## 3 Materials and Methods
### Data Acquisition
#### 3.2.1 Gura Apelor Dam  
There are several key considerations for data acquisition at this dam: the active environment of the dam's placement, the deformability characteristic to this type of dam, the public road on top of the dam, the visible surface deteriorations, and the subsidence trends highlighted in publicly available leveling data. Given all this, the objective of the surveys carried out on the Gura Apelor dam was to assess and highlight any deformations in the top layers of the dam's crest (e.g., subsidence and/or uplift) and explore the method'ssuitability for detecting discontinuities or inhomogeneities in the first few meters below the crest.  
The embankment dam has a clay core, and clay is usually unsuitable for GPR surveys. However, this type of clay (described at Section 2.2) should allow for good signal penetration, particularly due to the small fraction of water from pores, which can be a limiting factor for GPR's ability to detect features of interest [39].  
Therefore, considering all of the above, four profiles (GA1-GA4) were carried out longitudinally on the whole length of the dam's crest from one end of the dam to the other (Figure 6--left). The profiles were positioned symmetrically from the center of the dam, starting from the access road side towards the spillway gallery. In terms of distance between the profiles, two profiles are located in the middle with a distance between them of 1 m, positioned on each side of the middle line of the dam's crest (Figure 6), and two other profiles are positioned closer to the edges of the crest (1 m away from the kerb and 3 m away from the closest middle profile).  
The purpose of this profile arrangement along the crest was to obtain a general overview of the whole crest and assess the lateral continuity of any features of note. Having the profiles at 1 m away from the lateral sides of the crest also allows for minimizing any interferences in the data resulting from reflections of the nearby structures (fences, raised curb, etc.).  
The acquisition parameters (Table A1) included a gain function, negative close to the surface to reduce the effect of the high reflective features which normally appear in GPR data, and 2 more gain points to allow for collecting data with depth. All of the other parameters (range, samples, scan/unit, etc.) were adjusted to allow for signal penetration at a maximum depth possible in the given material (clay) without losing too much information from the surface as well.
two profiles was positioned in the central part of the crest, with the next one positioned on the side facing the downstream, 1 m away from the first profile (Figure 6--right). Two data sets were collected with the same antenna but different acquisition parameters (Table A1). The first set (S1) was aimed at obtaining more information from the depth, including both low-pass and high-pass filters, as well as a negative gain on the top layer with higher gain applied with depth, and a high stacking number (stacking = 64), whereas the second data set (S2) focused on information closer to the surface and had no filters applied, no stacking, and no gain. The metallic plates visible on the surface (Figure 6--right) were purposefully avoided, therefore ending up with a profile closer to the raised metallic fence.