3D Visualisation of New Hybrid-Rotational Ground Penetrating Radar For Subsurface Inspection of Transport Infrastructure

Ground Penetrating Radar (GPR) facilitates the detection and localisation of subsurface structural anomalies incritical 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 unclear visual output of anomaly spatial profiles, both for physical and simulated datasets. To tackle these limitations, new hybrid-rotational GPR utilises 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 visualisation workflow based on entirely open access tools, returning 3D spatial profiles of subsurface anomalies in (i) simulated, (ii) fixed-directional and (iii) hybrid-rotational GPR datasets. Work includes development of two unique hybrid-rotational GPR systems, for laboratory and in-field data collectionrespectively. Following initial 3D grid alignment and smoothing, data undergoes 3D Stolt migration, normalisationand proximal clustering via Hierarchical Density-Based Spatial Clustering of Applications with Noise (HDBSCAN). This returns segmented point subsets associated with suspected structural anomalies. Finally, 3D convex hulls are recovered using the QuickHull method. Detection and localisation performance is first appraised through numerical simulation in open-source software gprMax. Practical laboratory experimentation follows, with both commercial fixed-directional system and developed hybrid-rotational GPR, before in-field demonstration on a largescale, tunnel subsurface analogue. In each experiment, all targets were successfully identified within returned 3D visualisations of hybrid-rotational GPR datasets. Moreover, the spatial profiles were consistently observed to be accurately localised to within decimetre 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.