Ground Penetrating Radar Model Research Articles

Ground penetrating radar investigations to study active faults in the Norcia Basin (central Italy)

A ground penetrating radar (GPR) survey was conducted across the Quaternary intramountain graben of the Norcia basin (Italy) in an effort to locate an active fault zone and to investigate the shallow geological structures . Measurements over an exposed (trenched) fault identify a radar signature consisting of hyperbolic diffractions in correspondence with the main fault's position. The migrated profile shows a good spatial correlation with the known fault at a control site. The average wave velocity of radar impulses in the ground was obtained by comparing with the field scans of real traces two synthetic signals, one at the hanging wall and one at the footwall of the fault. This analysis made possible to estimate the thickness of the sedimentary layers involved in the fault mechanism and the stratigraphic throw of the fault itself. The combined use of GPR across the probable northern continuation of the fault, with the information obtained with the study in the exposed fault, was used to select the location of new trench excavations. The GPR, being a relatively easy, non-invasive and high-resolution technique, can thus be used in palaeoseismological investigations, particularly for a preliminary investigation where the geological context is poorly defined. ► aGPR survey was conducted across an active fault zone in Norcia Basin (Italy). ► The wave velocity of radar impulses were obtained with a 1D synthetic GPR modelling. ► GPR survey was used to select the location of new trench excavations.

Hydrologic trajectories in transient ground-penetrating-radar reflection data

A variable-rate infiltration experiment was conducted in a sandbox to demonstrate that distinctive patterns are produced in transient ground-penetrating-radar (GPR) data collected during wetting and drying events. The observed GPR response was found to be very consistent with the results of numerical simulations performed using finite-difference time-domain modeling of GPR coupled with a 1D unsaturated flow model (HYDRUS-1D) for which the sand hydraulic properties were determined independently using core samples. Despite this agreement, few methods are available that can efficiently analyze transient GPR data to make a quantitative link between observed responses and the hydraulic properties of soils. To address this problem, a computationally efficient method is proposed that is analogous to coherency analysis used in multioffset surveys. The new method isbased on the calculation of semblance along trajectories through transient GPR data. Each trajectory represents a specific GPR arrival, e.g., the ground wave and reflections from the wetting front and subsurface boundaries. The specific path of the trajectories is controlled by the hydraulic properties of the soil, just as the normal-moveout trajectories used to calculate semblance in multioffset data are controlled by wave velocity. Because the method is based on the output of 1D unsaturated flow models, it can be used for situations with complex hydrologic boundary conditions. Good agreement was found in this study between the calculated trajectories and the arrivals observed for both simulated and empirical GPR data. A sensitivity analysis performed in this study suggests that most parameters of the Mualem–van Genuchten soil model can be identified using this approach to coherency analysis of transient GPR data.

Effectiveness of 2-D and 2.5-D FDTD Ground-Penetrating Radar Modeling for Bridge-Deck Deterioration Evaluated by 3-D FDTD

Computational modeling effectively analyzes the wave propagation and associated interaction within heterogeneous reinforced concrete bridge decks, providing valuable information for sensor selection and placement. It provides a good basis for the implementation of the inverse problem in defect detection and the reconstruction of subsurface properties, which is beneficial for defect diagnosis. The objective of this study is to evaluate the effectiveness of lower order models in the evaluation of bridge-deck subsurfaces modeled as layered media. The two lower order models considered are a 2-D model and a 2.5-D model that uses the 2-D geometry with a compressed coordinate system to capture wave behavior outside the cross-sectional plane. Both the 2- and 2.5-D models are compared to the results obtained from a full 3-D model. A filter that maps the 3-D excitation signal appropriately for 2- and 2.5-D simulations is presented. The 2.5-D model differs from the 2-D model in that it is capable of capturing 3-D wave behavior interacting with a 2-D geometry. The 2.5-D matches results from the corresponding 3-D model when there is no variation in the third dimension. Computational models for air-launched ground-penetrating radar with 1-GHz central frequency and bandwidth for the detection of bridge-deck delamination are implemented in 2-, 2.5-, and 3-D using FDTD simulations. In all cases, the defect is identifiable in the results. Thus, it is found that in layered media (such as bridge decks) 2- and 2.5-D models are good approximations for modeling bridge-deck deterioration, each with an order of magnitude reduction in computational time.

Forward Time Domain Ground Penetrating Radar Modeling of Scattering from Anomalies in the Presence of Steel Reinforcements

Ground penetrating radar (GPR) for nondestructive testing is applied to civil infrastructure such as bridges and roadways. Conventional methods of processing and analyzing GPR data for civil infrastructure are often qualitative, using relative reflection amplitude from subsurface boundaries or reinforcing steel (rebars) as an indicator of health. A Finite Difference Time Domain (FDTD) simulation of GPR is used to generate a bridge deck response of a heterogeneous model of a healthy bridge deck. The result is a healthy deck background that can be removed from measured data to bring anomalies to attention. For the purpose of this article, the measured data is also simulated. In lieu of modeling the identified rebars as perfect electrical conductors (PECs), they are modeled as hard point source excitations to allow for examination of the effect that the scattered waves from the rebar can have on an anomaly. This is an important consideration for application of many inversion methods.

Ground penetrating radar imaging and time-domain modelling of the infiltration of diesel fuel in a sandbox experiment

Ground penetrating radar (GPR) is a non-destructive method which, over the past 10 years, has been successfully used not only to estimate the water content of soil, but also to detect and monitor the infiltration of pollutants on sites contaminated by light non-aqueous phase liquids (LNAPL). We represented a model water table aquifer (72 cm depth) by injecting water into a sandbox that also contains several buried objects. The GPR measurements were carried out with shielded antennae of 900 and 1200 MHz, respectively, for common mid point (CMP) and constant offset (CO) profiles. We extended the work reported by Loeffler and Bano by injecting 100 L of diesel fuel (LNAPL) from the top of the sandbox. We used the same acquisition procedure and the same profile configuration as before fuel injection. The GPR data acquired on the polluted sand did not show any clear reflections from the plume pollution; nevertheless, travel times are very strongly affected by the presence of the fuel and the main changes are on the velocity anomalies. We can notice that the reflection from the bottom of the sandbox, which is recorded at a constant time when no fuel is present, is deformed by the pollution. The area close to the fuel injection point is characterized by a higher velocity than the area situated further away. The area farther away from the injection point shows a low velocity anomaly which indicates an increase in travel time. It seems that pore water has been replaced by fuel as a result of a lateral flow. We also use finite-difference time-domain (FDTD) numerical GPR modelling in combination with dielectric property mixing models to estimate the volume and the physical characteristics of the contaminated sand.

Characterization of snow cover using ground penetrating radar for vehicle trafficability – Experiments and modeling

For navigational and field testing purposes, it is highly desirable to obtain information about the geometry and material properties of terrains over which vehicles will traverse, preferably in a non-contact manner. The Ground Penetrating Radar (GPR) has emerged as a leading remote-sensing method to probe subsurface properties of terrains; the method detects the difference of the relative permittivity of adjacent layers; the relative permittivity is a function of the density of the material. To the best of our knowledge, no studies using GPR on snow have been conducted for winter mobility/trafficability. In this paper, we used natural and sieved, single- and double-layered snow of which density was varied in the range from 150 to 450 kg/m 3. Given the time-varying GPR signal of snow layers, we were able to determine the depth and dielectric constant ( ε) of each layer using the layer-stripping method. To better interpret the signal from GPR system, which has its limitations, we also conducted modeling and simulation of GPR operating on layered dry snow; given depth and dielectric constant of snow, we simulated the time-history of GPR signal in a layered medium using the Finite Difference Time Domain (FDTD) program GPRMAX as well as the transmission line theory (TLT). For mechanical properties, density is the most commonly used parameter in characterizing the strength of snow. We also obtained hardness of snow as a function of density using Rammsonde. It was found that hardness and relative permittivity increase with density indicating the potential of correlating relative permittivity to hardness; however, the spread of the data is such that future work is needed to quantify statistically this relationship.

Ground-penetrating-radar response to fracture-fluid salinity: Why lower frequencies are favorable for resolving salinity changes

Time-lapse ground-penetrating-radar (GPR) surveys exploit signal-amplitude changes to monitor saline tracers in fractures and to identify groundwater flow paths. However, the relationships between GPR signal amplitude, phase, and frequency with fracture aperture and fluid electrical conductivity are not well understood. We used analytical modeling, numerical simulations, and field experiments of multifrequency GPR to investigate these relationships for a millimeter-scale-aperture fracture saturated with water of varying salinity. We found that the response of lower-frequency radar signals detects changes in fluid salinity better than the response of higher-frequency signals. Increasing fluid electrical conductivity decreases low-frequency GPR signal wavelength, which improves its thin-layer resolution capability. We concluded that lower signal frequencies, such as [Formula: see text], and saline tracers of up to [Formula: see text] conductivity are preferable when using GPR to monitor flow in fractured rock. Furthermore, we found that GPR amplitude and phase responses are detectable in the field and predictable by EM theory and modeling; therefore, they can be related to fracture aperture and fluid salinity for hydrologic investigations of fractured-rock flow and transport properties.

Implementation of ADI-FDTD subgrids in ground penetrating radar FDTD models

Realistic numerical modeling of ground penetrating radar (GPR) using the finite-difference time-domain (FDTD) method could greatly benefit from the implementation of subgrids – supporting finer spatial resolution – 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.

Numerical modelling and experimental verification of GPR to investigate ring separation in brick masonry arch bridges

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 GPR—these 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 “micro regions” 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.

Numerical modeling of ground-penetrating radar in 2-D using MATLAB

We present MATLAB codes for finite-difference time-domain (FDTD) modeling of ground-penetrating radar (GPR) in two dimensions. Surface-based reflection GPR is modeled using a transverse magnetic (TM-) mode formulation. Crosshole and vertical radar profiling (VRP) geometries are modeled using a transverse electric (TE-) mode formulation. Matrix notation is used in the codes wherever possible to optimize them for speed in the MATLAB environment. To absorb waves at the edges of the modeling grid, we implement perfectly matched layer (PML) absorbing boundaries. Although our codes are two-dimensional and do not incorporate features such as dispersion in electrical properties, they capture many of the important elements of GPR surveying and run at a fraction of the computational cost of more elaborate algorithms. In addition, the codes are well commented, relatively easy to understand, and can be easily modified for the user's specific purpose.

Superimposed ice regime of a high Arctic glacier inferred using ground-penetrating radar, flow modeling, and ice cores

[1] A 900 MHz ground-penetrating radar (GPR) profile, collected along 1.7 km of the centerline of a high Arctic glacier in Svalbard, is interpreted using shallow ice core data and thermomechanical flow modeling. Four distinct zones are visible in the image and correspond to areas of firn, recent and older superimposed ice, and ablation zone glacier ice on the ground. The areas of firn and superimposed ice are characterized by relatively steeply and gently dipping internal reflecting horizons (IRH), respectively, in the radar image. The IRHs arise from permittivity contrasts due to density variations, produced in firn by alternating firn and ice layers, and in superimposed ice by varying air bubble content. Recently formed and older superimposed ice can be distinguished in the GPR image. A three-dimensional flow model is used to indicate the long-term mass balance and flow history responsible for the spatial distribution and orientation of superimposed ice IRHs in the radar image. Results indicate that the IRH distribution can only be explained by invoking a down-glacier shift in mean 30 year equilibrium line altitude (ELA) by 20–30 m, compared with measured mean 30 year ELA. They also suggest that many of the superimposed IRHs are relict features, produced >100 years B.P., when the ELA was 100–150 lower than at present. This work advocates combined GPR and flow modeling as a unique tool for validating mass balance measurements and/or inferring the mass balance/dynamic history of a glacier beyond a recent measurement period.

Numerical Modeling of GPR to Determine the Direct Ground Wave Sampling Depth

The direct ground wave method of ground penetrating radar (GPR) has been suggested as a cost‐effective means of estimating field‐scale soil moisture variability for irrigation and water resource management. Knowing the sampling depth of the GPR direct ground wave (GW) is very important because it is critical to know the depth when measuring soil moisture in the field. Few studies have addressed this particular aspect of the GPR method. Numerical simulation of GPR electromagnetic waves using GPRMAX2D was performed for two‐layer soil models to estimate the direct GW sampling depth for soil moisture. Dry over wet soil layers and wet over dry soil layers were modeled by using appropriate dielectric permittivity values for each layer. Model runs were conducted for a gradually decreasing upper layer thickness. The GW sampling depth was estimated as the upper dry or wet layer thickness when the modeled GW velocity decreased or increased by 5% as affected by the lower wet or dry layer, respectively. It was found from this modeling exercise that the GW sampling depth changed with the antenna frequency as well as the moisture content of the upper layer. A very strong linear relationship (r2 = 0.98) was found between the wavelength and the sampling depth of the GPR direct GW.

Finite difference time domain method forward simulation of complex geoelectricity ground penetrating radar model

The ground penetrating radar (GPR) forward simulation all aims at the singular and regular models, such as sandwich model, round cavity, square cavity, and so on, which are comparably simple. But as to the forward of curl interface underground or “v” figure complex model, it is difficult to realize. So it is important to forward the complex geoelectricity model. This paper takes two Maxwell’s vorticity equations as departure point, makes use of the principles of Yee’s space grid model theory and the basic principle finite difference time domain method, and deduces a GPR forward system of equation of two dimensional spaces. The Mur super absorbed boundary condition is adopted to solve the super strong reflection on the interceptive boundary when there is the forward simulation. And a self-made program is used to process forward simulation to two typical geoelectricity model.

Modeling of ground-penetrating Radar for accurate characterization of subsurface electric properties

The possibility to estimate accurately the subsurface electric properties from ground-penetrating radar (GPR) signals using inverse modeling is obstructed by the appropriateness of the forward model describing the GPR subsurface system. In this paper, we improved the recently developed approach of Lambot et al. whose success relies on a stepped-frequency continuous-wave (SFCW) radar combined with an off-ground monostatic transverse electromagnetic horn antenna. This radar configuration enables realistic and efficient forward modeling. We included in the initial model: 1) the multiple reflections occurring between the antenna and the soil surface using a positive feedback loop in the antenna block diagram and 2) the frequency dependence of the electric properties using a local linear approximation of the Debye model. The model was validated in laboratory conditions on a tank filled with a two-layered sand subject to different water contents. Results showed remarkable agreement between the measured and modeled Green's functions. Model inversion for the dielectric permittivity further demonstrated the accuracy of the method. Inversion for the electric conductivity led to less satisfactory results. However, a sensitivity analysis demonstrated the good stability properties of the inverse solution and put forward the necessity to reduce the remaining clutter by a factor 10. This may partly be achieved through a better characterization of the antenna transfer functions and by performing measurements in an environment without close extraneous scatterers.

Optimization of the transmitter-receiver separation in the ground-penetrating radar

The finite-difference time-domain method is applied to simulate three-dimensional subsurface-scattering problems, involving a ground-penetrating radar (GPR) model consisting of two transmitters and a receiver. The receiving antenna is located in the middle of the two identical transmitters, which are fed 180/spl deg/ out of phase. This configuration implies the existence of a symmetry plane in the middle of two transmitters and the cancellation of the direct signals coupled from the transmitters at the receiver location. The antenna polarizations and their separations are arbitrary. The transmitter-receiver-transmitter configured GPR model is optimized in terms of the scattered energy observed at the receiver by varying the antenna separation. Many simulation results are used to demonstrate the effects of the antenna separation and the optimal separation encountered for a specific target and GPR scenario.

Transmitter‐receiver‐transmitter‐configured ground‐penetrating radars over randomly heterogeneous ground models

Ground‐penetrating radar (GPR) problems are simulated using the finite‐difference time‐domain (FDTD) method. The GPR model is configured with arbitrarily polarized three antennas, two of which are transmitting antennas fed 180° out of phase. The receiver is placed in the middle of two transmitters, where it receives no direct coupling from the transmitting antennas. The ground is modeled as a dielectric, lossy, and heterogeneous medium. The performances of the transmitter‐receiver‐transmitter‐configured GPRs above the heterogeneous ground models are investigated. The computational domain is terminated by perfectly matched layer (PML) absorbing boundaries. The PML is adapted to match both air and ground regions of the computation space.

Electric field of a horizontal antenna above a homogeneous half‐space: Implications for GPR

Inverse modeling and interpretation of subsurface structures depend on accurate knowledge of the undisturbed field. This is especially true in the analysis of radargrams, in which it is difficult to resolve the upper homogeneous medium from the less shallow scatterers. The available forward models based on plane‐wave and ray approximation are not accurate enough for this task. To improve resolution capabilities, we determine the undisturbed field using exact expressions for the electric field of a sine‐shaped ground‐penetrating radar (GPR) signal antenna above a homogeneous half‐space. In the frequency domain it consists of the sum of two improper integrals with complex integrands. Each integrand contains a kernel multiplied by a Bessel function of the first kind and of order zero or one. In the general case these integrals do not have a solution in closed form, and their integrands are poorly convergent. Therefore, to solve the integrals we must use a special formalism involving integrals around branch points. When we assume that both the transmitter and the receiver are on the boundary of the half‐space, there exist analytic solutions for the first integral without further restrictions and for the second integral for two special cases: free space and half‐space, neglecting displacement currents. We check our corresponding numerical results against these analytic solutions. In the time domain we represent the electric field as a function of transmitter‐receiver offset and time. For a purely dielectric half‐space the backtransformation of the first integral is analytical under the assumed simplification, allowing us to check the numerical results obtained with a fast Fourier transform (FFT) algorithm. These results allowed us to design radargrams for five different models of a homogeneous earth, and they are fundamental for interpretation and further research of GPR modeling.

GPR modeling study in a contaminated area of Krzywa Air Base (Poland)

Krzywa Air Base is a former Soviet military base which is highly polluted because of soil contamination during the dismantling of fuel tanks after the 1990 political changes in the U.S.S.R. The contaminated sites are indicated as magazines 1 and 2 in Figure 1. The area is located within the pre‐Sudetic block, where Tertiary and Quaternary deposits lie on Mesozoic crystalline basement. The deposits consist of sands of different grain sizes, gravels, clays, and interbedded silts, with a total thickness ranging from 80 to 100 m. In the extreme cases, the pollution is so intense that the spilled fuel can be extracted in the system of drainage wells. In general, the contamination can be correlated with the geologic structure. As a prelude to acquisition of field data, we simulate and evaluate the expected ground‐penetrating radar (GPR) responses for uncontaminated and contaminated layers.