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The origin of the echo-free zone

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Drews, R. , Eisen, O. , Hamann, I. , Kipfstuhl, J. , Lambrecht, A. , Steinhage, D. and Wilhelms, F. (2008): The origin of the echo-free zone , Symposium on radioglaciology and its applications, Madrid, SpainJune 2008. .
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Abstract:

In extensive parts of the Antarctic and Greeland ice sheet radio-echo sounding (RES) reveals a layer in the lowest hundreds of meters above bedrock more or less free of radio echos, known as the echo-free zone (EFZ).To identify its origin, we compare radio echos obtained around the EPICA drilling site in Dronning Maud Land (DML) with the microstructure and physical properties of the ECPIA-DML (EDML) ice core.We find that radio echos disappear in the depth range below about 2100~m, where the small-scale layering, visible from line-scanning profiles of the ice core, becomes progessively disturbed on the scale of centimeters and meters.The formation of RES horizons seems obviously directly related to the integreity of layering present in physical properties.The upper onset of the EFZ occurs in the vicinity of a change in crystal orientation fabric from a gridle to a single maximum distribution.We conclude in the here presented case that radio echos fade out where the coherency of the layers is lost due to disturbances caused by the ice flow.Important implications for glaciological applications are that the EFZ marks the depth in which the climate record preserved in the ice is likely affected on the cm-scale, and the concentration of shear in the basal layer of the ice sheet.

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