Velocity difference between the surface and base of Ice Stream B2, West Antarctica, from radar fading-pattern experiment

Abstract

AbstractAs an ice-sounding radar receiver and transmitter are moved together across the snow surface, the returning echoes change rapidly, forming the "spatial fading pattern". Because the fading pattern is determinedly the details of the ice-bed interface" and remains fixed in position relative to that interface while the ice moves, repeated measurements of the fading pattern can reveal the relative displacement between the snow surface and targets near the bed of the ice and/or in the ice. Six high-density grids, comprising flag lines approximately along and normal to the direction of ice flow on Ice Stream B2, West Antarctica, were repeatedly profiled with a digital 50 MHz radar system by triggering a burst of 128 pulses from the radar transmitter every 0.7 m. A highly directional microwave motion sensor and a manual event button provided registration marks along the lines. The original recorded amplitudes were resampled at a constant spatial interval by interpolation between adjacent traces to make possible an analytical comparison of fading patterns. Fading patterns recorded on different days were compared by cross-correlation analysis to estimate the horizontal displacement. Then near-surface crevasses, which locally blocked the transmitted energy, were used to correct for flag-positiomng errors. The results from the three largest grids are significantly different: differential motions between the surface and the bed range from 1 to -0.5 ma−1. We suspect that the differences are related to the complex systems of strains within the ice stream reported by Hulbe and Whillans (1997).