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- Volume 9, Issue 3, 2011
Near Surface Geophysics - Volume 9, Issue 3, 2011
Volume 9, Issue 3, 2011
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Employing ADI‐FDTD subgrids for GPR numerical modelling and their application to study ring separation in brick masonry arch bridges
Authors Nectaria Diamanti and Antonios GiannopoulosABSTRACTIn realistic numerical modelling of ground‐penetrating radar (GPR) and when parts of the computational domain need to be modelled in detail, the implementation of subgrids into the conventional finite‐difference time‐domain (FDTD) mesh could greatly economize on computational resources. A novel alternating‐direction implicit FDTD subgridding scheme is used to numerically simulate the GPR responses from delaminations located in brick masonry arches. The heterogeneity of these structures renders electromagnetic signals, which originate from the interaction between the GPR system and the bridge, often complex and hence hard to interpret. Therefore, GPR numerical models were created in order to study the attributes of reflected signals from various targets within the structure of the bridge. Results from a range of modelling scenarios are presented. The effect of varying the thickness of faults, their location in brickwork, as well as the effect of water ingress in hairline delaminations on GPR signals, are examined. Moreover, GPR vertical resolution and the presence of lossy brickwork are studied through various numerical models.
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Discontinuous Galerkin time‐domain method for GPR simulation of conducting objects
Authors L.D. Diaz Angulo, J. Alvarez, S.G. Garcia, A. Rubio Bretones and R. Gomez MartinABSTRACTIn this paper we describe the discontinuous Galerkin time‐domain method and apply it to the simulation of ground‐penetrating radar (GPR) problems in 3D. The method is first validated with analytical solutions and we show its superior behaviour when compared to the classical finite‐difference time‐domain method, widely employed in GPR simulation.
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Determining Fresnel reflection coefficients in 3D half‐space geometry by GPR multistatic data
Authors Raffaele Solimene, Antonietta D’Alterio and Francesco SoldovieriABSTRACTThis paper deals with a procedure for retrieving Fresnel reflection coefficients of a half‐space medium starting from measurements collected under a reflection mode multistatic configuration. In particular, the procedure consists in inverting a linear integral operator that links the reflected field measurements and the Fresnel reflection coefficients.
This contribution extends our previous work already developed for 2D scalar configurations. Here, we consider a 3D source and accordingly a dyadic representation of the reflection coefficient. Accordingly, the corresponding inverse problem amounts to solving a vector problem with a dyadic unknown. The amount of data necessary to solve the inverse problem is achieved by collecting different Cartesian components of the reflected field. Accordingly, the reconstruction problem is cast as the inversion of several scalar integral equations, which provides the transverse electric and magnetic Fresnel reflection coefficients as functions of the angle of incidence. Synthetic examples dealing with the case of a homogeneous half‐space soil are presented to show the effectiveness of the proposed procedure.
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Chlorides and moisture assessment in concrete by GPR full waveform inversion
Authors A. Kalogeropoulos, J. van der Kruk, J. Hugenschmidt, S. Busch and K. MerzABSTRACTCorrosion of rebar within reinforced concrete is a major problem for countries where salt is applied to roads for de‐icing. Concrete structures are periodically inspected in order to monitor possible damage caused by chloride‐induced corrosion of the reinforcement. However, the available drilling and visual inspections do not supply sufficient spatial information or can only be assessed in advanced stages of corrosion, respectively. Consequently, the condition of bridge decks can only be assessed with low certainty. Therefore, a spatially continuous and non‐destructive method detecting chloride in concrete structures is desirable. This paper describes a novel method to estimate material properties using the full‐waveform inversion of bistatic off‐ground ground penetrating radar data. In this way, all information present in the ground‐penetrating radar (GPR) traces is used, which enables the estimation of quantitative electromagnetic properties. A critical step for full‐waveform inversion is a proper characterization of our horn antenna GPR system by estimating the phase centre and the effective wavelet using measurements over a stainless steel plate. The inversion of GPR data measured over nine concrete specimens having different moisture and chloride contents returned a relative dielectric permittivity and a conductivity that included a frequency‐dependent component. As expected, the inversion results for almost all specimens showed for increasing chloride and humidity content specimens increasing conductivity and permittivity values, respectively. In contrast to traditional ray‐based techniques we were able to distinguish between moisture and chloride effects and to obtain quantitative values for the permittivity and conductivity. For increasing chloride content increasing frequency‐dependent conductivity values were obtained.
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Two‐dimensional linear inverse scattering for dielectric and magnetic anomalies
Authors Raffaele Persico and Francesco SoldovieriABSTRACTIn this paper, we deal with the problem of two‐dimensional electromagnetic linear inverse scattering from dielectric and magnetic anomalies buried in a half‐space. First, the formulation of the exact model of the electromagnetic scattering is given. Then, the problem is faced under a simplified model thus arising a linear inverse problem that is solved by resorting to the well assessed singular value decomposition tool. The reliability of the solution procedure is tested with synthetic data achieved by a finite‐difference time‐domain code. While the presented linear inverse scattering algorithm is able to detect and localize the targets with a good accuracy, it is not able to identify the electromagnetic nature of the targets in terms or dielectric or magnetic properties.
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Interpreting complex, three‐dimensional, near‐surface GPR surveys: an integrated modelling and inversion approach
Authors T.M. Millington, N.J. Cassidy, L. Nuzzo, L. Crocco, F. Soldovieri and J.K. PringleABSTRACTWith the increasing computational power of modern personal computers, sophisticated modelling and inversion techniques are becoming popular tools for the interpretation of high‐resolution, fully three‐dimensional GPR surveys. In this paper, we present the latest results of ongoing practical research into the development of novel, integrated, finite‐difference time‐domain (FDTD) numerical modelling and linear tomographic inversion methods for the interpretation and analysis of near–surface, 3D GPR data. The proposed approach utilizes the Born approximation solution to the inverse‐scattering problem and a truncated singular value decomposition (TSVD) to create the final, inverted reconstructions. A three‐dimensional, full‐field, O(2,4) accurate FDTD modelling scheme is used to generate the numerical‐based Green’s functions and incident fields for the inversion. As such, accurate antenna sources (including the influence of shields) and near‐field air/ground interface effects are inherently included in the inversion formulation. The performance of this integrated method is evaluated via a simulated, 3D, forensic‐based, test‐case example (a 900 MHz survey over a clandestine human burial target) including coherent noise from near‐surface clutter. Although the example is simplistic, the results show that the scheme works well, despite some assumptions in the inversion methodology. As such, useful information can be gained on the true form, depth, location and spatial interrelationships of the buried features and, therefore, improved interpretations can be obtained in a three‐dimensional context.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)