## 5 Criteria for an 'Effective' RTSSI Visualisation Framework  
So far, we have found heuristic arguments unsatisfactory for appraisal of RTI methods in a RTSSI context. Significant disparity exists between respective operating principles, deployment methodologies and output conveyance; not least in the inherent multifaceted and context-dependant grounds for suitability and performance comparison. Two specific examples would include: (i) the nature/variety of detectable features and (ii) assessment timescale. The basis for our criteria is twofold. First, we recognise each RTI method discussed exhibits at least one critical limitation (Table 1). Logically, an effective RTSSI visualisation framework would not share any, meaning an innovation directly addressing either would have significant impact on the current RTI hardware market. This motivates our novel formulation of well-defined but sufficiently general criteria for an 'effective' RTSSI visualisation framework. We illustrate the benefits visually using a conceptual network diagram (Figure 10). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Method** & **Critical Limitations for RTSSI** & \\ \hline Visual & \multirow{2}{*}{Lack necessary penetrative capability to directly visualise subsurface.} \\ Laser & & \\ \hline Therm. & \multirow{2}{*}{\ \begin{tabular}{c} Acoustic \\ Radar \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} Information loss (2D projections of 3D \\ features) limits survey faithfulness. \\ \end{tabular}  } & Skew from ambient temperature variations. \\ \cline{1-1} \cl  
Second, our creation of a Category Connection Matrix (CCM) from encountered literature (Table 2)--inspired by the hybrid workflow matrix presented in [209])--highlights emerging research trends, which we use to infer future research avenues in RTSSI. The categories extracted inform where research attention is currently most directed. Furthermore, collectively, motivation for all relevant studies is to contribute to producing the most effective RTSSI visualisation framework possible. Thus, each identified research category must align with at least one criterion. From this, we formulate our proposed criteria for an 'effective' RTSSI visualisation framework, which considers:  
Data Acquisition:  
1. **Completeness--**Uninterrupted scan coverage should be achieved to record the full extent of the influential regions of the tunnel subsurface. Current methods either lack deep penetrative capability or exhibit bind spots due high localisation of scans or geometry curvature. 2. **Duration--**Survey execution should balance acquisition speeds with recovered data quality (i.e., resolution, distortions) ensuring inspections and repairs cause minimal network disruption. A rapid low-quality scan limits inspection closure, but misinformed repairs take longer to fix and vice versa. Information Conveyance:  
Figure 10: Using criteria greatly simplifies comparison networks. (**a**) Heuristic comparisons are irregular and multi-directional networks. (**b**) Criteria inclusion collapses the network to be regularised and unidirectional. **Accessibility/Interpretive Clarity--**A railway network end-user who is not a specialist in the utilised RTSSI technique(s) (e.g., planner) should independently be able to understand and make informed decisions based on visualisation output (consider radargrams the antithesis to this, containing considerable but mostly incomprehensible information). 2. **Faithfulness--**Inconsistency between the physical subsurface geometry undergoing inspection and corresponding representation within the visualisation medium should be kept to a minimum. An example of unfaithful conveyance is how overhead structures can confusingly appear as below-ground features (airwaves) in radargrams [93, 210]. 3. **Interactivity--**Visualisations should react intuitively to end-user engagement in ways that make surveys more ergonomic, efficient and versatile. Again consider radargrams; time-slicing in pseudo-3 DGPR conveys depth more ergonomically than viewing am isolated B-scan.