C.R. BENTLEY, M.D. STENOIEN, S. SHABTAIE, C. LIU, and N. LORD, Geophysical and Polar Research Center, University of Wisconsin, Madison, Wisconsin 53706
Airborne collection of remote-sensing data over the "Trunk D" area, comprising ice stream D, the adjacent ridges, and portions of ice streams C and E, was conducted by the Support Office for Aerogeophysical Research (SOAR) between 9 December 1996 and 16 January 1997. We at the University of Wisconsin will be analyzing the radar-sounding and laser-altimeter data over a subset of the area (figure 1).
Clouds were the main obstacle to collecting laser-altimeter data; the surface return was obscured to some degree on 60 percent of the flights. Where the laser was blocked by clouds, the terrain clearance will be determined (to a lesser accuracy) from the radar soundings.
Over the UW area (figure 1) the ice thickness varies from 500 to 2,000 meters (m), well within the range of the SOAR 60-megahertz (MHz) ice-sounding radar. On the interstream ridges (including Siple Dome), where there is little surface scattering, the bottom echo is mostly strong and internal layers clear (figure 2). Surface clutter, however, totally masks the bottom echo over the ice-stream margins and interferes seriously with it elsewhere over the ice stream. Internal layers either are not present in the ice stream or are completely masked by the surface scatter.
Radar signals returning from the irregular surfaces in and under the ice are modified by diffraction. Consequently, when the radar system is moved across the snow surface, the form of the returning signal changes rapidly, producing the spatial "fading pattern." Repeated measurements of the fading pattern can reveal the relative displacement between the snow surface and the reflecting horizons.
Six grids near station Upstream B on ice stream B2 were surveyed with a digital 50-MHz ice-sounding radar two or three times each during the 1991-1992 field season (Bentley et al. 1992). Grids measured several hundred meters across; the spacing between lines was 30 or 50 m. The grids were defined by bamboo stakes em-placed at close, precisely positioned intervals.
The radar system was towed by a Tucker Sno-Cat at a speed of approximately 8 kilometers per hour. An automated system, comprising a microcontroller and a trailing bicycle wheel, triggered the radar system every 0.7 meters. A microwave motion sensor that detected the stakes and a manual trigger provided registration marks on the digital record.
The data spacing was not constant because
Therefore, to facilitate an analytical comparison of fading patterns, the orig inally recorded amplitude and phase were resampled at a constant spatial interval by interpolation between adjacent traces.
The amplitude and phase of the resampled fading patterns were compared analytically through cross-correlation analysis to estimate the displacement between surveys. The phase data were first unwrapped and filtered to remove initial phase differences and the effect of the ice-thickness gradient. The internal returns were used to remove the positioning error of the antennas, whose distance from the motion detector varied slightly from survey to survey—we equated the displacement between the apparent fading patterns from the bed and from selected internal horizons in the upper 70 percent of the ice (which we assume is not deforming) to the displacement between the surface and the bed.
Completed results at one of the grids from two sets of independent measurements over consecutive time intervals of 24 and 27 days, show that the basal irregularities travel 0.31.0 meter per year more slowly than the surface. Thus, at least 99.7 percent of the ice-stream movement of 440 meters per year (Whillans, Bolzan, and Shabtaie 1987) takes place by sliding and/or bed deformation; no more than 0.3 percent occurs by shear straining within the ice.
Work continues on two projects that use satellite-based active microwave sensors to view the west antarctic ice sheet.
The ice sheet of West Antarctica is conventionally divided morphologically into four classes with differing ice dynamics: the interior ice sheet, ice streams, domes, and ridges. There is, however, a fifth class that should be recognized—ice-sheet saddle zones, which lie between the domes, ridges, and ice streams.
An example is "Siple Saddle," which separates Siple Ice Dome from ridge C/D (figure 1). Radar soundings across the boundary between Siple Saddle and Siple Dome (figure 2) reveal no sign of an active or relict ice-stream margin there; although there is a slight increase in crevasse scatter over the saddle, it is strikingly less than that over the margin of ice stream D (figure 2) or C (Shabtaie and Bentley 1987). Instead, the radar data show a saddle in ice elevation; the surface profile is concave up between Siple Dome and ridge C/D and concave down between ice streams C and D. Satellite imagery of this area shows several curvilinear undulations in height within the saddle zone, running from ice stream C to ice stream D. We interpret those features to indicate that the saddle is currently widening both toward ridge C/D and toward Siple Dome. We believe that "Siple Saddle" is dynamically active and not in equilibrium.
This work was supported by National Science Foundation grants OPP 92-22092 and OPP 93-19043 and National Aeronautics and Space Administration contract NAS5-33015. (Contribution number 580 of the Geophysical and Polar Research Center, University of Wisconsin at Madison.)
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