Ground penetrating radar (GPR) is a non-destructive and continuous electromagnetic (EM) detection technique for civil and environmental parameter measurement applications such as pavement condition and soil property characterization. This technique is based on the measurement of the travel time and reflection amplitude of a short EM pulse, which are functions of medium properties. Most GPR measurements of sub-layer thickness are conducted based on the priori knowledge of dielectric constants of the pavement materials. And actually, the dielectric constant is an unknown but important parameter in the applications. For some applications, the dielectric constants are estimated based on manuals or tables that can only provide rough results not the real changes of pavement materials. In some other cases, the dielectric constants are estimated by using the surface reflectivity information. However, such method is not applicable for rough surface and ground-coupled GPR applications.
Compared to the air-launching GPR mode, the ground-coupled mode is more complicated because of the coupling effect between the antennas and ground. In this paper, numerical simulations about the wave propagation paths of the ground-coupled GPR are conducted. The simulation results reveal some interesting ray paths of GPRs in the ground-coupled mode. And based on the simulation results, new methods are introduced for calculating the pavement dielectric constant and thickness directly from the ground-coupled GPR data. Finally, applications and field test results for pavement evaluation are presented.
Many theoretical studies have been reported on applications of ground penetrating radar (GPR) system to detect the permittivity and thickness of subsurface layers. However, to develop a GPR system that can accurately measure the thickness and the permittivity simultaneously is not a straightforward task. The main difficulty of quantitative thickness measurement is that the reflected wave from the subsurface interface is very weak compared to the directly coupled waves. The reflected signal may be completely submerged into the strong direct waves. Secondly, the inversion computation from measured data is very noise sensitive. In this paper, we present the development of a frequency-modulated-continuous-wave (FMCW) radar for quantitative layer thickness measurement. A new mathematical model for the calculation of depth and permittivity from the measured electromagnetic data is presented. The new model is based on the time delay between the direct wave and the reflected wave recorded by a bistatic radar. The data inversion algorithm considers the influences from air-ground interface. It is found that neglecting the air layer effects as the case applied in seismic analysis, the inversion will not be correct. This is because the electromagnetic rays from the GPR take different propagation path from straight or curved ray in seismic-like analysis. Ray path searching must be included in the calculation algorithm. With the consideration of wave path, the experimental results agree well with the actual values either in field test of in laboratory test.