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 GPR to enhance the characterization of rock mass structures. A dynamic exploration method 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 is 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, and fractured zones. Optical imaging provides high-resolution orientation data at the borehole wall. Borehole GPR extends detection radially into the surrounding rock mass. Together, the two methods enable spatially enhanced characterization and partially mitigate 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 method effectively reduces borehole requirements compared to conventional grid layouts. Through the integrated analysis of optical imaging and GPR data, common anomalous features can be successfully identified. The method demonstrates practical applicability for detecting fractures with apertures greater than 1 cm and meter-scale cavities. Good consistency between the two techniques validates 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 and accelerates project timelines. It also provides a practical framework for the spatial characterization of rock mass discontinuities with minimal borehole requirements.