  Using a bistatic radar echo sounding (RES) system developed for calibration
of the RICE particle astrophysics experiment at the South Pole, we have studied
radio frequency (RF) reflections off the bedrock. The total propagation time of
~ns-duration, vertically (z-) broadcast radio signals, as a function of
polarization orientation in the horizontal plane, provides a direct probe of
the geometry-dependence of the ice permittivity to a depth of 2.8 km. We
observe clear birefringent asymmetries along z- in the lowest half of the ice
sheet, at a fractional level ~0.3%. This result is in contrast to expectations
based on measurements at Dome Fuji, for which birefringence was observed in the
upper 1.5 km of the ice sheet. This effect, combined with the increased radio
frequency attenuation expected near the bedrock, renders the lower half
thickness of South Polar ice less favorable than the upper half of the ice
sheet in terms of its ultra-high energy neutrino detection potential.
