3-D Visualization of New Hybrid-Rotational Ground-Penetrating Radar for Subsurface Inspection of Transport Infrastructure

Ground-penetrating radar (GPR) facilitates the detection and localization of subsurface structural anomalies in critical transport infrastructure (e.g., tunnels), better informing targeted maintenance strategies. However, conventional fixed-directional systems suffer from limited coverage—especially of less-accessible structural aspects (e.g., crowns)—alongside the unclear visual output of anomaly spatial profiles, both for physical and simulated datasets. To tackle these limitations, new hybrid-rotational GPR utilizes novel 360° orientable air-launched antennas to increase subsurface coverage, principally in tunnels. Prototype systems currently lack a versatile workflow to generate practical visual output for surveyors. This study develops a versatile visualization workflow based on entirely open-access tools, returning 3-D spatial profiles of subsurface anomalies in: 1) simulated; 2) fixed-directional; and 3) hybrid-rotational GPR datasets. Work includes the development of two unique hybrid-rotational GPR systems, for laboratory and in-field data collection, respectively. Following initial 3-D grid alignment and smoothing, data undergo 3-D Stolt migration, normalization, and proximal clustering via Hierarchical Density-Based Spatial Clustering of Applications with Noise (HDBSCAN). This returns segmented point subsets associated with suspected structural anomalies. Finally, 3-D convex hulls are recovered using the QuickHull method. Detection and localization performance is first appraised through numerical simulation in open-source software gprMax. Practical laboratory experimentation follows, with both commercial fixed-directional systems and developed hybrid-rotational GPR, before in-field demonstration on a large-scale, tunnel subsurface analog. In each experiment, all targets were successfully identified within returned 3-D visualizations of hybrid-rotational GPR datasets. Moreover, the spatial profiles were consistently observed to be accurately localized within decimeter-length scales of known target locations. Overall, the advances presented in this work both facilitate and demonstrate the significant practical potential of new hybrid-rotational GPR technology as a basis for future subsurface surveying of critical transport infrastructure.