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3rd EEGS Meeting
- Conference date: 09 Aug 1997 - 11 Aug 1997
- Location: Aarhus, Denmark
- ISBN: 978-94-6282-128-6
- Published: 09 August 1997
1 - 50 of 124 results
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Mapping groundwater areas
Authors T. Valli and A. MattsonThe volume of environmental research has increased in the Geological Survey of Finland (GTK) during the 1990's. The surveys of groundwater areas have played a remarkable role in them, because of the municipal needs for groundwater. The growing risk of pollution, environmental accidents etc. requires information about the flow and storage capacity of groundwater. Geophysical methods have tumed out to be a useful tool in giving regional information of bedrock topography and the thickness of overburden. In these studies GTK has worked as a consult and contractor for cities, municipalities and regional environment centers.
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Large-scale mapping of aquifers in the county of Aarhus using geophysical methods
Authors K. I. Sørensen and V. H. SøndergaardIncreasing problems with water quality have made ground water a very important issue in Denmark. In 1995 the Minister of Environment decided a 10-subjects scheme to improve ground water protection. One of the major issues is that all pesticides that can pollute the ground water should be forbidden. Another major issue is that counties in Denmark by the end of 1997 should be classified into 3 types of recharge areas. The classification includes: - Recharge areas of great drinking water interest, - Recharge areas of drinking water interest and - Recharge areas of limited drinking water interest The clasciffication is based on a comprehensive evaluation of the size and quality of the ground water resource. The new ground water resource plan provides guide-lines for the protection of the ground water in the county.
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Effect of "windows" mapped by geophysical methods on groundwater quality
Authors L. Thorling, K. I. Sørensen and V. ErnstenGeophysical methods turns up to be very useful, when vulnerability of ground waters is mapped in areas with a heterogeneous geological setting. In Denmark groundwater accounts for almost 100 % of the drinking water resource. Only groundwater that satisfy the quality standards of drinking water is used. Thus nitrate becomes a mayor threat in rural areas where nitrate loads generally exceed the quality limit of 50 mg/l. Previous work in Denmark concentrated on nitrate reduction under homogeneous geological condition/1/. In areas with thick day layers above the aquifer nitrate was found to penetrate about 3 - 8 m into the til! and never reached the groundwater /2/. In sandy areas nitrate was reduced in the aquifer by pyrite at the redox boarder where oxygen and nitrate were depleted at the same depth and below reduced conditions with iron and mangese was found. /3/ These homogeneous conditions accounts for about 50 % of the Danish ground waters, but in the rest of the country the geological settings are often very heterogeneous due to disturbance form ice movements during the last glaciation. Therefore both the top layers above the aquifer and the aquifer are very irregular with Iayers of altemating gram size from impermeable sediments with up to 15 % day minerals to coarse gravel deposits. The aim of this work is to study the effect of geological heterogeneity on groundwater quality and redox conditions in the aquifer. This work is part of the Danish Environmental Research programme (1992-1996).
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Borehole logging investigations for planning of remedial actions on water supply wells showing deterioration in groundwater quality
By K. KlittenComprehensive wireline logging investigation is necessary for providing the information needed for the evaluation of the possibility for relief of a deterioration in the quality of the water from a well, and for the choice and design of a remedial solution. Two different techniques are available as remedial solutions both aiming to improve the quality of the water produced from the well. One is a reconstruction of the well in order to close off the inflow of water with unwanted quality.
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Investigation of vadose zone flow mechanisms in unsaturated chalk using cross-borehole ERT
Authors L. Slater, A. M. Binley, M. D. Zaidman and L. J. WestGroundwater resources are vulnerable to surface-derived contaminants resulting from human activity. The vadose zone is a natural barrier between contaminant sources and groundwater stored in aquifers. The fate of contaminants currently in transit within the vadose zone (as determined by operative transport mechanisms) is of political, economic and social concern. Hence techniques are required to investigate solute transport mechanisms within the vadose zone.
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Using imcomplete geophysical datasets to monitor a conductive groundwater tracer experiment
More LessThe tedious and repetative nature of groundwater tracer experiment monitoring using geophysics can result in an incomplete dataset for subsequent interpretation. A fundamental cause is due to the inherent unpredictability of the rate and direction of movement of, and initial geometry of the tracer slug after injection. This problem is inherent to the experiment since if these parameters were accurately known, there would be no need to perform the tracer study. The incompleteness of the geophysical dataset is also directly affected by the balance between collecting enough data to solve the flow problem, and not wanting to collect redundant data due to the high cost of field data acquisition. This paper presents the results of interpreting incomplete geophysical datasets which were obtained to monitor the migration of a conductive saline groundwater tracer through a deltaic sand aquifer in Gray, Maine. In addition to providing a non-intrusive means for monitoring the temporal movement of the conductive tracer, interpretation of these geophysical data has also yielded a more detailed picture of the geologie and hydrogeologic setting explaining observed anisotropy in flow.
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Quantification of hydrogeological parameters by geophysical methods
By S. KomatinaWithin hydrogeological exploration, geophysical methods have a significant role in solving following problems: - possibilities of safe analyses of aquifer geometry and structural properties; - monitoring of aquifer contamination by mineral and organic deposits; - defining relevant indications on a hanging-wail permeability characteristics and aquifer hydrodinamic parameters; - indicating groundwater level; - analysis of groundwater chemistry; - determining clay facii in a complex of hanging-wall sediments; - exploration of velocity transfer for wet front and pollutant within.
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Can K-p relations be estimated through ground water flow model inversion
Authors D. Dam and S. ChristensenIn connection with construction of ground water models an extensive geophysical survey of the area is often made. In Denmark the model area is usually mapped by means of dense geoelectrical surface measurements. The geophysical surface measurements give an area coverage and are relatively inexpensive.
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High resolution travel-time tomography used to delineate a shallow waste disposal site
Authors E. Lanz, H. Maurer, J. Ansorge and A. GreenIn Switzerland, large quantities of waste are buried in glacial and glaciofluvial sediments. These same sedinients comprise two-thirds of the country's aquifers and thus contain important sources of drinking water. To assess the potential for environmental contamination, the boundaries of the buried waste sites (i.e. their lateral extents and depths) must be well known. The depths of the sites are of particular interest for judging if the groundwater table is within or below the waste.
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Laboratory and field results of the use of spectral induced polarization (SIP) method for detecting organic and inorganic contaminants
By H. VanhalaEnvironmental applications of the induced polarization (IP) and the spectral induced polarization (spectral IP or SIP) methods, i.e., the possibility to detect organic and inorganic contaminants in glacial soils, has been studied the Geological Survey of Finland (GTK) for some years. This presentation deals with the IP of natural and contaminated soils (the membrane polarization), and gives two field examples; an area with an organic (hydrocarbon) contamination (Fig. 1), and an area with an inorganic (heavy metal) contarnination.
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Environmental applications of airborne geophysics in Finland
Authors J. Lerssi, M. Kurimo, T. Jokinene, E. Lanne and R. PuranenGeological Survey of Finland (GSF) has carried out systematic airborne geophysical surveys in Finland since 1951. The second program started in 1972 using a flight altitude of 30-40 meters and line spacing of 200(100) meters (Kurimo, et al, 1986; Peltoniemi, 1982; Poikonen, 1991; Vironmäki, et al, 1982). Today 80 % of the country has been measured. The measurements have been made with fixed-wing aircraft The differential GPS is utilised in navigation. In low-altitude measurements the following systems are in use: magnetic, gamma radiation and vertical coplanar EM and VLF eleciromagnetic. Total magnetic field is measured with a wingtip gradiometer. The horizontal spacing between the sensors is 21 meters. Earth's gamma radiation is measured with a spectrometer which utilises a 25 1 NAI crystal detector. In the electromagnetic unit a vertical coplanar coil configuration is used. The frequency is 3112Hz (from 1995 also 14368 Hz) and coil separation is 21.4 m. GSF processes data with in-house software and profile, contour, shaded colour and grey scale maps are drawn (Kurimo, et al. 1986). The survey results are used in mineral exploration and geological mapping, recently also increasingly in environmental applications.
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Quantitative mapping of waste coal tar deposits by EM conductivity and VLF-resistivity surveys
Authors R. D. Ogilvy, P. I. Meldrum and J. NissenThe nature of coal tar deposits presents considerable difficulties for traditional site investigation metliods such as boreholes and cone penelrometer tests. These intrusive methods normally require access by heavy vehicles and would be at risk on a tar surface. However, the low viscosity of tar (in winter) means site characterisation may be undertaken by a geophysicist using lightweight instrumentation.
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Integrating electrical resistivity tomography and soil sampling methods to characterize a solid waste landfill area
Authors G. Morelli, G. Corsini, L. Di Tola, R. Pini, G. Vigna Guidi, M. Miletto, M. Ferguglia, G. Pogliano, G. Catanzariti, F. Franceschini, A. Scozzari and R. CorsiA municipal solid waste Iandfill, located 35 Km east of Pisa (Italy), was selected to experiment the application of Electrical Resistivity Tomography (ERT) to detect and delineate the plume of contamination caused by the absence of any natural or artificial protection for the underlying confined aquifer. ERT resuits have been supported and checked against geochemical data, hydrogeological data and measurements of soil physical properties.
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Identification of shallow targets through integrated 2-D and 3-D multi-fold georadar investigation: A case history
Authors M. Pipan, L. Baradello, I. Finetti, E. Forte and A. PrizzonA 3D multi-fold GPR investigation was carried out in the archaeological park of Aquileia in the framework of a survey proposed by the local superintendency of cultural heritage. Primary objective of the work was the identification of targets of potential archaeological interest in a polygon of approximately 2800 sqm in area, which borders the town cemetery to the north. The comparison of single-fold and multi-fold test profiles encouraged the design of a complete multi-fold grid. The 3-D option was chosen to correlate and map targets of limited dimensions. Targets of possible archaeological interest were identified at estimated depths not exceeding 200 cm in the part of the dataset processed to date.
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Dispersion analysis in radar tomography
Authors S. Tillard, S. Ahr, F. Hollender and F. JullienGround Penetrating Radar and Borehole Radar are commonly used for various applications in civil engineering, archeology, aquifer and soil studies... These prospection methods can also be used to study geological formations for underground waste disposal, for which a set of specifications has been defined by ANDRA. In electrically resistive host rock, for example granite, they allow the localization of discontinuities such as fractures or altered zones.
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Finite-difference ground-penetrating radar modeling in frequency-dependent media
Authors T. Bergmann, J. O. A. Robertsson and K. HolligerRealistic modeling of electromagnetic wave propagation in the radar frequency band requires a full solution of Maxwell's equations as well as a complete description of the material properties. We present a two-dimensional (2-D) flnite-difference time-domain solution of Maxwell's equations that allows to account for the frequency dependence of the dielectric permittivity and electrical conductivity typical of many near-surface materials. In close analogy with the viscoacoustic case, the governing equations are obtained by assuming dielectric and conducting relaxation functions. The finite-difference solution is second-order accurate in time and fourth-order accurate in space, conditionally stable, and computationally only marginally more expensive than its standard equivalent without frequency-dependent material properties. The algorithm is applied to a realistic problem illustrating the respective damping effects of conductivity and frequency-dependent dielectricity.
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Finite-difference modeling of ground-penetrating radar data: The zero-offset approximation
Authors T. Bergmann, K. Holliger and J. LeckebuschMost ground-penetrating radar suveys are acquired in quasi zero-offset mode, Le. source-receiver offsets are small compared to the target depth. The correct way to realistically model such an experiment would be to compute a corresponding sequence of common-offset radargrarns. In finite-difference time domain (FDTD) modeling this approach would be excessively expensive and hence is substituted by initiating a downward travelling plane wave at the earth's surface. However, this method of zero-offset modeling is only realistic for horizontafly layered, one-dimensional media, and thus is unsatisfactory in most realistic situations and defeats the very purpose of finite-difference rnodeling. Therefore, we propose an alternative approach based on the "exploding reflector" concept. This approximation is more realistic, even in highly complex media, and is only marginaily more expensive than the plane wave method.
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Borehole radar surveys in the Borth rock salt mine (FRG)
Authors T. Richter, T. Fechner, L. Corin, L. Halleux, P. Feller, K. Berns, U. Niewianda, R. Maly and G. v. LierdeThe Borth salt deposit (FRG) lies in the Zechstein basin, which covers a large part of Central Europe. The mine is located north-east of Duisburg. Although the deposit is over 150 metres thick, only the lower part is mined here. Mining activity (app. 800 metres below surface) is restricted to a 20 m thick layer of extremely pure white salt.
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Ground penetrating synthetic pulse radar performances
Authors J. Cariou, O. Blondel and E. FerrariFour years ago, the « Ministère de l'Equipement et des Transports », in collaboration with two other French companies (SNCF - Société Nationale des Chemins de Fer; and the RATP - Régie Autonome des Transports Parisiens) presented a paper concerning the development of a synthetic pulse radar. After demonstrating the feasibility of such a system at the end of the eighties, we have recently improved our knowledge and carried out comparisons with the pulse and synthetic radars existing on the market. The main purpose of this research is to try to improve the depth of investigation for the detection of discontinuity or cavities in the ground or in civil engineering structures.
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Forward modeling contribution in the GPR signatures interpretation of urban heterogeneities
Authors G. Grandjean, A. Bitri and J. C. GourryBRGM and the french Projet National Microtunnel funded a research project where Ground Penetrating Radar (GPR) was used to characterize the test-site of the Laboratoire Central des Ponts et Chaussées (France). The objective of this project was to find a method to better interpret the radar signatures by combining data processing to forward modeling, especially in the case of urban heterogeneities. We focused a large part of our work to adapt a sufficient quick and accurate algorithm to be efficient in industrial applications. Modeling algorithm, adapted from radar waves propagation theory in 2D heterogeneous dispersive media, is based on the upward extrapolation of a downgoing field in the frequency-wavenumber domain. Like for other modeling techniques such as ray-tracing (Goodman, 1994; Cai and McMechan, 1995; Powers, 1995), FDTD (Moghaddams et aL, 1991; Roberts, 1994) or pseudospectral methods (Carcione, 1996; Casper and Kung, 1996) some compromises have been introduced and must be kept in mind. In the following, we will first recall the modeling method, and after an example description from the LCPC test site, where real processed data are compared to synthetics, we will insist on the advantages and the weakness of the method.
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Shallow 3-D reflection seismic surveying of glacial sediments in the Swiss molasse basin
Authors F. Büker, H. Horstmeyer and A. C. GreenApplications of geophysical methods to resolve diverse engineering and environmental problems have become increasingly common over the past few years. Investigations of shallow targets in complex heterogeneous geological settings require methods capable of providing high-resolution infonnation. Although seismic reflection profihing has often been applied successfully in nearsurface studies, reliable knowledge of the true dip and lateral extent of structures cannot be obtained from sparse 2-D seismic data sets alone (Lanz et al., 1996). In order to map the stratigraphy and three-dimensional distribution of unconsolidated surficial sediments a 3-D highresolution reflection seismic survey has recently been conciucted within the Suhre Valley, central Switzerland.
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Modelling and imaging within the diffraction limit with application in engineering geophysics
Authors L. -J. Gelius, H. Westerdahl and F. N. KongIn engineering geophysics the task is often to locate and/or image different buried objects or structures. If we consider geotechnical investigations typically targets will be cavities, buried drums, tunnels, pipelines and rock pieces in soil. Correspondingly, in non-destructive testing we aim to characterize structural distortions or to image target objects embedded in a given structure.
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Tau-P mapping" based filtering techniques for linear noise attenuation in reflection data
Authors G. P. Deidda, R. Balia and L. SambuelliOne of the most relevant problems in reflection data processing is related to the suppression of localised coherent noises. In shallow reflection surveys typical examples among these noises are: a) the air-coupled wave, due to the extensive use of low-cost surface energy sources such as dropping weights, hainmers, rifles and so on, which produce, more or less extensively, a wide spectrum, high amplitude air wave; b) other linear noises, such as the direct and refracted waves which, in addition to the fact that their frequency band overlaps significantly that of the signal, are very close to the reflections themselves andlor are spatially undersampled.
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Underground tunnels detection and location by high resolution seismic reflection
Authors B. Piwakowski, J. C. Tricot, C. Leonard, N. Ouarradi and B. DelannoyIn spite of the widespread development of high-resolution seismic reflection techniques during recent years, the detection of underground cavities remains fundamentally unsolved. Except for some general principles, each case requires a particular study. Until now, the successful location of small (in regard to wavelength) cavities has been performed in an indirect way, i.e. by means of the detection of the cavity-induced perturbations of the seismic image of the regular geological structure (which should be known in advance) [1],[2],[3],[4] The direct detection of cavities by means of the reflection originating from the top of a void, though theoretically possible, gives no results in practice. [5]. This paper presents the successful application of the high resolution seisrnic reflection for the indirect detection and location of abandoned old chalk-pits, appearing in the north of France and being now unexploited. Some positions of these voids which form a network of tunnels are mapped, whereas others are unknown and represent a real danger for both population and building activities. The tunnels occur typically in chalk at a depth of 8-15 m and are followed by a water table and a man layer (Fig.1).
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A seismic reflection survey to help determine the ice thickness on the Lobbia Glacier, Adamello Massif, Italy
Authors L. Levato, L. Veronese, A. Lozej, E. Santuliana and E. TabaccoIn the framework of a geophysical study to investigate the ice thickness on the Lobbia Glacier located in the Adamello Massif, Italy, we recorded 1393m of 2D multifold seismic data, in summer 1996, to constrain a joint gravity survey covering an area of about 4 km2, Single fold seismic data (Tabacco, Pettinicchio, and Veronese 1995), and gravity data (Olivier et al., 1996), collected during previous surveys, provided an estimate of the maximum ice thickness. The main processing sequences inciude band-pass filtering, f-k filtering, residual static corrections, stack, deconvolution and migration. The contact between the ice and the granite substratum appears as a strong reflection at a maximum two way time of 105 ms.
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Three dimensional georadar acquisition with a real time positioning system
Authors F. Lehmann, M. van der Veen, A. C. Green, H. Horstmeyer and P. WildFor efficient acquisition of three-dimensional (3-D) subsurface information, quasi-continuous georadar and coordinate data are recorded simultaneously as the georadar antennas are continuously transported across survey areas. A self-tracking laser theodolite with automatic target recognition capabilities has been adapted to serve as a real-time positioning system for the acquisition of the 3-D data. This surveying system provides complete coordinate information with high accuracy. To take advantage of advanced processing procedures the georadar data must be transferred on to a regular grid. A fast two-dimensional Fourier transform technique has been designed for this task. The system has been tested successfully across a number of geological targets in Switzerland. For example, 3-D georadar data were recently collected and processed across a glaciofluvial environment of 25 m x 25 m within a single day. Using the full 3-D information of the data set, channel sediments as well as surface reflections could be clearly identified with the help of time slices.
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The improvement of geotechnical subsurface models through the application of S-wave reflection seismic exploration
Authors J. Brouwer, R. Ghose, K. Helbig and V. NijhofDue to its relatively high sensitivity to the distribution of elastic parameters, the seismic technique is quite suitable for geotechnical subsurface investigations. However, practical implementation of the P-wave exploration technique for soft-soil and shallow-groundwater conditions, as often encountered in the Netherlands, may often prove problematic; the highimpedance contrast at the groundwater table may induce strong reverberations and guided waves, resulting in a distortion of the seismic record at shallow depth. The use of buried sources may be of some advantage, but at the cost of a considerable decrease in the speed of operation.
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An efficient approach to shallow high-resolution seismic data acquisition: Preliminary results
Authors M. van der Veen, F. Lehmann, P. Wild and A. GreenHigh-resolution reflection seismic techniques are powerful tools for mapping shallow geological structures (Steeples and Miller, 1990; Lanz et aL, 1996). During the past decade, the quality of seismic data has improved significantly. This improvement can be ascribed to a better understanding of shallow seismic techniques and significant developments in technology. These advances not only have improved the data quality and increased the ability to record very high-fold data, but they also have increased significantly the logistical complexity of a typical shallow seismic survey; many more geophones now have to be planted and higher shot densities are required.
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The quantification and monitoring of data quality and the design of optimum sweeps
Authors J. Brouwer, P. Bakker, D. Kraaijpoel and K. Roy-ChowdhurryThe application of vibratory sources in seismic exploration principally allows us to tune the source signature to the (changes in) specific (sub-) surface conditions through a proper choice of the pilot signal. Objective design of source signatures is feasible only if we can define how the quality of the dataset is affected by the choice of pilot signal. This requires a description of the quality of the seismic data in terms of the pilot signal.
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Application of proton magnetic resonance for detection of fractured chalk aquifers from the surface
Authors A. V. Legchenko, J. M. Baltassat, A. Beauce and D. ChigotDirect measurement of Proton Magnetic Resonance (PMR) signal from water molecules guarantees a high reliability of the water detection. It is the main distinction of the PMR compared to other geophysical tools. Interpretation of experimental data reveals location of aquifers, theirs depth and water content. A mean size of pores of the water-saturated rocks can also be estimated.
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Small scale correlation between the electrical and the hydraulic conductivity
Authors F. Effersø and K. I. SørensenThere has been an ongoing discussion about the relationship between the hydraulic and the electrical conductivity. In order to test this relationship on a small scale a new method for in situ determination of the hydraulic conductivity has been developed at the Department of Earth Sciences. In a series of field tests in a sandy alluvium, estimates of the hydraulic conductivity were obtained in three closely spaced borings together with gamma and electrical logs. The results indicate that there is no evident relationship between the electrical conductivity and the hydraulic conductivity on a small scale.
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Application of statistical experiment design: Are our data sets "optimal"?
Authors H. Maurer and D. E. BoernerReliable characterization of subsurface targets is the primary objective of every geophysical exploration technique. Regardless of the physical properties being mapped, or the variant of the geophysical method employed, quantifying the resolution of the Earth is a common theme of all field experiments. Inverse theory has proven essential for this task since, for a linear relationship between model parameters and data, it provides an exact mapping between data errors and model uncertainty. In this regard, the amount of information that can be extracted from measured data (i.e. the success or failure of a geophysical experiment) depends explicitly on data quality. Yet measurement accuracy and precision alone do not define data quality. Data independence and sufficiency are also important and illustrate the need for ensuring the collection of appropriate data through experimental design.
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Electromagnetic inversion for environmental site characterization: Data quality versus image resolution
Authors D. L. Alumbaugh and G. A. NewmanNon-linear electromagnetic inversion schemes have been developed to produce 2D and 3D images of subsurface conductivity structure from electromagnetic geophysical data. The solutions are obtained by successive linearized model updates where full forward modeling is employed at each iteration to compute model sensitivities and predicted data. Regularization is applied to the problem to provide stabiity. The use of the inversion is demonstrated on a data set collected with the Apex Parametrics tMaxMin I-8S' over a section of conductive waste at the Idaho National Laboratory's Cold Test Pitt. The out-of phase data are of very good quality while the in-phase are rather noisy due to slight mispositioning errors. A resolution study on synthetic data indicates that the error present in the in-phase data causes images of far lower resolution with more artifacts than if the in-phase and out-of-phase components are of similar quality. Better resolution images result if the data are weighted proportional to frequency; this gives each frequency equal weight. The loss of resolution due to poor quality in-phase data is demonstrated in a 3D inversion of the MaxMin data which shows both artifacts forming outside of the area known to contain the buried waste, as well as an inability to resolve depths.
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Study of configuration dependent resolution of 3-D buried conductors
Authors E. Auken, G. M. Hoversten and H. F. MorrisonThe electromagnetic response of a buried conductor is a complex function of the currents induced and channeled into the conductor from the currents flowing in the background media. The ration between the channeling and induced currents is basically deterrnined by the currents in the background half-space that are available for channeling and on the magnetic fleld on the conductor. In order to "see" the target the secondary fields must exceed some threshold value which is dependent on the layered background response and on the source receiver configuration.
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The pulled array transient electromagnetic method
More LessThe mapping of subsurface resistivity structures using transient electromagnetic soundings, TEM, has gained an important role in hydrogeological investigations in Denmark. Regional mapping with TEM have produced resuits, which leaves no doubt as to the ability of the method to delineate freshwater sandy aquifers in well conducting hosts of Tertiary clays. These results have been achieved measuring in a dense grid and do demonstrated the necessity of estimating the data quality and variation by correlating adjecent data sets to reveal distortion and multidimensionality of the measurements. The fact that these investigations are often carried out in densely populated areas, where the cultural noise level is high, accentuates this necessity even more (Christensen and Sorensen, 1994).
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A combined Gauss-Newton and quasi-Newton inversion method for the interpretation of apparent resistivity pseudosections
Authors M. H. Loke and T. DahlinTwo-dimensional (2D) electrical imaging surveys (Griffiths and Barker 1993) are now widely used to map areas with moderately complex geology where conventional resistivity sounding surveys do not give sufficiently accurate results. In more complex areas threedimensional (3D) surveys (Ellis and Oldenburg 1994, Loke and Barker 1996b), which gives even more accurate results but at a greater cost, have also been used.
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Quasi-3D resistivity imaging - mapping of three dimensional structures using two dimensional DC resistivity techniques
Authors T. Dahlin and M. H. LokeSo far, one dimensional (1D) and two dimensional (2D) techniques have dominated resistivity surveying, and the latter only in recent years, while three dimensional (3D) techniques are still in their infancy. In order to build images of the 3D resistivity distribution of the ground, it is today common practice to merge the result from a number of sections acquired and inverted using 2D resistivity imaging techniques, which may be referred to as a quasi-3D technique.
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Separation of anisotropy and inhomogeneity influence by the spectral analysis of azimuthal resistivity diagrams
Authors D. K. Bolkshakov, I. N. Modin, E. V. Pervago and V. A. ShevninThe first aim of resistivitv survey is a study of any inhomogeneities. Frequently in practice the effect of anisotropy is displayed together with that of layering or inhomogeneities. It complicates data interpretation within the framework of anisotropic models, and distorts results of interpretation in the framework of layered or inhomogeneous media.
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Application of complex resistivity tomography to field data from a kerosene-contaminated site
More LessRecently several investigations on samples in the laboratory have proved the influence of hydrocarbon (HC) contamination on the complex electrical resistivity of soils and rocks (Olhoeft, 1992; Bömer et al., 1993; Vanhala, 1997). Beside the magnitude of resistivity additional information about the contaminant is obtained from the phase angle which may be measured by the frequency domain induced polarization method. However, for environmental HC pollution problems the importance of the complex resistivity method depends on its practical value for the detection, delineation and monitoring of subsurface contamination under field conditions. Here, an application of complex resistivity tomography to field data, collected on a kerosene-contaminated former military jet fuel depot, is presented.
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3D modeling of the Lobbia Glacier bedrock (Adamello Massif, Italy) using gravity and GPS measurements
Authors A. Rosselli, R. Olivier, E. Tabacco, R. Cefalo, B. Dumont, Ph. Logean, Ph. Rosset, M. Manzoni, M. Pavan and S. UrbiniAn important ingredient in establishing climate models for global climatic change studies is the volume of glaciers and its temporal evolution.
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3-D FE-modeling of a mass movement on basis of geophysical investigations
Authors E. Castillo and F. KohlbeckA Iandslide has been modeled by three different models utilizing the FE-Program BMINES. The combination of several geophysical methods provided the information about the 3D-geometry of the slip and the range of some material properties. Seismic refraction surveys, geoelectrical soundings and electromagnetic measurements have been used for the characterization of the slip, location, depth, and physical properties of material.
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Electrostatic measurements in complex artificial (anthropogenic) grounds
Authors C. Panissod, A. Jolivet and A. TabbaghThe electrostatic method constitutes a generalization of the well known electrical method. As the electrostatic poles are located in the air, not far from the ground, the method can be used whatever the type of ground surface (tarmac, slabs, concrete,...).
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An evaluation of electromagnetic methods for waste site assessment
Authors L. Pellerin, D. L. Alumbaugh and M. Cathy PfeiferA baseline study in electromagnetic (EM) exploration of the shallow subsurface (< 10 m) was performed at the Idaho National Engineering Laboratory (INEL) Cold Test Pit (CTP) for the purpose of identifying and evaluating existing commercial and experimental instrumentation, integrating those technologies with multidimensional interpretational algorithms, and identifying gaps in shallow subsurface EM imaging technology.
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Predictive mapping of mechanical properties of alterites through continuous electrical resistivity measurements
Authors J. C. Gourry, R. Wyns and F. LebertBasement rocks constitute 25% of the surface geology of France. Throughout the geological record, and particularly during the Early Cretaceous and Paleogene, these rocks underwent significant weathering to form alterites more than 50 m thick in places. Later erosion incised vaileys in the alterites, which is why, in schistose areas, one may find aggregate quarries very close to day workings (for brick manufacture), the former being in the valley bottoms (minimum weathering) and the latter on the plateaus (maximum weathering). The aim of this research work is to assess the mechanical properties of these surface alterites in order to draw up maps for use in civil engineering sectors, such as excavation work for major communicatjon routes (roads, etc.) and laying underground pipes and cables (gas, electricity, telecommunication lines).
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A high density sampling survey of shallow karst features using EM and magnetic profiling techniques
Authors R. J. Sporry, A. Marquesda Costa, G. Desta, J. Roy and W. A. HugensDuring field experiments on high density sampling shallow EM (Geonics EM31) surveying near Moura, Portugal, it was found that a local shallow terrain depression showed enhanced rainwater infiltration and high apparent conductivity values (Sporry et. al. 1995). Other conductivity anomalies suggested a pattem correlating with the general geological strike direction (NW-SE).
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Flow training dikes for the Wouri estuary in Doula. DC prospection of sand in shallow sea water
Authors S. N‘Seke, B. Bagnérès, R. Lagabrielle and J. -L. MattiuzzoThe Port of Douala, which is located high up the Wouri River Estuary, is reached by means of a 50 km long access channel. The 25 km upper part of this channel currently requires intense and costly maintenance dredging. 20 years of dredging experience gained through three previous programs bas led the Cameroon National Ports Authonty (CNPA) to consider that an altemative solution with higher investment costs (flow training dikes) and reduced maintenance (by more than 60%) is the most appropriate in terms of present benefits.
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Outlook on geophysical evaluation of the ground conditions in the Kruja medieval castle, Albania
Authors A. Frasheri, L. Kapllani, F. Dhima and S. PeçiThe Castle of Kruja is the symbol of the culture and Albanian history. This castie is related with the most glorious epoch of the Albanian National Hero Skanderbeg.
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Relations between hydrogeologic and geoelectric parameters. A case study: The Setúbal peninsula, Portugal
Authors R. M. D. Gonçalves, A. R. Andrade Afonso and L. A. Mendes VictorThe Setubal Peninsula is located southern of Lisbon City, in the midst of Portugal (fig. 1). This belt (area) belongs to the Tejo and Sado river basins and it's located between them. It comprises the most important aquifer system of the country. Geohydrological data from the thousand water extraction wells, and more than one hundred Vertical Electric Sounding (VES) surveys exist from that region (fig. 2). Merging of these different data yields better knowledge about constitution and protection of the system.
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Logging techniques measuring parameters of water flow in wells - a critical review
More LessDuring the prospecting of a hydrogeological structure, a series of boreholes (wells) is usually drilled with the goal to get sufficient information on aquifers, their depth position, and their reservoir properties.
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Lowering of the ground water table - does it affect the fresh/salt water interface, and can we measure it?
Authors T. Vangkilde-Pedersen, H. Olsen and P. M. DuchThe Danish supply of potable water is mainly based on ground water. During construction work in areas with a shallow ground water table, temporary lowering of the water table is often necessary. In coastal regions, if the lowering of the water table is close to existing water abstraction sites, the ground water quality can be threatened by changes in the depth to the freshwater/saltwater interface. With an example from Vestamager near Copenhagen, Denmark a combination of geophysical methods for monitoring the freshwater/saltwater interface is demonstrated. The example ilustrates the advantages of combining geophysical well logging in observation wells with time domain electromagnetic (TDEM) soundings.
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