Surface roughness and borehole radar imaging in three-dimensions

Abstract

Broadband VHF borehole radars (BHR) can be used as a tactical tools to map orebodies, faults and other marker horizons; to identify hazards in advance of mining and to stop unnecessary mine development. Ground penetrating radar (GPR) operations in boreholes have a number of advantages over those conducted in mine galleries. Boreholes give freedom from gallery reverberation and offer access to the third dimension. Mine boreholes are often drilled in fans, and can be used to reconstruct targets in 3D. Significant progress has been made in recent years in using synthetic aperture radar interferometry (InSAR) to reconstruct 3D images from sparse arrays. SAR systems work with backscatter echoes and specular/nadir reflections from smooth surfaces are deliberately avoided. Hard-rock boreholes are thin requiring slimline BHR systems. The fact that slimline BHR systems are not directional makes it difficult to avoid recording nadir data. Automatic methods of projecting data into 3D image space such as migration and InSAR make stringent demands upon rock homogeneity, translucence and the accuracy of borehole trajectories. These demands can be relaxed by kinematic mapping using geologically plausible 3D primitives such as cylinders, planes and hollows. Field data quality and object illumination constraints set the point of balance between object identification in image space and interactive interpretation in observation space. The balance sought must be able to take advantage of specular reflections, as BHR time sections are frequently dominated by nadir glare.