Ice-sheet bed 3-D tomography

Information on bed topography and basal conditions is essential to developing the next- generation ice-sheet models needed to generate a more accurate estimate of ice-sheet contribution to sea-level rise. Synthetic aperture radar (SAR) images of the ice-bed can be analyzed to obtain information on bed topography and basal conditions. We developed a wideband SAR, which was used during July 2005 to perform measurements over a series of tracks between the GISP2 and GRIP cores near Summit Camp, Greenland. The wideband SAR included an eight-element receive-antenna array with multiple-phase centers. We applied the MUltiple SIgnal Classification (MUSIC) algorithm, which estimates direction of arrival signals, to single-pass multichannel data collected as part of this experiment to obtain fine-resolution bed topography. This information is used for producing fine- resolution estimates of bed topography over a large swath of 1600 m, with a 25 m posting and a relative accuracy of approximately 10 m. The algorithm-derived estimate of ice thickness is within 10 m of the GRIP ice-core length. Data collected on two parallel tracks separated by 500 m and a perpendicular track are compared and found to have difference standard deviations of 9.1 and 10.3 m for the parallel and perpendicular tracks, respectively.