## 3 Materials and Methods
### Ground-Penetrating Radar Method  
Ground-Penetrating Radar (GPR) is a non-destructive imaging technique used in civil engineering, environmental engineering, and other geophysical applications. It works by emitting electromagnetic (EM) waves into the ground and analyzing the reflected signals to create an image of subsurface features. GPR is particularly suited for engineering applications as it is non-invasive, fast, and high-resolution. In dams, it can be used for concrete and rebar assessment [7; 8; 12], embankment investigation [34], leak detection [36], and monitoring of subsidence or other deformations [30; 37]. The GPR method uses electro-magnetic signals to detect underground features based on contrasts in the relative permittivity property of the materials. The basic principle behind GPR is straightforward: an antenna with a preestablished central frequency sends electromagnetic pulses into the ground, while another antenna detects the time and shape of the reflected signal. The signal penetrates into the subsurface and then reflects back to the surface; any interference (reflection, refraction, diffraction) creates a detectable signal, and these data can be used to create a profile of the subsurface (Figure 5). An antenna with a determined central frequency will send electromagnetic impulses into the ground, while another antenna will detect the reflections of the signals from the interference of two materials with different relative permittivity values. A key parameter of GPR antennas is the central frequency; common antenna frequencies typically vary between 50 MHz and 4 Ghz [38]. As it is often the case in geophysics, this parameter generates a trade-off between depth of investigation and resolution: higher frequency offers higher resolution but lower depth of penetration. In the case of studies on dam crests, a low frequency is typically preferable, as it offers an opportunity to inspect potential faults deeper in the structure. Although GPR measurements can also be carried vertically, on the structure of the dam [36], this is challenging logistically, and here only horizontal measurements on the crest of the dam were carried out. GPR is particularly suitable in engineering applications on paved surfaces because a good contact between the antenna and the surface is essential for high-quality data [38], and paved surfaces can provide good quality contact [38]. Man-made materials are also compact and often exhibit strong relative permittivity contrasts [38], which enables detection. Cracks  
Figure 5: Representation of the basic operation principles of the GPR data collection and functionality. and fissures are also expected to produce notable detectable contrasts if they are large enough to be detected by the footprint of the antenna. There are several examples in the literature where GPR is used for comparable applications. Given its high resolution and applicability on paved surfaces, Ground-Penetrating Radar (GPR) is excellently suited for detecting even minute faults in dam elements [34; 35; 37; 39]. In dam monitoring, GPR has already shown its potential to detect voids and cracks inside the dam's body from several dams located in China [10; 11] and areas of internal erosion in dams where the intense leakage was already a known problem on several dams located in Sweden [10], which did not fail due to the self-healing effects of the materials in the filters. However, even though the GPR method has a generally high resolution compared to other geophysical methods, it is very sensitive to site conditions [10] and especially sensitive to clays and areas with high water content which absorbs the radar energy, not allowing for the recording of any useful reflections from underground. Even though it is sensitive to clays, it has been shown that the GPR method could still provide significant information in some cases, described by geometrical changes and horizontal inhomogeneities when applied on embankment dams in Italy affected by surface cracks and deterioration [34].