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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
81 - 95 of 95 results
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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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