HR: 08:15h
AN: C41D-02 INVITED     [Abstracts]
TI: Characterizing Subglacial Interfaces With Airborne Radar Sounding Techniques
AU: * Peters, M E
EM: mattp@ig.utexas.edu
AF: The University of Texas, Institute for Geophysics, Jackson School for Geosciences, 4412 Spicewood Springs Rd., Bldg. 600, Austin, TX 78759 United States
AU: Blankenship, D D
EM: blank@ig.utexas.edu
AF: The University of Texas, Institute for Geophysics, Jackson School for Geosciences, 4412 Spicewood Springs Rd., Bldg. 600, Austin, TX 78759 United States
AU: Morse, D L
EM: morse@ig.utexas.edu
AF: The University of Texas, Institute for Geophysics, Jackson School for Geosciences, 4412 Spicewood Springs Rd., Bldg. 600, Austin, TX 78759 United States
AB: Ice sheets are sensitive indicators of global change including sea-level rise. An ice sheet's subglacial interface is an important factor controlling its dynamic behavior. In particular, the grounding zones of ice streams and subglacial lakes are complex systems involving the interaction of the moving ice mass with underlying materials such as liquid water, saturated lubricating tills, and rough or frozen bedrock sticky spots. Imaging and characterizing the subglacial environment of ice sheets is fundamental to understanding these complex systems. Airborne radar sounding is a powerful and well-known technique for studying ice sheets and glaciers and their contiguous underlying environments. We present results from data acquired in 2001 over the ice stream C grounding zone in West Antarctica, as well as over a hypothesized subglacial lake near the South Pole. These data were acquired using a uniquely configured coherent airborne radar system. Our focus has been to characterize the subglacial interface through radar echo analysis based on reflection and scattering theory. The radar system uses a programmable signal source linked to a 10 kW transmitter and a dual-channel coherent down-conversion receiver. The radar operates in chirped pulse mode at 60 MHz with 15 MHz bandwidth. High and low-gain channels allow for recording a wide dynamic range of echoes simultaneously and without range-dependent gain control. Data acquisition includes integrations of 16 returned radar signals about every 15 cm along-track. Pulse compression and synthetic aperture radar (SAR) processing were components of data analysis. Subglacial echoes are influenced by the physical properties of the interface such as the composition and roughness of the materials at the interface. Other important factors include dielectric losses and volumetric scattering losses from propagation through the ice as well as transmission and refraction at the air-ice interface. Unfocussed SAR narrows the along-track radar beam thus increasing resolution at the subglacial interface. Basal reflection coefficients are computed from these data and used for inferring materials at a smooth subglacial interface, most notably when significant quantities of liquid water are present immediately beneath the ice. Echo behavior based on reflection and scattering theory shows that diffuse scattering often dominates the echoes. Scattering analysis consists of Doppler frequency processing to determine the positions of effective scattering centers at the subglacial interface. The along-track offset from the radar to the scattering center indicates the extent of echo sources, thus relating to scattering from the subglacial interface. These results also provide roughness estimates ranging from less than 10 cm for smooth sea-water to 10 m for bottom crevasses. Additional scattering analysis involves imaging based on short integration distances to obtain a low-resolution wide-angle look at the interface. The combined results of the reflection and scattering analysis allows for the classification of distinct subglacial environments including smooth water, very smooth saturated sediments, accreted ice, rough interfaces with bottom crevasses (at a grounding line), and mixed conditions with partial liquid water or interstitial ice.
DE: 4540 Ice mechanics and air/sea/ice exchange processes
DE: 1827 Glaciology (1863)
DE: 0629 Inverse scattering
DE: 0694 Instrumentation and techniques
SC: Cryosphere [C]
MN: 2004 AGU Fall Meeting