## 2 Theoretical Background  
A GPR device typically consists of a transmitter, a receiver antenna, a data acquisition and measurement control unit, a distance measuring wheel, and, optionally, a GPS. The transmitter of a GPR device emits high-frequency electromagnetic impulses. These electromagnetic waves are partially absorbed, partially refracted, and partially reflected in the analysed medium. The returning electromagnetic signal is stored by the receiver unit depending based on the time received. The transmission of radar waves depends on the electric and magnetic characteristics of the analysed medium [26]. Among these characteristics, the two most typical are the relative electric permittivity (dielectric constant) and the specific conductance. The in situ relative permittivity of asphalt layers can be determined in different ways, with most studies applying the Surface Reflection (SR) method [27; 28]. The advantage of the SR method is that the measured AC surface reflection depends only on the parameters of the upper layer when the analysed layer is thicker than 30 mm [12]. There are values reported in the literature for the relative permittivity of layers in road pavement structures (asphal: 4-8, concrete: 9-11). By applying these values, layer thicknesses can be easily calculated without the need for any drilled core samples. The difficulty of this method is that the relative permittivity of road construction materials can be variable, depending on the moisture content, the mineral composition, the temperature, the compactness, and the measuring frequency. The reason is that materials used for road construction (soil, crushed stone, asphalt, and concrete) are mainly three-phased disperse systems from a chemical point of view, in which the proportions of liquid and gas phases depend heavily on weather factors. Moreover, parameters of the solid phase (bitumen) can change over time. Consequently, GPR measurements are not only suitable for determining layer thicknesses but also for estimating the bulk density and air void content of asphalt mixtures, as well as for monitoring the compactness and moisture content of pavement subgrade to detect road distresses like cracks [29; 30; 31; 32]. In these cases, an important prerequisite is knowledge of the moisture content and material quality of the disperse system under study, at least approximately; however, even if this condition is not fulfilled, the measured results still offer the possibility of conducting relative spatial comparison, which, by itself, can provide very useful information. [PERSON] (1997) [33] was one of the first European researchers to apply GPR to measure the density of asphalt pavement structures. His research work was based on the principle that the relative permittivity of a pavement structure is presumably a function of the relative permittivity of its constituents; therefore, changes in the volumetric proportions of these phase constituents can be estimated by measuring the relative permittivity of the total pavement structure. The constituents of an asphalt mix are bitumen, mineral aggregates, air, and sometimes water (Figure 1). The relative permittivity of the bitumen is usually between 2.6 and 2.8, the relative permittivity of the crushed dry aggregates is usually between 5.5 and 6.5, and the relative permittivity of air is approximately 1. Although the relative permittivity of water depends on its chemical characterization capacity, it can be assumed that, in the case of a new pavement structure, water bears no effect on the measurement of the structure's relative permittivity. The compaction of asphalt decreases with an increase in the air proportion (having a low relative permittivity) and increases with an increase in the volumetric proportions of the bitumen and aggregates; in the latter case, the relative permittivity of asphalt also increases. GPR data collected on different roads have proven that a decrease in the value of the relative permittivity indicates density problems.