Influence of the borehole and its filling medium on crosshole georadar data: experimental evidence and numerical simulations

The reliability of tomographic attenuation images depends critically on the a priori assumptions of the inverse procedures. One important assumption is that the radiation patterns can be represented by formulas for dipole antennas located in a homogeneous full-space. Modeling studies suggest that this assumption is valid for gas-filled boreholes, but is violated for water-filled ones. To test experimentally these numerical results we developed a special packer system. This allows us to isolate the effects of the borehole filling by comparing data gathers measured with (gas-filled hole) and without the packer system (water-filled hole). Our experimental results demonstrate that the radiative characteristics of borehole antennas do indeed differ substantially for gas- and water-filled boreholes and that the dominant effects are correctly predicted by simple modeling approaches.