A special packer system is developed that replaces the water in a
borehole with a gas and thus allows us to isolate the effects of
the borehole filling media on crosshole georadar measurements. We
demonstrate experimentally that the radiative characteristics of
borehole georadar antennas differ substantially for gas and waterfilled
boreholes. The main differences are a spectral shift to lower frequencies
and a less focused radiation pattern in the case of a waterfilled
borehole. These effects are correctly predicted by a basic finite
difference time?domain (FDTD) modeling approach, which offers the
prospect for improving the quality of amplitude inversions from crosshole
georadar data.