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Fifth International Conferention on Ground Penetrating Radar
- Conference date: 12 Jun 1994 - 16 Jun 1994
- Location: Kitchener, Canada
- Published: 12 June 1994
1 - 20 of 95 results
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Detection of Buried Objects by the GPR Method
Authors Naser Al-Shayea, Park Gilmore and Richard WoodsThe ground penetration radar (GPR) technique is compared with two seismic methods, the spectral-analysis-of-surface-waves (SASW) and the crosshole, for detection of buried objects. Tests were performed in a 7 m diameter by 2 m deep sand bin in the G. G. Brown Lab of the University of Michigan. The bin is filled with uniform silica sand compacted to a uniform density of about 16 kN/m3 . During the filling process, a three-cell void was buried at a depth of 305 mm to the top in the center of the bin. GPR, SASW and crosshole tests were performed with all three cells empty, one cell (center cell) empty, and all cells full of sand. The three experimental techniques are being compared with each otherfor efficacy of void detection.
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Modeling out-of-plane scattering effects
More LessAntennas used in ground penetrating radar systems transmit and receive electromagnetic energy over very wide antenna patterns. The typical result observed in a two-dimensional radar image over a point scatterer is the characteristic hyperbola as the radar pulse is reflected from the scatterer both before and after the antenna has passed over the point. With the point scatterer arbitrarily located in three-dimensional space, the resultant radar hyperbola is actually a section from a quadric conic surface. Simply modeling these surfaces to compute the section of the cone intersected by the radar image allows location of scatterers nearby but out of the plane of the radar image. This allows location of subsurface features that the radar antenna may not be able to pass directly over and image because of physical or logistical constraints. Location solutions are only unique when the same scatterer is observed in multiple parallel or perpendicular images. Knowledge of the possible existence of such out-of-plane features should be considered when interpreting or performing velocity migrations on two-dimensional ground penetrating radar images. In three-dimensional investigations, such modeling can be much less compute-intensive than either 2D or 3D migration and allow location of features that are outside the volume surveyed.
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Petrophysical causes of electromagnetic dispersion
Authors Gary R. Olhoeft and Dennis E. CapronAn electromagnetic pulse propagating through air or vacuum does not change shape, but one propagating through material may significantly alter shape with distance of propagatIOn. The change in shape is most often caused by frequency dependent material properties. All frequency dependences in material properties anse from energy loss mechanisms. Geometric spreading loss does not cause frequency dependence as the energy is not lost, but spread over the surface area of an expanding sphere about the antenna. Electrical energy loss mechanisms include intrinsic conduction thermal loss, orientational relaxation of the water molecule mechanical loss, and clay-mineral electrochemical loss. These result in complex frequency dependent dielectric permittivity. Magnetic energy loss includes magnetic domain and superparamagnetic relaxation losses, and others not well understood. These result in complex frequency dependent magnetic permeability. Frequency dependence may also result from heterogeneous distributions of these properties on spatial scales comparable to the electromagnetic wavelength in the material. The velocity of electromagnetic propagation in a material is determined by the speed of light in vacuum divided by the square root of the product of permittivity and permeability. Thus, the velocity of propagation is frequency dependent. In wet soils, this usually results in higher frequency components of a pulse attenuating and propagating faster than the lower frequency components, resulting in pulse broadening. A consequence of such changing shape in the propagating pulse is increased difficulty in performing pulse deconvolution and migration (phase coherent image reconstruction) as the pulse waveform is not everywhere the same shape and phase.
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A test site for quantification of GPR responses
Authors E. Pettinelli, J. D. Redman, A.L. Endres and A.P. AnnanExpansion of the use of GPR in many areas and improved instrumentation is leading to the need for GPR data with amplitude fidelity. To be more quantitative, the factors which control amplitude must be thoroughly understood. To this end, a controlled test site has been established to address some of these problems. Targets of simple geometry were buried in a uniform natural host material. Procedures for monitoring site electrical properties have been devised and tested. In the following paper we discuss the test site development and present initial experimental results.
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Modeling dispersive ground penetrating radar data
Authors Michael H. Powers and Gary R. OlhoeftLaboratory analysis of field samples shows that the relative dielectric permittivity and dielectric loss tangent vary with frequency in wet soils. Frequency dependent electrical properties are seen in field data as attenuation and broadening or dispersion of a pulse. For this reason, our version 2.0 modeling program accepts frequency-dependent parameters and accounts for the pulsebroadening effects of dispersion. In some soils containing magnetic particles, the magnetic permeability is also modeled as a frequency dependent complex quantity. The model only allows zero-offset, one-dimensional data. Effects not considered are system noise, random near-field coupling changes, polarization artifacts, scattering losses, and higher-dimensional effects such as antenna pattern. The success of this method of subsurface characterization is strongly influenced by the user's understanding of the soil properties.
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Finite-defference time-domain forward modeling of GPR data
Authors Roger L. Roberts and Jeffrey J. DanielsThe finite-difference time-domain (FDTD) method has been adapted to accurately model GPR data. The method is based on explicit finite-difference approximations of Maxwell's curl equations. The model is set-up by dividing a finite-size volume into grid cells on the order of one-tenth of a wavelength in dimensions. Electric and magnetic field vectors are positioned along the'edges and normal to the sides of each grid cell. Specification ofthe electrical and magnetic properties for each grid cell permits modeling of coaxial feed cables, antennas, antenna enclosures, the air-gap between the antenna and the ground, and electrical and magnetic heterogeneity within the ground. During program execution, a voltage impulse is input in modeled balanced coaxial cables feeding the transmit antenna. The program is executed over the desired number of time-steps to obtain a full trace of data from modeled coaxial cables attached to a receive antenna. Special absorbing boundary conditions (ABCs) are used on the outer boundaries of the FDTD grid to keep energy impinging on the boundaries from reflecting back into the grid. Model results are compared to published field pattern data and measurements made over targets buried in the OSU GPR test pit. The absolute amplitude of FDTD modeled target reflection data is within 3.3 dB of data obtained from pit measurements. Both the frequency content and waveform characteristics ofthe modeled data also agree well with the experimental data.
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Modelling antenna-ground interactions
By Greg TurnerThe properties of antennae change significantly when they are brought close to the ground surface. Consequently the optimum design of ground penetrating radar (GPR) systems is strongly dependent on a detailed understanding of the interaction of the antenna and the neighbouring ground surface. The Numerical Electromagnetics Code (NEC) is a computer program for antenna modelling which uses integral equations to model wire-like objects and closed surfaces and can model loading and ground effects. NEC provides an attractive alternative to laboratory or field testing of antennas close to the ground surface as antenna configurations and ground conditions can be changed easily. It has the capability to detennine the electromagnetic field strength above and below the ground surface and in close proximity to the antennas where GPR measurements are made. The input impedance of the antenna can also be calculated.
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Radar image reconstruction by discrete model fitting in a layered inhomogeneous medium
Authors Toshia Wakayama, Toru Sato and Iwane KimuraWe have been studying radar image reconstruction based on the discrete model fitting (DMF) method to realize high performance of subsurface sensing. In the present paper, the DMF is extended to treat the inhomogeneity of the medium, and the performance of the algorithm is verified by computer simulation. We consider a situation in which several antennas are used both for transmission and reception, and then many time series data are obtained with all combination of transmitters and receivers. The medium is assumed to consist of layers with different permittivities, and point scatterers are embedded in it. Parameters to be estimated by the model fitting are positions and radar cross sections of targets, and permittivities and depths of layers. In the model fitting, nonlinear leastsquares improves the model parameters so that the estimated received data computed by ray tracing agree with the observed data. Since the nonlinear least squares is an iterative method, appropriate initial values for the model parameters are estimated from information on the delay time ofreceived echoes. To enhance the ability of detection of targets and layers, the combination of the initial guess and the model fitting is iterated as the number of assumed targets and layers is increased. The proposed method can treat multiple scattering and a large discontinuity of a medium, which the conventional methods based on Born approximation cannot treat. Moreover, this algorithm takes inhomogeneity of the medium into account, it can estimate a target location more precisely than the conventional aperture synthesis technique.
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Some results of a GPR modelling test
By Huilian WangThe Application of the Ground Penetrating Radar (GPR) technique is now increasingly extending to the broad field of engineering geology. Discerning between various buried objects in variable forms and sizes on GPR maps and studying the relevant site techniques are important problems. In this paper, the author presents GPR modelling test results for GPR conducted over a series of metal and nonmetal cylinders, spheroids, finite plates and their compositions buried in a swimming pool. These models simulated buried pipes, culverts, cavities, and lithologic interfaces. They are the basic targets which could be encountered on a fieldsite. The author also describes technical conditions.
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Simulation of eletromagnetic wave propagation in three-dimensional media by an FDTD method
Authors Tsili Wang and Alan C. TrippWe have developed a finite-difference time-domain solution to Maxwell's equations for simulating electromagnetic wave propagation in three-dimensional media. The algorithm allows arbitrary variations of electrical conductivity and permittivity within a model. We use the Vee's staggered grid technique to sample the fields and approximate the spatial derivatives with optimized second-order finite differences everywhere except close to the computational domain boundary where we use conventional central differences instead. The pointwise computational time of the optimized second-order difference scheme is the same as that of the conventional fourth-order difference scheme, but the former has better dispersion characteristics. Although the optimized difference scheme imposes stricter limitations on the size of time steps allowed for an explicit time-marching scheme, a simple calculation shows that this scheme is more cost-effective, due to its lower required spatial sampling rate, than the conventional second- or fourth-order difference scheme. The temporal derivatives are approximated by second-order central differences throughout.
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Ray-Based Synthesis of Bistatic Ground-Penetrating Radar Profiles
Authors Jun Cai and George A. McMechan2-D bistatic ground-penetrating radar (GPR) profiles may be numerically synthesized by combining ray tracing (for the kinematic properties) with transmitter and receiver directivities, reflection and transmission coefficients, geometrical spreading, and attenuation coefficients (for the dynamic properties). The main limitations are that wave effects, such as diffractions, and offline (3-D) effects are not included. The algorithm is applied to iterative modeling of multioffset, multi-frequency GPR data acquired over an outcrop of fractured Austin Chalk in Dallas county in northeast Texas. Modeling is able to simulate the main time and amplitude behaviors observed in GPR reflections at 50, 100 and 200 MHz at each of 1, 3 and 5 meter antenna separations, from a single model.
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A space marching inversion algorithm for pulsed borehole radar in the time-domain
Authors Yongmin Zhang and Ce LiuAn iterative algorithm is developed to reconstruct the image of formation conductivity surrounding a borehole using time-domain data. The forward modeling employed in the algorithm is derived from the transmission line matrix (TLM) method, which is used to simulate electromagnetic waves propagating in formations with two-dimensional variations in cylindrical coordinates. A new structure of a transmission line node is used to simulate a coil-type transmitter antenna in a borehole. Since the inversion algorithm proceeds iteratively and the part of the formation involved in the inversion marches in space step by step, no optimization is necessary, and problems caused by optimization procedure such as inverting large-scale matrix and computation of Jacobian matrix numerically, are avoided. This method is especially useful in cases where the analytic gradient is not available. The inversion algorithm is tested in formations having both one- and two-dimensional conductivity variations with coil-type transmitters. Investigation depth and resolution for noise-free cases are also discussed in this paper.
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Using ground penetrating radar as an integral part of the formulation of maintenance decisions concerning flexible pavements
More LessIn this paper, GPR measurements, combined with deflection studies and extraction testing of asphaltic concrete cores were obtained at selected sites. The deviation between the GPR prediction and destructive coring was analyzed statistically. Using the cores location as objects and the results of extraction testing as attributes, cluster analysis was used to identify, if any, the attributes that affect the radar prediction of various types of flexible pavement. The paper partially demonstrates how GPR fits into the overall framework of pavement maintenance decisions and pitfalls associated with GPR in thickness predictions of fully and partially designed bituminous pavements.
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Radar testing of structural concrete
Authors John H. Bungey, Marcus R. Shaw, Stephen G. Millard and Cledwyn ThomasApplication of sub-surface impulse radar to durability and integrity assessment of concrete bridge decks and highway pavements has developed over many years. This experience has recently been successfully extended to a much wider range of applications related to structural concrete. Developments in commercially available field testing apparatus, including digital systems with colour display facilities, have facilitated major advances in signal processing and analysis of site results. Currently established applications include determination of major construction features; estimation of element thickness; location of reinforcing bars, voids, honeycombing, cracking, moisture and chloride contamination. These are all comparative in nature and can be achieved to varying degrees of reliability, but accurate sizing of buried features is more difficult. Other proposed applications including estimation of chloride concentrations and location of reinforcement corrosion require further investigation.
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Quantitative measurement of pavement structures using radar
Authors J. Les Davis, James R. Rossiter, Darel E. and Cece B. DawleyEnhancements have been made to ground penetrating radar (GPR) technology to offer pavement engineers quantitative, non-destructive thiclrness measurements of multiple layers in pavement structures. The enhancements include a self-calibrating capability at every measurement location, signal penetration to a depth of 2 metres, resolution of layers as thin as 50 mm, and semi-automated processing and interpretation software. More than 125 thiclrness comparisons, made in Canada, the United States and Finland over a variety of pavement structures, gave GPR measurements of asphalt thiclrness accurate to within +5% of thiclmess measurements obtained by coring.
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Development of a route segementation procedure using predicted layer thicknesses from radar measurements
Authors Emmanuel G. Fernando and Teng-Soo ChuaMassive quantities of data can be accumulated very quickly with current ground penetrating radar equipment and software (e.g., approximately 1 trace every 2 feet at 35 mph). Depending on the size of a given network, and the frequency of sampling, the processing of radar data can yield a voluminous amount of layer thickness information on the network surveyed. For example, given a sampling rate of 1 trace every 2 feet and a 100-mile network, layer thickness estimates for about 264,000 points along the network can be generated for each lane surveyed. To be useful for pavement management applications, a post-processing stage is important during which time, the given network is subdivided into homogeneous sections or segments based on the radar predictions. A computerized procedure for route segmentation in the post-processing stage is presented in this paper. The program developed uses the cumulative difference approach as the basis for delineation. In the procedure, the cumulative difference method is successively applied until no further delineations are possible based on userspecified criteria for minimum section length and minimum difference between means of adjacent segments. A number of other criteria are also used. The program has been tested and verified using pavement sections with known changes in pavement structure with satisfactory results.
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Implementation of ground penetrating radar for network-lever pavement evaluation in Florida
Authors Emmanuel G. Fernando, Kenneth R. Maser and Bruce DietrichIn 1991, the Florida Department of Transportation (FDOT) initiated a study aimed at systematically implementing ground penetrating radar (GPR) for developing a statewide database of pavement layer thicknesses and base material type. Phase I of the study involved a demonstration of current GPR technology. This was achieved by conducting radar surveys on short pavement sections (0.1 to 1.5 miles long) established by FDOT and comparing the radar predictions with coring information. The results of these comparisons established the feasibility of using current radar technology for the purposes defined by FDOT.
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Radar signal processing and analysis for evaluation of reinforced concrete bridge decks
Authors Udaya Halabe, Roger Chen, Vasudev Bhandarkar and Sami ZahidThis paper presents the findings of a study on the use of Ground Penetrating Radar (GPR) for nondestructive evaluation of concrete bridge decks. Several concrete bridge deck specimens of varying internal conditions such as with/without reinforcement and with air and water-filled cracks were cast in the laboratory. The individual radar waveforms from these specimens were compared to study the effect of anomalies (e.g., cracks) on the radar waveforms. In addition, a computer model was used to conduct waveform inversion on the radar waveforms for these specimens in order to distinguish between specimens with and without embedded cracks.
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Algorithms for synthesis and inversion of radar data from concrete bridge decks
Authors Udaya Halabe, Kenneth R. Maser and Eduardo A. KauselThere is an urgent need to develop methods for rapid identification of major deterioration in bridge decks and pavements. As traditional methods are slow and cumbersome, the focus has shifted to the use of modem nondestructive techniques such as Ground Penetrating Radar (GPR). GPR is a fast, non-contact technique, but interpretation of the radar datais difficult and requires complex analysis. Recently, great improvements in the analysis of radar data have been made and theoretical models for the prediction of subsurface condition of concrete structures have been developed. This paper describes models for predicting the velocity and attenuation of electromagnetic waves in concrete as a function of frequency, temperature, moisture content, chloride content and concrete mix constituents. The electromagnetic properties of concrete are predicted by aggregating the individual properties of its constituents: water, salt, air, cement paste, and aggregate solids. This mixture model, in conjunction with a rebar model developed to account for the reflection produced from reinforcing bars embedded within the concrete, has been utilized to synthesize radar waveforms for representative reinforced concrete bridge deck geometries. A least squares inversion procedure has been applied to the computer generated synthetic waveforms. This paper demonstrates the use of this inversion procedure to predict the spatial variations in volumetric'water content, salt content, and rebar cover.
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Correlation of some parameters in GPR measurement data with quality properties of pavements and concrete bridge decks
Authors P. Maijala, T. Saarenketo and P. ValtanenIn order to develop faster and more objective Ground Penetrating Radar data interpretation methods, the University of Qulu and the Finnish National Road Administration have been studying the possibilities of using certain parameters obtained from GPR data to describe the quality of asphalt and concrete structures in roads and bridges. The majority of the test surveys were performed with ground coupled antennae, which involved problems with antenna coupling and ringing. Consequently a special software was developed to reduce antenna ringing and background noise, to trace reflection interfaces and to calculate amplitude ratios. The amplitude ratio R1, R2 theory described by Chung and Carter (1990) for aircoupled antennae was also tested with ground-coupled ones. The software was tested with bridge and road data in which the thicknesses of the structures and their material properties were known.
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