Railway Tunnel SubSurface Inspection (RTSSI) is essential for targeted structural maintenance. ‘Effective’ detection, localisation and characterisation of fully concealed features (i.e., assets, defects) is the primary challenge faced by RTSSI engineers, particularly in historic masonry tunnels. Clear conveyance and communication of gathered information to end-users poses the less frequently considered secondary challenge. The purpose of this review is to establish the current state of the art in RTSSI data acquisition and information conveyance schemes, in turn formalising exactly what constitutes an ‘effective’ RTSSI visualisation framework. From this knowledge gaps, trends in leading RTSSI research and opportunities for future development are explored. Literary analysis of over 300 resources (identified using the 360-degree search method) informs data acquisition system operation principles, common strengths and limitations, alongside leading studies and commercial tools. Similar rigor is adopted to appraise leading information conveyance schemes. This provides a comprehensive whilst critical review of present research and future development opportunities within the field. This review highlights common shortcomings shared by multiple methods for RTSSI, which are used to formulate robust criteria for a contextually ‘effective’ visualisation framework. Although no current process is deemed fully effective; a feasible hybridised framework capable of meeting all stipulated criteria is proposed based on identified future research avenues. Scope for novel analysis of helical point cloud subsurface datasets obtained by a new rotating ground penetrating radar antenna is of notable interest.

Ground Penetrating Radar (GPR) facilitates the detection and localisation 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 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 collection respectively. Following initial 3D grid alignment and smoothing, data undergoes 3D Stolt migration, normalisation 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, 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 large-scale, 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.

The high fidelity display of digital medical radiographs requires devices with high detail (3000 X 3500 arrays, 120 micrometers pixels), high brightness (2000 cd/m²), and high dynamic range (400). Medical radiographic film meets these requirements when transilluminated with bright illuminators. Currently available electronic displays using CRT technology are not able to provide the needed fidelity. New flat panel emissive display technologies offer potential solutions, particularly plasma displays and vacuum microelectronic displays. The NCAICM at Sandia National Laboratories is focusing particular attention on emissive display technologies. Commercial flat panel, color systems with improved fidelity are now becoming available. Static, monochrome designs using vacuum microelectronic technology offer the potential to provide a high fidelity display for medical radiography which meets these requirements.

Effective visualisation of railway tunnel subsurface features (e.g. voids, utilities) provides critical insight into structural health and underpins planning of essential targeted predictive maintenance. Subsurface visualisation here utilises a rotating ground penetrating radar antenna system for 360° point cloud data capture. This technology has been constructed by our industry partner Railview Ltd, and requires the development of complimentary signal processing algorithms to improve feature localisation. The main novelty of this work is extension of Shrestha and Arai’s Combined Processing Method (CPM) to 360° Ground Penetrating Radar (360GPR) datasets, for first-time application in the context of railway tunnel structural health inspection. Initial experimental acquisition of a sample rotational transect for CPM enhancement is achieved by scanning a test section of tunnel sidewall - featuring predefined target geometry - with the rotating antenna. Next, frequency data separately undergo Inverse Fast Fourier Transform (IFFT) and Multiple Signal Classification (MUSIC) processing to recover temporal responses. Numerical implementation steps are explicitly provided for both MUSIC and two associated spatial smoothing algorithms, addressing an identified information deficit in the field. Described IFFT amplitude is combined with high spatial resolution of MUSIC via the CPM relation. Finally, temporal responses are compared qualitatively and quantitatively, evidencing the significant enhancement capabilities of CPM.