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 water-filled boreholes. The main differences are a spectral shift to lower frequencies and a less focused radiation pattern in the case of a water-filled 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.

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