A numerical investigation into the accuracy of determining dielectric properties and thicknesses of pavement layers using reflection amplitude GPR data

In processing GPR data from road surveys the amplitudes of reflection data have often been used in obtaining an estimate of the dielectric properties of pavement layers and consequently an estimate of the layers´ thicknesses. The values of the dielectric constants of the layers are estimated using a recursive procedure based on assumptions of plane wave propagation, one dimensional target geometry and that all media probed by the GPR are low-loss. In practice, this kind of data processing requires the use of a calibration procedure in the field which entails the use of a metal sheet placed on the surface of the ground. This is used in order to provide a reference reflected GPIl amplitude by assuming that the reflection coefficient of the metal sheet is known. This reference information is then used in the recursive determination of reflection coefficients of the layer interfaces which are then used to obtain values for the dielectric constants of the layers. Although, the assumption of one dimlensional geometry (layered earth) appears to be a good approximation for at least short sections of GPR road data the other assumptions involved in establishing the processing procedure may not be as valid and therefore they may have an effect on the accuracy of the final layer thickness calculations. This paper presents the results of a numerical investigation into the accuracy of such processing procedure. Both, a two dimensional and a three dimensional numerical simulators for GPIt have been employed to investigate the accuracy with which valuies of dielectric constants of layered models can be estimated fronn simulated reflection amplitude GPR data. This relates to the accuracy of the calculation of the layers´ thicknesses which it shoiild be noted that is the important engineering parameter that needs to be determined by the GPR survey. Although, )he two dimensional models employ a theoretical line source as a model for the GPR´s transducers the three dimensional ones- employ a complete model of a GPR antenna. The GPR simulators are based on the finite-difference time-domain method.