Integrated Borehole GPR and Optical Imaging for Field Investigation of Rock Mass Structures

Conventional drilling and coring methods are inherently limited to providing one-dimensional geological data, which hinders accurate characterization of the spatial distribution of rock mass structures and properties. Mechanical disturbances during drilling often cause core breakage, further compromising the fidelity of in-situ geological representation. This study proposes an integrated approach combining borehole optical imaging and ground-penetrating radar (GPR) for enhanced characterization of rock mass structures. A dynamic exploration methodology is introduced, defined as an adaptive drilling layout workflow based on phased information feedback. The fundamental concept, key assumptions, boundary conditions, and field implementation procedures of this dynamic survey are systematically described. The integrated method was applied to a high-speed railway investigation project in the Tengzhou section, Shandong Province, China, where six boreholes were surveyed using both techniques. Results demonstrate that fused analysis of borehole optical images and GPR data effectively reveals rock morphology, fracture distribution, joint systems, fractured zones, and geological features such as rock veins. The method's complementary strengths—optical imaging providing high-resolution orientation data at the borehole wall and GPR extending detection radially into the surrounding rock mass—enable spatially enhanced characterization while partially mitigating the azimuthal ambiguity inherent in single-borehole radar measurements. A triangular borehole survey scheme is shown to be feasible for locating subsurface anomalies. The proposed dynamic exploration method effectively reduces borehole requirements compared to conventional grid layouts while successfully identifying common anomalous features through integrated analysis of optical imaging and GPR data. The method demonstrates practical applicability for detecting fractures with apertures greater than 1 cm and meter-scale cavities, with good consistency between the two techniques validating the feasibility of this integrated approach. The method's limitations, including resolution constraints and detection omission risks, are explicitly acknowledged, and risk control strategies are proposed. Overall, the dynamic exploration approach reduces investigation costs, accelerates project time-lines, and provides a practical framework for spatial characterization of rock mass discontinuities with minimal borehole requirements.