## 2 Materials and Methods
### Algorithms for the Primary Processing of GPR Data
Modern research focuses on modeling the behavior of AGC under interference conditions and developing methods to enhance its robustness. The next step is the delineation of road surface layer boundaries, as illustrated in Figure 5.  
For further details on layer boundary selection, refer to [31]. The layer boundaries were identified based on the white color, corresponding to local minima. The boundaries of only three layers were successfully delineated, ranging from the blue layer to the pink layer, with the green layer representing air. The boundary between the third and fourth layers remains indistinguishable due to signal attenuation. Figure 4: Radar profile after applying AGC. Figure 3: Radar profile after applying mean subtraction. Profiles are classified into two types based on the morphology of the layer boundaries:
* An acceptable profile refers to a georadar section in which the reflected layer boundaries exhibit a relatively uniform and horizontal shape, lacking pronounced anomalies. Such boundaries correspond to the proper construction of road structures in compliance with regulatory standards for layer thickness and density;
* A problematic profile is characterized by localized irregularities such as bulges or concavities in the reflective boundaries, which may indicate substrate heterogeneity, the presence of defects (e.g., binder leaching, subsidence, or voids), or non-compliance with construction technology. Subsidence typically occurs when the underlying layer exhibits high moisture content or due to the presence of groundwater. Concavities can also form when accumulated moisture in the underground structure freezes during winter. The main acceptance criteria include  
* the stable shape of the reflective boundaries, free from distortions and displacements exceeding 10-15% of the average layer thickness;
* the absence of local signal amplification and attenuation;
* compliance of the layer geometry with design documentation or typical pavement structure specifications. We will examine examples of both acceptable and problematic profiles. Figure 6 illustrates a problematic profile where the boundary of a concave layer is evident. Figure 7 presents another problematic profile, characterized by the boundary of a convex layer, while Figure 8 demonstrates an acceptable profile. It is important to note that, by default, the CartScan software assigns a relative dielectric permittivity of 5 to each layer in Figure 5. Consequently, the depth values along the \(Oz\)-axis for the layer boundaries in Figures 5-8 exhibit inaccuracies. However, the a priori thickness of each layer can be manually adjusted, allowing for corrections to the layer thickness in the radar image profiles. Figure 5: Radar profile after delineating layer boundaries. According to road pavement standards, the top layer consists of fine-aggregate asphalt with a thickness of 0.07 m. This is followed by a coarse-aggregate asphalt concrete layer with a thickness of 0.08 m. Beneath these layers are crushed stone layers: one with a grain size of 4 mm and a thickness of 0.3 m and another with a grain size of 8 mm and a thickness of 0.25 m. Finally, the subgrade layer consists of natural soil. The dielectric permittivity of each layer can be estimated based on its thickness. This approach, which determines the dielectric permittivity using layer thickness, is referred to as the engineering method. Figure 8: Example of an acceptable profile. (**a**) An example of an acceptable profile obtained after processing; (**b**) an example of an acceptable profile with reconstructed layers. Figure 6: Example of a problematic profile (concave layer). (**a**) An example of a problematic profile obtained after processing; (**b**) an example of a problematic profile with reconstructed layers. Figure 2.0 Example of a problematic profile. (convex layer). (**a**) An example of a problematic profile obtained after processing; (**b**) an example of a problematic profile with reconstructed layers.