A 3-D Directional Borehole Radar Imaging Method for Rough Fractures: Numerical Simulation and Analysis

The emergence of directional borehole radar (DBR) has made it possible to obtain the 3-D morphology of fractures through a single borehole. The use of a uniform circular array (UCA) as the array receiving antenna is an important way to achieve directional detection; however, the direction of arrival (DOA) method and 3-D imaging method with DBR still have limitations in the imaging capability of rough fractures. We, therefore, propose a method of performing migration first and then DOA estimation next, combined with the moving window method (MWM), to achieve multitarget 3-D imaging, where the DOA estimation adopts the idea of applying the multiple signal classification (MUSIC) algorithm directly. We use the multigrid finite difference time domain (FDTD) method for numerical simulation to verify the effectiveness of the proposed 3-D imaging method. Subsequently, the response of DBR to rough fractures was studied, and the results showed that the roughness characteristics of fractures would significantly change the imaging results but also bring more fracture feature information; we can even detect the rough fracture under unfavorable dip angle. The research provides a theoretical basis for the detection of fractures under complex conditions and has practical application value in the future.