An Application of Reflected Guided Borehole Radar Waves to Detection of Subhorizontal Structures

Borehole radar (BHR) waves will be severely attenuated when a borehole is filled with high-conductivity (HC) media. As a result, BHR side-reflections are significantly weakened, i.e., the capability to detect the targets far from the borehole is restricted. However, V-shaped reflected guided waves (RGWs) are commonly observed in BHR data from such HC boreholes. They are often regarded as unwanted contamination to conventional BHR surveys and should be avoided or suppressed. However, we advocate that the RGWs, as a type of reflected wave traveling along the borehole, contain geological information about the surroundings of the borehole and can be used to locate and recognize geological boundaries, such as lithological interfaces and fractures that intersect the borehole. Unlike conventional BHR surveys mainly imaging the structures near-parallel to the borehole, the RGWs can effectively sense near-normal ones. In addition, we can use RGWs to distinguish between echoes from fractures and those from bedding planes. This is achieved by recognizing different phase characteristics of RGWs for these structures: the RGWs from both sides of a fracture have the same phase, whereas the ones from a stratigraphic interface have opposite phases. Synthetic and real data are used to demonstrate the feasibility of detecting subhorizontal structures by RWGs. RGWs extend BHR measurements and allow a better understanding of subsurface structures. They help to predict and reduce mining risks and monitor potential geological hazards.