I. van den Bosch

Modeling of ground-penetrating Radar for accurate characterization of subsurface electric properties

The possibility to estimate accurately the subsurface electric properties from ground-penetrating radar (GPR) signals using inverse modeling is obstructed by the appropriateness of the forward model describing the GPR subsurface system. In this paper, we improved the recently developed approach of Lambot et al. whose success relies on a stepped-frequency continuous-wave (SFCW) radar combined with an off-ground monostatic transverse electromagnetic horn antenna. This radar configuration enables realistic and efficient forward modeling. We included in the initial model: 1) the multiple reflections occurring between the antenna and the soil surface using a positive feedback loop in the antenna block diagram and 2) the frequency dependence of the electric properties using a local linear approximation of the Debye model. The model was validated in laboratory conditions on a tank filled with a two-layered sand subject to different water contents. Results showed remarkable agreement between the measured and modeled Greens functions. Model inversion for the dielectric permittivity further demonstrated the accuracy of the method. Inversion for the electric conductivity led to less satisfactory results. However, a sensitivity analysis demonstrated the good stability properties of the inverse solution and put forward the necessity to reduce the remaining clutter by a factor 10. This may partly be achieved through a better characterization of the antenna transfer functions and by performing measurements in an environment without close extraneous scatterers.

Estimating soil electric properties from monostatic ground‐penetrating radar signal inversion in the frequency domain

[1] A new integrated approach for identifying the shallow subsurface electric properties from ground-penetrating radar (GPR) signal is proposed. It is based on an ultrawide band (UWB) stepped frequency continuous wave (SFCW) radar combined with a dielectric filled transverse electric and magnetic (TEM) horn antenna to be used off the ground in monostatic mode; that is, a single antenna is used as emitter and receiver. This radar configuration is appropriate for subsurface mapping and allows for an efficient and more realistic modeling of the radar-antenna-subsurface system. Forward modeling is based on linear system response functions and on the exact solution of the three-dimensional Maxwell equations for wave propagation in a horizontally multilayered medium representing the subsurface. Subsurface electric properties, i.e., dielectric permittivity and electric conductivity, are estimated by model inversion using the global multilevel coordinate search optimization algorithm combined sequentially with the local Nelder-Mead simplex algorithm (GMCS-NMS). Inversion of synthetic data and analysis of the corresponding response surfaces proved the uniqueness of the inverse solution. Laboratory experiments on a tank filled with a homogeneous sand subject to different water content levels further demonstrated the stability and accuracy of the solution toward measurement and modeling errors, particularly those associated with the dielectric permittivity. Inversion for the electric conductivity led to less satisfactory results. This was mainly attributed to the characterization of the frequency response of the antenna and to the high frequency dependence of the electric conductivity.

Electromagnetic inversion of GPR signals and subsequent hydrodynamic inversion to estimate effective vadose zone hydraulic properties

We combine electromagnetic inversion of ground penetrating radar (GPR) signals with hydrodynamic inverse modeling to identify the effective soil hydraulic properties of a sand in laboratory conditions. Ground penetrating radar provides soil moisture time series that are subsequently used as input in the hydrodynamic inverse procedure. The technique relies on an ultrawide band (UWB) stepped frequency continuous wave (SFCW) radar combined with an off-ground monostatic transverse electromagnetic (TEM) horn antenna. Ground penetrating radar signal forward modeling is based on the exact solution of the three-dimensional Maxwell equations for describing free wave propagation and on linear systems in series and parallel for describing wave propagation in the antenna. Water flow in the sand is described by the one-dimensional Richards equation using the Mualem-van Genuchten parameterization. Both model inversions are formulated by the classical least-squares problem and are performed iteratively using advanced global optimization techniques. Compared with time domain reflectometry (TDR), results demonstrated the appropriateness the GPR integrated approach to measure soil moisture remotely. In particular, the approach was found to be less sensitive to the inherent small-scale heterogeneities. Hydrodynamic inversion of soil moisture data led to hydraulic parameters agreeing reasonably well with direct measurements. The observed discrepancies were attributed to the different characterization scales and samples. The overall integrated approach offers great promise to map the effective hydraulic properties of the shallow subsurface at a high spatial resolution.

GPR design and modeling for identifying the shallow subsurface dielectric properties

A ground penetrating radar (GPR) system for identifying the shallow subsurface dielectric properties is proposed. It consists in a stepped frequency continuous wave (SFCW) radar operating in the 0.8-4 GHz ultrawide band combined with a dielectric filled TEM horn antenna to be used off ground in monostatic mode. This radar configuration is of practical interest since it responds to subsurface mapping requirements and allows for an efficient and realistic modeling of the radar-antenna-subsurface system. Forward modeling of the system is based on linear system response functions and on the exact solution of the three-dimensional Maxwell equations for damped wave propagation in a horizontally multilayered medium representing the shallow subsurface. The model is validated under controlled laboratory conditions. This model is destined to be inverted to reconstruct the depth dependent shallow subsurface dielectric properties from field observations.

Accurate and efficient modeling of monostatic GPR signal of dielectric targets buried in stratified media

This paper presents an accurate model of a monostatic stepped-frequency continuous-wave (SFCW) ground-penetrating radar (GPR). The model takes into account the multiple reflections occurring between the soil, target and antenna, which is a transverse electromagnetic (TEM) ultra-wide band (UWB) horn. The antenna radiation pattern is accounted for by a Huyghens cosinusoidal distribution of electric and magnetic current located on the aperture. The model is validated by experiments, involving dielectric targets embedded in a sandbox. These experiments validate altogether the radar modeling, as well as the MoM and the dyadic Greens functions (DGFs) used in the numerical algorithms.

Full-wave inversion of off-ground monostatic GPR signal focused on the surface reflection for identifying surface dielectric permittivity

S. Lambot*, L. Weihermullert, I. van den Boscht, M. Vancloosters and E.C. Slob* *Delft University of Technology Mijnbouwstraat 120, 2628 RX Delft, The Netherlands s.lambot @citg.tudelft.nl, e.c.slob@citg.tudelft.nl t Agrosphere Institute, ICG IV, Forschungszentrum lulich GmbH 52425 Jiilich, Germany l.weihermueller@ fz-juelich.de hlicrowave Laboratory, UniversitC catholique de Louvain Place du Levant 3, 1348 Louvain-la-Neuve, Belgium denbosch@emic.ucl.ac.be §Department of Environmental Sciences and Land Use Planning, UniversitC catholique de Louvain Croix du Sud 2 Box 2, 1348 Louvain-la-Neuve, Belgium vanclooster@geru.ucl.ac.be

Buried target signature extraction from ground-penetrating radar signal: measurements and simulations

Ground-penetrating radar (GPR) proves to be a very valuable tool in the field of humanitarian demining, especially for the detection of plastic land-mines. Recently, a monostatic stepped-frequency continuous-wave (SFCW) GPR, together with a conceptual model of the radar-antenna-soil system, has been developed for the characterization of the electromagnetic parameters of soil, i.e. dielectric permittivity (epsilon), magnetic permeability (mu) and electric conductivity (sigma). This approach is extended here to the extraction of the GPR signal and to modelling the signatures of buried targets. The equivalence principle is used to decompose the GPR signal into its soil and target-in-soil components, as well as to model the radar-soil-target system. It permits the soil contribution to be subtracted from the total GPR signal to provide the signature of the buried target. This signature is compared to simulations. For a proof of the concept, the GPR return signal from a buried metal sphere has been simulated using the Method of Moments and it shows good agreement with its measured counterpart. We also have extracted clean frequency- and time-domain signatures of a PMN-2 plastic mine embedded in a multilayered medium, subject to various water contents. The method is also applied to a B-scan above a buried conducting cylinder. Finally, a study of the main sources of errors in the extraction of the signature of a buried target shows that mistakes in antenna height measurement lead to errors more important than those due to misestimating the relative dielectric permittivity of the soil.

Accounting for multiple reflections and antenna radiation pattern in gpr signal modeling and experimental validation

This paper presents an accurate model of a monostatic stepped-frequency continuous-wave (SFCW) ground-penetrating radar (GPR). The model takes into account the multiple reflections occurring between the soil, target and antenna, which are a transverse electromagnetic (TEM) ultra-wide band (UWB) horn. Two equivalent current distributions representing the antenna radiation pattern are considered: a dipole of electric current located at the phase center of the antenna, and a Huyghens cosinusoidal distribution of electric and magnetic current located on the aperture. The model is validated by experiments, for which the targets are embedded within increasingly complex backgrounds: in free space, above a metal plane, and finally buried in a sandbox. These experiments validate altogether the radar modeling, as well as the MoM and the dyadic Greens functions (DGFs) used in the numerical algorithms.