The effect of conductive borehole water on borehole radar

A borehole containing conductive water produced an anomalous radar response in otherwise highly resistive host rocks, dominated by a first arrival with a fast rise and long slow tail. The fast rise may be followed by one or two high amplitude, relatively slow oscillations, before the slow tail. The expected reflectors were superimposed on the slow tail, except where the amplitude of the tail had saturated the radar receiver. Numerical modeling of the scenario was undertaken using a Finite-Difference Time-Domain code with rotational symmetry. A wire dipole was modeled, without resistive loading. Even without loading, the modeled results correspond to the experimental results. Modeling suggests that the conductive water forms an extension to the arms of the antenna, dramatically altering the time domain response of the antenna. While conductive water does have some screening effect on the radar antenna, lowering the signal strength of the transmitted pulse, its main effect is to change the coupling between the antenna and the surrounding rock. Unexpected wave modes can then be set up causing anomalous responses in early time. Modeling suggests that minimal isolation between the antenna and the borehole can remove the anomalous response.