Action TU1208

Journal Papers stemming out from Working Group 3 (with acknowledgement to COST Action TU1208)

Project 3.1 – Electromagnetic modelling for GPR (from the most recent paper to the oldest)

[wg3-p1-j9] C. Warren, S. Sesnic, A. Ventura, L. Pajewski, D. Poljak, A. Giannopoulos, “Comparison of Time-Domain Finite-Difference, Finite-Integration, and Integral-Equation Methods for Dipole Radiation in Half-space Environments,” Progress in Electromagnetic Research M (PIER M), vol. 57, pp. 175-183, 2017, doi: 10.2528/PIERM17021602 (Croatia, Italy, United Kingdom; STSM Outcome; OPEN ACCESS)

Abstract: In this paper we compare current implementations of commonly used numerical techniques — the Finite-Difference Time-Domain (FDTD) method, the Finite-Integration Technique (FIT), and Time-Domain Integral Equations (TDIE) — to solve the canonical problem of a horizontal dipole antenna radiating over lossless and lossy half-spaces. These types of environment are important starting points for simulating many Ground Penetrating Radar (GPR) applications which operate in the near- field of the antenna, where the interaction between the antenna, the ground, and targets is important. We analysed the simulated current at the centre of the dipole antenna, as well as the electric field at different distances from the centre of the antenna inside the half-space. We observed that the results from the simulations using the FDTD and FIT methods agreed well with each other in all of the environments. Comparisons of the electric field showed that the TDIE technique agreed with the FDTD and FIT methods when observation distances were towards the far-field of the antenna but degraded closer to the antenna. These results provide evidence necessary to develop a hybridisation of current implementations of the FDTD and TDIE methods to capitalise on the strengths of each technique.

[wg3-p1-j8] F. Mangini and N. Tedeschi, “Scattering of an electromagnetic plane wave by a sphere embedded in a cylinder,” Journal of the Optical Society of America A, vol. 34(5), pp. 760 – 769, May 2017, doi: 10.1364/JOSAA.34.000760 (Italy)

Abstract: In this paper, we face the problem of the scattering of a plane wave by a sphere embedded in an infinitely long circular cylinder. The problems of scattering by both a sphere and a cylinder are canonical problems in optics. However, the scattering problems involving different objects with different geometries have not been solved analytically in the literature: only asymptotic or approximated solutions are available. The problem of scattering by cylinders and spheres concurrently present can be of great importance in several areas, from optical microscopy to biomedical applications, and from metamaterials to civil engineering applications. To solve the problem, the incident wave is expressed as a superposition of cylindrical harmonics. The scattered wave by the cylinder, being a cylindrical wave as well, has been expressed as a superposition of spherical harmonics in order to take into account the interaction with the sphere. The theoretical procedure returns a linear system of equations for the computation of the unknown coefficients of the series. A numerical code is presented to compute the scattered field, where a suitable truncation criterion for the series expansions has been proposed. Comparisons with a finite-element method have been presented to validate the results.

[wg3-p1-j7] D. Poljak, S. Sesnic, S. Lallechere and K. El Khamlichi Drissi, “Stochastic post-processing of the deterministic boundary element modelling of the transient electric field from GPR dipole antenna propagating through lower half-space,” International Journal of Computational Methods and Experimental Measurements, vol. 5(5), pp. 678 – 685, 2017, doi: 10.2495/CMEM-V5-N5-678-685 (Croatia, France; OPEN ACCESS)

Abstract: The paper deals with time domain-deterministic stochastic assessment of a transient electric field generated by a ground penetrating radar (GPR) dipole antenna and transmitted into a lower half-space. The deterministic time domain formulation is based on the space-time Hallen integral equation for half-space problems. The Hallen equation is solved via the Galerkin–Bubnov variant of the Indirect Boundary Element Method (GB-IBEM) and the space-time current distribution along the dipole antenna is obtained, thus providing the field calculation. The field transmitted into the lower medium is obtained by solving the corresponding field integrals. As GPR systems are subjected to a rather complex environment, some input parameters, for example the antenna height over ground or earth properties, are partly or entirely unknown and, therefore, a simple stochastic collocation (SC) method is used to properly access relevant statistics about GPR time responses. The SC approach also aids in the assessment of corresponding confidence intervals from the set of obtained numerical results. The expansion of statistical output in terms of mean and variance over a polynomial basis, via the SC method, is shown to be a robust and efficient approach providing a satisfactory convergence rate.

[wg3-p1-j6] C. Warren, A. Giannopoulos, I. Giannakis, “gprMax: Open source software to simulate electromagnetic wave propagation for Ground Penetrating Radar,” Computer Physics Communications, vol. 209, pp. 163-170, December 2016; doi: 10.1016/j.cpc.2016.08.020 (United Kingdom; OPEN ACCESS)

Abstract: gprMax is open source software that simulates electromagnetic wave propagation, using the Finite-Difference Time-Domain (FDTD) method, for the numerical modelling of Ground Penetrating Radar (GPR). gprMax was originally developed in 1996 when numerical modelling using the FDTD method and, in general, the numerical modelling of GPR were in their infancy. Current computing resources offer the opportunity to build detailed and complex FDTD models of GPR to an extent that was not previously possible. To enable these types of simulations to be more easily realised, and also to facilitate the addition of more advanced features, gprMax has been redeveloped and significantly modernised. The original C-based code has been completely rewritten using a combination of Python and Cython programming languages. Standard and robust file formats have been chosen for geometry and field output files. New advanced modelling features have been added including: an unsplit implementation of higher order Perfectly Matched Layers (PMLs) using a recursive integration approach; diagonally anisotropic materials; dispersive media using multi-pole Debye, Drude or Lorenz expressions; soil modelling using a semi-empirical formulation for dielectric properties and fractals for geometric characteristics; rough surface generation; and the ability to embed complex transducers and targets.

[wg3-p1-j5] D. Poljak, S. Antonijević, S. Šesnić, S. Lalléchère, K. E. K. Drissi, "On deterministic-stochastic time domain study of dipole antenna for GPR applications," Engineering Analysis with Boundary Elements (Elsevier), vol. 73, pp. 14-20, December 2016; doi: 10.1016/ j.enganabound.2016.08.011 (Croatia, France)

Abstract: A deterministic-stochastic transient study of Ground Penetrating Radar (GPR) dipole antenna radiating in a presence of a two-media configuration is carried out in the paper. A deterministic direct time domain formulation is based on the corresponding space-time Hallen integral equation. The numerical solution is carried out via the improved space-time variant of the Galerkin-Bubnov Indirect Boundary Element Method (GB-IBEM). The Stochastic-Collocation (SC) method is then applied to determine accurate confidence intervals due to the random variations of GPR input parameters. Once obtaining the current along the dipole antenna, it is possible to calculate other parameters of interest for GPR dipole antenna behavior, such as the field reflected from the interface of two media, or the field transmitted into a lower half-space. Some illustrative numerical results for the transient current along the dipole antenna and transient electric field transmitted into the lower half-space are given.

[wg3-p1-j4] S. Šesnić, S. Lalléchère, D. Poljak, P. Bonnet, K. E. K. Drissi, "Stochastic collocation analysis of the transient current induced along the wire buried in a lossy medium," WIT Transactions on Modelling and Simulation, vol. 59, pp. 47-58, 2015; doi: 10.2495/CMEM150051 (Croatia, France)

Abstract: The paper deals with the stochastic collocation analysis of a time domain response of a straight thin wire scatterer buried in a lossy half-space. The wire is excited by a plane wave transmitted through the air-ground interface. Transient current induced at the centre of the wire, governed by corresponding Pocklington integro-differential equation is determined. This configuration, as is the case with many electromagnetic compatibility (EMC) issues, suffers from uncertainties in various parameters, such as ground properties, wire dimensions, position, etc. The obtained results yield additional statistical information thus enabling more accurate and efficient analysis of buried wire configurations.

[wg3-p1-j3] D. Poljak, V. Dorić, "Transmitted field in the lossy ground from ground penetrating radar (GPR) dipole antenna," WIT Transactions on Modelling and Simulation, vol. 59, pp. 3-11, 2015; doi: 10.2495/CMEM150011 (Croatia)

Abstract: The paper deals with the evaluation of transmitted electric field in the ground due to the GPR dipole antenna. The frequency domain formulation is based on the integro-differential equation of the Pocklington type. The influence of the earth air interface is taken into account via the simplified reflection/transmission coefficient arising from the Modified Image Theory (MIT). The space-frequency Pocklington equation is solved via the GalerkinBubnov variant of the Indirect Boundary Element Method (GB-IBEM) and the corresponding transmitted field is obtained by numerically computing field integrals. Some preliminary results for the electric field transmitted into material media are presented.

[wg3-p1-j2] F. Tosti, A. Umiliaco, "FDTD Simulation of the GPR Signal for Preventing the Risk of Accidents due to Pavement Damages," International Journal of Interdisciplinary Telecommunications and Networking, vol. 6(1), pp. 1-9, January-March 2014; doi: 10.4018/ijitn.2014010101 (Italy)

Abstract: It is well known that road safety issues are closely dependent on both pavement structural damages and surface unevenness, whose occurrence is often related to ineffective pavement asset management. The evaluation of road pavement operability is traditionally carried out through distress identification manuals on the basis of standardized comprehensive indexes, as a result of visual inspections or measurements, wherein the failure causes can be partially detected. In this regard, ground-penetrating radar (GPR) has proven to be over the past decades an effective and efficient technique to enable better management of pavement assets and better diagnosis of the causes of pavement failures. In this study, one of the main causes (i.e. subgrade failures) of surface damage is analyzed through finite-difference time-domain (FDTD) simulation of the GPR signal. The GprMax 2D numerical simulator for GPR is used on three different types of flexible pavement to retrieve the numerical solution of Maxwell’s equations in the time domain. Results show the high potential of GPR in detecting the causes of such damage.

[wg3-p1-j1] F. Frezza, L. Pajewski, C. Ponti, G. Schettini, N. Tedeschi, "Through-wall electromagnetic scattering by N conducting cylinders," Journal of the Optical Society of America A, vol. 30(8), pp. 1632-1639, Aug. 2013; doi: 10.1364/JOSAA.30.001632 (Italy)

Abstract: A spectral-domain analysis is presented for the scattering by perfectly conducting cylindrical objects behind a dielectric wall. The solution is developed with an analytical-numerical technique, based on the cylindrical wave approach. Suitable cylindrical functions and their spectral representations are introduced as basis functions for the scattered fields, to deal with their interaction with the planar interfaces bounding the wall. The numerical solution is given in TE and TM polarizations states, and in both near- and far-field zones. The model yields an accurate computation of direct scattering that can be useful for through-wall-imaging applications. A stack of three different dielectric media is considered in the theoretical model. In the numerical results, the upper medium, where the incident field is generated, is assumed to be filled by air, the central layer represents the wall, and the lower medium, which contains the scatterers, is air filled, too. Also general problems of scattering by buried objects can be simulated, being the cylinders buried in a medium of arbitrary permittivity, placed below a dielectric layer.

Project 3.2 – Imaging and inversion techniques for GPR (from the most recent paper to the oldest)

[wg3-p2-j11] A. Ristic, Z. Bugarinovic, M. Vrtunski, Miro Govedarica, "Point Coordinates Extraction from Localized Hyperbolic Reflections in GPR Data,” Journal of Applied Geophysics, vol. TBD, pp. TBA, 2017; doi: 10.1016/j.jappgeo.2017.06.003 (Serbia)

Abstract: In this paper, we propose an automated detection algorithm for the localization of apexes and points on the prongs of hyperbolic reflection incurred as a result of GPR scanning technology. The objects of interest encompass cylindrical underground utilities that have a distinctive form of hyperbolic reflection in radargram. Algorithm involves application of trained neural network to analyze radargram in the form of raster image, resulting with extracted segments of interest that contain hyperbolic reflections. This significantly reduces the amount of data for further analysis. Extracted segments represent the zone for localization of apices. This is followed by extraction of points on prongs of hyperbolic reflections which is carried out until stopping criterion is satisfied, regardless the borders of segment of interest. In final step a classification of false hyperbolic reflections caused by the constructive interference and their removal is done. The algorithm is implemented in MATLAB environment. There are several advantages of the proposed algorithm. It can successfully recognize true hyperbolic reflections in radargram images and extracts coordinates, with very low rate of false detections and without prior knowledge about the number of hyperbolic reflections or buried utilities. It can be applied to radargrams containing single and multiple hyperbolic reflections, intersected, distorted, as well as incomplete (asymmetric) hyperbolic reflections, all in the presence of higher level of noise. Special feature of algorithm is developed procedure for analysis and removal of false hyperbolic reflections generated by the constructive interference from reflectors associated with the utilities. Algorithm was tested on a number of synthetic and radargram acquired in the field survey. To illustrate the performances of the proposed algorithm, we present the characteristics of the algorithm through five representative radargrams obtained in real conditions. In these examples we present different acquisition scenarios by varying the number of buried objects, their disposition, size, and level of noise. Example with highest complexity was tested also as a synthetic radargram generated by gprMax. Processing time in examples with one or two hyperbolic reflections is from 0.1 to 0.3 s, while for the most complex examples it is from 2.2 to 4.9 s. In general, the obtained experimental results show that the proposed algorithm exhibits promising performances both in terms of utility detection and processing speed of the algorithm.

[wg3-p2-j10] M. Salucci, L. Poli, N. Anselmi, A. Massa, “Multifrequency Particle Swarm Optimization for Enhanced Multiresolution GPR Microwave Imaging,” IEEE Transactions on Geoscience and Remote Sensing, vol. 55(3), pp. 1305-1317, March 2017, doi: 10.1109/TGRS.2016.2622061 (France, Italy)

Abstract: An innovative inverse scattering (IS) technique for the simultaneous processing of multifrequency (MF) ground-penetrating radar (GPR) measurements is proposed. The nonlinear IS problem is solved by profitably integrating a customized MF version of the particle swarm optimizer (PSO) within the iterative multiscaling approach (IMSA) to jointly exploit the reduction of the ratio between unknowns and uncorrelated data with a pervasive exploration of the multidimensional search space for minimizing the probability that the solution is trapped into local minima corresponding to false solutions of the problem at hand. Both numerical and experimental test cases are reported to assess the reliability of the MF-IMSA-PSO method toward accurate GPR tomography as well as improvements with respect to the competitive state-of-the-art inversion approaches.

[wg3-p2-j9] M. Sun, C. Le Bastard, N. Pinel, Y. Wang, J. Li, J. Pana, Z. Yud, “Estimation of time delay and interface roughness by GPR using modified MUSIC,” Signal Processing (Elsevier), vol. 132, pp. 272–283, March 2017, doi: 10.1016/j.sigpro.2016.05.029 (France, China; COOPERATION WITH IPC; INDUSTRY INVOLVEMENT; TU1208 SP Special Issue)

Abstract: In civil engineering, roadway structure evaluation is an important application which can be carried out by ground penetrating radar. In this paper, firstly a signal model taking into account the influence of interfaces roughness (surface and interlayer) is proposed. In order to estimate the time delay and interface roughness, we propose a method composed of 2 steps: 1) a modified MUSIC algorithm is proposed for time delay estimation; 2) the interface roughness is estimated by using Maximum Likelihood method (MLE) with the estimated time delays. The proposed algorithms are tested on data obtained by a method of moments (MoM). Numerical examples are provided to demonstrate the performance of the proposed algorithm.

[wg3-p2-j8] M. Salucci, L. Poli, A. Massa, “Advanced multi-frequency GPR data processing for non-linear deterministic imaging,” Signal Processing (Elsevier), vol. 132, pp. 306–318, March 2017, doi: 10.1016/j.sigpro.2016.06.019 (France, Italy; TU1208 SP Special Issue)

Abstract: In this paper, the quantitative imaging of the dielectric characteristics of unknown targets buried in a lossy half-space is performed by suitably processing wide-band ground penetrating radar (GPR) measurements. An innovative multi-frequency (MF) fully non-linear inverse scattering (IS) technique exploiting the integration of a conjugate-gradient (CG) solver within the iterative multi-scaling approach (IMSA) is proposed. Representative results from numerical test cases are presented to provide the interested readers with some indications on the effectiveness, as well as the current limitations, of the proposed approach when directly compared to a state-of-the-art frequency-hopping (FH) based method formulated in the same framework. Such a validation points out that if, on the one hand, the proposed MF strategy is computationally more efficient than the FH one, on the other hand, it turns out to be less reliable and accurate in several situations.

[wg3-p2-j7] M. Salucci, L. Tenuti, L. Poli, G. Oliveri, A. Massa, “A computational method for the inversion of wide-band GPR measurements,” Journal of Physics: Conference series, vol. 756, Article No. 012008, pp. 1-7, October 2016, doi: 10.1088/1742-6596/756/1/012008 (France, Italy; OPEN ACCESS)

Abstract: An innovative method for the inversion of ground penetrating radar (GPR) measurements is presented. The proposed inverse scattering (IS) approach is based on the exploitation of wide-band data according to a multi-frequency (MF) strategy, and integrates a customized particle swarm optimizer (PSO) within the iterative multi-scaling approach (IMSA) to counteract the high non-linearity of the optimized cost function. If from the one hand the IMSA provides a reduction of the ratio between problem unknowns and informative data, on the other hand the stochastic nature of the PSO solver allows to "escape" from the high density of false solutions of the MF-IS subsurface problem. A set of representative numerical results verifies the effectiveness of the developed approach, as well as its superiority with respect to a deterministic implementation.

[wg3-p2-j6] M. Sun, C. Le Bastard, N. Pinel, Y. Wang, J. Li, “Road surface layers geometric parameters estimation by ground penetrating radar using Estimation of Signal Parameters via Rotational Invariance Techniques method,” IET Radar, Sonar & Navigation, vol. 10(3), pp. 603-609, March 2016; doi: 10.1049/iet-rsn.2015.0374 (France, China; COOPERATION WITH IPC; INDUSTRY INVOLVEMENT)

Abstract: In civil engineering, ground penetrating radar is widely used to assess the roadway structures. This study evaluates the influence of interface roughness. A modified estimation of signal parameters by rotational invariance techniques algorithm is proposed with a spectral smoothing technique for efficiently estimating the time delay, the permittivity of the layers and the interface roughness. To reduce the impact of noise, a preprocessing method called propagator is used to estimate the noise variance. The algorithm is tested on simulated data obtained by the rigorous numerical method called PILE (propagation inside layer expansion). Numerical simulations are conducted to assess the performance of the algorithm. The simulation results show that the proposed algorithm can estimate road parameters with small relative root mean square error.

[wg3-p4-j5] L. Mertens, R. Persico, L. Matera, S. Lambot, "Automated Detection of Reflection Hyperbolas in Complex GPR Images With No A Priori Knowledge on the Medium," IEEE Transactions of Geoscience and Remote Sensing, Vol. 54(1), January 2016, pp. 580-596; doi: 10.1109/TGRS.2015.2462727 (Belgium, Italy)

Abstract: In this paper, we propose an automated detection algorithm for well- and ill-shaped ground-penetrating radar re- flection hyperbolas for complex media, calibrated with human recognition principles. The algorithm detects the apex of the hyperbolas by fitting an analytical function of a hyperbola to the profile edge dots detected with a Canny filter. The existence of a hyperbola is determined using a set of carefully chosen criteria calibrated in order to fit the ambiguity zone for the human brain. The inherent misshapedness of field hyperbolas is further consid- ered by defining a buffer zone around the theoretical hyperbola. First, the method was tested in the laboratory over tree roots and PVC pipes and on field images over tree root systems. Both time- and frequency-domain radars were used on-ground. After around 1–3 min of computation time for 10 000 edge dots in a MATLAB environment (single 1.96-GHz processor), the results showed rates of false alarm and nondetection of maximum 20% and 28%, respectively. In comparison with the semiautomated hyperbola detection provided by a commercial software, these rates were lower. Second, we conducted a sensitivity analysis to estimate the validity of the fitting of a hyperbola equation neglecting the object radius. The fitting was close, but the derivation of the relative permittivity from the analytical equation neglecting the radius led to high errors. In conclusion, owing to the low computational time and its good performances, the proposed algorithm is suitable for complex environments.

[wg3-p2-j4] M. Sun, C. Le Bastard, Y. Wang, N. Pinel, “Time-Delay Estimation Using ESPRIT With Extended Improved Spatial Smoothing Techniques for Radar Signals,” IEEE Geoscience and Remote Sensing Letters, vol. 13(1), pp. 73-77, January 2016; doi: 10.1109/LGRS.2015.2497378 (France, China; COOPERATION WITH IPC; INDUSTRY INVOLVEMENT)

Abstract: In the electromagnetic field, radar is widely used to measure or estimate the media parameters or to detect targets through obstructions. For horizontally stratified media, the layer thickness can be deduced from the time delays of backscattered echoes and the dielectric constants. The high-resolution method estimation of signal parameters via rotation invariance techniques (ESPRIT) has been proposed for time-delay estimation. In practice with a radar, backscattered echoes are correlated. In order to apply the ESPRIT method, in this letter, we propose to use two adaptive improved spatial smoothing techniques with the propagator method for fighting against the correlation between the echoes. The proposed solution does not use any approximation. Numerical examples are provided to show the performance of the algorithm.

[wg3-p2-j3] M. Salucci, G. Oliveri, A. Massa, "GPR Prospecting Through an Inverse-Scattering Frequency-Hopping Multifocusing Approach," IEEE Transactions on Geoscience and Remote Sensing, vol. 53(12), pp. 6573-6592, December 2015; doi: 10.1109/TGRS.2015.2444391 (France, Italy)

Abstract: An innovative information-acquisition approach to 2-D ground-penetrating radar (GPR) prospecting is presented. A microwave inverse-scattering nested scheme combining a frequency hopping (FH) procedure and a multifocusing (MF) technique is proposed. On the one hand, the FH scheme effectively handles multifrequency GPR data, whereas on the other hand, MF techniques have been proven to be effective tools in reducing the occurrence of multilocal minima affecting GPR investigations. This allows the use of a local search technique based on the conjugate gradient method to iteratively solve the inverse problem at hand. Selected results are reported and analyzed to give some insights to the interested readers on the advantages and limitations of such an approach when handling synthetically generated and experimental GPR data.

[wg1-p1-j2] S. Nounouh, C. Eyraud, A. Litman, H. Tortel, "Near-subsurface imaging in an absorbing embedding medium with a multistatic/single frequency scanner," Near Surface Geophysics (EAGE), vol. 13(3), pp. 211-218, June 2015; doi: 10.3997/1873-0604.2014046 (France)

Abstract: Probing the near subsurface in the presence of absorbing media is a very challenging problem. Within that framework, we analyze the capabilities of a mono-frequency/multistatic set-up for detecting shallowly buried targets. As the antennas constitute an important part of the probing device, an accurate method for modelling the antennas behaviour is proposed. This modelling, performed thanks to a correct balanced set of elementary sources, is then incorporated in the calculation of the scattered field, performed with a home-made Finite Element Method software. Efforts have also been put into the measurement procedure. The measured fields are thus post-processed with an efficient method which takes profit of the spectral bandwidth properties of the scattered field. These fields serve as input data for the inversion algorithm, an extension of the DORT method to elongated targets. This qualitative and fast imaging procedure, which exploits the spectral properties of the multi static scattering matrix, has been adapted to the present stratified configuration. Imaging results of shallowly buried targets embedded in a high losses medium are presented to assess the well-behaviour of the proposed methodology.

[wg3-p2-j1] S. Meschino, L. Pajewski, G. Schettini, "A SAP-DOA Method for the Location of Two Buried Objects," International Journal on Antennas and Propagation (Special Issue on “Inverse Scattering and Microwave Tomography in Safety, Security, and Health”), vol. 2013, Article ID 702176, 10 pp., 2013; doi: 10.1155/2013/702176 (Italy; OPEN ACCESS)

Abstract: A localization technique for buried metallic and dielectric objects is proposed and tested. An array of isotropic antennas investigates a scenario with cylindrical targets buried in a dielectric soil. The targets are in the near field of the array, and a Sub-Array Processing (SAP) approach is adopted: the array is partitioned into subarrays, and Direction of Arrival (DoA) algorithms are used to process the electromagnetic field received by each subarray and estimate the dominant arrival direction of the signal. By triangulating all the estimated DoAs, a crossing pattern is obtained. It is filtered by a Poisson-based procedure and subsequently elaborated by the 𝑘-means clustering method in order to distinguish between targets and background, estimate the number of targets, and find their position. Several simulations have been performed to compare different DoA algorithms and to test the localization method in the presence of two buried cylinders. Different values of the permittivity of the involved dielectric materials have been considered; the target positions and size have also been varied. The proposed procedure can be useful for ground-penetrating radar applications, near-surface probing, and for the detection and localization of defects in a hosting medium.

Project 3.3 – Intrinsic models for describing near-field antenna effects (from the most recent paper to the oldest)

[wg3-p3-j2] A. De Coster, A. P. Tran, S. Lambot, "Fundamental Analyses on Layered Media Reconstruction Using GPR and Full-Wave Inversion in Near-Field Conditions," IEEE Transactions on Geoscience and Remote Sensing, vol. 54(9), pp. 5143-5158, September 2016; doi: 10.1109/TGRS.2016.2556862 (Belgium, United States; COOPERATION WITH IPC)

Abstract: An innovative information-acquisition approach to 2-D ground-penetrating radar (GPR) prospecting is presented. A microwave inverse-scattering nested scheme combining a frequency hopping (FH) procedure and a multifocusing (MF) technique is proposed. On the one hand, the FH scheme effectively handles multifrequency GPR data, whereas on the other hand, MF techniques have been proven to be effective tools in reducing the occurrence of multilocal minima affecting GPR investigations. This allows the use of a local search technique based on the conjugate gradient method to iteratively solve the inverse problem at hand. Selected results are reported and analyzed to give some insights to the interested readers on the advantages and limitations of such an approach when handling synthetically generated and experimental GPR data.

[wg3-p3-j1] F. André, S. Lambot, "Intrinsic Modeling of Near-Field Electromagnetic Induction Antennas for Layered Medium Characterization," IEEE Transactions on Geoscience and Remote Sensing, vol. 52(11), pp. 7457-7469, November 2014; doi: 10.1109/TGRS.2014.2312816 (Belgium)

Abstract: We present a closed-form equation for intrinsic modeling of near-field electromagnetic induction (EMI) antennas for planar layered media characterization. Resorting to a decomposition of the backscattered EM field into elementary distributions over the antenna aperture, the EMI transmitting and receiving antennas are modeled using infinitesimal magnetic dipoles and field points, and characteristic frequency-dependent global reflection and transmission coefficients. Low-frequency propagation of the EM fields in the medium is described using 3-D planar layered media Green's functions. We performed measurements with a loop antenna situated at different heights, ranging from near-field to far-field conditions, above water of known electrical conductivity to determine its intrinsic properties, and a range of salinity conditions was applied to subsequently validate the proposed model. The EMI system was set up using a vector network analyzer equipped with a prototype EMI antenna specifically designed for this application. The model showed good accuracy for reproducing the observed data, and model inversion provided good estimates of the medium electrical conductivity. Yet, insensitivity of the EMI signal to water electrical conductivity was encountered for low salinity due to the presence of a copper sheet as the bottom boundary condition of the experimental setup. Moreover, the efficiency of the antenna decreased rapidly as antenna height above water surface increases, leading to increasing discrepancies between estimated and measured water electrical conductivity values as the antenna moves away from the water surface. Although some technical improvements are still needed, the proposed approach is promising for quantitative estimation of soil electrical conductivity from EMI data.

Project 3.4 – Data processing for GPR (from the most recent paper to the oldest)

[wg3-p4-j7] F. Benedetto, F. Tosti, “A signal processing methodology for assessing the performance of ASTM standard test methods for GPR SYSTEMS,” Signal Processing (Elsevier), vol. 132, pp. 327–337, March 2017, doi: doi:10.1016/j.sigpro.2016.06.030 (Italy, United Kingdom; TU1208 SP Special Issue)

Abstract: Ground penetrating radar (GPR) is one of the most promising and effective non-destructive testing techniques (NDTs), particularly for the interpretation of the soil properties. Within the framework of international Agencies dealing with the standardization of NDTs, the American Society for Testing and Materials (ASTM) has published several standard test methods related to GPR, none of which is focused on a detailed analysis of the system performance, particularly in terms of precision and bias of the testing variable under consideration. This work proposes a GPR signal processing methodology, calibrated and validated on the basis of a consistent amount of data collected by means of laboratory-scale tests, to assess the performance of the above standard test methods for GPR systems. The (theoretical) expressions of the bias and variance of the estimation error are here investigated by a reduced Taylor's expansion up to the second order. Therefore, a closed form expression for theoretically tuning the optimal threshold according to a fixed target value of the GPR signal stability is proposed. Finally, the study is extended to GPR systems with different antenna frequencies to analyze the specific relationship between the frequency of investigation, the optimal thresholds, and the signal stability.

[wg3-p4-j6] A. Benedetto, F. Tosti, L. Bianchini Ciampoli, F. D’Amico, “An overview of ground-penetrating radar signal processing techniques for road inspections,” Signal Processing (Elsevier), vol. 132, pp. 201-209, March 2017, doi: 10.1016/j.sigpro.2016.05.016 (Italy, United Kingdom; TU1208 SP Special Issue)

Abstract: Ground-penetrating radar (GPR) was firstly used in traffic infrastructure surveys during the first half of the Seventies for testing in tunnel applications. From that time onwards, such non-destructive testing (NDT) technique has found exactly in the field of road engineering one of the application areas of major interest for its capability in performing accurate continuous profiles of pavement layers and detecting major causes of structural failure at traffic speed. This work provides an overview on the main signal processing techniques employed in road engineering, and theoretical insights and instructions on the proper use of the processing in relation to the quality of the data acquired and the purposes of the surveys.

[w3-p4-j5] N. Economou, "Time-varying band-pass filtering GPR data by self-inverse filtering," EAGE Near Surface Geophysics (NSG), vol. 14(2), pp. 207-217, April 2016, doi: 10.3997/1873-0604.2016015 (Greece; TU1208 NSG Special Issue)

Abstract: Even though ground penetrating radar data signal processing has already been studied by many researchers, more research is needed and expected from automatic ground penetrating radar data analysis. An automatic band-pass filtering procedure can lead to sufficient real-time data interpretation as signal buried in noise could be amplified. Ground penetrating radar traces are highly nonstationary, requiring time-varying processing techniques. An algorithm, based on self-inverse filtering, which is a special case of inverse filtering, was implemented. It is a ground penetrating radar trace filtering approach and is implemented by applying inverse filtering in each time sample in the time-frequency domain. Applied on a synthetic trace, this algorithm performed better than a stationary band-pass filter and empirical mode decomposition family methods, whereas its application on real ground penetrating radar data from two different sites enhanced reflections buried in noise without the need to test different high-frequency band stops and with minimum distortion of the signal and the initial temporal resolution of the data.

[wg3-p2-j4] J. Li, C. Le Bastard, Y. Wang, G. Wei, B. Ma, M. Sun, “Enhanced GPR Signal for Layered Media Time-Delay Estimation in Low-SNR Scenario,” IEEE Geoscience and Remote Sensing Letters, vol. 13(3), pp. 299-303, March 2016; doi: 10.1109/LGRS.2015.2502662 (France, China; COOPERATION WITH IPC)

Abstract: In this letter, a new method is proposed to enhance the ground-penetrating radar (GPR) signal for time-delay estimation in a low signal-to-noise ratio. It is based on a subspace method and a clustering technique. The proposed method makes it possible to improve the estimation accuracy in a noisy context. It is used with a compressive sensing method to estimate the time delay of layered media backscattered echoes coming from the GPR signal. Several simulations and an experiment are presented to show the effectiveness of signal enhancement.

[wg3-p4-j2] M. Varela-González, M. Solla, J. Martínez-Sánchez, P. Arias, "A semi-automatic processing and visualisation tool for ground-penetrating radar pavement thickness data," Automation in Construction (Elsevier), vol. 45, pp. 42-49, September 2014; doi: 10.1016/j.autcon.2014.05.004 (Spain)

Abstract: Ground-penetrating radar (GPR) is a recommendable and cost-effective non-destructive technique for measuring the thickness of pavement layers because data acquisition can take place at normal traffic speeds. On the other hand, the large amount of data collected is difficult to process. Given that processing is conducted by qualified practitioners, it is a key to obtain software tools that allow for accurate thickness measurements and fast processing times. This paper presents a new semi-automatic program for the processing and visualisation of GPR data to measure pavement thicknesses. The results showed that an optimisation in the execution time allowed for a near-immediate response in data processing even when dealing with large data sets. Different data set lengths, ranging from 100 m to 20 km, were analysed, and the processing times required to complete the entire process were examined taking into account three different hardware configurations (i3, i5 and i7 processors). In all cases, the processing times did not exceed 30 s. An additional test was performed to evaluate the reproducibility of the algorithm on a well-defined and preconditioned concrete asphalt course. Furthermore, the visualisation application allows for the georeferencing of the field GPR data by using additional GPS data.

[wg3-p4-j1] M. Manataki, A. Vafidis, A. Sarris, "Application of empirical mode decomposition methods to ground penetrating radar data," First Break (EAGE), vol. 32, pp. 67-71, August 2014 (Greece)

Abstract: Empirical Mode Decomposition (EMD) is a relatively new technique introduced by Huang et al., (1998) for analysing non-linear and non-stationary time series. The decomposition is based on a signal’s local extrema, which define different oscillation modes present in the signal. What EMD does is the separation of those differ- ent oscillatory modes into a finite and usually small number of stationary sub-signals called Instrinsic Mode Functions (IMFs). EMD suffers from mode mixing which limits the frequency separation among the different modes and makes the physical meaning of the IMFs unclear.

The introduction of Ensemble Empirical Mode Decomposition (EEMD) by Wu and Huang (2009) as an EMD-based noise assistance method improved the modes separation by eliminating the mode-mixing problem. The signal is not fully reconstructed by the IMFs calculated from EEMD. The latter lead Torres et al. (2011) to propose another modification, the Complete Ensemble Empirical Mode Decomposition (CEEMD). CEEMD is also a noise assisted and adaptive method where the original signal can be fully reconstructed. EMD-based algorithms have been applied to seismic reflection data (Battista et al., 2007; Bekara and Van der Baan, 2009). Battista et al. (2009) used EMD to remove wow noise from Ground Penetrating Radar (GPR) data. Chen and Jeng, (2011), applied EEMD to enhance GPR data and provided promising results. Here, we compare the empirical mode decomposition methods using both synthetic and real GPR data. In particular, we examine: (1) the separation of high-frequency wavelets from the low frequency ones and (2) the noise level that yields better decomposition for EEMD and CEEMD. We also examined the capability of these decomposition methods to remove random and coherent noise from real GPR data.

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