Using Seismic Anisotropy to Detect Mean Ice Crystal Orientation in the Jakobshavn Glacier

We collected multi-offset seismic reflection data on Jakobshavn Glacier, Greenland, in order to investigate englacial fabric development. Data were recorded from a stationary array of eight 100 Hz vertical geophones while 0.5 kg explosive sources buried 10 m below the surface were deployed at offsets ranging from -1760 to +1600 m. Source spacing was 160 m yielding 22 variable angle recordings with incidence angles ranging from -24 to +20 degrees. Analysis of the measured ray velocities reveals ice velocity variation as function of the angle of incidence. This velocity anisotropy along with englacial reflectivity present strong evidence of a preferred orientation of ice crystals near the base of Jakobshavn Glacier. Analysis of the seismic wave velocity dependence on ice fabric yields a mean ice crystal orientation distributed within a 32 degree cone from vertical. These measurements are consistent with laterally extensive complex ice fabric development reported over the same region of Jakobshavn Glacier.