Abstract: Very few researchers have developed numerical models of ground-penetrating radar (GPR) that include realistic descriptions of both the antennas and the subsurface. This is essential to be able to accurately predict responses from near-surface, near-field targets. We have developed a detailed 3D finite-difference time-domain models of two commercial GPR antennas â a Geophysical Survey Systems, Inc. (GSSI) 1.5-GHz antenna and a MALÃ Geoscience 1.2-GHz antenna â using simple analyses of the geometries and the main components of the antennas. Values for unknown parameters in the antenna models (due to commercial sensitivity) were estimated by using Taguchi's optimization method, resulting in a good match between the real and modeled crosstalk responses in free space. Validation using a series of oil-in-water emulsions to simulate the electrical properties of real materials demonstrated that it was essential to accurately model the permittivity and dispersive conductivity. When accurate descriptions of the emulsions were combined with the antenna models, the simulated responses showed very good agreement with real data. This provides confidence for use of the antenna models in more advanced studies.
Abstract: Realistic numerical modeling of ground penetrating radar (GPR) using the finite-difference time-domain (FDTD) method could greatly benefit from the implementation of subgrids â supporting finer spatial resolution â into the conventional FDTD mesh. This is particularly important, when parts of the computational domain need to be modeled in detail or when there are features or regions in the overall computational mesh with values of high dielectric constant supporting propagation of waves at very short wavelengths. A scheme that simplifies the process of implementing these subgrids into the traditional FDTD method is presented. This scheme is based on the combination of the standard FDTD method and the unconditionally stable alternating-direction implicit (ADI) FDTD technique. Because ADI-FDTD is unconditionally stable its time-step can be set to any value that facilitates the accurate calculation of the fields. By doing so, the two grids can efficiently communicate information across their boundary without requiring to use a costly time-interpolation scheme. This paper discusses the performance of ADI-FDTD subgrids when implemented into the traditional FDTD method, using different communication schemes for the information exchange at the boundary of the two grids. The developed algorithm, can handle cases where the subgrid crosses dielectrically inhomogeneous media. In addition, results from the comparison between the proposed scheme and a commonly employed purely FDTD subgridding technique are presented.
Abstract: Successful remediation of sites contaminated with dense non-aqueous phase liquids (DNAPLs) requires adequate characterisation of the volume and extent of the DNAPL source zone. Ground penetrating radar (GPR) has been proposed to characterise the subsurface distribution of DNAPLs; however, its effectiveness for real applications remains unproven. The objective of this study was to evaluate the potential of GPR to map realistic DNAPL-spill scenarios within heterogeneous subsurface environments, and to monitor the progress of subsequent remedial efforts. This was investigated by creating a novel link between two published numerical models: DNAPL3D-MT to generate a realistic DNAPL scenario, and GPRMAX to simulate a sequence of GPR surveys applied at the surface. A published volumetric mixing model permitted conversion of an evolving hydrogeological domain into a bulk permittivity domain. A field-scale, two-dimensional, surface release of a chlorinated solvent DNAPL into heterogeneous silty sand was employed as a demonstration case, including complete DNAPL remediation by dissolution and its mapping by time-lapsed 100 MHz surface GPR scans. Qualitative and quantitative interpretation of the results reveal that realistic GPR scans are simulated, generating a complex GPR response that is sensitive to both variations in DNAPL saturation and intrinsic subsurface heterogeneity. As a result, deconvoluting the response in the absence of a pristine site scan remains challenging. However, with the aid of newly developed GPR analysis tools presented here, including the combination of âsequential scan subtractionâ with ânormalized radar trace and radar section sum of squaresâ, changes in DNAPL distribution (mass reduction and remobilization) are demonstrated to be quantifiable. Thus, it is concluded that periodically monitoring the time-lapsed remediation of the source zone with GPR is particularly promising.
Abstract: Cryoconite holes form on ice due to enhanced ablation around particles deposited on the surface, and are present in the ablation area of glaciers worldwide. Here we investigate the use of Ground Penetrating Radar (GPR) as a non-destructive method to monitor and map cryoconite holes. We compare GPR data obtained from the Jutulsessen blue ice area in Dronning Maud Land, Antarctica, with modeled GPR data. The modeled GPR response to cryoconite holes is numerically calculated by solving Maxwell's equations with a 3D Finite-Difference Time-Domain (FDTD) scheme. The model includes a realistic shielded bowtie antenna and dimensions and constituent parameters of cryoconite holes excavated in the field. We have performed what-if scenarios with controlled variation of single parameters. We show that GPR can be used to determine the horizontal extent, depth and whether a cryoconite hole is frozen or contains liquid water, information unavailable from visual surface inspection. The cryoconite thickness can, for completely frozen holes, be determined to within a 1/4 of the GPR center frequency wavelength. The exact water content is not readily extractable because the GPR response is influenced by many other factors such as: cryoconite thickness, shape and roughness, as well as antenna ground coupling.
Abstract: We use ground-penetrating radars (GPRs), firn cores, and electromagnetic finite-difference time-domain (FDTD) numerical modeling to characterize the GPR response to a frozen high-arctic firn pack. As a result of extensive summertime percolation, the firn pack comprises a high fraction of ice layers, lenses, and vertical glands. We show that the GPR response on the firn pack mainly depends on the following: (1) the thickness of the ice layers; (2) the distance between layers; (3) the layer roughness; and (4) the presence or absence of elliptical ice lenses. Using 3-D FDTD modeling, we show that the GPR is not sensitive to typical ice glands, which implies that the GPR underestimates firn heterogeneity, such that firn stratigraphy in percolation and wet-snow zones could be incorrectly interpreted as being better preserved than it actually is. We find that thin ice layers (< 0.05 m) or multiple thin ice layers give a strong response. Thicker ice layers typically give a weaker backscatter per unit area, mainly due to the lack of interference of the reflections from the upper and lower interfaces, but are, due to their continuity, easily trackable. Ice layers with a thickness comparable to the GPR wavelength give 180deg phase-shifted upper and lower reflections and are, in general, separated by a band of low GPR response, due to the lack of permittivity contrast within the ice layers. Despite the ice lensesâ relatively short horizontal correlation length, as inferred from cores, bands of high-amplitude clutter caused by these features can be traced over several kilometers in GPR profiles.
Abstract: A new implementation of the perfectly matched layer absorbing boundary for finite-difference time-domain grids is presented. The approach which is based on the complex co-ordinate stretching perfectly matched layer (PML) formulation uses the complex frequency shifted stretching function and is based on the simple concept of the recursive evaluation of an integral avoiding the calculation of time derivatives. This recursive integration PML is simple to implement, efficient and exhibits a modest gain in performance over the convolutional PML without requiring any extra computational resources or an increase in the algorithmic complexity of the PML implementation.
Abstract: The application of ground-penetrating radar (GPR) as a non-destructive technique for the monitoring of ring separation in masonry arch bridges was studied. Numerical modelling techniques were used to simulate tests using GPRâthese numerical experiments were backed up and calibrated using laboratory experiments. Due to the heterogeneity of these structures, the signals coming from the interaction between the GPR system and the bridge are often complex, and hence hard to interpret. This defined the need to create a GPR numerical model that will allow the study of the attributes of reflected signals from various targets within the structure of the bridge. The GPR numerical analysis was undertaken using the finite-difference time-domain (FDTD) method. Since âmicro regionsâ in the structure need to be modelled, subgrids were introduced into the standard FDTD method, in order to economize on the required memory and the calculation time. Good correlations were obtained between the numerical experiments and actual GPR experiments. It was shown both numerically and experimentally that significant mortar loss between the masonry arch rings can be detected. However, hairline delaminations between the mortar and the brick masonry cannot be detected using GPR.
Abstract: Ground-penetrating radar (GPR) modelling is employed to study the electromagnetic wave scattering that emanates from a rough subsurface interface. The numerical analysis is achieved by using a finite-difference time-domain numerical modelling algorithm. For the 2D GPR models, the rough interfaces are generated using both Gaussian and fractal statistics. For a given root mean square interface roughness height, we study the effect that the change in correlation length has on the incident wavefield for a randomly generated surface with a Gaussian roughness spectrum. Similarly, for an interface generated using fractal statistics, we examine the effect of fractal dimension for two different values of maximum wavenumber. For the Gaussian case, the correlation length is the most important parameter in defining the target signature. But, when fractals are used, an increase in clutter is observed as the fractal dimension increases. In addition, the clutter is more pronounced when the highest value of maximum wavenumber is used. Detection performance is evaluated by utilising 3D ground-penetrating radar models. Comparing the 3D models with and without the presence of a rough subsurface interface, increased wavefield attenuation and strong depolarization effects were observed from a fractal subsurface terrain.
Abstract: The perfectly matched layer (PML) is nowadays considered as the best optimum absorbing boundary condition available. However, the PML with the classical stretching tensor has certain limitations. Strangely, these limitations have rarely been addressed in elastic wave modelling. For example, substantial reflections occur when strong evanescent waves are propagating parallel to the interface. To circumvent problems like this, the complex frequency shifted stretching tensor has been introduced in electromagnetic modelling. In this paper we show that the convolution PML with this stretching tensor as used in electromagnetic modelling can be adapted for elastic wave modelling. Numerical results of a model where the presence of evanescent waves is predominant show that the PML based on the complex frequency shifted stretching tensor can improve the performance of the absorbing boundary layer considerably.
Abstract: In finite-difference time-domain (FDTD) modeling of elastic waves, absorbing boundary conditions are used to mitigate undesired reflections that can arise at the model's truncation boundaries. The perfectly matched layer (PML) is generally considered to be the best available absorbing boundary condition. An important but rarely addressed limitation of current PML implementations is that their performance is severely reduced when waves are incident on the PML interface at near-grazing angles. In addition, very low frequency waves as well as evanescent waves could cause spurious reflections at the PML interface. In electromagnetic modeling, similar problems are circumvented by using a complex frequency-shifted stretching function in the PML formulas. However, in elastic-wave modeling using the conventional PML formulation â based on splitting the velocityand stressfields â it is difficult to adopt a complex frequency--shifted stretching function. We present an alternative implemen-tation of a PML that is based on recursive integration and does not require splitting of the velocity and stress fields. Modeling re-sults show that the performance of our implementation using a standard stretching function is identical to that of the convention-al split-field PML. Then we show that the new PML can be modi-fied easily to include the complex frequency-shifted stretching function. Results of models with an elongated domain show that this modification can substantially improve the performance of the PML boundary condition. An efficient implementation of the new PML requires less memory than the conventional split-field PML, and, therefore, is a very attractive alternative to the con-ventional PML. By adopting the complex frequency-shifted stretching function, the PML can accommodate a wide variety of model problems, and hence it is more generic.
Abstract: This paper deals with the fundamentals of ground penetrating radar (GPR) operation and presents a software tool that can be used to model GPR responses from arbitrarily complex targets. This software tool called GprMax is available free of charge for both academic and commercial use and has been successfully employed in situations, where a deeper understanding of the operation and detection mechanism of GPR was required. Examples from both 2D and 3D models are presented which demonstrate the use of GprMax. The tool can be downloaded from www.gprmax.org or by contacting the author.
Abstract: Sensors often are piezoelectric crystal transducers that convert movement (a variation of pressure) into an electrical voltage. Several non-destructive techniques involve the use of transducers, such as sonic testing, tomography, acoustic emission and pulse-impact echo. There are different types of transducers according to their different aims and applications, but in all cases the mounting of a sensor is an essential requirement in order to record good quality dataâa good acoustic coupling between the transducer and the surface of the structure has to be ensured. It is common practice to use Cyanoacrylate adhesive glue (e.g. superglue) for most applications, but the authors found its use problematic during temporary installations due to the difficulties encountered to remove the sensor at the end of the experiment. For this reason, a study has been carried out to investigate possible alternative couplant materials. Eight different materials have been selected, and their amplitude of response in terms of time-domain and the frequency-domain has been compared. A final evaluation based on several pre-defined criteria has then been obtained, showing the feasibility of âplasticineâ as a valid alternative to superglue.
Abstract: A review of established techniques used to determine radar wave velocities within different materials is presented. A new semi-intrusive method for determining the velocity of impulse radar energy through materials is proposed based on the refraction of electromagnetic waves as they transmit through layers of different materials. Inserting a metal bar into the material and traversing the surface of it enables the impulse radar velocities of the material under investigation to be readily interpreted from the radargram. This new method provides a quick, readily interpreted and practical tool for ground penetrating radar operators to use.
Abstract: Some concern exists over the safety and durability of the 600 post-tensioned bridges in the UK, and the much larger number worldwide. The objective of the work reported herein was to identify voiding in the metallic tendon ducts in these bridges. Voiding can give rise to two sets of problems: (a) possible ingress of chlorides, which would cause corrosion; and (b) a lack of redistribution of stress within the beam. It was against this background that it was important to first of all identify the extent of voiding in post-tensioned bridges.
The new technique of ultrasonic tomography was used for the trials reported in this paper. Two test beams were examined: a 10 m long beam at the Transport Research Laboratory (TRL), Crowthorne, UK and a short test beam constructed at Stanger Science and Environment, Elstree, UK. The ducts in the TRL beam were 40 mm in diameter. This is smaller than would normally be encountered in a post-tensioned bridge beam. A more usual duct diameter would be 100â110 mm with a cover of around 125 mm. The second test beam at Stanger Science and Environment, Elstree contained 100-mm diameter ducts.
The time-of-flight tomography data obtained demonstrated that it is a potentially highly successful method of investigating post-tensioned concrete beams. The method is somewhat time consuming and so should be used in conjunction with a simpler testing method, e.g. sonic impact-echo, which identifies areas of interest. The smaller the ducts to be investigated, the smaller the required distances between testing stations. This therefore significantly increases the testing time.
Abstract: The use of pulsed radar for investigating the integrity of structural elements is gaining popularity and becoming firmly established as a nondestructive tests method in civil engineering. Difficulties can often arise in the interpretation of results obtained, particularly where internal details are relatively complex. One approach that can be used to understand and evaluate radar results is through numerical modeling of signal propagation and reflection. By comparing the results of a numerical modeling with those from field measurements, engineers can gain valuable insight into the probable features embedded beneath the surface of a structural element. This paper discusses a series of numerical techniques for modeling subsurface radar and compares the precision of the results with those taken from real field data. It is found that the more complex problems require more sophisticated analysis techniques to obtain realistic results with a consequential increase in the computational resources to carry out the modeling.
Abstract: A large-scale magnetic survey was conducted in the archaeological area of Makrygialos. The site was threatened due to the construction activities carried out in the area, as part of the national highway re-route project. Geophysical prospection contributed to the archaeological evaluation of the site, which was based mainly on the salvage excavations that took place prior to and after the geophysical survey.
Magnetic prospecting was applied on a routine base, in order to cover a large area in a short period of time. Also, magnetic susceptibility was used to acquire detailed information of the stratigraphy of the ditches revealed by the excavations. The Le Borgne contrast was calculated and was used as an index of the magnitude of the magnetic anomalies.
Geophysical data were processed by a number of filtering techniques, including the removal of regional trends and Hanning smoothing. Fourier transformation was applied and bandpass filtering procedure was based on the examination of the power spectrum of the data. In addition, two-dimensional inversion filtering was performed at specific parts of the data set, in an effort to rectify the significant geophysical anomalies of the site and obtain more information about their width and magnetization. The results of the geophysical survey were able to highlight a system of three curvilinear ditches, which excavation data suggested were probably dug during the Neolithic period. Various linear and geometrical anomalies, related to subsurface structures, are included among the other geophysical features encountered at the site. The geophysical prospecting techniques were able to map more than 60,000 m2 of the site, a large portion of which has now been destroyed by the construction activities for the national road. In this way, geophysical maps can be used as a valuable source of information for the future study of the site. The present case study illustrates the impact of geophysical exploration in the management of archaeological sites threatened by large-scale construction projects.
Abstract: A modification to the near-field to far-field transformation scheme by Luebbers et al. for the finite difference time domain (FDTD) method is introduced. For a worst case scenario, results indicate an improvement in the accuracy of the calculated far-field responses and radar cross-section (RCS) in comparison to the original algorithm.
Abstract: Both in the transmission line matrix (TLM) and the finite difference time domain (FDTD) methods, absorbing boundary conditions (ABCs) are used to truncate the computational domain for the simulation or open boundary problems. A comparison of the performance of ABCs applied to TLM and FDTD is presented. The results indicate a significant improvement in the performance of an ABC when applied to TLM compared to the performance of the same ABC when applied to FDTD. An explanation for this improvement in the performance is given.
Abstract: The implementation of subgrids in the traditional finite-difference time-domain (FDTD) method is often required, especially when structures of fine geometry need to be modeled. Since the FDTD method is conditionally stable, different time-steps should be employed in the main grid and in the subgrid. To overcome the requirement for time interpolation at the boundary between the two grids, an unconditionally stable method, the alternating-direction-implicit (ADI-FDTD) method, has been used in the subgrid. As a result both the main FDTD grid and the subgrid use the same time-step. This paper presents an investigation into the performance of an ADI-FDTD subgrid when it is implemented into the conventional FDTD method.
Abstract: In processing GPR data from road surveys the amplitudes of reflection data have often been used in obtaining an estimate of the dielectric properties of pavement layers and consequently an estimate of the layers' thicknesses. The values of the dielectric constants of the layers are estimated using a recursive procedure based on assumptions of plane wave propagation, one dimensional target geometry and that all media probed by the GPR are low-loss. In practice, this kind of data processing requires the use of a calibration procedure in the field which entails the use of a metal sheet placed on the surface of the ground. This is used in order to provide a reference reflected GPIl amplitude by assuming that the reflection coefficient of the metal sheet is known. This reference information is then used in the recursive determination of reflection coefficients of the layer interfaces which are then used to obtain values for the dielectric constants of the layers. Although, the assumption of one dimlensional geometry (layered earth) appears to be a good approximation for at least short sections of GPR road data the other assumptions involved in establishing the processing procedure may not be as valid and therefore they may have an effect on the accuracy of the final layer thickness calculations. This paper presents the results of a numerical investigation into the accuracy of such processing procedure. Both, a two dimensional and a three dimensional numerical simulators for GPIt have been employed to investigate the accuracy with which valuies of dielectric constants of layered models can be estimated fronn simulated reflection amplitude GPR data. This relates to the accuracy of the calculation of the layers' thicknesses which it shoiild be noted that is the important engineering parameter that
needs to be determined by the GPR survey. Although, )he two dimensional models employ a theoretical line source as a model for the GPR's transducers the three dimensional ones employ a complete model of a GPR antenna. The GPR simulators are based on the finite-difference time-domain method.
Abstract: GPR detection of voids in post-tensioned concrete bridge beams. [Proceedings of SPIE 4758, 376 (2002)]. Antonios Giannopoulos, Paul Macintyre, Scott Rodgers, Mike C. Forde. Abstract. Not available.
Abstract: As test sites for ground penetrating radar (GPR), archaeological sites offer a set of demanding (high-resolution) application contexts in a situation where repeated studies may be made over a range of non-trivial targets (i.e. not always clearly defined or predictable, hence requiring diagnosis and evaluation, rather than mere detection). When the points above are further taken into account, they offer excellent opportunities for the test, evaluation and development of equipment, modelling approaches, and interpretational/processing tools. The modelling approach discussed is the transmission line matrix method (TLM). This method represents the volume of interest as a mesh of two-wire transmission lines and provides a formalism where Maxwell's equations governing propagation of an electromagnetic wave through a variable media can be described in a discrete circuit theory form. A discrete-time approach is taken where scattering takes place at the nodes of the mesh due to discontinuities in the line impedances: the propagation of the waves can then be established over time, with one time-step being defined by the node-node travel time. The approach can be successfully applied to the use of GPR systems in a wide variety of contexts and can cope with complex subsurface structures.
