Ground Penetrating Radar 2018, Volume 1, Issue 2, GPR-1-2-3,   https://doi.org/10.26376/GPR2018009

Electromagnetic modelling and simulation of a high-frequency Ground Penetrating Radar antenna over a concrete cell with steel rods

Alessio Ventura and Lara Pajewski

Full text: PDF [3.2 MB, open access]

Supplementary Materials: ZIP [21.7 MB, open access]

Abstract:   This work focuses on the electromagnetic modelling and simulation of a high-frequency Ground-Penetrating Radar (GPR) antenna over a concrete cell with reinforcing elements. The development of realistic electromagnetic models of GPR antennas is crucial for accurately predicting GPR responses and for designing new antennas. We used commercial software implementing the Finite-Integration technique (CST Microwave Studio) to create a model that is representative of a 1.5 GHz Geophysical Survey Systems, Inc. antenna, by exploiting information published in the literature (namely, in the PhD Thesis of Dr Craig Warren); our CST model was validated, in a previous work, by comparisons with Finite-Difference Time-Domain results and with experimental data, with very good agreement, showing that the software we used is suitable for the simulation of antennas in the presence of targets in the near field. In the current paper, we firstly describe in detail how the CST model of the antenna was implemented; subsequently, we present new results calculated with the antenna over a reinforced-concrete cell. Such cell is one of the reference scenarios included in the Open Database of Radargrams of COST Action TU1208 “Civil engineering applications of Ground Penetrating Radar” and hosts five circular-section steel rods, having different diameters, embedded at different depths into the concrete. Comparisons with a simpler model, where the physical structure of the antenna is not taken into account, are carried out; the significant differences between the results of the realistic model and the results of the simplified model confirm the importance of including accurate models of the actual antennas in GPR simulations; they also emphasize how salient it is to remove antenna effects as a pre-processing step of experimental GPR data. The simulation results of the antenna over the concrete cell presented in this paper are attached to the paper as ‘Supplementary materials.’

Keywords:  Ground Penetrating Radar (GPR); electromagnetic modelling; Finite-Integration technique (FIT); antennas; TU1208 Open Database of Radargrams; concrete.

Introduction

Electromagnetic simulations of Ground Penetrating Radar (GPR) [1] scenarios including realistic models of the antennas are not yet common. Accurate models of GPR antennas have been only occasionally developed during the past two decades [2]-[9]; rarely, they have been combined with realistic models of complex environments [10]. In most cases, GPR electromagnetic simulations use hertzian dipoles or lines of current to represent the transmitting antennas; the physical structure of the receiving antennas is usually not included in the models and the electric field impinging on the receivers is calculated [11]-[16]. This simplified approach is customarily adopted because easier to implement and computationally cheaper; in fact, nowadays running realistic models of GPR scenarios is still a challenging task, notwithstanding computing power is increasing and becoming more accessible.

In this paper, we employed commercial software implementing the Finite-Integration technique (FIT) [17] (CST Microwave Studio) for modelling and simulating an antenna representative of a widely used high-frequency commercial device manufactured by Geophysical Survey Systems, Inc. (GSSI). All necessary information about the antenna was taken from Dr Craig Warren’s PhD Thesis [6], where the freeware tool GprMax3D [18] was used to develop a Finite-Difference Time-Domain (FDTD) model of the same antenna. It has to be noted that, in [6] and here, the numerical model does not exactly replicate the commercial antenna because the electromagnetic properties of some antenna materials are unknown, due to commercial sensitivity; the undisclosed values were estimated in [6] (the match between the real and synthetic crosstalk responses of the antenna in free-space was maximized, by using Taguchi's optimisation method). It is also worth mentioning that the FDTD model developed in [6] is currently included in the library of antennas of the open-source software gprMax [19, 20], therefore gprMax users can easily include this antenna into their simulations without having to build it step-by-step. The CST model that we developed was successfully validated via comparisons with synthetic and experimental data available in [6], in cooperation with colleagues from The University of Edinburgh (United Kingdom); such data were obtained with the antenna immersed in free space and in lossy dielectric environments, with and without a circular-section metallic target and some results of the performed comparisons were presented in a conference paper [21].

In Section 2 of the present paper, we describe in detail how we developed the CST Microwave Studio model of the antenna; this information was not included in [21]. Then, in Section 3, we present new results that we obtained by simulating the antenna over a reinforced-concrete cell. Such cell is one of the reference scenarios included in the Open Database of Radargrams of COST Action TU1208 [22] and hosts a series of five circular-section steel rods, having different diameters and/or embedded at different depths into the concrete [23]. We compare results obtained by using the realistic CST antenna model, and results obtained by representing the transmitting antenna with a line of current and by neglecting the physical structure of the receiving antenna. The aim of this comparison is to confirm and further highlight the importance of including realistic models of the actual antennas in GPR simulations, whenever the objective of the simulations is to accurately replicate a real GPR response, or to exploit the simulation results into an inversion process. Moreover, the comparisons presented in this paper emphasize once more how strong are antenna effects, and therefore, how salient it is to develop methods for removing them as a pre-processing step of GPR data. The results of our simulations are attached to the paper as ‘Supplementary materials.’

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References

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Unrestricted use, distribution, and reproduction in any medium of this article is permitted, provided the original article is properly cited.   Please cite this article as follows: A. Ventura and L. Pajewski, "Electromagnetic modelling and simulation of a high-frequency Ground Penetrating Radar antenna over a concrete cell with steel rods,"  Ground Penetrating Radar, Volume 1, Issue 2, Article ID GPR-1-2-3, July 2018, pp. 52-70, doi.org/10.26376/GPR2018009.

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