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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 - 50 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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Bridge deck and pavement evaluation using BBL's ground penetrating radar technology
Authors Guy L. Mailhot and Michel ParentThis paper describes ground penetrating radar technology developed by Beauchemin-BeatonLapointe. Inc. as a practical tool in the evaluation of bridge decks and pavements. Described in the paper is the radar equipment used by BBL which includes the 2.5 GHz "Hom Type Antenna" recently developed by GSSI, the sophisticated and state-of-the-art software developed in-house to acquire, process, analyze the data and to present the results in a practical format using AutoCADTM. compatible drawings. Also discussed is the inspection vehicle and set-up used to collect the radar data. Presented in the paper are certain results of an actual survey using the developed technology on the Jacques Cartier Bridge Deck in Montreal, pavement evaluation work performed and currently underway (thickness measurement and void detection) as well as the results of experimental tests performed at McGill University. Also described is the integration of BBL's radar technology with existing and complementary infrared thermography technology for bridge deck evaluation, which enables a complete integration of radar, infrared thermography, and standard video images in order to provide a complete and accurate assessment of bridge decks.
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GPRSIM Forward Modeling Software and Time Slices in Ground Penetrating Radar Surveys
Authors Dean Goodman, Yasushi Nishimura and Kouji TobitaSimulation of ground penetrating radar using an exact ray tracing interactive software, the dispersive nature ofmaterials and their impact they could have on simulating ground penetrating radar, and a useful description and examples on how to create radar time slices that can help to illuminate target features are presented.
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Highway speed radar for pavement thickness evaluation
More LessAccurate knowledge of pavement layer thicknesses is important for accurate backcalculation of pavement layer properties as well as for network-level pavement condition inventories. Often this information is unknown, and records are inaccurate, or difficult to access. This paper describes a self-calibrating Ground penetrating Radar (GPR) system for highway speed pavement layer thickness evaluation. The radar system incorporates air-coupled horn antennas, PC based digital data acquisition, and software for computing pavement properties from raw waveforms. A software system called PAVLAYER~ has been developed and extensively tested for pavement thickness evaluation. This system has demonstrated an accuracy ranging from 5 to 10 percent for asphalt layer thickness, and 10 to 15 percent for base layer thickness. These accuracy figures are based on test data from 100 pavement sections and correlation with data from 594 cores and test pits. The paper describes the details of the hardware and software components and the analytic methods used in the pavement thickness evaluation system. Also presented are descriptions of, and results from, field evaluation programs in which the radar results were correlated with ground truth. These programs have been carried out with federal and local highway agencies in 20 different states in the U.S., in the U.K. and in Germany. Each of these programs have involved radar surveys of pavement sections representing different ages, pavement designs and pavement condition. Extensive coring has been carried out to confirm the radar predictions, and the correlation between core and radar data are presented. The reported results also show how the accuracy of the layer thickness calculations are affected by different variables and how other pavement structure and condition information is revealed in the radar data. Specific results are presented showing repeatability, and the influence of the survey speed, moisture on the pavement surface, surface treatments and thin overlays, and pavement age and structure.
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A comperative study of ground probing radar and impact echo nodestructive techniques for detecting and locating buried objects
Authors M. Momayez, A. Sadri and F.P. HassaniThe comparison of two nondestructive testing methods for detecting flaws and locating buried objects in concrete is the focus of this paper. Applications of Ground Probing Radar (GPR) and Impact Echo (IE) techniques are examined. In addition, this investigation shows thatwhen GPR and IE are used in tandem, extensive information about the integrity of concrete can be obtained in a short period of time. GPR is an electromagnetic reflection technique utilizing nonionized microwaves whereas IE makes use of transient stress wave reflections. Although both techniques are based on wave reflection, the nature of the information carried in the reflection signals from GPR and IE is different and complimentary when combined together. In this study, a large concrete slab was prepared with various types of objects such as rebars, metal and plexiglass plates, rocks and thin plastic sheets simulating cracks placed in it. It is shown that GPR can be used to detect flaws and buried objects rapidly while IE can locate flaws and buried objects accurately and provide reliable data on the mechanical properties of concrete. The combined use of Ground Probing Radar and Impact Echo techniques is an effective tool for concrete monitoring and quality control.
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Pavement evaluations using ground penetrating radar in Texas
Authors T. Scullion, C.L. Lau and Y. ChenThis paper describes the OPR system developed and implemented for the Texas Department of Transportation (TxDOT). The system includes a Penetradar PS-24 OPR system together with a data acquisition and signal processing system developed by the Texas Transportation Institute. The entire system is housed in a data acquisition vehicle which also includes distance measuring equipment and a synchronized video recording system. The OPR has been used on several major pavement projects in Texas. The antenna is air launched and suspended between 250 and 350 mm above the pavement and has been shown to operate adequately at close to highway speeds (50 kph). In this paper the signal processing algorithms will be described together with results from a series of case studies. The signal processing system tracks the reflections and measures their amplitudes and arrival times on a trace by trace basis. Methods of cleaning up the signal with end reflection subtraction and of accommodating antenna bounce have been built into the software. From typical OPR reflections from pavements the layer dielectrics and thicknesses can be calculated for a maximum of 4 distinct layers. The moisture content of typical granular base material has been related to the relative dielectric value of the layer. The detection of pavement defects, such as voids, has been related via decision rules to the measured amplitudes, dielectrics and thicknesses and their variance along a highway. Field measurements and ground truth information will be reported on several in service pavements in Texas. These applications include layer thickness determination, void detection and locating areas of asphalt stripping.
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GPR for snow pack water content
Authors A.P. Annan, S.W. Cosway and T. SigurdssonGPR offers a simple and rapid means of providing information for mapping snow thickness. Snow thickness data are of great importance in hydrological studies where snow melt can lead to floodIng. In many countries with hydroelectric power generation, water storage in the snow pack represents a key input to reservoir level control planning. At first glance, the snow thickness problem would appear trivial, and indeed measurement of snow thickness can be straight forward. Unfortunately, the economic applications of snow thickness data require equivalent water content (not snow thickness)!! To effectively use snow thickness information, it is necessary to develop systematic procedures for transforming GPR snow thickness observations to water content. In this paper we describe the steps required to achieve the water content objective. An illustration of this approach and associated difficulties is provided using multi fold GPR data acquired on an area of multi-year snow in Norway.
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Geotechnical investigations of insulated permafrost slopes along the Norman wells pipeline using ground penetrating radar
Authors Brian Moorman, Alan Judge, Margo Burgess and Tom FridelDuring construction of the Norman Wells pipeline, 56 permafrost slopes were covered with a 0.5 m to 1.8m insulating blanket of wood-chips in an attempt to retard the rate ofpermafrost thaw. Warmer mean annual pipe operating temperatures (OC to 3.5C) than anticipated have led to larger thaw bulbs than predicted beneath many insulated slopes. Initially, manual probing with an active layer probe was used to monitor the development of the thaw bulbs. However, as the depth-of-thaw increased to over 2 m in some areas, manual frost probe measurements became time consuming. A series of experiments were conducted to test the ability of ground penetrating radar to determine the thermal, lithologic, and structural characteristics ofwood-chip slopes. On many ofthe slopes, gridded surveys were conducted enabling three dimensional subsurface analyses and the construction of isopleth maps of the depth-of-thaw. In this paper we show examples of lithologic mapping, including qualitative indications of the ice-content of soils, mapping the boundaries between soil units and imaging internal bedding structures within units. The thermal structure of the subsurface is examinedby interpolating the depth of frost layers on radar profiles, and by mapping the depth ofmultiple frost layers with isopleth maps. An example of mapping the lateral of extent of subsurface cavities is also discussed. In general, the depth of signal penetration ranged from as great as 10min frozen sand to as little as 3 m in unfrozen clayey soil. Surveys at 50 MHz contained the most information and the depth-of-thaw was best mapped when it was deeper than 1.5 m below the surface.
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Airborne snow thickness radar
Authors Louis Lalumiere, James Rossiter and Simon PrinsenbergA ground penetrating radar (GPR) system for measuring snow thickness has been developed as a component of an operational airborne sea ice thickness sensor, Ice Probe. Ice Probe is a helicopter borne sensor utilizing an electromagnetic (EM) induction sensor and a laser profilometer to determine the combined thickness of ice and snow in real-time. With a GPR system to provide snow thickness information, Ice Probe can report ice and snow thicknesses separately.
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The use of ground penetrating radar to search for persons buried by avalanches
Authors Jürgen Niessen, Erich Kliem, Erhard Pöhlking and Klaus-Peter NickAlthough, in areas at risk from avalanches, skiers outside the safe ski-runs should carry an electronic transmitter with them in case they are buried by an avalanche, there is still an urgent need to locate buried persons exactly by means of an independent sensor in the presence of other objects. A demonstration of the ability of ground penetrating radar to help to find people under a snow cover was carried out in the German Alps under fairly realistic conditions. Of course, these measurements were not performed with real victims: the Bavarian Mountain Rescue Service prepared a professional test area for the trials. To prevent the occurrence of avalanches, the Bavarian Avalanche Warning Service regulary checks suspicious snow slopes with the aid of probes. The critical snow layers which are responsible for the production of avalanches were also investigated with the radar.
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Use of ground penetrating radar to study the interaction between permafrost and airport infrastructure on Baffin Island N.W.T.
Authors J.A. Pilon, M. Pilkington, M. Allard and J. FrydeckiIn this paper we present the results of Ground Penetrating Radar (GPR) surveys conducted at three airports on Baffin Island, N.W.T. These airports are Pangnirtung, Broughton Island and Clyde River and are operated by the Arctic Airport Division of the Department of Transportation of the Government of the North West Territories. They were selected because they are scheduled for improvements and resurfacing (Broughton Island and Clyde River) to correct for the occasional "soft spots" on the runways and are experiencing problems related to permafrost (Pangnirtung) where the runway lighting system undergoes annual failures in mid to late winter. The results of the GPR surveys show that in Pangnirtung the problem is related to frost cracking above active ice wedges beneath the runway, whereas the surface instability problems at the other two sites appear to be related to the presence of saline permafrost at some depth beneath the runways.
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The integration of radar, magnetic, and terrain conductivity data in an archaeological investigation of a Hudson Bay's company fur trade post
Authors Paul Bauman, Roderick Heitzmann, John Porter, Janan Sallomy, Mike Brrewster and Chris ThompsonIn 1799, the site of Rocky Mountain House, Alberta, stood at the southwestern comer of the northern fur empires of the North West Company and their competition, the Hudson's Bay Company. Between 1799 and 1875, four forts were constructed by these companies at Rocky Mountain House. The third of these forts, which stood from 1835 until 1861, has been only partially excavated. The goal of this investigation was to evaluate the utility of geophysics as an archaeological aid in studying historic sites in Western Canada. The specific objectives were to identify the position of the rooms and palisades of the 1835 fort, and to locate burial sites associated with this and other forts. A challenging obstacle in describing the 1835 fort using geophysics is the fact that in 1861, the Blackfoot peoples burned the fort to the ground. Information from three geophysical methods are discussed. Ground penetrating radar data provided the greatest detail in describing fort construction and burial locations. The GPR data, however, would have been of limited use if not viewed in conjunction with the information provided by magnetic gradient, and terrain conductivity plots of the particular sites. The integration of the various methods provided an overall plan of the original construction of the 1835 fort. While GPR most clearly identified approximately 50 graves at one large cemetery, terrain conductivity mapping was superior in locating probable coffins at a second, smaller burial site. Beyond being a good example of an archaeogeophysical data set, the significance of this paper is the clarity with which it is shown the need to integrate a number of geophysical techniques at investigations of historic sites in Western Canada.
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Relationships between Dielectric and Hydrogeologic Properties of Sand-Clay Mixtures
Authors Michael D. Knoll and Rosemary KnightA series of laboratory experiments was performed to investigate relationships between dielectric properties and hydrogeologic properties such as lithology, porosity and water saturation of granular materials. Mixtures of sand, clay, air and water were prepared to vary hydrogeologic parameters systematically. Lithologic variation was simulated by varying the relative proportion of sand and clay in the mixtures. Compaction was simulated by packing different volumes of sand and clay (in the same relative proportion) into the sample holder. Water saturation was varied by injecting water into the pore space and by evaporative drying. The experimental data (i.e., measurements of porosity, clay content, water saturation, dielectric constant, electrical conductivity and frequency) may be used to test dielectric mixing formulas and effective medium theories. Results show that much of the scatter in dielectric property-hydrogeologic property crossplots is due to systematic variation in interrelated hydrogeologic and/or dielectric parameters. For instance, the relationship between dielectric constant and porosity for dry or saturated materials may be multi-valued if there are also changes in clay content. However, different mechanisms of porosity reduction may be associated with characteristic dielectric responses. Porosity reduction by compaction results in an increase in dielectric constant for dry materials and a decrease in dielectric constant for saturated materials. Porosity reduction by pore-filling (i.e., changing lithology) results in either an increase or a decrease in dielectric constant depending upon the volume fractions, microgeometries and surface properties of the components. These results provide insight into the types of changes one may expect to see in ground penetrating radar data from different geologic settings, and hence should be of interest to those involved in modeling, inversion and stratigraphic interpretation of ground penetrating radar data.
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White marble dam, North Adams, Massachusetts
Authors Doria Kutrubes, Mark Blackey and Thomas JenkinsGround penetrating radar (GPR) was used to determine the thickness and configuration of a 156 year-old white marble block dam, believed to be the only one of its kind in North America. The dam, currently in disrepair, is within Natural Bridge State Park and is owned and managed by the Commonwealth of Massachusetts, Department of Environmental Management (MADEM). System calibrations on exposed marble blocks determined that average .GPR signal propagation velocity is approximately 0.105 mlns (0.344 ft/ns). The dam's thickness east of the sluice is approximately 1.8 meters (6 feet) at the spillway crest and 2.7 meters (9 feet) at the toe. In contrast, the dam's crest west of the sluice has a measured thickness of only 1.2 meters (5 feet). The marble dam appears to be constructed directly on top of bedrock, and bedrock may be incorporated into the dam's structure in some areas. By identifying the dam's thickness and upstream-face geometry, this GPR survey helped MADEM's consulting engineers to stabilize the dam and design a dredging program for the silted-in upstream impoundment while preserving the structure's historical integrity.
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Geophysical investigations with the georadar in the middle of Athens at Syntagma square and the discovery of the subterranean river Eridanos
Authors Stavros P. Papamarinopoulos and Marina G. PapaioannouDuring extensive geophysical exploration in the middle of Athena for the location of archaeological remains, an unexpected geophysical finding was revealed. The 80 l\AHz monostatic antenna of the georadar was utilized around the Square. At Metropoleos Street, in the extension of Philellenon Street and parallel to it, in five different and parallel vertical sections, the image of a river was depicted. The minimum of the river bed is at 6 m below the asphalt and its width is about 10 m. There is another possible palaeobed 30 m wide at 14 m depth below the level of the pedestrians. This river is the legendary Eridanos mentioned by ancient writers such as Hesiod, Plato, Strabo and Pausanias. It was associated with the earliest traditions of the Hellenes and its location and direction is associated with important buildings of classical Athena. Up to now its very existence, position and direction was doubtful. The river seems to originate from the Lecabettus hill rolling towards lower elevation, intersecting Syntagma Square and meandering towards the West part ofthe city.
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Detection of bedrock fractures and lithologic changes using borehole radar at selected sites
Authors John W. Lane, F.P. Haeni and John H. WilliamsBorehole-radar methods were used to map fractures and lithologic changes in metamorphic and igneous rock at sites in New Hampshire and Massachusetts, and in sedimentary rock at a site in lllinois. Interpretations of single-hole and cross-hole surveys were correlated with other geophysical and hydrologic data. Single-hole borehole-radar surveys using directional antennas were conducted in New Hampshire and Massachusetts. The surveys detected reflectors, which are interpreted as individual fractures or fracture zones that extend laterally up to 40 m (meters) from wells. Average radar velocities at the sites are 0.113 mlns (meters per nanosecond) in gneiss and 0.123 mlns in granite. Single-hole directional surveys were also conducted in lllinois. Reflectors that are interpreted as fracture zones and lithologic changes in dolomite were detected up to 25 m from the wells, and reflectors that are interpreted as solution zones were detected up to 15 m from the wells. The radar velocity in rock at this site is 0.081 mlns. Changes in lithology and permeability appear to correlate with differences in the amplitude and arrival time of the radar pulse that travels through the bedrock directly between transmitter and receiver. Cross-hole radar tomography surveys were conducted at the Massachusetts and lllinois sites to determine velocity and attenuation changes in bedrock between wells. Low-velocitylhighattenuation zones identified can be interpreted as lithologic boundaries and (or) water-saturated fracture zones.
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Analysis of a directional borehole radar signal with a time-frequency distribution
Authors Sato Motoyuki, Ebihara Satoshi and Niitusma HiroakiThe authors developed a directional borehole radar which is equipped with conformal micro array antennas on a conducting cylinder. We estimate the incident angle of incoming reflected wave by measuring the induced surface current at several points on the cylinder. Field measurement was carried out in Kamaish mine, Japan. Measurement was made with a network analyzer based system to obtain broad-band information (O.3MHz - 3GHz). We calculated transient reflection signal from the measured data in the frequency domain by FFT algorithm. However, we found that the measured signal suffers from the direct coupling between transmitting and receiving antennas. Direct coupling is one of the most common problems in ground penetrating radar, it sometimes shades the reflected wave from the surrounding material. Applying time-frequency analysis to the measured signal, we obtained a time-frequency distribution (Choi-Williams distribution). Phase velocity of guided waves along this structure was theoretically analyzed and we could identify them in this time-frequency distribution of the received signal. Consequently, we could extract reflected waves, which contain information inside the surrounding rock.
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Application of ground penetrating radar on Colloseum pillars
Authors M. Bernabini, E. Brizzolari, L. Orlando and G. SantellaniOne very important aspect in restoring monuments is to determine the decay degree of the material forming their structures. To this end three pillars of Colosseum consisting of travertine blocks have been investigated by Ground Penetrating Radar (GPR), in order to test the efficency of various measurement technique and to obtain information on geometry and fracturing monument structural elements. Mesaurements were made each 5 cm, on horizontal and vertical profIles over some faces of investigated pillars. In order to improve resolution and interpret the reflections obtained, seismic-type processing using deconvolutions, filtering in frequency and in wavenumber, migrations etc. has been applied. In parallel, a specific processing programme for SIRlO (Radan III) and a programme of seimic processing have been used. Among various methods applied, the best result have been obtained using FK velocity filter on stack sections. A detailed geometric tridimensional reconstruction of pillars, starting from external geometry, has been allowed us to directly correlate external discontinuity with sigle electromagnetic events. The working procedure has given detailed information on blocks internal geometry and on presence of unknown voids and fractures. This esperimental study has confirmed the efficency of radar in identifying discontinuities in construction elements. Once again, the inadequacy of standard radar processing programmes has been highlighted.
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Polarimetric borehole radar measurements in granite
Authors Sato Motoyuki, Miwa Takashi and Niitsuma HiroakiPolarimetric borehole radar information can be obtained from a combination of co-polarization and cross-polarization reflection measurements in a borehole. Most conventional borehole radars use axially-oriented electric dipole antennas for transmission and reception, resulting in a response only to the co-polarized signal component of the a reflected wave. However, more information can be obtained if the cross-polarized component of the reflected wave is also measured. Cross-polarization borehole radar measurements are presented in which an axiallyoriented slot antenna is used as the receiving antenna and an electric dipole is used as the transmitting antenna. The antenna was excited by a short DC pulse to obtain broadband pulse-echo operation. Measured signals were transmitted through a broad-band optical signal link. Field measurement using the prototype radar sonde was carried out in granite rock. The measured signals showed that detection of the short-range reflections can be improved with crosspolarization measurements since direct coupling between the transmitting and receiving antennas is considerably reduced. We also observed the polarization dependency of a reflectivity of a water containing fracture. For instance, reflection from a layer containing gravels showed significant depolarization. This additional information obtained from the polarimetric borehole radar measurement will give us more knowledge of cracks such as surface roughness and water contaminant.
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Litho-structural characterization in granitic rocks using single-hole and crosshole radar techniques
Authors Kevin Stevens, Richard Everitt, Paul Street and Ganpat LodhaAn experiment was initiated to study the solute transport characteristics of a region of 'Moderately Fractured Rock' between the 130 m and the 240 m levels as part of the continuing geoscience research at ABCL's Underground Research Laboratory (URL). Two coplanar, subhorizontal characterization boreholes, MF-l and MF-2, each approximately 200 m long, were drilled into a region of moderately fractured granite. Detailed single-hole and crosshole geophysical surveys including reflection and transmission borehole radar were performed in these two boreholes. Boreholes MF-l and MF-2 are separated vertically by a domain of xenolithic granite, two domains of pink gneissic granite, and a discontinuous domain of grey gneissic granite. Two regional low-dipping, intrablock fracture zones (FZ-3 and FZ-2) with associated sub-vertical fracturing also straddle the region. Single-hole radar reflection and crosshole radar transmission tomography surveys using the RAMAC borehole radar system, with 22 and 60-MHz antennas, were used to investigate the litho-structural characteristics of the rock mass. Structural projections 'of FZ-3 and FZ-2 in the plane of the boreholes suggest a good correlation between radar reflectors and the fracture-zone interfaces at distances of 10 m to 60 m from the boreholes. The crosshole radar tomographic images show a decrease of the radar wave velocity of approximately 10% within the experiment region. Discrete regions of below and above average radar wave velocity have been interpreted to reveal the approximate positions of intense fracturing, mafic xenoliths and possibly zones of unfractured granite. Information obtained from these borehole radar data sets has been included in the construction of a litho-structural model of the experiment region.
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Evaluation of experimental data from a GPR system for detection and classification of buried mines
Authors Bertil Brusmark, Staffan Abrahamson, Dan Axelsson, Anders Gustafsson and Hans StriforsWe present the software and instrumentation developed to assess the potential of a GPR system to detect and classify buried mines. Preliminary results from the evaluation of detection algorithms on experimental data are displayed. The data have been registered using our recently constructed experimental setup consisting of a sandbox and a computer controlled antenna positioning system.
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Evaluation and three-dimensional representation of ground-probing radar measurements
More LessGPR measurements are made in both boreholes and drifts for exploration. Measurements with directional antennae not only allow the distance to the reflection points to be determined but also the angle with respect to the azimuth of the plane defined by the reflection point and the two antennae. If a directional antenna cannot be used, the location of the reflection surface can be determined by special placement of the antenna in a vertical plane perpendicular to the axis of the drift. The migration method we use makes it possible to determine the point of reflection from the traveltime and the angle defined above. A planar element perpendicular to the normal to the reflection surface can be assumed through each point of reflection. If these elements are connected with each other, a zone is obtained representing the location of the reflector. The calculation of the angle, the filter processes, signal attenuation compensation, and other kinds of data processing, including determination of the traveltime ("picking"), is done using an interactive computer program we have developed. A software interface has been developed to transfer the reflection points to a 3-D graphics system so that projections, cross sections, and spatial representations can be prepared.
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Processing ground penetrating radar data
Authors Steven C. Fischer, Robert R. Stewart and Harry M. JolGround penetrating radar (GPR) data are processed using seismic analysis techniques. Processing operations performed include signal saturation, spherical divergence gain recovery, spiking deconvolution, bandpass filtering, velocity analysis, elevation corrections, NMO correction, and f-k migration. As examples we first use 0.5 km lines of 50 MHz and 100 MHz, single-fold data, with a CMP gather from William River delta, Lake Athabaska, Saskatchewan. Next, a 120m, 100 MHz, single-fold line from Ft. Smith, N.W.T. is processed using similar steps. Finally, 100 m of 100 MHz, single-fold data, with a CMP gather, from Brigham City, Utah, are analyzed. Filter bandwidths chosen for the 50 MHz and 100 MHz measurements were 10/20-75/100 MHz and 20/30-150/180 MHz, respectively. Velocity spectra for NMO corrections were determined for William River delta and Brigham City based on CMP gather velocity semblances and interpreted arrivals. Velocities range from 0.3 m/ns to 0.07 ms/ns (air to wet sand respectively). Higher velocity surface statics, caused by dry topography, were corrected for after interpreting shallow arrivals. The Slave River velocities are similar to William River delta suggesting similar r'ock type with higher water saturation and/or lower porosity. Dry sands at Brigham City resulted in radar velocities of 0.14m/ns. Application of seismic processing techniques significantly improved these GPR sections.
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A SFCW Polarimetruc Ground Penetration Radar
Authors Alan Langman and Michael R. InggsExtraction of target information from the measurement of the state of polarization of the scattered wave has been a promising area of research for a number of years. This paper discusses the theory and implementation of a stepped frequency CW polarimetric subsurface radar. The extended Prony method is used to obtain high resolution range profiles from which the scattering matrices are extracted. The results of laboratory trials are presented. These include measurements of the scattering matrix for a pipe and plate conducting targets, buried in a large sand box. Polarization responses for these targets are, plotted to emphasize the use of the scattering matrix for target discrimination.
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Resampling in frequency domain, a method for interpolation of time series
Authors Rudolf Fruhwirth, Roswitha Müller and Rupert SchmöllerResampling is a process often used in geophysical applications. Especially in the field of seismic and radar data processing it is applied mainly to time series. In most cases the resampling process influences the contents of time series both, in the time domain and in the frequency domain. If a time series is resampled to a larger sampling interval the frequency spectrum will vary since the Nyquist frequency ifN) decreases and the frequency components higher then the new Nyquist frequency fold into the area below. Vice versa, by resampling to a smaller sampling interval, fNy is increasing and usually an unknown spectrum, depending on the used method will be added. Resampling is performed by interpolation. The simplest way is to use a linear function. On the other hand polynomial functions or cubic or rational splines lead to more satisfying results, but all of these methods influence the frequency spectrum. If the interpolation is performed in the frequency domain, the spectrum of the resampled data set can be controlled in such a way that it is unchanged. The principle of this method is based on the fact that the sampling interval of a time series in the time domain automatically defines the Nyquist frequency. But vice versa, in the frequency domain full control over the sampling interval is given by alteration Of/Ny' This work presents the above method and demonstrates that a discrete time series can be transformed into a continuous time series by moving/Ny to infinity and that it is very easy for that reason to resample to larger or smaller sampling intervals which are not integral multiples of the original sampling interval. The problem of aliasing is also discussed and practical examples for application of this method to GPR data are mentioned.
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Image synthesis and perception of ground probing radar
Authors Ameen Maluf, Laeeque Daneshmend, Moe Momayez and Ferri HassaniA Ground Probing Radio Detection And Ranging (RADAR) system is used to extract subsurface scans. The principles and practices employed in this research relate to a Ground Probing Radar system operating with high frequency (lGHz) electromagnetic waves with adjacent transmitter and receiver antennas. Based on the theory of Kalman Filters, multi-sensor fusion is used to establish probabilistic models of individual sensor estimates and embed theses descriptions in a team-theoretic framework to finally describe the interactions between different sensors. Applied iteratively, consistent sensor estimates will converge to a qualitative image reconstruction. We investigate the possibility of deriving a functional model of image processing and representation to enhance the visual perception of subsurface images. The research is directed towards qualitative image integration and synthesis based on artificial neural networks pattern classification methods and front-end multi-sensor fusion techniques.
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Signal and image classification in georadargrams
Authors Klaus-Peter Nick, Thomas Cimiotti, Uew Papzinger, Ljudmilla Kleinmann and Jrgen LaugksThe given tasks in our joint project are the detection and location of hyperbolas generated by pipes and cables in the subsurface when illuminated with a common pulse radar, and to find classification indications of the object's nature. Classification not only includes informations about the material and cross-section of an object, but also its three-dimensional extension in the subsurface. Thus, the processing of radar data is divided into several successive steps. First, the hyperbola has to be detected in the radargram and the approximate location of the pixels belonging to the hyperbola has to be detected. We perform this by means of the Hough transform algorithm. The result of the Hough transform and its degree of automation depend greatly on the preprocessing of the data transformed. To apply the correct preprocessing and processing strategy, classification of the radargrams by texture arialysis is investigated. After the detection of the hyperbola, an analysis of the reflected wavelets can provide some clues about the object's nature. Although absolute classification, which can be derived straightforwardly from simulations, will be successful only in a few exceptional cases, signal analysis is very useful in three-dimensional tracking of objects. We were able to demonstrate this by using different statistical correlation methods to identify whether hyberbolas in parallel transects were generated from the target or not. A simple but often successful method of detecting and following long objects is the presentation of parallel transects as a time slice.
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Eccentricity-migration: A method to improve the imaging of pipes in radar reflection data
Authors Christian Stolte and Klaus-Peter NickHyperbolas are a widely observed feature in radar reflection data. Zero-offset migration of these hyperbolas rarely improves the image. Often energy is sme~red out and weak reflections disappear. The reasons are perturbations in the local geology influencing the velocity field, uncertainties introduced by uneven movement of the antennas, the bistatic character of the antenna as well as the geometry of the buried objects. In this work the target geometry is incorporated into migration to improve the subsurface image. An algorithm is presented, which automatically determines the diameters of buried pipes without prior knowledge of the medium-velocity for electromagnetic propagation. In a second step the algorithm utilizes these determined diameters of the buried pipes to enhance the radar image through migration. Conventional migration (KIRCHHOFF, f-k, FD) reconstructs the geometry of subsurface features from one parameter alone: the velocity field. Eccentricity-migration incorporates another parameter into migration: the deviating eccentricity of the hyperbola. Therefore, we chose the name "eccentricity-migration". The reflection image of a line diffractor in a radargram is uniquely characterized by its numerical eccentricity N E = V2, the ratio of the focal length of the hyperbola to the apexto- origin distance. For a pipe, N E is smaller than V2. Therefore, if the temporal and spatial resolution of the data are sufficient and changes in medium-velocity are small, then the pipe-diameter can be uniquely extracted from the shape of the reflection hyperbola alone. A further advantage of E-migration is its increased sensitivity to changes in velocity. Based on a known pipe-geometry, either a priori or extracted through E-migration, a more detailed velocity field can be derived and the subsurface image in the vicinity can be furthermore improved.
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Recognition and removal of surface scattering in GPR data
Authors Roger Young and Jingsheng SunGround Penetrating Radar (GPR) data may show strong noise events due to surface scattering by obstacles such as trees and boulders. The surface scattering can be very strong due to low attenuation in the air. A procedure is developed for identification and removal of these events. It is based on the fact that the velocity of radar waves in the air is .3 mlns, but the velocity in the earth is much lower, approximately .1 mlns.
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GPR frequency selection
Authors A.P. Annan and S.W. CoswayOne of the most important variables in GPR is selection of the operating frequency. Proper frequency selection can make the difference between survey success or failure. The most important aspect for a user is to understand the basic principles involved so that adequate survey planning can be carried out. Frequency selection depends on clearly defining the survey objective. This requires that the site and the survey conditions be evaluated prior to the survey. If not done, survey failure or success will be impossible to predict. Factors to be considered are site geology, surface obstructions, host geology, electrical properties, target size and depth of exploration. Given the above information, various constraints on frequency can be developed. From these frequency estimates, upper and lower bounds are placed on the frequency likely to be effective for the particular problem. In addition, the expectations of the end user of the survey results can be established thereby avoiding surprises.
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Ground penetrating radar considerations for optimizing the data collection scenario
More LessAs research utilizing ground penetrating radar (GPR) systems is undertaken a wide range of problems and conditions is undertaken. Typically, before research of this type is funded, the client needs some degree of assurance that the research in question has a "good" chance of success. In this paper we address the topic in three steps. First, we discuss what one can reasonably determine from the client's description of the problem, utilizing pictures, sketches, etc., which indicate the chances of successfully solving the problem. Second, we discuss the minimum measurements that can be made on-site in order to improve the probability of success without actually performing the survey. Finally, we discuss the procedures/methods which we have developed which directly relate to the problem being addressed so that success will be assured while keeping the data collection time, the data processing time, and the data analysis time all listed below, to a minimum.
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