Ground Penetrating Radar Wave Propagation Research Articles

An Experimental Study of GPR Near-Field Problem in Depth Ranging of Buried Objects

We evaluate the often neglected ranging (or cover depth) error of an object located within the Fresnel Region (or called radiating near-field) and reactive near-field region in ground penetrating radar (GPR). The experiments were conducted in concrete specimens with various rebar cover depths from 10 to 80 mm, which imitates any object imaged by GPR from reactive near-field, Fresnel region to far-field region. Velocities of each individual data point of the hyperbolic reflections of the embedded reinforcement in radargram are calculated by developed algorithms based on common offset antenna and multiple trilaterated ray paths. Two different velocity algorithms, based on semi- and full-trilaterated ray paths, were applied to estimate the wave travelling velocity as a prerequisite of cover depth measurement. The algorithms were firstly validated using a high-frequency 2-GHz antenna to verify the travelling speed of radar waves in air, which is equal to the speed of light. Then, the same antenna was used to estimate the cover depth by measuring the time of flight and velocity based on the two velocity algorithms. The results reveal that the ranging accuracy is highly in doubt in reactive near-field and Fresnel regions but is largely improved when the object locates in far-field region where GPR wave propagation becomes plane-type. The measurements, riding on accurate modelling of ray-path’s trilateration models, provide evidence that the normal linear scale of GPR time-to-depth conversion overlooks the effects of near-field/Fresnel region. We therefore suggest that the GPR near-field/Fresnel region and far-field boundary must be taken into account before any attempt of time-to-depth conversion and depth estimation of objects by GPR are carried out.

Adding realistic noise models to synthetic ground‐penetrating radar data

ABSTRACTCost‐effective computing capabilities have paved the road for the use of numerical modelling to develop advanced methods and applications of ground‐penetrating radar (GPR). Realistic synthetic data and the corresponding modelling techniques, respectively, should consider all subsurface and above‐ground aspects that influence GPR wave propagation and the characteristics of recorded signals. Critical aspects that can be realized in modern GPR modelling tools include heterogeneous and frequency‐dependent material properties, complex structures and interface geometries as well as three‐dimensional antenna models, including the interaction between the antenna and the subsurface. However, realistic noise related to the electronic components of a GPR system or ambient electromagnetic noise is often not considered, or simplified by assuming a white Gaussian noise model which is added to the modelled data. We present an approach to include realistic noise scenarios as typically observed in GPR field data into the flow of modelling synthetic GPR data. In our approach, we extract the noise from recorded GPR traces and add it to the modelled GPR data via a convolution‐based process. We illustrate our methodology using a modelling exercise, where we contaminate a synthetic two‐dimensional GPR dataset with frequency‐dependent noise recorded in an urban environment. Comparing our noise‐contaminated synthetic data with field data recorded in a similar environment illustrates that our method allows the generation of synthetic GPR with realistic noise characteristics and further highlights the limitations of assuming pure white Gaussian noise models.

Second-order symplectic PRK scheme for ground-penetrating radar simulations in dispersive materials

An accurate and effective forward model is helpful in interpreting measured ground-penetrating radar (GPR) data. In this paper, an explicit second-order symplectic partitioned Runge–Kutta scheme (SPRK) with Higdon's absorbing boundary condition is developed to simulate GPR wave propagation in dispersive media. For this purpose, the first-order Debye dispersion model is successfully applied to the second-order SPRK method. A single-layer dispersive medium model and a three-layer pavement structure model are simulated, in order to verify the accuracy and efficiency of the second-order SPRK method, as well as to clarify the effect of dispersion on the GPR echo signal. The numerical simulation results demonstrate that the proposed algorithm not only can maintain the simulation accuracy, but can also save about 19% of the calculation time, when compared with the FDTD algorithm, in a dispersive medium. We also obtain the GPR profile of a pavement structure with a circular cavity disease and an underground pipe structure in a dispersive medium, and the simulation results show that the dispersion characteristics of the materials lead to high absorption of the GPR electromagnetic waves. Finally, field experiment data are used to further verify the applicability of the algorithm. The numerical simulations and experiments indicate that the dispersion of the medium must be considered in GPR numerical simulation, and the authenticity of the simulated image must be ensured.

Analyzing the behavior of GPR wave propagation in zinc contaminated soil combining the dielectric properties—experimental study

There is a very strong link between the behavior of ground penetrating radar (GPR) wave propagation in zinc-contaminated soil and the dielectric properties of soil. This relationship can be of significant use in the practices of quick detecting the degree of pollution in zinc-contaminated soil. In this research, measurements were conducted on the zinc-contaminated soil samples with different soil index properties (i.e., zinc ion concentration, wet density and moisture content). The radar reflection wave data of the common midpoint and common-offset sounding mode were obtained by using the 600 MHz antenna, and the relative permittivity was measured using the vector network analyzer. The attribute analysis of radar reflection wave shows that the wave velocity is affected by wet density and moisture content, but independent of zinc ion concentration. Both the amplitude and the peak frequency decrease with the increase in zinc ion concentration, wet density and moisture content. For the soil dielectric properties, the metal ions can change the conductivity of solution in soil, affecting the imaginary part of relative permittivity, but with little effect on the real part. The positive correlations between the relative permittivity with density and moisture content are caused by the variation of three-phase composition of soil. Besides, the measured soil dielectric properties and the radar reflected wave attributes confirm each other, which can well explain the change rules of electromagnetic wave velocity, amplitude and central frequency. The presented results can increase understanding and confidence on GPR for quantitative monitoring and detecting of zinc-contaminated soil.

Sequential and coupled inversion of horizontal borehole ground penetrating radar data to estimate soil hydraulic properties at the field scale

Horizontal borehole ground penetrating radar (GPR) measurements can provide valuable information on soil water content (SWC) dynamics in the vadose zone, and hence show potential to estimate soil hydraulic properties. In this study, the performance of both sequential and coupled inversion workflows to obtain soil hydraulic properties from time-lapse horizontal borehole GPR data obtained during an infiltration experiment were compared using a synthetic modelling study and the analysis of actual field data. The sequential inversion using the vadose zone flow model HYDRUS-1D directly relied on SWC profiles determined from the travel time of GPR direct waves using the straight-wave approximation. The synthetic modelling study showed that sequential inversion did not provide accurate estimates of the soil hydraulic parameters due to interpretation errors in the estimated SWC near the infiltration front and the ground surface. In contrast, the coupled inversion approach, which combined HYDRUS-1D with a forward model of GPR wave propagation (gprMax3D) and GPR travel time information, provided accurate estimates of the hydraulic properties in the synthetic modelling study. The application of the coupled inversion approach to measured borehole GPR data also resulted in plausible estimates of the soil hydraulic parameters. It was concluded that coupled inversion should be preferred over sequential inversion of time-lapse horizontal borehole GPR data in the presence of strong SWC gradients that occur during infiltration events.

Analysis of GPR Wave Propagation in Complex Underground Structures Using CUDA-Implemented Conformal FDTD Method

Ground penetrating radar (GPR), as a kind of fast, effective, and nondestructive tool, has been widely applied to nondestructive testing of road quality. The finite-difference time-domain method (FDTD) is the common numerical method studying the GPR wave propagation law in layered structure. However, the numerical accuracy and computational efficiency are not high because of the Courant-Friedrichs-Lewy (CFL) stability condition. In order to improve the accuracy and efficiency of FDTD simulation model, a parallel conformal FDTD algorithm based on graphics processor unit (GPU) acceleration technology and surface conformal technique was developed. The numerical simulation results showed that CUDA-implemented conformal FDTD method could greatly reduce computational time and the pseudo-waves generated by the ladder approximation. And the efficiency and accuracy of the proposed method are higher than the traditional FDTD method in simulating GPR wave propagation in two-dimensional (2D) complex underground structures.

The Three Dimension First-Order Symplectic Partitioned Runge-Kutta Scheme Simulation for GPR Wave Propagation in Pavement Structure

Numerical simulation of three-layer layered electromagnetic waves is key problem for nondestructive testing of ground penetrating radar (GPR) pavement. In this paper, the difference iterative scheme of three-dimensional first-order symplectic partitioned Rung-Kutta is derived, which is applied to pavement detection of ground penetrating radar by using Higdon ABC boundary condition. Incident waves are considered as line sources. The accuracy and efficiency of the proposed algorithm are verified by the traditional 3D-FDTD algorithm. The results indicate that the accuracy and efficiency between the two methods are consistent. Unlike the traditional 3D-FDTD algorithm, the CPU time of the proposed method is reduced by 30%. The diseases location of the pavement structure is directly reflected by the numerical simulation result of the proposed method. This provides a three-dimensional symplectic partitioned Rung-Kutta algorithm, which can be applied to the forward simulation of GPR. It provides a three-dimensional symplectic partitioned Rung-Kutta algorithm, which can be applied to the forward simulation of GPR. The accurate electromagnetic response information of the target can be obtained by the proposed method.

Assessing the pavement subgrade by combining different non-destructive methods

The subgrade provides support to the pavement system and assures an effective distribution of traffic loads in depth. Therefore, a failure in the subgrade will have consequences on the entire pavement behaviour.This work presents an integrated approach for the analysis of the road subgrade condition by combining different Non-Destructive Testing (NDT) techniques. Different Ground Penetrating Radar (GPR) systems, both antennas configuration and frequencies, were tested in order to achieve the best methodology for subgrade cracking detection. Additionally, NDT load tests were performed with two deflectometers, Falling Weight Deflectometer (FWD) and Light Weight Deflectometer (LWD), aiming to determine the elastic modulus of the subgrade and consequently detect damaged areas.The tests were conducted at a real scale test section built to simulate pavement foundation layers consisting of clay soil subgrade, frequently used in African countries. The main tests performed are presented and analysed in this paper. Troubleshooting’s are referred mainly related with GPR wave propagation on clayey materials, due to high absorption. Recommendations are made regarding the use of GPR antennas as air-coupled antennas lead to a better identification of pavement layer interfaces while ground-coupled antennas were preferable to detect anomalous areas, namely cracking and debonding. The results showed good agreement between both NDT methods (GPR and load tests) in the identification of the anomalous areas and were validated with some in-situ cores extracted.

2D spectral element modeling of GPR wave propagation in inhomogeneous media

We present a spectral element method, for simulation of ground-penetrating radar (GPR) in two dimensions. The technique is based upon a weak formulation of the equations of Maxwell and combines the flexibility of the elemental-based methods with the accuracy of the spectral based methods. The wave field on the elements is discretized using high-degree Lagrange interpolation and integration over an element is accomplished based upon the Gauss–Lobatto–Legendre integration rule. As a result, the mass matrix and the damping matrix are always diagonal, which drastically reduces the computational cost. We first develop the formulation of 2D spectral element method (SEM) in the time-domain based on Maxwell's equations. The presented formulation is with matrix notation that simplifies the implementation of the relations in computer programs, especially in MATLAB application. We discuss the differences between spectral element method and finite-element method in the time-domain. Also, we show that the SEM numerical dispersion is much lower than FEM. To absorb waves at the edges of the modeling domain, we implement first order Clayton and Engquist absorbing boundary conditions (CE-ABC) introduced in numerical finite-difference modeling of seismic wave propagation. We used the SEM to simulate a complex model to show its abilities and limitations. As well as, one distinct advantage of SEM is that we can easily define our model features in nodal points, because the integration points and the interpolation points are similar that makes it very flexible in simulation of complex models.

Development of a new algorithm for accurate estimation of GPR's wave propagation velocity by common-offset survey method

The development of a new algorithm to estimate electromagnetic (EM) wave propagation velocity (v) with shielded ground penetrating radar (GPR) antenna with high-frequency of 2GHz is presented in this paper. Common offset method (fixed transmitter-receiver distance) is used for data collection, the antenna's separation distances and the corresponding triangulation geometry are also considered. The distribution of discrete velocities at each point oblique to the embedded target object is calculated by the proposed method. It is similar to the curve fitting process of hyperbolas in most software. The method has been validated in air and in concrete media and compared with the traditional hyperbola geometry method (ASTM D6432-11). The results show that errors of the new algorithm is significantly lower than the hyperbolic method. Besides, effects of analyzing reflected signals with different object depths are discussed. Based on the result, differentiation of velocity variations in concrete as a result of water seepage and leakage will be studied in next stage.

Ultra‐wide band antenna designs and numerical system modelling for forward‐looking GPR

ABSTRACTThis paper presents ultra‐wide band (UWB) horn feeder and parabolic reflector antenna designs, which are considered highly suitable for forward‐looking vehicle‐mounted ground‐penetrating radar (GPR) systems and time‐domain numerical modelling of impulse GPR signal radiation, wave propagation and target scattering. In this context, the partial dielectric‐loaded Vivaldi shaped TEM horn (PDVA), TEM horn fed double‐ridged horn (PDTEM‐RHA) and their compact versions are investigated, designed simulated and measured. An offset parabolic reflector antenna prototype with a compact PDTEM‐RHA feeder is proposed to reach hyper‐wide band impulse radiation performances from 500 MHz up to 15 GHz for a multi‐band forward‐looking GPR operation that can provide deep subsurface detection with high‐resolution imaging, due to a big advantage of narrowed beam widths. The antenna gain and radiation pattern performances are demonstrated with measurement results.

Application of Symplectic Partitioned Runge-Kutta Method and Total-Field Scattered-Field Formulation to Simulation of GPR Data

The second-order Lobatto IIIA-IIIB symplectic partitioned RungeKutta (SPRK) method, combining with the first-order Mur absorbing boundary condition, is developed for the simulation of ground penetrating radar wave propagation in layered pavement structure. For 2-dimetional case, a significant advantage of this method is that only two functions need to be calculated at each time step. The total-field/scattered-field technique is used for plane wave excitation. Numerical examples are presented to verify the accuracy and efficiency of the proposed algorithm. The results illustrate that the reflected signal calculated by the SPRK method is in good agreement with that obtained using the finite difference time domain (FDTD) scheme, but the CPU time consumed by proposed algorithm is reduce about 20% of the FDTD scheme. In addition, an actual field test is conducted to evaluate the further performance of the SPRK method. It is found that the simulated waveform fits well with the measured signal in many aspects, especially in the peak amplitude and time delay.

A Precise Integration Method for Modeling GPR Wave Propagation in Layered Pavement Structure

Construction of electromagnetic wave propagation model in layered pavement structure is a key step in back analysis of ground penetrating radar (GPR) echo signal. The precise integration method (PIM) is a highly accurate, eff... | Find, read and cite all the research you need on Tech Science Press

The First-Order Symplectic Euler Method for Simulation of GPR Wave Propagation in Pavement Structure

Construction of electromagnetic wave propagation model in layered pavement structure is a key problem for applying ground penetrating radar (GPR) to the road quality detection. A first-order explicit symplectic Euler method with Higdon absorbing boundary condition is presented to simulate GPR wave propagation in 2-D pavement structure. The incident wave is considered as line source and plane wave source, respectively. The total-field/scatter-field technique is used to simulate plane wave excitation. Numerical examples are provided to verify the accuracy and efficiency of the proposed algorithm. It can be observed that the symplectic Euler method achieves almost the same level of accuracy as the finite-difference time-domain scheme, while saving CPU time considerably.

A parallel 3‐D staggered grid pseudospectral time domain method for ground‐penetrating radar wave simulation

We presented a parallel 3‐D staggered grid pseudospectral time domain (PSTD) method for simulating ground‐penetrating radar (GPR) wave propagation. We took the staggered grid method to weaken the global effect in PSTD and developed a modified fast Fourier transform (FFT) spatial derivative operator to eliminate the wraparound effect due to the implicit periodical boundary condition in FFT operator. After the above improvements, we achieved the parallel PSTD computation based on an overlap domain decomposition method without any absorbing condition for each subdomain, which can significantly reduce the required grids in each overlap subdomain comparing with other proposed algorithms. We test our parallel technique for some numerical models and obtained consistent results with the analytical ones and/or those of the nonparallel PSTD method. The above numerical tests showed that our parallel PSTD algorithm is effective in simulating 3‐D GPR wave propagation, with merits of saving computation time, as well as more flexibility in dealing with complicated models without losing the accuracy. The application of our parallel PSTD method in applied geophysics and paleoseismology based on GPR data confirmed the efficiency of our algorithm and its potential applications in various subdisciplines of solid earth geophysics. This study would also provide a useful parallel PSTD approach to the simulation of other geophysical problems on distributed memory PC cluster.

A review of practical numerical modelling methods for the advanced interpretation of ground‐penetrating radar in near‐surface environments

ABSTRACTAs ground‐penetrating radar (GPR) technology improves, there is an increasing demand for more sophisticated and detailed interpretational tools. Numerical modelling has become one of the most popular advanced analysis methods and there is a wide range of electromagnetic modelling approaches available to the GPR user. The finite‐difference time‐domain (FDTD) technique is one of the most common, as it provides modellers with a robust, flexible, yet accurate, modelling scheme that is capable of simulating GPR wave propagation in complex, three‐dimensional, heterogeneous, lossy, subsurface environments. Through the use of a well‐constrained example, the realistic modelling of near‐surface GPR is evaluated including the practical limitations, computational procedures and application constraints of the FDTD method. The results show that, despite some obvious deficiencies, even a relatively basic FDTD model can provide important additional information for the advanced interpretation of observed GPR data.

An analysis of the multi-fold method for GPR exploration

Multi-fold technology is widely applied in seismic exploration as a method of enhancing useful signals and suppressing noise interference to greatly increase the signal to noise ratio (S/N). The authors introduce it to ground-penetrating radar (GPR) surveys and compare the experimental results to the conventional profiling method to demonstrate the feasibility and advantages of the technique for GPR exploration. Based on the experimental data, the authors summarize the GPR wave propagation rules and the parameters of multi-fold acquisition and processing. It is proven to be a useful attempt to enrich the GPR survey technology.