ABSTRACT. From temperature measurements down through the 3001m deep borehole at the North Greenland Icecore Project (NorthGRIP) drill site, it is now clear that the ice at the base, 3080 m below the surface, is at the pressure-melting point. This is supported by the measurements on the ice core where the annual-layer thicknesses show there is bottom melting at the site and upstream from the borehole. Surface velocity measurements, internal radio-echo layers, borehole and ice-core data are used to constrain a timedependent flow model simulating flow along the north-northwest-trending ice-ridge flowline, leading to the NorthGRIP site. Also time-dependent melt rates along the flowline are calculated with a heat-flow model. The results show the geothermal heat flow varies from 50 to 200 mW m^2 along the 100 km section of the modeled flowline. The melt rate at the NorthGRIP site is 0.75 cm a^1, but the deep ice in the NorthGRIP core originated 50 km upstream and has experienced melt rates as high as 1.1cm a^1.

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Abstraet. We performed experiments to collect radar backscatter data at K. (13-4GHz) and C bands (5'3GHz) over simulated sea ice at the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) during the 1990 and 1992 winter seasons. These experiments were conducted over bare saline ice grown in an indoor tank and an outdoor pond facility. The radar data were calibrated using a complex vector calibration scheme to reduce systematic efTects. In conjunction with the radar measurements we measured ice physical properties.These measurements demonstrate that the dominant backscatter mechanism for bare saline ice is surface scattering. Both the copolarizcd and cross-polarized measurements compare favourably with the predictions of surface scattering models at two frequencies.During the 1992 indoor tank experiment we applied four successive layers of snow (about 2·5 ern each) to the saline ice sheet after the ice thickness had reached about 12cm. The backscatter at normal incidence dropped by 15dB and the backscatter at 45' increased by II dB with the introduction of the first snow layer. The application of three more layers, each of approximately 2·5 cm depth. did not alter the radar signature significantly. By modelling and direct observation we found that the initial change in the signature was caused by a roughening of the surface at the snow-ice interface and the change in dielectric contrast at the snow-icc interface.

ABSTRACT. The University of Kansas developed a coherent radar depth sounder during the 1980s. This system was originally developed for glacial ice-thickness measurements in the Antarctic. During the field tests in the Antarctic and Greenland, we found the system performance to be less than optimum. The field tests in Greenland were performed in 1993, as a part of the NASA Program for Arctic Climate Assessment ( PARCA ). We redesigned and rebuilt this system to improve the performance.The radar uses pulse compression and coherent signal processing to obtain high sensitivity and fine along-track resolution. It operates at a center frequency of 150 MHz with a radio frequency bandwidth of about 17 MHz, which gives a range resolution of about 5 m in ice. We have been operating it from a NASA P-3 aircraft for collecting ice-thickness data in conjunction with laser surface-elevation measurements over the Greenland ice sheet during the last 4 years. We have demonstrated that this radar can measure the thickness of more than 3 km of cold ice and can obtain ice-thickness information over outlet glaciers and ice margins.In this paper we provide a brief survey of radar sounding of glacial ice, followed by a description of the system and subsystem design and performance. We also show sample results from the field experiments over the Greenland ice sheet and its outlet glaciers.