## 3 Materials and Methods
### Data Processing  
The data processing has been carried out using the specialized GPR data processing software, ReflexW (v.9.5), and follows a sequence of filters and gain function applications. Even though the processing flow has some similarities between the data collected on the two dams, the use of filters has been adapted to the characteristics of each dam (Table A2). The rationale behind the steps for data processing was to improve the signal/noise ratio. First, we remove unwanted noise from the data with the application of 1D filters (which act on individual traces) and 2D filters (applied on all the traces), after which the gain functions were applied to amplify and highlight areas of interest; the last step consisted of migrating the data to convert from the time domain to the distance domain, and reducing the aspect of the hyperbolas moving the radargrams closer to the shape of the features in reality. The objective of the data processing steps chosen was to maximize signal/noise ratio to highlight data closer to the central frequency and eliminate information from deeper layers where the data are unclear due to the high attenuation.  
The processing flow followed similar steps and functions for the data collected on both dams, where the radargrams recorded a range of 130 ns in the case of the Gura Apelor dam and 180 ns in the case of Herculane dam. The steps are as follows:  
1. Trace removal using the remove range function, removing the first section from the data (only in the Herculane dam radargrams) covering the data collected over the spillways, which was interfering with the highlighting of other smaller features due to the strong reflection and reverberation effect on the data;
2. Adjusting the zero time with the move startime function;
3. Removing low-frequency noise using the subtract-mean (dewowo) function;
4. Removing low and high frequencies outside the range of the antenna frequency using a butterworth bandpass filter;
5. Removing system-based coherent noise ringing by subtracting of an average trace with the background removal function;
6. Amplifying signals using the gain function, energy decay;
7. For converting from the time domain to the distance domain, the fk-Stolt migration tool was used (only for the Herculane dam), using a velocity of 0.1 m/ns.  
No migration has been carried out on the data from the Gura Apelor dam due to the complex structure and the high variation in velocities throughout the whole radargram. Velocity analysis was carried out using the hyperbola fitting tool in order to approximate velocities in certain areas of interest.  
In the case of the Gura Apelor dam, the profiles were approximately 450 m long, which makes visualization difficult for certain features. Therefore, the files were cut on several intervals in order to analyze and highlight features of interest. For the Herculane dam on the other hand, even though the profiles are not as long, at approximately 170 m, the trace removal tool has been used to remove the data collected over the spillway area, which was interfering with other anomalies visible in the data due to their strong visual effect on the data. The remove range tool was also used to extract areas from different intervals to focus on smaller, more specific areas.