in deep fjords that cannot be surveyed with deep radar sounders. The results provide more definite view of the bed topography of these major glaciers system than available previously, meanwhile, at a spatial resolution of 750 m along the trough and with an average precision of about 60 m. However, more precise rock information or supplementary data e.g., magnetic data is needed because of the difficulty in associating with space-varying geology/density.
This study provides simple guidelines for utilizing gravity data to obtain glaciers bed topography and then to understand glaciers' evolution in ongoing warm climate for both ocean and atmosphere.