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.