## 3 RTI Data Acquisition Methods
### Radar Methods  
Ground Penetrating Radar (GPR) directs radio pulse emissions at the tunnel intrados, which penetrate and partially backscatter off strong dielectric gradients in the subsurface associated with features of interest [10; 70; 71; 72; 73]. Pulsed Radar (PR) samples consecutively emit wideband waveforms to measure backscatter in the time domain. Step-Frequency Continuous Wave (SFCW) radar incrementally sweeps an emission sinusoid through a predefined frequency band; the Fourier Spectrum of the returning signal is directly ascertained in the frequency domain by frequency-wise inspection of return signal strength [74]. In RTI, systems fall under three categories:  
Figure 3: TOSCA-FI Software Platform: 2D heatmap overlays on a 3D digital bridge model [60].  
* **Trolley-Mounted**[75, 76, 77] (Figure 4a)--Units commonly feature interchangeable air-coupled antenna of differing frequencies to facilitate trade-off between penetration depth and output image resolution [28]. However, motorisation is infrequent, scans are unidirectional (typically railbed only) and offer no protection to operatives;
* **Handheld**[78, 79, 80, 81]--Compact ground-coupled scanners guided by hand can achieve real-time scanning of curved tunnel sidewalls and crown. Typically restricted by limited penetrative depth (d < 50 cm), coverage speed (under m\({}^{2}\) h\({}^{-1}\)) and gantry requirement to reach high surfaces make units impractical for full RTI;
* **Vehicle-Mounted**[82, 83, 84, 85, 86] (Figure 4b)--Multidirectional fixed antenna units attached to locomotives, rolling stock or RRVs. Although capable of data capture at speeds ranging from to 50-30 km h\({}^{-1}\), fixed directionality guarantees blind spot and air-coupling reduces achievable penetrative depth.  
Radargrams (B-scans) traditionally convey survey output, which although encoding multiple feature characteristics (e.g., depth, extent, orientation, heterogeneity, etc.) [70, 73], demand extensive digital filtering [87, 88, 89, 90, 91] and may still contain abundant false-artifacts (e.g., ringing effects from rails, airwaves from overhead surfaces) [92, 93, 94], making them notoriously unintuitive. Increasing clarity via orthogonal intersection [95, 96] (Figure 5a) and parallel stacking [97, 98] (Figure 5b) of B-scans to form C-scans is now well-established practice in many commercially available GPR processing software packages [99, 100, 101, 102, 103]. Collectively, we denote this pseudo-3 DGPR. We stipulate 'pseudo' to emphasise the inherent information loss resulting from 2D projections of 3D tomography. Beneficially, C-scan datasets can exploit time-slicing [104, 105, 106] (Figure 6a,b), in situ transparency filtering [107] and false-colouration [108] to improve conveyance of 3D forms. More recent investigations into true-3D volumetric reconstruction [98, 109, 110, 111, 112, 113, 114, 115, 116, 117] (Figure 6c) show promise for advances  
Figure 4: A selection of leading GPR systems. **(a)** Ballast fouling inspection trolley with selectable-frequency GPR antenna module [77]. **(b)** Loco-mounted fixed directional air-coupled GPR antennas.  
towards practically viable fully immersive GPR-based subsurface inspection surveys (undertaken in fully digitised virtual survey environments) [118, 119, 120, 121, 80, 122]. Whilst conceivable and under trial, achieving mainstream commercial deployment will require blind spot alleviation through adoption of rotary scan motion complementary to tunnel curvature, possibly similar to the superposition of concentric cylindrical 'look-ahead' radargrams pioneered by the TULIPS system [123] for tunnel excavation monitoring. Pursuit of blind spot elevation is therefore of critical importance if comprehensive RTSSI profiles are to be captured.