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Detection of in-situ ice fabric anisotropy near the WAIS divide using polarimetric radar

Detection of in-situ ice fabric anisotropy near the WAIS divide using polarimetric radar. Kenny Matsuoka, Charles Raymond, Howard Conway, and Shuji Fujita. Acknowledgements. On-ice crews for 2005-6 field season. NSF – OPP #0440847; PI: Raymond RPSC WAIS camp Science Construction

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Detection of in-situ ice fabric anisotropy near the WAIS divide using polarimetric radar

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  1. Detection of in-situice fabric anisotropy near the WAIS divide using polarimetric radar Kenny Matsuoka, Charles Raymond, Howard Conway, and Shuji Fujita

  2. Acknowledgements On-ice crews for 2005-6 field season • NSF – OPP#0440847; PI: Raymond • RPSC • WAIS camp • Science Construction • Comm shop Larry Homen • National Institute of Polar Research, Tokyo • UW Space GrantPavan Vaswani Joe Donovan Ryan Peter

  3. Past Current Future strain history  preferred fabric  deformation Ice-crystal alignments (ice fabric) Two commonfabric patternsfound in ice cores Projection to the horizontal plane

  4. Permittivity e: A A e||c= 1.01e_|_c B B Reflectivity R: RA >> RB RA-RB = 10 ~ 20 dB Anisotropy in ice permittivity induces anisotropic radio echo Anisotropic reflectivity Horizontal plane Birefringence (a path effect) Two principal polarization components A and B travel at different speeds, and can arrive at the receiver with different phase. A B

  5. Frankensled II Polarimetric radar method Starburst measurements 60-MHz and 179-MHz pulse-modulated radars 30 m 12 azimuths (15o intervals)

  6. 180o-periodic echoes  Anisotropic (uniaxial) reflectivity 90o-periodic echoes  Birefrigence (or biaxial reflectivity) Azimuthal variations in the echo Rmax-Rmin 179 MHz is more sensitive to fabric-based reflection. Echo drop Apparent echo features can be complex, if both effects happen together.

  7. Study Area: WAIS divide 19 circles: Sites for polarimetric radar measurements Contour: Surface topography Color image: Bed topography (SOAR, BEDMAP)

  8. Echo intensity (dBm) Uniaxial echo found at 179 MHz Data taken with different pulse widths show the same feature Site: Adivide

  9. High vs. Low radar frequencies (60 M) (179 M) Apparent uniaxial echo only at 179 MHz suggests thatanisotropic reflectivity is induced by ice fabric. Site: Adivide

  10. Depth dependence ofecho-drop orientation 179 MHz; Site: BW30

  11. 90o Biaxial, orthogonal, signal drops 60 MHz 60 MHz 179 MHz Site: AE30

  12. Azimuth of biaxial echo dropsin 60 MHz data (preliminary results) Azimuths of the drops: 45o off principal axes of the ice fabric 19 circles: Radar measurements Contour: Surface topography Color image: Bed topography(SOAR, BEDMAP)

  13. Summary • Features of the ice fabric can be measured by polarimetric radar over large areas. • In some locations, principal axes of the ice fabric are aligned with surface slope. • In some locations, more complex features are found that could arise from spatial or temporal complexity in ice flow. Coming field season:- Polarimetric radar measurements of shallower ice - Strain grid measurements

  14. Take-home messages • For ice core scientists:Measure ice fabric of the Divide Core at high vertical intervals from shallow depths. • For ice core drillers:Measure azimuth of each ice core. • For ice-flow modelers:Radar measurements may provide a proxy of fabric-induced enhancement factor in ice flow. • For radar nuts:Let’s develop state-of-the-art polarimetric radar.

  15. Thank you! Kenny Matsuoka matsuoka@ess.washington.edu

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