## 5 Conclusions  
In this paper, we describe two 2D GPR surveys carried out on the Herculane concrete dam and the embankment (rockfill with a clay core) Gura Apelor dam in Romania, marking the first time the GPR method is used on these dams. We attempt to explore the method's feasibility on both types of dams in Romania by using fast non-invasive longitudinal profiles across the dams' crest to highlight areas of possible damages or deteriorations and to determine any possible limitations of the method. Although we focus on dams in Romania, the method is applicable to similar environments in other parts of the world as well. The solid concrete surfaces of both dams ensured consistent close contact between the antenna and the surface. This facilitated excellent signal penetration during the Ground-Penetrating Radar (GPR) surveys and suggests that dam crests are a suitable environment for such surveys. The GPR technique carried out on the Gura Apelor dam proved to be very useful in providing an overview image of the first 10 m below the crest's surface, even though embankment dams are usually more challenging in terms of GPR applications due to their high clay content which absorbs the electromagnetic signal. In this case, choosing the 200 MHz antenna proved to be a good choice, offering a useful compromise between the resolution and depth of penetration. Surveys on the crest of the dam manage to highlight a subsidence area which appears to correlate with the vertical displacements recorded through annual topographical leveling surveys for dam monitoring, enhancing the detail and understanding of the dam's structure, including aspects not visible on the surface. As well as the subsidence area, the GPR also highlights different layers, which most probably represent the different materials used on top of the crest of the dam, to seal the core and to offer support and foundation for the precast concrete blocks covering the crest to be used as a public road. Even though these are constant across all of the profiles and the area marked as subsidence shows an inhomogeneity character at the lowest point of the horizon no. 1, it is not clear what may be the cause of the subsidence; one set of GPR data collected on a single day cannot provide information about the variation in time. Therefore, even though these longitudinal profiles on the dam's crest represent a fast approach to create or provide an overview of the internal structure beneath the dam's crest, there is a need for such surveys to be carried out regularly and in different environments, as well as integrating the data with other methods (UAV--LiDAR, photogrammetry, other geophysical methods, topo-geodetic displacement surveys, etc.) for increasing the understanding of the development, structure, and causes of this aspect. The concrete dam, Herculane, highlighted some of the difficulties that can emerge when acquiring GPR data in this context when using lower-frequency antennas like 200 MHz. There is a tradeoff as lower frequency antennas may not provide valuable information regarding minute features such as rebars; however, it was possible here to highlight areas in the crest where the constituent rebars at the surface appear to be affected by humidity (most probably through the surface cracks) which might even be affected by corrosion. This aspect is very important in maintaining the dams in a good state, as untreated degradation at the surface has a tendency to evolve and affect larger surfaces in time, therefore increasing separation costs. Other possible areas of interest have been highlighted, and a possible inhomogeneity appears at lower levels; however, the main outcome is related to potential corrosotions and damages at the near surface related to the cracks visible on the dam's crest surface. There is, however, a need for more regular profiles to validate the finding as well as more certainty regarding possible damage in the dam's body related to the inhomogeneity. A priori knowledge in concrete dams is not a guarantee.