Englacial Characterization of a Temperate Glacier Using 3D Multioffset Multi-channel GPR

In this study, we used ground penetrating radar to image the geometry and spatial distribution of englacial voids. It is well established that radar velocity is correlated with water content in ice. Accurate laterally continuous velocity measurements are most efficiently acquired with continuous multi-offset data. Previous acquisition of such data was prohibitive due to technological limitations. Recent advances in GPR technology, including faster electronics and multi-channel capability, have made multi-fold acquisition feasible. In spring 2006, we acquired a 100 m x 100 m multi-azimuthal, multi-offset 3D grid. We used 25 MHz antennas with offsets ranging from 5 m to 150 m with 15 different offsets. These data were acquired at the transition from low water input during winter, to relatively high water input and rapid glacier motion during the spring and early summer. During late summer, 2007, we also collected a second dataset consisting of a multi-azimuthal common midpoint survey. Both surveys generated data along multiple azimuths, allowing us to examine velocity anisotropy. Variations in the velocity with respect to direction indicate preferential void orientation. Determining the distribution, direction and size of the voids is vital to understanding englacial flow and its affect on overall glacial hydrology. Preliminary velocity analysis shows anisotropy with the preferred fracture direction.