Geophysical constrains to improve synchronization of deep ice-core records

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Eisen, O. , Wilhelms, F. , Steinhage, D. and Schwander, J. (2005): Geophysical constrains to improve synchronization of deep ice-core records , 22. Internationale Polartagung, Deutsche Gesellschaft für Polarforschung, 18. bis 24. September 2005, Jena .
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We present a geophysical method to synchronize deep ice-core records by combination of radio-echo sounding and numerical modeling ofelectromagnetic reflections. Most continuous internal reflection horizons are known to form isochrones and can be followed over large distances. With an ice core at either end of the profile, the reflection horizons present time markers that are used to synchronize the ice-core records. Electrical properties along an ice core serve as input to a numerical model which simulates the propagation of electromagnetic waves in the ice and reproduces the reflection characteristics of the radar profile near the ice core. The depth of origin of reflections are identified in two steps: firstly, pronounced series of reflections are used to calibrate the electromagnetic wave speed; secondly, individual peaks in conductivity in the input record are removed, thus also removing the correspondingreflections in the synthetic radargram. Our numerical modeling approach improves the accuracy with which the reflector origins are identified compared to the usual method where reflector traveltimes (respective depths) and ice-core profiles are merely compared. A pilot study at the EPICA drilling site in Dronning Maud Land, Antarctica, shows that it is possible to locate the origin of internal reflections with an accuracy of 0.5 m in a depth of 2000 m and more. The method imposes little constrains on the input records, making it applicable to a number of drilling sites. Both, dielectric profiling and electrical conductivity measurements can be used as electrical input. Moreover, as the method also calibrates the wave speed, only a coarse density profile is required. Application to the deep-drilling locations in Antarctica will improve the relative synchronization and will help to answer the question of phase relation of climate changes observed in the ice-core records at different locations.

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