detection of in-situ ice fabric anisotropy near the wais divide using polarimetric radar kenny...
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Detection of in-situice fabric anisotropy
near the WAIS divide using polarimetric radar
Kenny Matsuoka, Charles Raymond,
Howard Conway, and Shuji Fujita
Acknowledgements
• 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
On-ice crews for2005-6 field season
Joe Donovan Ryan
Peter
Ice-crystal alignments (ice fabric)
Two commonfabric patterns
found in ice cores
Past Current Futurestrain history preferred fabric deformation
Projection to the horizontal plane
Anisotropy in ice permittivity induces anisotropic radio echo
Horizontal plane
A A
B BReflectivity R:
RA >> RB
RA RB = 10 ~ 20 dB
Anisotropic reflectivity
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
||c = 1.01_|_c
Permittivity :
Polarimetric radar method
Frankensled II
60-MHz and 179-MHzpulse-modulated radars
12 azimuths (15o intervals)
Starburstmeasurements
30 m
Azimuthal variations in the echo
Echo drop
Rmax Rmin
180o-periodic echoes Anisotropic (uniaxial) reflectivity
90o-periodic echoes Birefrigence (or biaxial reflectivity)
Apparent echo features can be complex, if both effects
happen together.
179 MHz is more sensitive to fabric-based reflection.
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 MHzData taken with different pulse widths show the same feature
Site: Adivide
Ech
o in
tens
ity (
dBm
)
High vs. Low radar frequencies
Apparent uniaxial echo only at 179 MHz suggests thatanisotropic reflectivity is induced by ice fabric.
Site: Adivide
(179 M) (60 M)
Depth dependence ofecho-drop orientation
179 MHz; Site: BW30
Biaxial, orthogonal, signal drops
Site: AE30
90o
60 MHz 60 MHz 179 MHz
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)
Summary
1. Features of the ice fabric can be measured by polarimetric radar over large areas.
2. In some locations, principal axes of the ice fabric are aligned with surface slope.
3. 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
• 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.
Take-home messages
