Anisotropic backscatter in ice-penetrating radar data: potential mechanisms and implications

• Main Authors: Drews, Reinhard, Eisen, Olaf, Rack, Wolfgang, Steinhage, Daniel, Weikusat, Ilka
• Format: Conference Object
• Language: unknown
• Published: 2010
• Subjects: Antarc*; Antarctic; ice core; Polarforschung
• Online Access: https://epic.awi.de/id/eprint/22658/; https://epic.awi.de/id/eprint/22658/1/Dre2010a.pdf; https://hdl.handle.net/10013/epic.35328; https://hdl.handle.net/10013/epic.35328.d001

Airborne and ground-based radar have been used extensively in the past to measure ice thickness and to investigate the internal structure of ice sheets in terms of layering. The main reflection mechanisms for internal reflections are changes in density, conductivity, and crystal orientation fabric, which alter thepermittivity of the ice. Linking the different mechanisms to the individual reflection horizons enables thededuction of glaciological parameters like accumulation rates or age-depth estimates. If no sample material from snow pits or ice-cores are available, multi-frequency and multi-polarization measurements must be applied to distinguish between the different reflection mechanisms. The backscattered power of horizons caused by changes in conductivity varies with the center frequency whereas in the case of horizons originating from changing crystal orientation the backscattered power is dependent on the polarization plane of the carrier signal.In this study we examine a sample data set near the German summer station Kohnen (drill site for theEPICA-EDML ice core) on the Antarctic plateau. The data were acquired with an airplane sliding on ground, producing varying incident polarization with a circular profile and several cross profiles with different headings. We find that the backscattered power changes with varying antenna orientation (i.e. polarization). In the upper third of the ice column the backscatter has two maxima with a 180° symmetry. The maxima align with the direction of minimal surface strain. At approximately 900 m depth the anisotropy is shifted by 90° in heading azimuth, with the maxima now being parallel to the maximum in surface strain. This dataset is unique, as airborne systems (primarily designed for the sounding of ice thickness) are usually not used for ground-based applications. The observed anisotropy appears clearly and is intriguing as the reason for it is entirely unknown. As primary suspects we consider the role of changing crystal orientation and ellipsoidal shaped air ...