## 3 Experiment Analysis of Transition Zones
### Introduction of GPR  
GPR uses a radio wave source to transmit a pulse of electromagnetic energy into the inspected medium [33]. Its principle in railway measurements is shown in Figure 2. GPR is an effective and non-invasive tool for mapping railway structures and analysing subsurface conditions. The reflected energy, originating from the interfaces between materials of different dielectric properties, is received and recorded for analysis. GPR data consists of changes in reflection amplitude, changes in the arrival time of specific reflections, and signal attenuation [32; 38]. The method provides a continuous profile of the thickness and properties of railway structures, which can be used to analyse the quality of track substructures, such as the moisture susceptibility of ballast and subballast, fouling of ballast, layer deformation, and mud pumping [39; 40; 41]. GPR measurements for this case study were performed using a GSSI SIR-30 GPR system, manufactured by Geophysical Survey Systems Inc., Nashua, NH, USA, including a 400 MHz antenna attached to a VR-Track Ltd Tka-8 maintenance engine, as illustrated in Figure 3.  
The GPR antenna can be lifted to 0.3 m above the sleeper level and the maximum survey speed is up to 160 km/h, depending on data collection settings. The data collection rate was controlled by a distance measurement instrument. During the survey, GPS coordinates and digital video were recorded using Rail Doctor(tm) Camlink software, developed by Roadscanners Oy, Rovaniemi, Finland. The processing, visualization, interpretation, and analysis of the GPR data are performed using the Rail Doctor software (version 3.2), developed by Roadscanners Oy, Rovaniemi, Finland,2017. The measurement system has been calibrated against sample analysis [38] and pit tests [39]. A screenshot from the recorded video is shown in Figure 4a, wherein a transition zone consisting of embankment and a bridge can be found. A measurement result from GPR at the corresponding location is shown in Figure 4b. The right axis of Figure 4b indicates the depth, in meters, beneath the top of sleepers (or below rails). The ballast layer is located from 0 m to 0.6 m below rails, where the ballast at 0-0.25 m is between sleepers and 0.25-0.6 m is below sleepers. The subballast is located from 0.6 m to 1.2 m, and below 1.2 m is embankment and subgrade. It should be noted that sleepers are filtered during the data processing to obtain a better visualization of the data. The lateral axis of Figure 4b is the distance or location, which is in the form of \"km + m\". The two sides of the bridge in a transition zone are named as the embankment-bridge side and the bridge-embankment side in the paper depending on the train moving direction. The moving directions of the results presented in this paper are all from left to right. Therefore, the embankment-bridge side is always on the left side of bridges (before the bridges) and the bridge-embankment side on the right side (after the bridges). Figure 3: Railway engine with GPR system: (**a**) schematic diagram; (**b**) photograph. Figure 2: Measuring principle of ground penetrating radar (GPR). \(A_{1}\) stands for the amplitude of the reflection between the sleeper and ballast; \(A_{2}\) stands for the amplitude of the reflection between the ballast and subballast; \(t_{1}\) stands for the travel time in ballast; \(t_{2}\) stands for the travel time in subballast. Dielectric properties decide the amplitude, arrival time, and signal attenuation of reflections, and they are different with a higher content of fines and water [42]. The dielectric constant and the velocity of the signal in various materials are shown in Table 1[43]. This difference can be detected by analysing the GPR signal [38,43,44]. As a result, the distribution of the relative moisture susceptibility in ballast and subballast can be obtained.