Kenny Matsuoka, Charles Raymond, Howard Conway, and Shuji Fujita 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 Comm shop Larry Homen National Institute of Polar Research, Tokyo UW Space Grant Pavan Vaswani Joe Donovan Ryan Peter

Ice-crystal alignments (ice fabric) Two common fabric patterns found in ice cores Projection to the horizontal plane Past Current Future strain history  preferred fabric  deformation

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

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

Azimuthal variations in the echo 180o-periodic echoes  Anisotropic (uniaxial) reflectivity Rmax - Rmin 179 MHz is more sensitive to fabric-based reflection. Echo drop 90o-periodic echoes  Birefrigence (or biaxial reflectivity) When the azimuth of the polarization plane is altered, we can see two typical features. … Apparent echo features can be complex, if both effects happen together.

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

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

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

Depth dependence of echo-drop orientation 179 MHz; Site: BW30

Biaxial, orthogonal, signal drops 60 MHz 60 MHz 179 MHz Site: AE30

Azimuth of biaxial echo drops in 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)

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

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.

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

Radio-wave propagation within ice Glaciological characteristics Mechanisms Attenuation Chemistry + temperature Birefringence Polarization dep. Ice crystal alignments Reflection Polarization dep. Ice crystal alignments Density Chemistry + temperature

Radio echoes are frequency, polarization-plane dependent 60 MHz Frequency-dependent 179 MHz Polarization dependent First of all, I would like to show radar data from Antarctica that shows frequency, and polarization-plane dependent features. 90o difference in the polarization plane azimuth Matsuoka et al. (2003) 179 MHz

Strong fabric at deeper depths induces azimuthal variations to 60 MHz as well.

Birefringence effects estimated with a Greenland summit ice core Crystal-alignment data from the GISP2 ice core were used Crystal alignments data courtesy: Larry Wilen (Ohio Univ.) Birefringence is significant almost everywhere in the ice sheets

Spatial variation of the fabric suggested by the radar Matsuoka et al. (2004)

Anisotropic high-scattering zone Matsuoka et al. (2004) Only in the region with significant horizontal strain Only when the polarization plane is aligned to the horizontal compression axis. Fast-flowing ice distorts the scattering zone

What can we learn from the radio-echo intensity? Pr : echo intensity R : reflectivity B : birefringence L : loss (attenuation) z : depth (e: permittivity) S : radar system parameters Matsuoka et al. (2003) Frequency f and polarization-plane b dependences of the echo intensity can be caused by reflectivity and birefringence