Simulation and Analysis of Conductively Guided Borehole Radar Wave

In a borehole radar (BHR) survey, a steel cable or a metallic drill pipe attached to a BHR can act as a waveguide, and often gives measured radargrams with strong, obliquely striped events. These guided BHR wave events are commonly considered as interference noise for conventional BHR surveys and have not been fully investigated. To better understand these events, a 2-D cylindrical finite-difference time-domain method is used in this paper for simulation and investigation of the guided BHR wave propagation. Snapshots of wavefields and response profiles for different configurations are simulated and analyzed to understand the generation and interaction process of the guided waves with the borehole and the surrounding formation. The simulation results confirm that if a conventional BHR is attached with a conductive waveguide, the BHR wave can be coupled with the waveguide to generate guided BHR waves propagating along the borehole. The backward-looking or forward-looking guided BHR reflections from the strata discontinuities are recorded on BHR profiles. The generation of forward-looking events makes it feasible to image ahead of a drill bit in real time while drilling using a conventional BHR. A simple formula to estimate the location of the interface using the forward-looking events is given. Based on the synthetic data with a BHR 3-7 m away from the interface, it was demonstrated that the location of the interface can be accurately predicted with an error of about 1 cm. Finally, the effect of the length of the waveguide on the guided waves is also discussed.