Type of Culminating Activity
Master of Science in Geophysics
John H. Bradford, Ph.D.
Glacial hydrology plays an important role in the motion and melt of glaciers. The transport of water through glacial ice is still poorly understood. Previous studies show that much of the water is stored and routed through planar voids within the glacier. From borehole observations and previous radar surveys, the voids appear to have a preferential orientation. Transverse waves, including radar waves, and shear seismic waves, travel at different velocities when polarized at different orientations relative to aligned inclusions. I conducted two georadar surveys on Bench glacier, in the Chugach Mountains, AK. Bench glacier has been the field site of a collaborative effort to characterize these englacial voids, and their role in glacial hydrology. Since georadar velocity is commonly used to estimate glacial water content, it is important to know the orientation of the voids relative to the georadar olarization, in any estimation of the water content. From mixing models with aligned inclusions, I was able to estimate the degree of velocity anisotropy expected from water filled voids in glacier ice. Surveys over the same area conducted with different polarizations, yielded velocity estimates that were measurably different.
The first survey was conducted in 2006. This was a large multi-offset multiazimuth 3D survey. The grid was surveyed in three directions to measure the differences in the radar wave velocities due to orientation. This survey not only provided estimates of velocities in over 3000 locations, but also produced a 3D volume showing englacial structure. The distribution of velocities in each direction was statistically different and showed an anisotropic velocity field that agreed with the theory and previous observations on Bench glacier. The second survey was a common midpoint survey conducted in 2008. This survey was designed to quickly test for and give an estimate of subsurface anisotropy. I collected common midpoint gathers with three different antenna configurations in five different directions. By doing so, I was able to sample the same part of the glacier with waves with different polarizations. Again this survey showed measurable anisotropy with the fastest velocity occurring when the wave was polarized perpendicular to the long axis of the voids.
Both surveys show measurable anisotropy greater than the uncertainty in the velocity estimates. If preferentially aligned inclusions are suspected, it is necessary to assume an anisotropic velocity model. By assuming an isotropic velocity model, volumetric water content estimates ranged from 0% to 8%. By assuming an anisotropic model, the water content estimate is better constrained and found to be 1.2%. In this work, I demonstrate how to survey and determine velocity anisotropy resulting from aligned inclusions and the importance of an anisotropic velocity model for estimating water content in temperate glaciers.
Nichols, Joshua D., "Characterization of Englacial Voids Using Radar Velocity Anisotropy" (2010). Boise State University Theses and Dissertations. Paper 157.