The effect of wet drilling in kaolinitic strata on borehole radar performance
Unexpectedly weak signal levels, in both radar and cross-hole modes, were obtained during a recent VHF borehole radar experiment in an arid, alluvial environment. An investigation revealed that the kaolinitic strata, which hosted the vertical boreholes, were moistened by drilling fluid during site preparation. It was therefore conjectured that the weak received signals were due to attenuation caused by wet, conductive annuli surrounding the boreholes in an otherwise dry medium. To examine this premise, the VHF electrical properties (permittivity and conductivity) of representative dry and wet samples from the different strata were measured. These data were then used in a 3D, electrodynamic Finite Difference Time Domain (FDTD) code to model different annular configurations of dry and wet strata in and around boreholes of different dimensions. In the model, a 1.2 m center-fed dipole was placed coaxially in the various boreholes and excited by a Gaussian pulse. This allowed quantification of the reduction in electric field strength due to the wet drilling technique. The results correspond well with the measured attenuation observed during the borehole radar surveys. The paper concludes with recommendations for the preparation of borehole radar surveys of paleochannels covered by kaolinitic strata.