Modeling the radio echo reflections inside the ice sheet at Summit, Greenland

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.