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56th EAEG Meeting
- Conference date: 06 Jun 1994 - 10 Jun 1994
- Location: Vienna, Austria
- ISBN: 978-90-73781-05-4
- Published: 10 June 1994
81 - 100 of 537 results
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Finite difference depth migration with micro spatial filters
More LessFinite difference operators that are accurate and very spatially narrow may be constructed by optimizing the implicit 45° finite difference operator, analytically computing the required inverse operator for an inhomogeneous medium, and then truncating the resulting operator in a way which preserves accuracy and stability.
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A Unified approach to seismic reflection imaging
Authors M. Tygel, P. Hubral and J. SchleicherGiven a seismic record of either displacement or pressure seismograms and assuming a laterally inhomogeneous, isotropic three-dimensional (3-D) macro-velocity model, we provide a complete theory of seismic reflection imaging. It consists of (a) a Kirchhoff-type weighted diffraction stack to transform (migrate) the seismic data from the record space (i.e., the time-distance domain) into the image space (i.e.. the depth-distance domain) and (b) a weighted isochrone stack to transform (demigrate) the migrated seismic inlage back into the record space.
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Imaging complex structures using band-limited Green's functions
By D. NicholsMany geophysical processes can be formulated in terms of the Green's functions of the wave equation. A process such as pre-stack Kirchhoff migration requires the estimation of the Green's function for every surface (source or receiver) location. The cost of calculating and storing the full Green's function is often prohibitive. Calculating one Green's function involves solving the full two-way wave equation for an impulsive source at each surface location. The storage required for the full Green's function is very large, the Green's function has an extra dimension, depth, compared to surface seismic data.
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Pre-stack depth migration with compensation for absorption
Authors R. Sollie, R. Mittet and K. HokstadThe propagation of seismic waves in real media is in many respects different from propagation in an ideal solid. The anelasticity of the medium will cause dissipation of seismic energy, thus decreasing the amplitude and modifying the frequency content of the propagating wavelet. We present a method for including absorption effects into prestack finite-difference migration schemes. We will outline the theoretical foundation for the method, describe the design of the extrapolation filter, and thereafter test the method on synthetic data. The aim of this work is to show that a prestack depth migration scheme that compensates for absorption is both feasible and gives improved images.
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Wave field extrapolation in the space-wave number domain
Authors F. J. Dessing and C. P. A. WapenaarThe forward model of seismic data that is used within our seismic research project DELPHI [1] is based on a number of operator multiplications.
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Sampo EM studies of Keivitsa nickel-copper deposit
Authors T. Jokinen, T. Valli and T. MatunenThe Geological Survey of Finland (GSF) has found and is exploring the Keivitsa' ore deposit in northern Finland, maybe the largest sulphide deposit ever found in Finland. This multimetal deposit contains nickel, copper, platinum group elements and gold.
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Underground tunnels detection and location by high resolution seismic reflection
Authors B. Piwakowski, J. C. Tricot and B. DelannoyThe seismic techniques are applied successfully for seismic imaging of geological structures having great dimensions compared to the wavelength (layers), but their application for detecting smaller objects as tunnels and cavities remains still limited. This paper presents the successful application of the high resolution seismic reflection for the detection and localisation of the remains of the old chalk-pits, appearing in the north of France and being now unexploited. Same positions of these voids which form a network of tunnels are mapped (Fig.1 .a), whereas others are unknown and represent a real danger for both population and building activities. General geological well logging data in this region is shown in, Fig. 1.b. The the tunnels occur typically at a depth of 8 - 15 m and are followed by a water table.
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Autonomous underwater vehicle - A new tool for ocean exploration
Authors B. Sichler, D. G. Masson, J. L. Michel and V. RenardFour main kinds of tools are used in the study of oceanic environments surface instruments, deep towed instruments at a' variety of depths (including ROVs), manned submersibles which are free from the surface influence, and bottom stations which are particularly useful for long-term monitoring. Autonomous Underwater Vehicles (AUVs) are relatively new to the marine scientific community: their independence from the surface and their unmanned nature gives them both advantages and disadvantages over more conventional tools. Because they are not linked to a ship their energy supply is restricted, but at depth, their operation is not disturbed by the sea state. Because they are unmanned, they need many automatic control systems, but all the features linked with the safety and comfort of observers may be safely omitted from their design . The speed of the vehicle is two to four times higher than that of conventional deep-tows, but one consequence is that the data collected must be largely processed after recovery. Many kinds of AUVs already exist. Generally they have been built for very specific tasks and their mass ranges from a few kilograms to several tons. A few of them have reached operational status in deep sea applications (e.g. Epaulard in France, AUSS in USA, MT88 in Russia). More recently, a small low cost AUV has been used for CTD measurements under the Arctic ice.
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High resolution seismic methods applied to subsidence evaluation and solution mine design
Authors J. A. C. Meekes, L. J. J. van Vliet and S. P. WijnSolution mined cavities in sedimentary deposits can cause serious subsidence problems. The occurence of subsidence, espcially sinkhole formation and ground collapse, is partly due to insufficient knowledge of the local geology and of the subsidence mechanisms. Susbsidence prediction improves safety and reduces environmental risk of existing solution mines.
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The use of geophysical techniques for the detection of - Partially collapsed - Mine shafts
Authors J. Brouwer and V. NijhofDuring the past centuries the Southernmost part of the Netherlands has been known for extensive marl recovery. Systems of galleries and mine shafts exist over - assumed - areas of several hundred km². At some locations mine galleries have recently collapsed causing severe damage to the roads and buildings in the surrounding urban areas. The reason for the sudden collapse of parts of the mining system was supposed to lie in leakage of drainage pipes, causing a decrease in carrying capacity of the marl. Due to the collapsed galleries parts of the system of mining shafts have become unreachable making it but impossible to determine the exact lateral exterit of the collapse features and the galleries from within the mining system and leaving only surface methods to locate the shafts and to determine the potential hazard for similar collapse features.
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Multichannel processing of very high resolution seismic
Authors Y. -H. de Roeck, B. Marsset, J. Meunier, M. Noble and R. GiraultThe installation of offshore structures requires a fine survey of the near subsurface to detect the nature of the ground and eventual obstacles. Seismic reflection methods are the most commonly used. But up to now at least, these methods were limited to single channel acquisition, sometimes digital (in the best cases) due to technology limitations. A fire cycle of 0.5 or 1 sec and a sampling rate of 5kHz could not allow multichannel recording. The advent of new technologies allows the use of the multichannel concept for superficial subsurface exploration and brings: (1) a significant improvement of signal to noise ratio, (2) real depth information of the interfaces using velocity analysis, (3) the possibility to acquire data in rougher sea conditions because of its multiplicity.
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Simultaneous inversion of group- and phase-velocity data in engineering seismics
Authors R. Misiek, L. Dresen and Ch. SchneiderIn engineering seismics it is important to get complete information about the lithostructural parameters of near surface layers. By a simultaneous inversion of the groupand phase-velocity dispersion data of Rayleigh surface waves it is possible to determine these parameters and, in contrast to refraction seismics, especially those of low velocity layers and the S-veloctities of all near surface layers. Using both standard P-wave refraction seismics and surface wave methods, only a few changes in the refraction field layout are needed to get high quality information about the lithostructural parameters of the near surface layers. A separate S-wave refraction survey is unnecessary.
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Highly reliable broadband three-component piezoseismometers for engineering and mining digital observation systems
By V. M. FremdDigital PC-based systems of seismic observations are now widely used. In these systems the signals from remote seismometers are transmitted to central point equipped with a universal PC for information recording, acquisition, processing and storage. Such third generation digital system provides necessary seismometric channels parameters, efficient data processing and good operation and financial characteristics. In the presente of accessible PC and telemetric devices the main problem is the optimal matching highly reliable seismometers.
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Sampo EM interpretation in 3D structures
Authors M. Oksama, T. Jokinen and H. SoininenSampo is a wide-band electromagnetic method. Transmitter is a vertical magnetic dipole and receiver measures three perpendicular magnetic components. Standard interpretation of Sampo results is based on a layered earth or thin plate model (Soininen and Jokinen, 1991 ; Dyck et al., 1981).
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Induced polarization effects in transient EM data for structural mapping
Authors B. A. James and A. ZerilliInduced Polarization (IP) effects in transient electromagnetic (TEM) measurements have not been extensively addressed by practitioners of TEM methods for structural mapping. This discussion focuses on illustrating IP effects pertinent to central loop sounding measurements. Examples of field data from several localities are given to illustrate these effects.
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A Rapid solution for electromagnetic scattering - The first step to 3D imaging
Authors P. W. Walker and R. W. GroomOur ability to understand and interpret the Earth's conductivity structure has been limited by our ability to model it. Modelling the electomagnetic response of complex structure can be computationally very intensive, and because of this, such structure is often represented by models that are too simpte to be realistic. Examples of this are common. One is in the helicopter electromagnetic (HEM) method, where the ground is assumed to be either a homogenous halfspace, or at best, horizontally stratified. Another example is in magnetotellurics, where the ground is often assumed to be either one or two dimensional, and the source excitation a plane wave.
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System design for acquisition of precise electromagnetic data
More LessThe advent of new, sophisticated inversion techniques for interpreting crosswell subsurface electromagnetic data brought with it stringent demands for improvements in the accuracy and fidelity of the measurements. In particular, it is now possible to form a tomographic image of the subsurface section bounded by two vertical boreholes if the acquired data is noire free over a large bandwidth. Theoretical considerations and the results of numerical experiments presented by Lee and Xie (1993) indicate that representative tomograms can be constructed using high accuracy, wideband, time domain electromagnetic data which is transformed to a wave domain prior to analysis. The image is constructed using a non linear ray tracing technique normally reserved for seismic velocity tomography. In the present case however, this technique can be used to obtain an image of the subsurface distribution of electrical conductivity.
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Controlled source EM tomography - An improved imaging technique for magnetic susceptibility and resistivity
Authors S. Sakashita, H. Shima and S. GasnierHigh resolution imaging technique has been required in environmental and engineering studies as well as in reservoir characterization and mineral exploration. Geotomography methods have been studied since 1980's to obtain higher resolution.
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Development of multi-component multi-frequency data acquisition system for borehole EM tomography
Authors S. Gasnier, H. Shima, S. Sakashita and A. AbdelhadiIn the framework of the geophysical investigations between two drilt holes, a new EM instrument has been developed in the 4Hz to 18KHz frequency range. This system consists of a downhole transmitter probe located in one drill hole and a three-component receiver probe located in the second drilt hole. A reference cable coming from the transmitt er probe enables to measure the in-phase and the out-of-phase parts of each three components with respect to the primary field (fig. 1).
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Automatic inversion of AEM data into multilayer parameters and its application to groundwater exploration in the Namib Desert
Authors K. P. Sengpiel, B. Fluche, R. Lenz, B. Rottger and W. VossAs a contribution to the Water Master Plan of Namibia, the Federal Institute for Geosciences and Natural Resources (BGR) conducted a helicopter-borne geophysical survey in an area of 5300 km2 in the Namib Desert in 1992. The survey area is bounded in the North by the ephemeral Kuiseb River and in the West by the Atlantic Ocean.
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