Numerical and experimental surveys on the GPR early-time signal features for the evaluation of shallow-soil permittivity

Experimental investigations and numerical simulations are considered with the aim of analyzing the features related to the recently-proposed 'early-time signal' (ETS) method, which could allow for efficient non-destructive testing (NDT) of shallow subsurface by means of simple fixed bistatic configurations of Ground Penetrating Radar (GPR). The possibility of monitoring the soil dielectric-constant and conductivity features (i.e., the real and imaginary parts of the permittivity) through the first-arrival signal attributes in ground-coupled radars is first analyzed with an accurate and versatile approach based on an electromagnetic (EM) Computer-Aided-Design (CAD) tool. Our realistic simulation of the scenario for ETS operation includes the design of the appropriate GPR antennas, the choice of the transmitted waveforms, and the description of the non-homogeneous environment. Thus, it is possible to predict how the various physical parameters (chosen input signals, type, location, and distance of the radiators, etc.) affect the ETS characteristics. This efficient analysis also enables for the identification of which are the more revealing signal attributes to give predictable correlation with the ground permittivity values, and what kind of functional relations can be outlined among the involved parameters. Related experimental investigations from ad-hoc laboratory-scale measurements with commercial GPR systems are also carried out and discussed in connection with what derived theoretically as regards the effects both on the dielectric constant and on the conductivity of the media. Essential information is hence provided for the reliability of this innovative approach in practical testing cases.