## 1 Introduction  
Climate change has emerged as one of the most pressing societal challenges of the 21 st century, and transitioning to low-carbon energy sources is an essential aspect of tackling this challenge [1]. The deployment of new energy sources is a major lever for decarbonization [2], but established forms of energy, such as existing hydropower dams, are also crucial for this purpose [3].  
In Romania, dams have been a core energy supplier for decades. There are currently 545 known hydropower dams in the country [4] that provide over 10% of the country's primary energy [5] and over a quarter of the country's electricity [6].  
While hydropower dams represent a sensitive subject due to the required tradeoff between clean energy and river conservation, maintaining them represents a very important aspect, not only because they represent massive constructions, but also because of Romania's commitment to the COP 21 Paris Agreement, where Romania pledged to achieve the 2 degrees target by 2050, and hydropower energy represents a form of green energy that is essential for this goal. This largely involves maintaining existing dams rather than building new ones.  
Current legislation does not mandate geophysical surveys as a requirement for tracking displacements, deformations, and degradation, even though the law requires the dams to be regularly monitored using a variety of methods (most significantly, annual topo-geodetical surveys). Therefore, there is a need for testing and finding the best methodologies for applying new methods, especially geophysical methods, which have been shown to provide a variety of useful information which can aid with diagnosing different types of degradation of the dams.  
Aging and degradation are well-known problems associated with dams [7; 8; 9], but early geophysical detection of potential cracks, fissures, and other faults in the dam structure could offer awareness on such potential faults and could direct early intervention that could reduce costs and prevent further degradation. The use of geophysical methods in dam site monitoring must be reliable and robust both in earthfill [10] and in concrete dam sites, as well as both on the dams themselves [11; 12] and on the tailings and other areas around the dams [13; 14].  
However, the GPR method has not been exploited to the fullest in this context. While the literature features significant examples of application of GPR applied in civil engineering to infrastructure like roads, for example [15; 16; 17; 18], the potential of GPR on dams is still insufficiently explored in the context of hydropower dams.  
To our knowledge, this is the first time GPR surveys were applied on the Gura Apelor and Herculane dams in Romania, and marks the first attempt to use this method to collect fast data on the crest of the dams, using longitudinal profiles with the purpose of exploring their suitability for the different types of dams normally found in Romania. We aim to provide information which can be integrated with other monitoring methods to provide a fast assessment of any inhomogeneities and deteriorations in the shallow part of the dam's crest, as well as to explore limitations of the method on these types of dams.