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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
21 - 40 of 124 results
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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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