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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
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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Detecting and mapping recent faults with a ground penetrating radar in the alluvial fans of the Arava valley, Israel
Authors Uri Basson, Yehouda Enzel, Rivka Amit and Zvi Ben-AvrahamGround Penetrating Radar (GPR) field experiments were conducted to detect recent faults that may be connected to the Dead-Sea Rift, on several all uvial fans in the southern Arava Valley, Israel. The existence of fault-scarps and lineaments interpreted as surficial faults there, indicated the Late Pleistocene and Holocene seismic activity of the rift. High resolution GPR profiles were carried out to map the shallow stratigraphy of the alluvial deposits and to detect subsurface recent faults. The GPR experiments were a part of integrated neotectonic research, which combines geophysical and geomorphological complementary methodsto investigate recent faulting events. During the study we used GPR and trench excavations to detect and study faults which rapture the surface, and faults with no visible surficial expression. The profiles that were conducted to cross the faults perpendicularly have produced reflections that were not sufficiently different from reflections of stratigraphic discontinuities that are common in the alluvial fans. On the other hand, a dramatic indication interpreted as a fault has been obtained in the profiles that were conducted along a wide faulted zone discontinuity, and along a strike-line of a fault, which is significantly wider then other lateral discontinuities. Consequent to locating a suspected faulted zone, high-resolution GPRmapping of its typical stratigraphy can be made. In spite of the complex stratigraphy of the alluvial fans, which produces abundance of pitfall reflection anomalies, we introduce a powerful way to identify the faults with GPR. The results of the GPRmapping can help focusing the trench excavations that are essential for understanding the details ofthe paleo seismic faulting events. Prospecting offaulted zones with GPR can reduce the excavation activities to a minimum, save time, effort and damage to the landscape.
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Erosion of ejecta at meteor crater: constraints from ground penetrating radar
Authors John A. Grant and Peter H. SchultzGround penetrating radar (GPR) provides a rapid, non-destructive means for investigating the shallow subsurface around a variety of terrestrial landforms also found on other planets. Impact craters represent one of the most ubiquitous planetary features and GPR data at Meteor Crater, Arizona, delineate stratigraphic relationships between the ejecta and associated alluvial deposits in the shallowest 1-3 m ~f the subsurface. Analyses of these data indicate that the continuous ejecta remain better preserved than was defined previously via surficial mapping. Radar penetrates a thin veneer of alluvium and colluvium that masks the actual distribution. Returns from uniform, finegrained alluvium filling regional drainages blocked by the crater confirm their generally low transport capacity through time. GPR data and sampling further reveal that some subtle ridges extending from the crater are not the result of debris flow deposition of eroded ejecta. Instead these ridges are formed by preserved ejecta deposits draping pre-impacttopography. Collectively, these results establish that the continuous crater ejecta are in a surprisingly pristine state of preservation in contrast with visual inspection. More generally, this study demonstrates the high potential of GPR in future planetary applications. Ease of miniaturization makes GPR ideally suited for planetary rover missions to the terrestrial planets and Galilean satellites where it could help resolve a range of questions relating to their histories of surface processes.
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Three-dimensional geometry of fluvial gravel deposits from GPR reflection patterns; a comparison of results of three different antenna frequencies
Authors Peter Huggenberger, Edi Meier and Beres MilanGround-probing radar (GPR) proftles have been recorded in fluvial gravel deposits to investigate the two and three dimensional aspects of inhomogeneities. Because reflection patterns depend strongly on the frequency of the input signal, the measurements have been perfonned with a variety of antenna frequencies: an OYO Georadar 1 system with a pair of 250 MHz antennae and a Sensors & Software Inc. pulseEKKO IV system with pairs of 100 MHz and 200 MHz antennae. The GPR profiles which have been recorded with different frequencies are compared. The reflection patterns on the GPR proftles illustrate different aspects of subsurface inhomogeneities. It is demonstrated that the 250 MHz antennae of OYO and the 200 MHz antennae of pulseEKKO IV portray the small scale variability of the gravel deposit, whereas the reflection proftles recorded with the pulse EKKO IV 100 MHz system show reflection patterns which can be assigned to the principal set-boundaries of the sedimentary units. Recently developed software packages provide some good processing opportunities such as filtering, trace editing and migrating in the time domain. The 3-D representation of GPR sections enables the recognition of the geometry of sedimentary units. This information could be useful in the process of quantification of geophysical data as input in hydraulic models.
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Modelling GPR reflections from magnetic susceptibility and electrical conductivity variations
Authors O. Lzaro-Mancilla and E. Gmez-TrevioTypical GPR sections show a multiplicity of reflection signals, whose origin is usually considered to be related solely to variations in the electrical pennitivity of soil materials. The possibility that some of these reflections could be related to variations in electrical conductivity, is commonly ruled out on the basis of the operating frequency of current GPR instruments. Magnetic susceptibility variations , on the other hand, are commonly neglected because in most situations they affect velocity much less than electrical permitivity . In general, however, neither GPR operate at their optimum frequency nor can magnetic susceptibility be neglected at large. The objective of the presented work is to asses the relative importance of the last two properties in GPR studies. In general, actual field conditions usually fall between two extremes . One is the case of the very conductive ground where GPR is not applicable; the other is a completely transparent medium with perfect reflections. The intermediate cases may include very resistive media but with high enough conductivity contrasts to produce measurable reflections. We test this hypothesis along with the possibility that some of the observed reflections in GPR sections could also be due to natural- magnetic susceptibility variations. We consider a plane-wave source in the presence of a horizontally layered model whose three electromagnetic properties vary from layer to layer. Our results indicate that reflections due to electrical conductivity and magnetic susceptibility variations, can under natural conditions be of the same order as those associated with electrical pennitivity.
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Ground penetrating radar of lakeshore spits in nothwestern Saskatchewan, Canada: variable internal structure
Authors Harry M. Jol, Derald G. Smith and Richard MeyersGround penetrating radar (GPR) field experiments were carried out on three different lakeshore spits located in remote northwestern Saskatchewan, Canada. The objective was to test present ideas regarding the origin of spits, particularly thickness, depositional migration direction and lake level stability. To determine depositional processes and direction of the spits, we first identified radar facies (groups of reflection with similar patterns) which from previous experiments have shown correspond with internal sedimentary structures and facies. From the results we have identified two different depositional processes which form spits not yet reported in the literature. Big Buffalo spit (4 km long) in Peter Pond Lake shows dipping reflections near the spit end indicating active longshore processes and overwash deposits. Ted Island spit, in Churchill Lake, is a complex 350 m long spit with internal reversed cross stratification sequences which infer alternating depositional trends from two different directions. The Big Island spit in Lac Ile-a-la-Crosse forms on the south side of a dumlinoid-shaped island and is 650 m long. Reflections show a deep infilled basin between the island and spit end, as well as dipping reflections (5_9) within the recurved spit end. Loss of signal return from below the spit strata in all three spits is attributed to a lithofacies change from sand to mud or diamicton.
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Ground penetrating radar investigation of a wetlands replacement site
Authors John Madsen, Susan McGeary, David Krantz, Samuel McIntire, William Daniels and Curtis StorlazziGround penetrating radar (GPR) techniques were used with conventional lithostratigraphic information and water table elevations to develop a model for the hydrogeologic setting of a -0.5 km2 wetlands replacement site in southern Delaware, USA. The GPR profiles are a valuable component in the modeling because they provide a continuous, high-resolution image of the subsurface that cannot be provided by conventional coring and piezometric approaches. GPR data was gathered using a Sensors & Software, Inc. pulseEKKO IV unit using 50 and 100 MHz antennas. Over 2.3 km of common offset profiles were collected using the 100 MHz antennas with a transmitter-receiver separation of -1 m (3 ft) and a station spacing of -0.3 m (1 ft). 50 MHz data was also gathered to compare subsurface images using lower frequency antennas. Commonmid- point records were collected at -60 m (200 ft) intervals along the profiles. The quality of data collected with the GPR is excellent. The upper portions of the profiles contain a water table reflection (-1 m depth) that can be traced throughout the site.. Below the water table coherent reflections were obtained to depths greater than 10m. We have collected 7 vibra-cores to depths ranging from 2 m to 6 m along the profiles to correlate GPR reflections with changes in subsurface lithologies. Sediments encountered during vibra-coring consist primarily of fine to medium sands, silty clay sand, silty clay to clay, and a thin layer or quartz granules to pebbles. In addition to the vibra-cores conventional subsurface hydrogeologic information at the site includes sediment samples from borings, hand-auger cores, and water table elevations from piezometers.
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Internal structure of a pacific coast barrier spit using ground penetrating radar
Authors Richard Meyers, Derald Smith, Harry Jol and M.B. HayGround penetrating radar (GPR) field experiments were carried out on the modem barrier spit at Willapa Bay, Washington (north ofthe Columbia River mouth). The barrier spit is influenced by a 3.6 m tidal range (Spring), as well as high wave energy and longshore transport depositional processes. The spit is 36 kIn long by 2-5 kIn wide and is composed of fine-grained beach and eolian sand. The spit has a freshwater aquif~r recharged by rainfall. To determine paleodepositional processes of the barrier spit, radar facies (reflection patterns) which correspond with sedimentary facies and depositional' processes were identified. Next, interpretations of depositional processes using the geomorphic association between facies and process were made. The objective was to reconstruct the overall sequence of paleo-depositional patterns and processes which have formed the Willapa barrier spit during the late Holocene (last 5000 years). A puiseEKKO™ IV radar system was used with 25, 50, 100 and 200 MHz antennae. GPR transects were carried out along beaches and roadways across the spit and where necessary were corrected for topography. Processing and plotting of the radar profiles was carried out using puiseEKKO™ IV software packages. GPR surveys indicate a shingle-like accretionary depositional pattern of beach and shoreface reflections which dip towards the ocean at about 1 degree. At the distal portion of the barrier reflections dip in the direction of longshore transport (north). Nearby radiocarbon-dated wood in beach sediment suggests that the spit began to form 4000 years ago. The loss ofradar signal from below 10-15 m deep may indicate a major lithofacies change. Very difficult penetration by vibracoring of spit sediment suggests that GPR reflections may be partially attributed to the tight packing of the individual sediment particles along nearly straight inclined reflections, as well as slight changes in sediment grain size at bedding planes deposited by major winter storm events.
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Ground penetrating radar in a hydreological investigation of the Oak Ridges Moraine, Ontario
Authors J.A. Pilon, H.A.J. Russel, T.A. Brennand, D.R. Shapre and P.J. BarnettThis paper presents a summary ofthe results obtained with Ground Penetrating Radar at three ofthe eleven sites surveyed during the summers of 1992 and 1993 as part of the hydrogeological investigation ofthe Oak Ridge Moraine. After topographic correction ofthe data, we were able to accurately determine the location ofthe water table. Areal surveys at the St. John and Bolton Fann sites allowed comprehension of changes in water table topography. In addition, we obtained substantial subsurface structural information which improves our understanding of sediment architecture and genesis. The data from the Brighton and Bolton Fann sites were acquired near aggregate quarries which allowed direct correlation between the radar records and the sedimentary exposure. The results presented here highlight the level of geological details detected by the GPR surveys. Even though further studies will be conducted in 1994 and 1995, we are already able to demonstrate that GPR is an excellent geophysical technique for geologic (textural and structural), and hydrogeologic studies in appropriately resistive surficial sediments.
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Application of ground penetrating radar in coastal stratigraphic studies
Authors Sytze van Heteren, Duncan FitzGerald and Paul McKinlayIn recent years, ground-penetrating radar (GPR) has been applied in a rapidly increasing number of stratigraphic studies. Its ability to produce continuous subsurface profiles (under favorable circumstances) has resulted in a better understanding of the shape, extent, and internal structures of facies belonging to several different depositional environments. Thus far, however, GPR has been used in few coastal regions, mainly because of the widespread occurrence of signal-attenuating units. Facies producing signal attenuation include fine-grained estuarine and lagoonal clays and coarser-grained units that contain saltwater. The present study, utilizing a Geophysical Survey Systems Inc. System 3 GPR, focuses on a number of barrier beaches and spits along the New England coast. Most of these barriers are sandy to gravelly; they include a reservoir of fresh groundwater that increases in thickness toward the center of the barrier. To a certain degree, the volume of fresh water is related to the size of the barrier. The occurrence of fresh groundwater in a mainly sandy matrix has turned out to be favorable for GPR application in several instances, with the best results being obtained in the centermost portions of wide and high barriers. Signature quality decreases rapidly toward both the sea and the back-barrier marshes and tidal flats. In both directions, the freshwater-saltwater boundary is increasingly closer to the surface. GPR has proven to be useful in several aspects of the study of coastal barriers. It is an important tool in identifying and characterizing different stratigraphic units that make up a barrier. GPR is also highly useful in determining sediment budgets of coastal systems. Finally, GPR can be applied to determine the occurrence and the maximum extent of peat layers underneath a barrier. Interpretation of GPR profiles, in conjunction with more conventional methods of barrier study, provides a data base that can be used to analyze a range of physical parameters which may have left their mark in a barrier, such as relative sea-level changes, paleotidal range, sediment source and supply, and the effect of storms.
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Trial of tunnel radar for cavity and ore detection in the sudbury mining camp, Ontario
Authors P.K. Fullagar and D. LivelybrooksThe utility of ground penetrating radar (GPR) in metalliferous mines for cavity detection and orebody delineation is under investigation at Ecole Polytechnique. As a first stage in this investigation, fixed offset Pulse Ekko IV radar profiles were read in underground mine drifts at Sudbury in August 1993. At Stobie Mine a 3.7m diameter ventilation shaft was detected unequivocally using unshielded 100MHz antennas, at ranges between 12 and 16m, and an orepass was located at a range of 10m. This demonstrated that GPR is effective in 5m x 5m mine drifts, provided target travel times are greater than about 80ns, corresponding to a range of approximately 5m in Stobie metasediments and greenstones. Reverberation within the drift could obscure reflections from closer targets. Attempts to map massive sulphide ore contacts within 10m of drift walls at Lower Coleman Mine were unsuccessful. This outcome is attributed to sulphide blebs and stringers in the host rock which increase the intensity and longevity of reverberations within the drift (by enhancing the reflectivity of the walls) and which also scatter and attenuate any radar signals propagating to and from the contact. "Geologically minor" sulphide concentrations can thus seriously compromise the effectiveness of radar. Further ore delineation work should be focussed on massive sulphide bodies which make sharp contact with barren host rocks.
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Study of cavity depth estimation behind concrete tunnel lining using GPR
Authors Satoshi Maekawa and Thomas J. FennerIn considering the maintenance and repair of existing tunnels, it is necessary to determine the concrete lining thickness as well as the scope and depth of cavities behind concrete. Since ground penetrating radar (GPR) can rapidly provide high resolution continuous profiles and, is capable of detecting cavities and measuring concrete thickness, results in the frequent use of this method for these applications. Tunnel inspections however, present special problems due to traffic control and difficult access to the tunnel liner, especially in large diameter tunnels. Accurate assessments of cavity volumes is essential in estimating injection quantities during grouting operations. This requires complete coverage of radar profiles with accurate positioning information. As a result a special tunnel inspection vehicle, RAPIDAS, was developed for data acquisition. RAPIDAS is a vehicle mounted with four hydraulic booms. It can position up to four antennas simultaneously to the desired locations for radar measurements. It permits simultaneous and continuous profile measurements while the vehicle is moving. RAPIDAS is equipped with multiple channel radar systems to obtain high quality radar data in a short period of time. Special post processing software was also developed for quantitative analysis of the acquired radar data. For the calculation of cavity depths we have applied the least squares method between the basic wavelet and the observed wavelet in the frequency domain to obtain the reflection coefficient series. The effectiveness of the software was confirmed in model experiments and in field tests. For data processing we used both the newly developed software and RADAN (software ofthe GSSI Corporation). Post processing included background removal, amplitude adjustment and migration to emphasize the reflection wavelet from the reverse side oflining. Next, from features such as the magnitude of the amplitude and inversion of the phase, the scope of the cavity was extracted. Then a reflection coefficient series analysis was carried out with respect to the cavity location. The travel time in the cavity was obtained thereby confirming the cavity depth.
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Application of GPR for a more efficient mine planning
Authors M. Momayez, A. Hara, F.P. Hassani and A. SadriThe Canadian mining industry is faced with international competition, low base metal prices and diminishing mineral reserves. To remain competitive on the international market, the industry must reduce the cost of mining and increase productivity while maintaining a high standard of safety. To do so, there is a great need for improved methods that would allow the detection of geological structures in advance of mining and the monitoring of pillar integrity to reduce dilution and increase safety. This will facilitate planning for optimum exploitation of the mine and to increase production at lower costs. The present paper discusses an example of more efficient mine planning using new technologies such as Ground Probing Radar (GPR). Here, GPR technology is used at the 2500 level of the Kidd Creek mine in Timmins, Ontario to (l) monitor the stability of the sill pillar, (2) locate the presence of disseminated sulfide pockets in the sill pillar for extracting the mineral content and (3) monitor the stope backs and wall structures to evaluate the effect of on-time filling sequences.
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GPR surveys inside an hydroelectric water-supply tunnel to investigate the rock-concrete interface and the fractures affecting the host rocks
Authors Mauro Piccolo and Annalisa ZanelliOn request of the Italian National Electrical Agency, the Company IDROGEO carried out a G.P.R. survey inside an old water-supply tunnel 14 km long belonging to an hydroelectric power plant located in the North East of Italy. The aim of the survey was the geo-structural investigation of the rock formations surrounding the tunnel with particular interest in the mapping of cavities and fractures associated to the water occurrences and circulation. A detailed investigation was also requested to detect the presence of voids at the concrete-rock interface. The tunnel crosses different rock formations belonging to the Alpine sequence with the presence of evaporitic formations affected by strong tectonic deformations. More than 7,000 meters of G.P.R. profiles were recorded by using a GSSI SIR 10 equipped with 100 and 500 MHz antennas with simultaneous data recording on two channels. The survey at 500 MHz .was aimed at the precise determination of the concrete thickness and at the detection of the voids at the concrete-rock interface, whereas the use of 100 MHz transducers permitted the detection of larger unconformities and cavities up to a distance of 15-20 metres. The identified structural elements were divided into 5 groups: - lack of contact and delaminations at the concrete-rock interface - geostructural elements - open fractures - voids and unconformities - honeycomb alterations The survey also permitted the location of some old artifacts whose position and nature were uncertain.
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Ground penetrating radar applications on high grade gold deposits at the Sixteen to One Mine, California
More LessIn general, application of modem technology to ancient mineral deposits is a learning process that can be expensive, time consuming and draining to a company's precious human resources. To stay abreast of new technology and remain competitive in an increasingly environmentally demanding society, hi-tech research has become of primary concern to the management of this company. The principles of Ground Penetrating Radar (GPR) were evaluated at the Sixteen to One mine and found to have good potential for locating high grade pockets underground. This paper presents a description of ore occurrences at the mine, the company's motivation for testing GPR, and its suitability for, locating high grade gold pockets. Also presented are underground test results, and a discussion of findings and suggestions for this application and similar ones in the future. The geologic features at this mine are somewhat unique, and it is doubtful that specific results can be readily adapted to mines elsewhere. Intertwined with technical facts and figures the reader will discover a human factor found to be important in the successful application of this technology and all others. The inquisitive personality in conjunction with a "nuts and bolts" academic personality has proven to be a successful combination for the evaluation of GPR It is however, no substitute for proper planning.
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FDTD2D+ - A finite-difference, time-domain radar modelling program for two dimensional structures
Authors D. Livelybrooks and P.K. FullagarThe two and a half dimensional finite difference program, FDTD2D, of Moghaddam et al. computes the radar responses for an arbitrary 2-D conductivity, permittivity and permeability model with one or more embedded current source(s). After Fourier transformation in the strike direction, the program solves for the six electromagnetic fIeld components as functions of (x, y, k, t), and then inverse transforms them. Fields are calculated via a set of finite-difference, time domain equations on a staggered grid following Yee. Time derivatives are also represented by central differences, and electric and magnetic fields are solved for equal but staggered steps in time. The regular model grid is terminated using the "absorbing" boundary conditions of Liao et al.
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Application of GPR for geological mapping, exploration of industrial mineralization and sulphide deposits
More LessThe ground penetrating radar (GPR) technique is applied to the localization and characterization ofrock types, mineralizations and their associated boundaries, and to the identification of features inside the rock. This is made possible through the determination of variations of electrical properties within the rock units. GPR has been extensively tested at several localities in Sweden and Denmark. The main testing area in Sweden has been several sites in the Skellefte field. The Skellefte field is well known as the biggest sulphide mining district in Scandinavia. The testing area consists mainly of volcanic rocks surrounded by granites of various ages. Test measurements at the Skellefte mining areas are compared directly to geological variations in the rock as they have been exposed through the exploration activities. Electrical property contrasts between orebodies and the host rock gives possibilities of determining the distribution of shallow mineral resources. A combination of the variation in reflection pattern, reflection amplitude and penetration depth is used to determine the boundaries between different rock units. Reflection measurements have been used to identify both fracture zones as well as alteration zones and to determine their location and orientation. Lithological variations of limestone have been studied within the Faxe Kalk exploration area in Sjelland, Denmark. Divisions of the limestone into areas of coral reefs and lagoon deposits are made possible through distinctive reflection patterns and differing penetration depths between the rock units. The effect of tectonics can be studied by means of various reflection patterns from flint horizons within the lagoon deposits. The GPR results are used as a basis for geological modelling and as a foundation for mineral exploration activity.
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Twenty years of Ground-probing radar in salt and potash mines
More LessRadar has been used for twenty years for underground exploration of the boundaries and internal structures of salt deposits in Germany. The method and equipment, developed by the Federal Institute for Geosciences and Natural Resources (BGR) in cooperation with the German mining company Kali und Salz AG in the early 1970s, make it possible to determine the distance to stratigraphic boundaries up to 1000 m from the antenna, especially in anhydrite, claystone, and basalt, and between joints filled with water or brine. The equipment is light weight and battery powered; it can be used almost anywhere in a mine without any preparatory measures. Therefore, the method is very economical. The location of the reflecting discontinuities may be determined using additional directional antennae. For exploration preparatory to mining, various probe systems have been developed for use in drillholes. In cooperation with Prakla-Seismos AG, a special GPR probe was developed for deep boreholes. Salt domes have been explored down to depths of 3000 m with'this probe. A probe has been developed for use in boreholes in mine drifts. This probe is designed so that it is safe to use in mines in which there is the risk of explosions. It is equipped with a directional antenna so that the spatial orientation of the reflecting planes can be determined. The technical aspects of the equipment are discussed and the results of exploration in German and Canadian salt deposits using the radar method are used as examples.
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RAD-SEIS: Direct acces for ground-penetrating radar to reflection seismological techniques
More LessConstant-offset georadar profiles look similar to stacked common mid-point seismic sections. Yet, technologies used for the field aquisition, processing and interpretation of seismic reflection data are significantly advanced with respect to what is common today in ground-penetrating radar. To take advantage of seismic techniques, the recently developed RAD-SEIS system allows: (a) direct digital recording in standard seismic SEGY data format; (b) continous profiling with accurate automatic antennae positioning; (c) programmable aquisition of constant-offset and multi-offset sections and cubes. The upgrade of the relatively inexpensive analog GSSI SIR-3 unit is achieved by adding a 16-bit digitizing board to a portable 486 personal computer and by employing a newly designed method for automatically triggering the system at constant spatial intervals as small as 1 centimeter. Programming of the 486 computer, using software tools provided with the digitizing board, allows the data to be recorded directly in SEGY format. In the multi-offset mode, an end-on "source-receiver" spread geometry, typical of that employed in reflection seismic surveying, is used. At the processing facility, the georadar field data are transferred from the personal computer via a network link directly into a workstation-based seismic processing and interpretation package. 300 Mbytes of field data, typical of a single day's multi-offset profiling, can be transferred in less than 60 minutes. Although turnaround time for processing large georadar datasets is relatively short, analog paper hardcopies, which are recorded directly in the field, remain an essential means for quality control and can help guide the surveying strategy.
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Subsurface sensing for the autonomous retrieval of buried objects
Authors Herman Herman and Anthony StentzUsing surface and subsurface sensing, we have developed a perception system for autonomous retrieval of buried objects. The subsurface sensing system uses Ground Penetrating Radar (GPR) to detect and localize buried objects. An industrial robotic arm is used to position the GPR antenna, and a 2-D laser rangefinder system generates an elevation map which is used to guide the robotic arm. Using this setup we have automated the GPR data collection process. An image processing algorithm is used to locate the object of interest in the GPR data. After the object is located, we use sense and dig cycle to retrieve the object. During this loop the excavator alternately removes a layer of soil and takes a subsurface scan. The electromagnetic (EM) wave propagation velocity in the soil can be computed by comparing the data from the previous subsurface scan with the current one. With each successive iteration, the estimates of the object's size, shape, and location improve. This loop is repeated until the object is within certain distance and able to be retrieved. This "sense and dig cycle" enables the system to handle layered soil to some extent since at every cycle the EM propagation velocity estimate is updated. The computationally intensive parts of the processing are run in parallel on multiple processors to achieve near real-time performance. The system so far has been used to detect, locate and retrieve small buried objects in the testbed.
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Ultra-wideband, high RCS, active calibration targets for foliage/ground penetrating SARs
Authors George Moussaly, Theodore Grosch, F. Jud Heinzmann and Philip FialerThis paper describes the design, development, test, and operation of compact, active electronic targets having high and constant values of radar cross section (RCS of 20 to 40 dBsm). The active targets were specifically designed to be used for reference calibration of airborne synthetic aperture radars (SARs) used in foliage and ground penetrating (FOPEN, GPEN) applications. The use of active (vs. passive) calibration targets in this application was driven by the need to achieve physically compact reference targets with high, constant RCS over the ultra-wide bandwidths and for relatively long radar wavelengths typical of FOPEN/GPEN radars. Two target types were developed. One type operates from 30 MHz to 90 MHz and was designed to respond to a stepped frequency radar waveform. The other target type operates from 100 MHz to 500 MHz and was designed to respond to an impulse radar waveform. For each target type, two different models were built: one for above ground operation and the other for burial below ground. A description of the active targets is provided along with an example of the design methodology which included electromagnetic modeling of the target antennas above and below the ground. Lessons learned during the target development process are discussed. A description of field tests using the active targets is presented along with a comparison of modeled and measured target RCS and an example of the active target imaged by an airborne SAR.
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Recording GPR data by means of a video camera recorder
Authors Gerald B. Rupert and L. John TylerA technique is presented for recording synchronized radar images and a video record of antenna position. This requires a GSSI Model 38/39 video display unit (VDU) or equivalent and a video camera recorder (camcorder) possessing a high fidelity stereo sound track. For this system, a continuous history of antenna position is recorded on the video channel; and simultaneously, radar data is recorded on the audio track. This permits the precise correlation of radar events with antenna position. The resulting data set can be viewed on dual monitors or on a single monitor with "picture in a picture" capabilities. As with other recording systems, the radar data can be down loaded directly into a computer for processing.
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Automatic signal processing of front monitor radar for tunnelling machines
Authors Toru Sato, Ken-ya Takeda, Takashi Nagamatsu, Toshio Wakayama, Iwane Kimura and Tetsuya ShinboIt is planned to install a front monitor radar on the surface of the rotating drill oftunnelling machines in order to detect obstacles such as casing pipes of vertical borings. The conventional aperture synthesis technique can no more be applied to such cases because the radar image of a pipe does not constitute a hyperbola as is the case for linear scanning radars. We have developed a special purpose signal processing algorithm with the aid of the discrete model fitting method, which can be used for any pattern of scanning. The details of the algorithm are presented together with the results of numerical simulations and test site experiments.
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Computerized scanner system SARA
Authors Peter Ulriksen and Christian StolteA meaningful 3-D data presentation requires a high geometric fidelity in the collection of the GPR data. Since Synthetic Aperture Focusing Techniques require several samples per space time wavelength, an enormous amount of data must be collected, which precludes manual movement of the antenna. To permit this new GPR concept, a computer controlled scanner has been built. It consists of a 5 m long linear movement unit with a ball screw connected to a servo motor. A second servo motor is connected to pairs of small wheels used to move the linear unit in the transverse direction. The stroke of the linear unit is 4.5 m and within that length the antenna can be positioned at submillimeter accuracy. At full speed the antenna travels over the linear unit in 12 s. The maximum vertical deflection with a 5 kg load is 1 mm. The weight of the scanner is approx. 75 kg with no load. After a period of evaluation of various concepts, a second scanner will be built to facilitate bistatic measurements. That way the 2-dimensional back scatter field can be determined, generating data suitable for processing in 3-D seismic packages. Using the previously developed 5 channel GPR controller, MRS, and 2 orthogonally polarized antennas on each scanner, it is possible to obtain the polarization matrix. Since MRS allows each connected antenna to work either as a transmitter, a receiver or both it is possible to measure the back scattered field around one point of transmission although the scanners can not cross. Since the computer that acquires all the data and displays it is the same that controls the scanner, it is also possible to draw marks on the ground with a spray can connected to the scanner. To visualize the great quantity of 3-D data, acquired with this scanner, the help of 3-D display and interpretation software was found indispensable. Such software is commonly used in the field of oil exploration to interpret three-dimensional subsurface structures. Before interpretation, the 3-D data cube was prepared for visualisation with several seismic data processing routines including filtering, deconvolution/envelope and migration. Depending on the data quality , "a priory" knowledge, and complexity of the subsurface structure the processed data was found to display a clearer image than the raw data. Several visualization techniques as time slices (windows), cube-, chair-display and flying carpet were compared and evaluated towards their usefulness in enhancing the subsurface image. An animation of the 3-D data was found indispensable for the interpretation of the data and is presented here.
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High speed georadar data acquisition for groundwater exploration in The Netherlands
More LessGeoradar has proved to be a valuable new geophysical technique for non-destructive groundwater exploration in The Netherlands. Under favourable conditions detailed continuous images are obtained to depths of 40 m. To increase the commercial attractiveness of georadar, especially for water supply companies, a high-speed data acquisition system was devised. Sensors & Software Inc. developed a high speed pulseEKKO system and TNO designed and built a polyethylene carrier for the antennas; this carrier is trailed by the "Mule": an All Terrain Vehicle that can move at the required constant low velocities of about 2 km/hour. At this speed a fast portable field computer allows sufficient stacking at common time windows. Continuous georadar data acquisition rates are achieved 10 times higher than conventional manual measurements. The system is distance triggered by pulses produced by an electronic odometer. The carrier is flexible and multi-functional: on loose soil it is dragged, whereas on metalled and rough roads wheels are attached, bringing the antennas at adjustable levels above the ground. The antennas may be rotated 90 degrees, and thus be attuned to the expected strike of geological structures. The carrier can accommodate three antenna-frequencies: 200, 100, and 50 MHz. Prior to the design of the mobile georadar system, tests were run along a profile to examine the influence of antenna elevation and car proximity. This paper shows some of the results of the test surveys, presents the final design of the high speed data acquisition system and illustrates its performance by renewed measurements along the test line. Moreover, some remarkable results recorded by the new system are shown to demonstrate the effectiveness of the high speed georadar system.
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Data acquisition systems for ground penetrating radar with example applications from the air, the surface and boreholes
Authors David Wright, Jerry Bradley and Thomas GroverData acquisition for short-pulse (impulse) ground penetrating radar (GPR) has evolved as new technology has become available but commonly involves equivalent time sampling a radio-frequency signal to produce an audio-frequency replica of the signal. An airborne radar system developed by the U.S. Geological Survey (USGS) in 1978 recorded the sampled signal on analog tape. Low data rates made waveform addition (stacking) for signal-to-noise improvement impractical. Advances in data acquisition and recording that have occurred within the last two decades have made digital recording the norm in modern GPR's. Waveform addition for signal-tonoise enhancement is common. However, all commercial short-pulse GPR's known to the authors still use equivalent time sampling prior to digitizing. Fast single-shot digitizers are a viable alternative to equivalent time sampling in some applications.
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Instantaneous polarization match: Identifies amplitude changes due to anomalous polarization in GPR data
Authors Rob D. Luzitano and Tad J. UlrychInstantaneous polarization match identifies amplitude anomalies due to the altering of wavelet polarization by an interface or the intervening medium. The match factor (p) varies from 0.0 to 1.0 for cross and parallel polarization, respectively, thereby scaling the antenna's effective aperture and thus the received power via the radar equation. Polarization match is defined mathematically from the dot. product of antenna and wavelet polarizations. Calculation of p requires two component data, i.e. transmit and receive antennas parallel (E=) and perpendicular (E.i)' Assumptions regarding the antennas were minimized by using a general equation containing antenna polarization terms which we estimated from a lake bottom reflection. The uncertainty in p values arises primarily from random noise and timing errors. Due to inherent variations in noise level, the standard deviation varies from about 0.06 for low matches (< ~ 0.35), rising at middle match values to about 0.065, then decreasing to about 0.015 at matches greater than 0.95.``
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The development of an advanced GPR system at the Univeristy of Houston
Authors Shiqun Xie, Di Lan, Jin Wang, Yongmin Zhang, Richard Liu and David Shattuck!An advanced ground penetrating radar system is being developed at the University of Houston. The objective of this research is to study the possibility of improving GPR performance by improving hardware structure, data handling, signal processing, and data interpretations of GPR systems. The other goal of this work is to study ways to reduce the complexity of operating the GPR system, namely, parameter set up, field operation, and data processing and interpretation. A prototype GPR system is being designed and implemented in the Department of Electrical Engineering at the University of Houston. Based on the traditional GPR design, the new GPR hardware system features compact size, easier operation, with improved spatial resolution and penetration depth. Circuit modifications are made on the transmitter, sampling head, controller, and data transmission components. A smaller version of resistively loaded printed-circuit-antennas are used for transmitting and receiving probes. Fiber optic links and wireless links are used as data transmission channels. In the data storage and processing station, software has been developed to enhance the radar image. A complex cepstrum computation and the direct wave cancelation can be performed by using the built-in software, along with the standard windowing, filtering and displaying functions. An user friendly control software is developed based on Microsoft Windows and Visual Basic packages. Since the GPR system is still under development, only primary data will be shown. Data inversion is also attempted using a direct-timedomain inversion algorithm recently developed by the authors.
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The application of ground penetrating radar in Israel: selected case histories
Authors Alex Beck and Amit RonenRadar technology has been used in Israel for the past two years by the Institute for Petroleum Research and Geophysics (IPRG) for many applications such as civil engineering, archaeology, infrastructure, environmental problems and geology. For such applications, we use the SIR-10 system equipped with 100, 500 and 1000 MHz antenna. This article discusses four examples of the application of GPR technology in Israel. The first instance concerns karst phenomena under highways, including open cracks and spaces at a depth of 3-4 meters in a fairly homogeneous dolomite rock, which could endanger the stability of the highway. The survey was conducted using a 500 MHz antenna towed by a vehicle. The second example addresses the detection of a tunnel dug in the subsurface by smugglers at the Israel-Egypt border and which was used to transfer merchandise, money and drugs. A radar survey carried out over a stretch of some 600 meters revealed an underground tunnel, 3.5 meters below the surface and about 1 meter in diameter, in a sandy-clayey environment. The third example concerns the inspection of an old 2 km runway using a 1 GHz ground coupled antenna towed by a vehicle. The s~rvey revealed an undulating subsurface strata at a depth of 50-110 em, probably the old runway asphalt surface laid some 50 years ago. The fourth case deals with the detection of oil spills at an oil refinery in a sandy environment using 100 MHz and 500 MHz antenna. The oil spills detected by GPR were in. good correlation with actual oil contaminated samples in drills. Further drills in areas suspected of being oil contaminated were planned on the basis of the GPR results. Most of the oil spills were located in close proximity to oil pipes which were probably to some degree eroded or broken.
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Major natural gas pipeline projects in Germany using GPR
Authors J. Czarnowski, S. Heinzel, P. Brühl, C. Staib, M. Robeck, G. Frank and Dr. R. FruhwirthIn July 1990 DORSCH CONSULT, Munich was engaged by Wintershall AG, Kassel to provide engineering consulting services for one of the longest natural gas pipeline projects in Gennany, the MIDAL pipeline (MITTE DEUTSCHLAND ANBINDUNGSLEITUNG/CENTRAL TIEON LINE) project and the STEGAL pipeline (Saxony-Thuringia Natural Gasline) . .For the first time in a project of this nature the Company employed geophysical methods on a large scale in the preliminatry investigation of the pipeline routes. The application of ground penetrating radar/GPR, in combination with other electromagnetic soundings, yielded rapid and reliable results. Without this ability to quickly and reliably interpret the GPR data on site, the project's success would have been in question.
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The application of ground penetrating radar to detection of shallow faults and caves
Authors Shikun Deng, Zhengrong Zuo and Huilian WangGround penetrating radar (GPR) is the electromagnetic equivalent of the reflection seismic technique for high-resolution shallow geological mapping (Davis and Annan,1989). The records obtained from GPR reflection method are similar to those obtained from single-trace reflection seismic one in appearence. So, some seismic interpretation concepts may be used in the interpretation of GPR data. As a means of engineering geophysical exploration, GPR technique has its unique virtues: it has good performance on less suffering from the enviroment influence in measurements; it has higher resolution than other geophysical technique when penetration is available; and it is a nondestructive detection method with fullautomatization, portability and efficiency. In this paper, the application of GPR to detecting shallow faults and caves were introduced with three case histories, which showed the feasibility of GPR for detailed prospecting of the shallow geological problems and inspection of quality of old engineering structures.
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Georadar-model and in-situ investigations for inspection of railway tracks
Authors C. Göbel, R. Hellmann and H. PetzoldThe reasons for damage to railroad tracks often lie in the subgrade. At present investigations of tracks are carried out selectively and schematically by drilling and digging (every 100 m). By using the GPR it is possible to give a comprehensive assessment concerning the condition ofthe complete profile ofthe track
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Application of seismic processing techniques to discontinuity mapping with ground-penetrating radar in crystalline rock of the Gotthard massif, Switzerland
More LessThe Gotthard Pass (2108m above sea level) is one of the main thoroughfares across the Swiss Alps, connecting northern Switzerland to the southern Ticino region. Near the summit of the pass ground-penetrating radar (GPR) data were collected across an accessible "slab" of slightly inclined granitoid rock of 50m length and varying width up to a maximum of 25m. An abandoned quarry face provided depth information on the rock mass down to 15m. The GPR survey conducted by two persons included several single-fold constant-offset profiles and a 14-fold multi-offset profile. The nominal center frequency of the antennae was l00MHz. All data were recorded in digital form with the RAD-SEIS system, based on a GSSI SIR-3 unit and a personal computer. Gained and bandpass-filtered georadar sections show numerous distinct reflections to the maximum recording time of 950ns, corresponding to depths of nearly 60m. Some strong reflections extend the full 50m - length of the recording profiles. Apparent dips of reflections vary from 0 to 20, the principal ones being in the 0 to 5 range. Normal moveout (NMO) velocities, derived by interactive velocity picking of common midpoint gathers, are fairly constant at 0.12m/ns throughout the recorded section. Stacking of the multi-offset profile data reduces noise at large two-way times and increases continuity of the deeper reflections. Migration of the constant-offset profiles and the stacked section focusses the abundant diffraction patterns, yielding much clearer images. Comparison of the GPR sections with the rock face, leads to the conclusion that the principal reflections are due to moisture-filled fractures with widths of less than a centimeter.
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The geophysical investigation of drilling obstacles for microtunneling projects by means of GPR
Authors M. Guenther and A.F. KathageUp to now the experience gained shows that the GPR measuring process is best suited to be used for geophysical pre-study of microtunnelling routes. Such pre-studying of drilling routes often means that the GPR-technology has to be used to the limits ofits performance capacity. Inspite ofthe high values of signal absorption in loamy soils as well as loss due to scattering in inhomogeneous piling, the task is to detect utilities as far as possible without exception and to localize precisely changes in soil structures underground. Considering the relatively high degree of lateral and vertical variation of GPR-sections in the direct surroundings of urbane infrastructures, the GPR-geophysicist is faced with a demanding task, whereby the experience gained under similar circumstances play an important role. Diverse detection tasks are shown, representative for the area of Southern Germany and Austria. Ranging from the detection of pipes in 4-channel antennae configurations, to the location of geological layers and drilling obstacles, it demonstrates a spectrum of typical cases and a survey of the application possibilities of the GPR-technology for pre-studying of drilling routes in Southern Germany and Austria.
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Shallow bedrock profiling using GPR
Authors C. Robillard, P. Nicolas, P. Amirat, M. Gariepy and F. GoupilGeophysical surveys have been conducted to profile the bedrock since the early stages of geophysics. However the accuracy of the results at shallow depth became relatively large. The development of the ground penetrating radar, over the last two decades offered an attractive method for shallow bedrock profiling. This paper outlines the methodology used and discusses the results of such surveys performed in France in various locations and geological set up in terms of efficiency, accuracy and reliability. We also give the client's economic point of view before concluding on the usefullness of the ground penetrating radar for shallow bedrock profiling.
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The use of ground penetrating radar for monitoring water movement in road structures
Authors Timo Saarenketo, Teuvo Nikkinen and Sakari LotvonenThe Ground Penetrating Radar technique was used to monitor water movement in road structures and the subgrade in 1992-93 as a part of the Road Structure Frost and Drainage Research Project conducted by the the Finnish National Road Administration. The purpose was to observe how water infiltrates from the road shoulder into different types of road structure, especially during the critical season of the year, ie. the autumn rainy season and the period in the spring, when the frost is thawing. The survey was performed by inserting a 3 m long tube into the road shoulder at the sites where the road structures and groundwater conditions were studied carefully by drilling and sampling and by GPR and then introducing 1-3 m3 salt water with a chloride content of 4 % in autum 1992 and 8-10 % in spring 1993 into the road structure and ground through the tube.The movement of water in the ground was monitored by visual inspection and by groundwater indicators and GPR. The results showed that GPR can be used for monitoring water discharge and movement in the road structure and subgrade. The factors which affect water movement were found to be the gradient of the road and subgrade, the direction of groundwater flow and the grain size and material properties of the structural courses and subgrade. The results also showed very clearly . that water of ,high density will penetrate through the fresh groundwater layer without being entirely mixed with it. The high chloride water was also found to thaw frozen road structures.
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Detection of an abandoned mining channel under a residential complex by ground penetrating radar
Authors J.N. Vaish and S.C. GuptaGround Penetrating Radar (GPR) investigations were conducted in a 2-storeyed newly built residential complex. The complex had 124 flats in 31 blocks spread over 1.2 hectare. Though conventional geotechnical investigations had revealed the SPTvalues
ranging 11-72and refusal in boring met at 0.5 - 4.0 m depth, no indication of any underground cavity or channel was observed. The flats were ready for occupation when in Dec. 1990, a small portion near the stair case between two flats caved-in after moderate rains in the region. Assuming it an isolated void, the filling and compaction was done. Around Aug.1991, the region experienced intermittent rains and a bigger portion between two blocks caved-in to about 2.5 m depth. It caused severe damage to the adjoining flat; its kitchen portion suffered subsidence and wide spread cracks developed in load bearing walls. The event called for non-destructive high resolution geophysical investigations. Since mining of Kaolin and Mica was carried out in the surrounding region several decades ago, it was considered that some abandoned mining channel might be buried under the damaged blocks. Thus GPR investigations were preferred in the first instance.
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Advances in the development of step frequency ground penetrating radar
Authors David A. Noon, Dennis Longstaff and Richard J. YelfIn this paper, we describe the background and developing research in Step Frequency Ground Penetrating Radar (SFGPR), and its potential capability in comparison with the more conventional impulse radar. We also describe our own development of a SFGPR system. Although the SFGPR technique has been investigated since 1972, its potential has not been fully exploited for field applications. The technology needed to implement the technique has, until now, lagged behind that of impulse hardware. However, due to the advances in frequency synthesisers and signal processing, it is now timely that a critical comparison of field performance between the two GPR techniques is made. In Australia, we began a project in 1993 to develop an experimental SFGPR system and to evaluate its performance against impulse radar. The system developed at this stage has been specifically designed for high-resolution, shallow-depth applications to map geological interfaces and to detect buried objects. Early results have demonstrated that a better performance can be more easily realised with the step frequency technique compared with swept-FM or impulse radar for these applications. This paper attempts to convey a fuller understanding of SFGPR by describing how the implementations of the step frequency and impulse techniques are different, although they adhere to the same principles of requiring wide bandwidth for resolution and high energy for penetration depth. We suggest that the controlled transmission of highly coherent signals produces a benefit over impulse radars. Other benefits arise through a more efficient use of transmitter power and received signal power. We present images obtained by our radar on an experimental test pit showing its closerange and high-resolution capability. We also comment on further improvements which the technique allows.
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Application of ground penetrating radar in locating underground utilities in Taipei MRT system
Authors Chi-Wen Yu, Wen-Chung Ko and Guo-Rong LeeThe Neihu extention line of Taipei Mass Rapid Transit system is proposed to run along the main broadway of the Neihu district of Taipei metropolis in elevated structure. The 10 kilometers long route will underlain numerous public underground utilities, mainly consisted of electric cable duct, tele-communication lines, water pipes, gas pipe, sewerage culverts etc. To avoid the interference between the underground utilities and pillar foundations, a non-invasive and fast survey program for locating the underground utilities is necessary in carrying out the detailed design of individual pillar foundation before construction. Since it is impractical, both in time and economic consideration, to excavate trench for each pillar foundation site in the design stage, Ground Penetrating Radar (GPR) technique is suggested to be utilized in locating the underground utilities. GPR is proven to be a competent survey technique for locating these underground utilities. This paper describes the field operation and results of a verification survey program. From preliminary survey results and trenching data, it appears that GPR is a promising tool for such purpose.
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Radar detection of thin layers of hydrocarbon contamination
Authors William Barber and Rexford MoreyComputer modeling, laboratory experiments and a field program were used to evaluate the ability of GPR to detect thin layers of hydrocarbon contamination on or near the top of the water table. The Sihvola mixing formula, complex dielectric constant of the constitutive components, and calculations of the complex impedance as a function of frequency, porosity and liquid fraction were used in the model. Radiated wavelets, convolved with the reflection coefficient were used to predict GPR reflected wavelets when the thickness of hydrocarbon varied from zero to two pulse lengths. Results suggest for thin layers with distinct electromagnetic boundaries, the reflection coefficient can provide an indication of the presence of hydrocarbon. Using TOR with a coaxial sample container holding a column of soil with a water table, we propagated an electromagnetic pulse through the column of soil while a thin layer of diesel fuel was introduced at the water table. The oil tension saturated zone created a gradual transition between moist soil and oil-saturated soil. This gradual transition zone in the region of the oil saturated sand did not generate the sharp electromagnetic discontinuity. Subsequently, there was not a strong return of the electromagnetic pulse. In the field, GPR data was gathered along lines in areas with known contamination. Monitoring wells, soil borings and total petroleum hydrocarbon data provided ground truth. While GPR imaged a number of factors related to site assessment, we did not see the effect of the thin layers of contamination.
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Imaging subsurface contaminants using inverse scattering techniques
Authors Pawan Chaturvedi and Richard PlumbUse of ground-penetrating radar in high-resolution imaging of subsurface targets has been investigated extensively in recent years. Imaging of subsurface contaminants is one of the applications that has attracted more attention in recent years with the increasing awareness about the environment. In this paper, a technique for imaging of subsurface contaminants is presented. The imaging problem is formulated as an electromagnetic inverse scattering problem. A nonlinear .technique used for solving the problem is outlined. This technique is based on solving the imaging problem iteratively. A forward and an inverse scattering problem is solved at each step of this technique. The forward problem is solved using the finite-difference time-domain (FDTD) technique whereas the inverse problem is formulated as a constrained optimization problem. The solution to the inverse problem is obtained by using the method of regularization. One of the problems associated with applying this technique for subsurface imaging applications is the evaluations of the integral of the Green's function for an inhomogeneous dielectric half-space. The FDTD technique is used to evaluate the Green's function which is then integrated numerically. Several simulations are performed with typical background and object properties. A ground-air dielectric half-space is considered for all simulations. For subsurface contaminant imaging, this techinque provides good-quality images for typical objects and ground properties with just a single iteration. This technique results in good reconstructions even with an inhomogeneous ground. The use of FDTD for solving the forward scattering problem provides the capability to model complex object geometries.
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Reconnaisance studies of moisture in the subsurface with GPR
More LessThe ground wave in GPR can be used for reconnaissance studies of moisture in the subsurface. Wave velocity depends on moisture via the dielectric properties of the material. When using a small antenna separation, the ground wave may be superimposed by the air wave. On the other hand, the ground wave decays fast with distance. As a consequence, the ground wave can be observed in the radargram only within a limited antenna separation. We begin the field studies with a wide angle reflection and refraction 0NARR) measurement to identify the ground wave, to determine its velocity and to fmd the optimal antenna separation. This separation is kept throughout a profile when tracing out the lateral variation of ground wave velocity. The free volumetric water content is determined from the dielectric properties of subsurface matter by an empirical relation of Topp et al. (1980). Field data are usually influenced by several factors, such as antenna frequency, antenna separation and small inhomogeneties near the surface. Moisture in the subsurface can be determined more accurately using lower antenna frequencies in a humid subsurface than vice versa. We used the Pulse EKKO IV radar system with 50 and 200 MHz antennas for the field measurements. Reconnaissance studies of moisture with GPR ground wave are non-destructive, free of radiation hazard, fast and suitable for both soft and hard subsurface.
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Radar tomography for environmental geotechnology: field and simulation tests
Authors Fan-nian Kong, Harald Westerdahl, Tore Lasse By and Nils-Otto KitterodThe ground wave in GPR can be used for reconnaissance studies of moisture in the subsurface. Wave velocity depends on moisture via the dielectric properties of the material. When using a small antenna separation, the ground wave may be superimposed by the air wave. On the other hand, the ground wave decays fast with distance. As a consequence, the ground wave can be observed in the radargram only within a limited antenna separation. We begin the field studies with a wide angle reflection and refraction 0NARR) measurement to identify the ground wave, to determine its velocity and to fmd the optimal antenna separation. This separation is kept throughout a profile when tracing out the lateral variation of ground wave velocity. The free volumetric water content is determined from the dielectric properties of subsurface matter by an empirical relation of Topp et al. (1980). Field data are usually influenced by several factors, such as antenna frequency, antenna separation and small inhomogeneties near the surface. Moisture in the subsurface can be determined more accurately using lower antenna frequencies in a humid subsurface than vice versa. We used the Pulse EKKO IV radar system with 50 and 200 MHz antennas for the field measurements. Reconnaissance studies of moisture with GPR ground wave are non-destructive, free of radiation hazard, fast and suitable for both soft and hard subsurface.
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Integrated studies of Swiss waste disposal sites: results from georadar and other geophysical surveys
Authors Eva Lanz, Laura Jemmi, Roger Mller, Alan green, Andr Pugin and Peter HuggenbergerGeoradar, electrical resistivity and magnetic data have been recorded across a composite landfill site in northern Switzerland. The landfill comprises industrial waste and domestic garbage deposited in former gravel pits. An integrated interpretation of the georadar and electrical resistivity data, constrained by surface and limited borehole geological information, has allowed the host surficial sedimentary units to be mapped over a wide area~ Of particular importance for an improved understanding of the local hydrogeological conditions are the location and approximate depth extent of gravel channels and lenses. All three data sets have been used to define the boundaries of the industrial waste, but the borders of the domestic garbage were only well-delineated on the georadar profIles and magnetic data. In addition to providing new details on the location and dimensions of the waste disposal sites, the geophysical data have confirmed suspicions that a strip of undisturbed land bifurcates the composite landfill and have supplied critical information on the location and depth of ferrous and non-ferrous metallic objects.
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Ground penetrating radar surveys for near surface charactarization: example from the Canada Creosote Site, Calgary
Authors Don Lawton, Harry Jol and Derald SmithSeveral ground penetrating radar (GPR) traverses were collected across a contaminated site near downtown Calgary, Alberta. At this location, known as the Canada Creosote Site, dense nonaqueous phase liquid (DNAPL) accumulated in the ground during creosote treatment of railroad ties and power poles between 1924 and 1962. In the late 1980's, these liquids were observed to be leaking into the Bow River just west of the downtown core, and a large-scale investigative program was begun by Alberta Environment. It was considered that DNAPL was accumulating in bedrock lows beneath surficial glacial tills, gravels and recent river sediments which are up to 10 m thick. About 1.5 km of GPR data were collected, including profiles along both north and south banks of the Bow River. It was found that good energy penetration was obtained through gravels beside the river, with radar reflections from bedrock recorded from depths of up to 5 m. However, on the flood plains above the river banks, energy penetration was significantly less probably due to the presence of near-surface clays and fill materials which are highly attenuative. For GPR profiles at river level, good agreement was obtained between drillhole data, refraction seismic profiles and interpreted GPR sections. Of particular interest is an amplitude anomaly in the GPR data that was mapped over a limited area. The drillhole data at this location showed DNAPL contamination in bedrock fractures.
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Detection of LNAPL pools with GPR: theoretical modelling and surveys of a controlled spill
Authors J.D. Redman, S.M. DeRyck and A.P. AnnanLNAPLs (light non-aqueous phase liquids), such as gasoline, kerosene and other hydrocarbon fuels, that have leaked into the subsurface, are a serious groundwater contaminant. Successful remediation of contaminated sites requires knowledge of the location of the LNAPL in the subsurface. Geophysical techniques such as surface and borehole EM, DC resistivity and GPR techniques have had limited success in delineating LNAPLs in the subsurface. To provide a comprehensive understanding of the factors controlling anomalous responses in GPR surveys above LNAPL contaminated zones, we have modelled the theoretical GPR response and performed GPR surveys of a controlled kerosene spill.
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