Glaciers and ice sheets modulate global sea level by storing water deposited as snow on the surface, and discharging water back into the ocean through melting. Their physical state can be characterized in terms of their mass balance and dynamics. To estimate the current ice mass balance, and to predict future changes in the motion of the Greenland and Antarctic ice sheets, it is necessary to know the ice sheet thickness and the physical conditions of the ice sheet surface and bed. This information is required at fine resolution and over extensive portions of the ice sheets.
The ice sheet has two major interfaces: the upper surface interface, between the air and the snow or ice; and the basal interface, between the ice and bedrock or subglacial water. In between, there are internal layers that originate from slight density changes or ancient volcanic deposits. Due to the broad antenna pattern of the sounding radar system, each image resolution cell will contain signals from the left and from the right of the antenna array, and originating from the surface and from the bed. To resolve these signals and to achieve swath sounding capability, an array of receiving antennas in the cross track direction is used. A tomographic algorithm has been developed to take raw data collected by a multiple-channel synthetic aperture sounding radar system over a polar ice sheet and convert those data into two-dimensional (2D) ice thickness measurements. Prior to this work, conventional processing techniques only provided one-dimensional ice thickness measurements along profiles.
In this innovative development supported in part by NASA ESTO, airborne tomographic ice sounding technology was used to successfully image the reflectivity and topography of the surface as well as the reflectivity of the ice sheet base and ice sheet thickness. From the surface topography and ice thickness measurements, the 3D basal topography can be computed. For the first time, one is able to “see” through kilometers-thick ice sheets and measure the 3D bottom topography and its scattering properties, across a several-kilometers-wide swath. Validation with independent measurements indicates that this technique provides accurate topographic measurement of ice sheet surface and bed, and can be used for local ice sheet bed mapping.
The tomographic sounding processing system is composed of several major modules: a sub-aperture, back-projection azimuth compression with ray-bending correction; a MUSIC/ML arrival angle estimation to estimate surface/bed return arrival angles; and post-processing modules including data regrid and DEM (digital elevation model) mosaic. It produces the ice thickness map and the bedmap as the final product.
This work was done by Xiaoqing Wu, Ernesto Rodriguez, and Anthony Freeman of Caltech; and Ken Jezek of Ohio State University for NASA’s Jet Propulsion Laboratory. NPO-48638