The depth of reflecting layers in Arctic ice sheets has been determined by electromagnetic echo sounding, using a varying distance between transmitter and receiver to determine the radar wave velocity. The depth of the radar reflecting layers is compared with a profile of electrical conductivity measurements (ECMs) from the Greenland Ice Core Project (GRIP) ice core, in order to determine the velocity of the radar waves in the ice cap. By using several reflecting layers, it is possible to isolate the firn correction of the wave velocity and to estimate the accuracy of the calculated electromagnetic wave velocity. The measured firn correction is compared with the correction calculated from the density profile, and a comparison between the depth profiles of ECM and radar based on the corrected electromagnetic wave velocity is presented. This profile shows that acid layers, which originate from major volcanic eruptions, show up as reflecting radar horizons.

Since 1994 the Alfred Wegener Institute (AWI) has operated an airborne radio-echo sounding system for remote-sensing studies of the polar ice caps in Antarctica and in Greenland. It is used to map ice thicknesses and internal layernigs of glaciers, ice sheets and ice shelves, and is capable of penetrating ice thicknesses of up to 4 km. The system was designed and built by AWI in cooperation with Aerodata Flugmeßtechnik GmbH, Technische Umversitat Hamburg-Harburg and the Deutsches Zentrum fur Luft- und Raumfahrt e.V. The system uses state-of-the-art techniques, and results in high vertical (5 m) as well as along-track (3.25 m) resolution. The radar signal is a 150 MHz burst with a duration of 60 or 600 ns. The peak power is 1.6 kW, and the system sensitivity is 190 dB. The short backfire principle has been adopted and optimized for antennae used on Polar2, a Dormer 228-100 aircraft, resulting in an antenna gain of 14 dB each. Digital data recording allows further processing. The quality of the recorded data can be monitored on screen and as online analogue plots during the flight.

[1] Radio echo surveys to determine the thickness of ice sheets often record reflections from inside the ice. To increase our understanding of these internal reflections, we have used synthetic seismogram techniques from early seismic modeling to construct two models. Both models were one-dimensional; the first considered only primary reflections, while the second included both primary and multiple reflections. The inputs to both models were a radio pulse and data from the Greenland Ice Core Project (GRIP) core of length 3028 m. The ice core data consisted of a profile of the high-frequency conductivity, calculated from dielectric profile (DEP) measurements, and a smooth profile of the real permittivity. The models produced synthetic radargrams which are the energy reflected from conductivity variations as a function of the two-way travel time. Both models gave similar results, indicating that multiples do not alter the travel time of the reflections, i.e., no O'Doherty-Anstey effect at our time resolution. One of the results was then processed to simulate the reflected energy passing through the receiver circuit of a radio echo system and then compared with a recorded trace. The processed result contained many of the larger reflections recorded below about 500 m, including nearly all the features from depths greater than 1500 m, in particular, several interstadial events in the Wisconsin age ice. Since high-frequency conductivity variations are dominated by chemical changes which are caused by deposition on the surface of the ice sheet, it is possible to conclude that the reflections deep inside the Greenland ice sheet can be treated as isochrons.

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