Ground Penetrating Radar Antenna Research Articles

Numerical Simulations and Laboratory Tests to Explore the Potential of Ground-Penetrating Radar (GPR) in Detecting Unfilled Joints in Brick Masonry Structures

The objective of this work is to define the sensitivity of the ground-penetrating radar (GPR) signal to detect deep unfilled joint defects in the inspection of brick masonry structures and, in particular, to look for deep unfilled joint defects. This definition will help the manager to quantify the volume of mortar to be reinjected in case of reinforcing work. As a first approach, a numerical modeling of a GPR antenna with a central frequency of 1.5 GHz is used to define the sensitivity of radar waves to detect unfilled joint defects. The simulations are carried out in a separated bistatic configuration. For each transmitter position, several signal acquisitions are implemented using a regularly spaced crescent pattern for the receivers. A specific algorithm for the processing of the simulated signals has been developed that uses the inverse methods applied in the time domain and specifically a method of phase focusing to locate the defects. The processing analyzes the travel times of the reflected signals by making the assumption that each point of the modeled space is a scattering point. The calculation of the travel time, which helps to identify the signal corresponding to each point of space, is made by using an estimated speed of the direct wave between transmitter and receivers in the material, which is then regarded as representative of the whole of the simulated environment. A parametric study allowed limits to be set in terms of size, orientation and depth of the defect. Early results are promising and show that unfilled joints can be detected in the depth of the masonry structure with good accuracy. The last stage of this work is to test the validity of these algorithms on a full scale model with different kinds of unfilled joint defects before using them on real structures.

Application of ground penetrating radar for coarse root detection and quantification: a review

Because of the crucial role coarse roots (>2 mm diameter) play in plant functions and terrestrial ecosystems, detecting and quantifying the size, architecture, and biomass of coarse roots are important. Traditional excavation methods are labor intensive and destructive, with limited quantification and repeatability of measurements over time. As a nondestructive geophysical tool for delineating buried features in shallow subsurface, ground penetrating radar (GPR) has been applied for coarse root detection since 1999. This article reviews the state-of-knowledge of coarse root detection and quantification using GPR, and discusses its potentials, constraints, possible solutions, and future outlooks. Some useful suggestions are provided that can guide future studies in this field. The feasibility and accuracy of coarse root investigation by GPR have been tested in various site conditions (mostly in controlled conditions or within plantations) and for different plant species (mostly tree root systems). Thus far, single coarse root identification and coarse root system mapping have been conducted using GPR, including roots under pavements in urban environment. Coarse root diameter and biomass have been estimated from indexes extracted from root GPR radargrams. Coarse root development can be observed by repeated GPR scanning over time. Successful GPR-based coarse root investigation is site specific, and only under suitable conditions can reliable measurements be accomplished. The best quality of root detection by GPR is achieved in well-drained and electrically-resistive soils (such as sands) under dry conditions. Numerous factors such as local soil conditions, root electromagnetic properties, and GPR antenna frequency can impact the reliability and accuracy of GPR detection and quantification of coarse roots. As GPR design, data processing software, field data collection protocols, and root parameters estimation methods are continuously improved, this noninvasive technique could offer greater potential to study coarse roots.

GPR in road investigations – equipment tests and quality assurance of new asphalt pavement

Ground penetrating radar (GPR) was successfully used in road applications in the Nordic countries as a nondestructive test procedure during the last decade. In order to standardise the procedures, boost cooperation and share knowledge between Finland, Sweden and Norway, a joint Mara Nord project was launched in 2010, involving national transport administrations, universities, consultant companies and GPR manufacturers. One outcome of the project will be recommendations to guidelines on different GPR applications. Within the frame of the project test, measurements of the most common GPR units and antennas were carried out. The results show that all tested equipment produce good results, and differences between the varying techniques are highlighted. Quality assurance of new asphalt pavements can be performed with the help of GPR, by relating the measured dielectric value to the void content of the asphalt pavement. The current method used in Finland is analysed. Recommendations and proposals for developing the quality assurance are presented. The project is scheduled to end by summer 2012.

Interpretation of Ground Penetrating Radar Image Using Digital Wavelet Transform

The ultra wide bandwidth nature of ground penetrating radar antenna has made a raw data acquired with the tool prone to unwanted noise and hence low signal to noise ratio. Quantitative interpretation of the data is desirable for radar image quality enhancement. This study used multiresolution analysis to process radar image at different levels of decomposition. Daubechie wavelets family was used to decompose the image into 4-different levels of details. Level 3 diagonal details and level 4 horizontal details provide a noise-free visualization of the subsurface discontinuities. This led to the detection of buried utility and unique identification of its spatial location. The depth of the buried utility was estimated based on the image scale. The work demonstrates the effectiveness of Daubechie wavelet transform analysis as yet another technique of utility survey data acquired with GPR.

Effect of antenna‐medium coupling in the analysis of ground‐penetrating radar data

ABSTRACTPhysically‐based ground‐penetrating radar (GPR) data processing is essential for quantitative characterization of soils and materials. A novel near‐field GPR antenna model coupled with layered media Green's functions was used to investigate the effect of antenna‐medium coupling in the analysis of GPR data. The radar antennas are modelled using an equivalent set of infinitesimal electric dipoles and characteristic, frequency‐dependent, global reflection and transmission coefficients. These coefficients determine through plane wave decomposition, wave propagation between the radar reference plane, point sources and field points. We calibrated an actual commercial 400 MHz time‐domain antenna, from which synthetic GPR data sets were generated. We observed that, depending on the model configuration, antenna effects may affect the topography of the objective function in full‐waveform inverse problems. In addition, antenna‐medium coupling has a significant impact on the medium surface reflection, whether in terms of amplitude or propagation time (which usually defines the so‐called time zero). We also showed that an effective source cannot be used for simulating near‐field radar data as the antenna‐medium coupling strongly depends on medium properties. In this respect, numerical experiments demonstrated promising perspectives for simultaneous estimates of medium permittivity and conductivity from antenna‐medium coupling.

A Time Domain Equivalent Source Model of an Impulse GPR Antenna Based on Measured Radiation Fields

Ground Penetrating Radar (GPR) is a valuable tool for determining bridge deck health. The ability to simulate scattering from bridge deck elements and the complex interactions between them, as well as from changes due to the presence and relative location of defects is important for understanding observed responses. These simulations can be performed using electromagnetic computational modeling techniques such as Finite-Difference Time Domain (FDTD). In order to accurately model the GPR investigation, it is necessary to have a time domain equivalent source model that can launch and receive electromagnetic waves into the computational space that replicates the signals transmitted and received by the physical GPR antenna. However, due to complexity of design and proprietary information, the GPR unit is typically very difficult, or even impossible, to fully model with sufficient detail. For bridge deck applications, simulation in two-dimensions adequately captures much of the three-dimensional scattering. Two-dimensional simulations have significant computational savings over three-dimensions, and are more feasible to be iteratively implemented to solve inverse problems. The work presented here uses experimental results and presents a computational approach to determine the characteristics suitable for excitation of a two-dimensional FDTD model.

Wideband Antenna With Conductive Textile Radiators for a Dual-Sensor Subsurface Detection System

A wideband antenna with conductive textile radiators (WACTR) is investigated for a pulsed dual-sensor subsurface detection system, which is composed of a ground penetrating radar (GPR) sensor and an electromagnetic induction (EI) sensor. It is demonstrated that undesired clutter of the EI sensor arising from the metallic mass and conductivity of the GPR antenna radiators can be removed by choosing textile material for the antenna without degradation of the antenna's performance. To the best of our knowledge, this is the first application of textile material used in antennas for such an aim. The antenna also has a new feeding structure to excite the textile patches, which offers a mechanically more rigid solution and a wider excitation surface. Some measured and simulated antenna characteristics are presented for the antenna located in the dual-sensor search head (DSSH). The ground effects on the antenna performance are additionally investigated as a complementary study. The detection results of some selected targets buried in soil by means of the proposed WACTR are presented. The targets are detected even in the non-metallic and deep-buried cases and it is also demonstrated that the GPR and EI sensors are complementary sensors.

Simulation Test of Equipment of Non-Contact Detection with GPR

The composite lining model of railway tunnel is set up and then simulation test is presented by non-contact detection with the simple detection equipment of ground penetrating radar (GPR) to study the main technique parameters of the equipment. The results show that the lining can be detected by the way, and the quality of GPR scan obtained in the simulation test decreases with the detection distance increasing. In the GPR scan obtained when keeping ground-coupled shielding antenna of 400MHz 20cm away from the ground(the distant from the bottom of GPR antenna to the surface of lining specimen), a variety of work conditions that has been preset in the experimental design can be identified, moreover, the location and range of both uncompacted backfill and cavity behind the lining etc. approach those that can be get when the antenna sticking to the lining, thus providing satisfactory qualitative interpretation of GPR scan. In addition, the deviation range of GPR non-contact detection is close to that when the antenna sticking to the lining, thus ensuring the detection value accuracy of lining thickness.

Analysis of Horn Antenna Transfer Functions and Phase-Center Position for Modeling Off-Ground GPR

The antenna of a zero-offset off-ground ground-penetrating radar can be accurately modeled using a linear system of frequency-dependent complex scalar transfer functions under the assumption that the electric field measured by the antenna locally tends to a plane wave. First, we analyze to which extent this hypothesis holds as a function of the antenna height above a multilayered medium. Second, we compare different methods to estimate the antenna phase center, namely, 1) extrapolation of peak-to-peak reflection values in the time domain and 2) frequency-domain full-waveform inversion assuming both frequency-independent and -dependent phase centers. For that purpose, we performed radar measurements at different heights above a perfect electrical conductor. Two different horn antennas operating, respectively, in the frequency ranges 0.2-2.0 and 0.8-2.6 GHz were used and compared. In the limits of the antenna geometry, we observed that antenna modeling results were not significantly affected by the position of the phase center. This implies that the transfer function model inherently accounts for the phase-center positions. The results also showed that the antenna transfer function model is valid only when the antenna is not too close to the reflector, namely, the threshold above which it holds corresponds to the antenna size. The effect of the frequency dependence of the phase-center position was further tested for a two-layered sandy soil subject to different water contents. The results showed that the proposed antenna model avoids the need for phase-center determination for proximal soil characterization.

Creating finite-difference time-domain models of commercial ground-penetrating radar antennas using Taguchi’s optimization method

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 — a Geophysical Survey Systems, Inc. (GSSI) 1.5-GHz antenna and a MALÅ Geoscience 1.2-GHz antenna — 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.

Quantifying field‐scale surface soil water content from proximal GPR signal inversion in the time domain

ABSTRACTWe applied inverse modelling of zero‐offset, air‐raised ground‐penetrating radar (GPR) data to measure soil surface water contents over a bare agricultural field. The GPR system consisted of a vector network analyser combined with a low‐frequency 0.2–2.0 GHz off‐ground monostatic horn antenna, thereby setting up an ultra‐wideband stepped‐frequency continuous‐wave radar. A fully automated platform was created by mounting the radar system on a truck for real‐time data acquisition. An antenna calibration experiment was performed by lifting the whole setup to different heights above a perfect electrical conductor. This calibration procedure allowed the flittering out of the antenna effects and antenna‐soil interactions from the raw radar data in the frequency domain. To avoid surface roughness effects, only the lower frequency range of 0.2–0.8 GHz was used for signal processing. Inversions of the radar data using the Green’s functions were performed in the time domain, focusing on a time window containing the surface reflection. GPR measurements were conducted every 4 m along a transect of 100 m. In addition, five time‐domain reflectometry measurements were randomly recorded within the footprint of the GPR antenna. A good agreement was observed between the GPR and time‐domain reflectometry soil water content estimates, as compared to the previous study performed at the same test site using a higher frequency 0.8–1.6 GHz horn antenna. To monitor the dynamics of soil water content, a pair of time‐domain reflectometry probes was installed at 8 cm depth near the footprint of the GPR antenna and both time‐domain reflectometry and GPR measurements were carried out for a period of 20 days. A good agreement of the trend was observed between the time‐domain reflectometry and GPR time‐lapse data with respect to several precipitation events. The proposed method and truck‐mounted setup appear to be promising for the real‐time mapping and monitoring of surface soil moisture contents at the field scale.

Structural and stratigraphic control on the migration of a contaminant plume at the P Reactor area, Savannah River site, South Carolina

Geophysical methods, including a shallow seismic reflection (SSR) survey, surface and borehole ground-penetrating radar (GPR) data, and electrical resistivity imaging (ERI), were conducted at the Savannah River site (SRS), South Carolina, to investigate the shallow stratigraphy, hydrogeophysical zonation, and the applicability and performance of these geophysical techniques for hydrogeological characterization in contaminant areas. The study site is the P Reactor area located within the upper Atlantic coastal plain, with clastic sediments ranging from Late Cretaceous to Miocene in age. The target of this research was the delineation and prediction of migration pathways of a trichloroethylene (TCE) contaminant plume that originates from the northwest section of the reactor facility and discharges into the nearby Steel Creek. This contaminant plume has been migrating in an east-to-west direction and narrowing away from the source in an area where the general stratigraphy along with the groundwater flow dips to the southeast. Here, we present the results from a stratigraphic and hydrogeophysical characterization of the site using the SSR, GPR, and ERI methods. Although detailed stratigraphic layers were identified in the upper approximately 50 m (164 ft), other major findings include (1) the discovery of a shallow (23 m [75 ft] from the ground surface) inverse fault, (2) the detection of a paleochannel system that was previously reported but that seems to be controlled by the reactivation of the interpreted fault, and (3) the finding that the hydraulic gradient seems to have a convergence of groundwater flow near the area. The interpreted fault at the study site appears to be of upper Eocene age and may be associated with other known reactivated faults within the Dunbarton Triassic Basin. The coincident use of the SSR and ERI methods in conjunction with the complementary 50-, 100-, and 200-MHz GPR antennas allowed us to generate a detailed geologic model of the shallow subsurface, suggesting that the migration of the TCE plume is constrained by (1) the paleochannel system with respect to its migration direction, (2) the presence of an inverse fault that may also contribute to the paleochannel growth and structural evolution, and (3) the local groundwater flow volume with respect to its longer and narrower shape away from the source updip stratigraphic bedding.

Comparing detection and location performance of perpendicular and parallel broadside GPR antenna orientations

GPR (Ground Penetrating Radar) results are shown for perpendicular broadside and parallel broadside antenna orientations. Performance in detection and localization of concrete tubes and steel tanks is compared as a function of acquisition configuration. The comparison is done using 100 MHz and 200 MHz center frequency antennas. All tubes and tanks are buried at the geophysical test site of IAG/USP in São Paulo city, Brazil. The results show that the long steel pipe with a 38-mm diameter was well detected with the perpendicular broadside configuration. The concrete tubes were better detected with the parallel broadside configuration, clearly showing hyperbolic diffraction events from all targets up to 2-m depth. Steel tanks were detected with the two configurations. However, the parallel broadside configuration was generated to a much lesser extent an apparent hyperbolic reflection corresponding to constructive interference of diffraction hyperbolas of adjacent targets placed at the same depth. Vertical concrete tubes and steel tanks were better contained with parallel broadside antennas, where the apexes of the diffraction hyperbolas better corresponded to the horizontal location of the buried target disposition. The two configurations provide details about buried targets emphasizing how GPR multi-component configurations have the potential to improve the subsurface image quality as well as to discriminate different buried targets. It is judged that they hold some applicability in geotechnical and geoscientific studies.

3D polarimetric GPR coherency attributes and full-waveform inversion of transmission data for characterizing fractured rock

Ground-penetrating radar (GPR) can detect and describe fractures to help us characterize fractured rock formations. A fracture alters the incident waveform, or wave shape, of a GPR signal through constructive and destructive interference, depending on the aperture, fill, and orientation of the fracture. Because the electromagnetic (EM) waves of GPR are vectorial, features exhibiting strong directionality can change the state of polarization of the incident field. GPR methods that focus on changes in waveform or polarization can improve detection and discrimination of fractures within rock bodies. An algorithm based on coherency, a seismic attribute that delineates discontinuities in wavelet shape, is developed for polarimetric GPR. It uses the largest eigenvalue of the time-domain scattering matrix when calculating coherence. This algorithm is sensitive to wave shape and is unbiased by the polarization of GPR antennas. Polarimetric coherency works better than scalar coherency in removing the effects of polarization on field data collected from a fractured limestone plot used for hydrologic experimentation. Another method, for time-domain full-waveform inversion of transmission data, quantitatively determines fracture aperture and EM properties of fill, based on a thin-layer model. Inversion results from field data show consistency with the location of fractures from reflection data. These two methods offer better fracture-detection capability and quantitative information on fracture aperture, dielectric permittivity, and electrical conductivity of the fill than traditional GPR imaging and scalar-coherency attributes.

Deployment Analysis of the Lenticular Jointed Antennas Onboard the Mars Express Spacecraft

DOI: 10.2514/1.36890 Extensive analytical and experimental activities were carried out that culminated in successful deployments in May and June of 2005 of three lenticular, jointed booms that formed a first of its kind of ground penetrating radar antenna onboard the Mars Express spacecraft. These activities went well beyond the normal required tasks due to a postlaunch realization that the stowed booms retained a high level of stored energy. This high level of stored energy resulted in an uncontrolled boom deployment rather than a predictable boom deployment. Experimentally measured straight section properties and hinge properties were incorporated into specialized modeling techniques that were then used to simulate the boom lenticular joints. System level models were exercised to understand the boom deployment dynamics and spacecraft level implications including spacecraft attitude control and possible entanglement. Discussion includes a comparison of deployment simulation results to measured flight data taken during the three boom deployments. Important parameters that govern lenticular joint behavior are outlined and a short summary of lessons learned and recommendations is included to better understand future applications of this technology.

Laboratory characterization of a GPR antenna for high-resolution testing: Radiation pattern and vertical resolution

High-resolution ground-penetrating radar (GPR) evaluations of structures are usually carried out using antennas with high nominal centre frequencies (between 1 and 2 GHz). A comprehensive characterization of such an antenna would make it possible to determine the capabilities of a system and obtain accurate data interpretations. This paper describes the experimental determination of the radiation pattern of a commercial 1.6 GHz antenna, which forms part of a comprehensive experimental characterization. Radiation patterns are closely related to spatial resolution, so the horizontal and vertical resolutions are evaluated first. The footprint of the antenna is then measured in air using simple devices at different distances. The final result is the approximate spatial radiation pattern of the emitted energy, measured in air. Finally, these measurements are also taken in sand to obtain the radiation pattern and footprint of the antenna in this medium.

CMP Antenna Array GPR and Signal-to-Clutter Ratio Improvement

Ground-penetrating radar (GPR) is recognized as a promising sensor for detecting buried landmines. In this case, the GPR antenna(s) must be elevated above the ground. However, this requirement results in heavy surface clutter. It is therefore necessary to overcome the effect. A commonly used procedure of time gating and background averaging cannot suit to small shallow nonmetallic landmine beneath a rough ground surface. In this letter, we proposed techniques to enhance the target signal through common midpoint (CMP) antenna array and data processing techniques, including velocity spectrum and CMP multifold stacking. The method has been tested using experiment data over a rough ground under which small plastic antipersonnel landmines is shallowly buried. The result shows the signal-to-clutter ratio was dramatically improved.

Measurement radius of reinforcing steel bar in concrete using digital image GPR

Ground penetrating radar (GPR) is currently one of the most efficient sensors used for the detection of dielectric cylindrical objects buried under concrete. Physical and theoretical modeling and experimental results of buried reinforcing steel bar (rebar) are given and studied using measurements of radargram data. This allows for reinforcing steel of radii (1.6 cm, 1 cm) to be detected and estimated from the radargrams. A physical model is presented for the electromagnetic signature of a buried reinforcing steel bar, which takes into account the radius of the rebar. This is achieved by subjecting GPR radargrams to a series of digital image processing stages, followed by different power reflectivity within the energy zone during the motion of the GPR antenna along the reinforced concrete surface. Power reflectivity for vertically oriented migration traces was generated. The distance between variant power reflectivity and the long dimension radius of an energy footprint can be considered when calculating the radius of reinforcing steel bar. The results indicate that, this model is capable of estimating the reinforcing steel bar radius to within 7%.

PRACTICAL ALGORITHMS TO FOCUS B-SCAN GPR IMAGES: THEORY AND APPLICATION TO REAL DATA

It is well known in B-scan ground penetrating radar (GPR) imagery that the underground scatterers generally exhibit defocused, hyperbolic characteristics. This is mainly due to the data collection scheme and the finite beam width of the main lobe of the GPR antenna. To invert this undesirable effect and obtain focused images, various migration or focusing algorithms have been developed. In this paper, we survey the performance of our recent focusing algorithms, namely; hyperbolic summation (HS) and frequency-wavenumber (w-k) based synthetic aperture radar (SAR) focusing. The practical usage of these focusing methods were tested and examined on both simulated and measured GPR data of various buried targets. The simulation data set is obtained by a physical optics shooting and bouncing ray (PO-SBR) technique code. Measurements were taken by a stepped frequency continuous wave (SFCW) radar set-up. Scattered C-band field data were measured from a laboratory sand box and from outdoor soil environment. The proposed focusing methods were then applied to the B-scan GPR images to enhance the resolution quality within these images. The resultant GPR images obtained with the proposed algorithms demonstrate enhanced lateral resolutions.

FDTD analysis of ground-penetrating radar antennas with shields and absorbers

One of the most critical hardware components of a ground-penetrating radar (GPR) is the antenna system. Important parameters of antennas, such as antenna bandwidth, radiation waveform and cross coupling determine the GPR system performance. The modified TEM horn antenna with distributed resistor load is presented in this paper, and the radiation properties of the antenna with the shields and absorbers are studied through the three-dimensional finite-difference time-domain (FDTD) scheme. Simulations show that the direct signal coupled from the transmitter is decreased by means of the shields and absorbers. Therefore, using the antenna in the GPR system can improve the signal-to-clutter ratio and the dynamic range of the system.