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53rd EAEG Meeting
- Conference date: 26 May 1991 - 30 May 1991
- Location: Florence, Italy
- ISBN: 978-90-73781-03-0
- Published: 28 May 1991
21 - 40 of 315 results
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Estimation of elastic parameters from AVO effects in t-p domain
Authors J. Helgesen and M. LandrøThe objective of AVO techniques is to extract elastic layer parameters and lithological characteristics of subsurface layers from seismic measurements. Many of the works reported in this field has been of qualitative nature, e.g. using AVO techniques to distinguish between hydrocarbonrelated and lithology-related bright spots.
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A contribution on the study of the production mechanism of solfide mineralization self potential
Authors G. A. Skianis and T. PapadopoulosThe production mechanism of sulphide mineralisation self potential is still open to question. The majority of geophysicists support basically the dipole model. According to this model electrochemical reactions take pIace in the system of the mineralized body and the surrounding medium and as a resuIt an excess of negative and positive charges (of the same magnitude) are formed at the upper and lower part of the ore body, respectively. Thus an electrical field (geobattery) is formed the potential of which can be measured at ground surface. There have been proposed various models to describe the electrochemical reactions which take place in and around the dipole (Sato-Mooney 1960, Habashi 1966, Thornber 1975, Bolviken 1979, Sivenas-Beales 1982.).
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A New approach to data acquistion processing and control for noisy geoelectric voltage recordings
Authors C. Satriano, V. Cuomo, V. Lapenna, M. Machiatto, D. Patella and C. SerioIn this paper we are concerned with the problem of picking out the useful signal from noisy voltage recordings in geoelectrical prospecting. The voltage time series, recorded at the measuring probes of the electrodic array is made up of the sum of a deterministic component, the response of medium to the energizing current, and a random noise component.
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Measurements of D.C. small signals masked by noises
By L. AlfanoThe measurement of a man made signal S, masked by natural noises of mean strenght N, is common to many geophysical methods; but the Author will take into account only the case of the geoelectrical D.C. current surveys about which he reached a sufficient experience. The problem arose when deep sounding where carried out by means of Dipole-Dipole arrays and with long distances between the two dipoles,up to twenty kilometers.
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Geoelectrical survey of structures closed in depth with two-dipping discontinuities
Authors E. Cardarelli and M. BernabiniIn a previous study presented to Geophisycal Prospecting we considered the case of bidimensional structures in which a conductive medium is between two resistive seminfinite media. The two dipping discontinuity surfaces were considered to be parallel (or divergent) planes (open structures).
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Interpretation of airborne geophysical measurements in sedimentary environments by image processing
Authors A. Gulyas, M. Bodrogt, R. Csatho, G. Y. Büttner and G. CsillacDetailed airborne EM, magnetic and radiometric mapping was carried out in the Transdanubian Central Range (Hungary) over an area of more than 240 km² in the past 3 years. The geology of the area is rather complicated. The Transdanubian Central Range is built up mostly Mesozoic and Tertiary sedimentary rocks, mafic volcanic rocks are also present. As the investigated geological objects are quite small e.g. bauxite lenses with a diameter of a few 10 m - the line spacing of the flying was 50 m.
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Analysis of D.C. logging through metal casing
Authors C. Schenkel and H. F. MorrisonThe measurement of formation resistivity through metal casing can aid in characterizing existing reservoirs for effective recovery of oil and gas, as well as geothermal heat, without the cost and time of drilling new wells. Measuring the resistivity of adjacent fonnations may be useful to locate oil that was missed during the original logging prior to insertion of the casing. The method is also useful to monitor changes in resistivity caused by subsurface processes such as injection or leakage of contaminants from a waste site, flooding operations for enhanced oil recovery, or extraction processes of geothennal production. Several patents have recently been issued which describe methods and devices that are capable of measuring formation resistivity through casing, (Kaufman, 1989; Vail, 1989a, 1989b; Gard et al., 1989). Currently, it is only known that Vail has developed and tested such a device, called Through-Casing Resistivity TooI (TCRT). Kaufman (1990) investigated the behavior of the potential and its derivatives for a borehole with casing based on models of an infinite-length conductive pipe in a homogeneous medium. Schenkel and Morrison (1990) presented numerical results for a point souree of current in or beyond the end of a fixed length of casing in a homogeneous half-space. This study has now been extended using a surface integral equation approach can calculate the potentials for various electrode configurations in the presence of a finite-length casing and a layered medium.
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First application of lP method in Iran to boundary detection of deeply buried salty and sweet water aquifers
Authors E. Yousefi and J. KimiaghalamThe induced polarization (commonly known as lP) method is one of the most effective tools in mineral exploration. As it is known the induced polarization is based on the electrochemical phenomenon of overvoltage, that is on the establishment and deection of double layers of electrical charge at the interface between ionic and electronic conducting materials when an electrical current is caused to pass across the interface (H.O. Seigel).
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Source signature estimation using plane wave decomposition
Authors V. Shtivelman and D. LoewenthalRecently a new deterministic technique for source signature estimation in one-dimensional models of media was introduced (Shtivelman and Loewenthal, 1988; Loewenthal and Shtivelman, 1989). The technique was developed for a general case of multilayered models with arbitrary locations of the source and two closely spaced receivers (provided the source is above the receivers) and can he applied to both marine and land data. The technique is based on the application of two sequential extrapolation processes and does not require explicit separation of the wave field into up and downgoing components. Lately, the method has been extended to the two-dirnensional problem (Loewenthal and Shtivelman, 1990).
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Multiple elimination via inversion velocity analysis and shifted hyperbolas
Authors G. Canadas, S. Desaunay and E. de BazelaireThe inversion processes introduced in the conventional velocity analysis yield better velocity stack gathers. Improved resolution means more accurate picking and better quality of the stacked signal. However, two problems are encountered when using these processes for multiple attenuation: 1) the high cost of inversion and 2) the construction of stacked traces from the inverted velocity-stack gather. Solutions are brought about by using shifted hyperbolas (de Bazelaire, 1988) and by implementing non-conventional stack methods.
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Wave equation multiple suppression using constrained cross equalization
By D. J. MonkA method for improving the attenuation of water layer multiple energy is suggested. The improvement is achieved using wave equation extrapolation to generate an initial model of the multiple energy, and then constraining the way the time variant wavelet is modified to fit the observed multiple energy. Reconciling the initial multiple model wavelet with the input data is a critical part of this process, and several techniques have been suggested previously by other authors. The approach used here is to fit time, amplitude and phase of the wavelets, by adapting the model trace using a weighted sum of four traces which can each be derived from the initial multiple model trace.
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Multiple suppression by multi-channel predictive deconvolution
Authors M. T. Taner, R. F. O‘Doherty and E. BaysalSince its inception deconvolution has become one of the standard tools of seismic data processing. It is used to suppress reverberations, wavelet shaping and suppression of short and long period multiples, most of the instances, all at the same time. Failure of single channel predictive deconvolution of suppressing multiples longer than 200 millisecond period led to developments of alternate methods, most of which involve with the multichannel processes. Prediction across the channels in the radial direction (Taner, 1980) and using slant stack traces (Alam and Austin, 1981) have been successful for sirnpler water bottom geometries. Of the multi-channel processes for general multiple suppression the ones using the wave equation continuation to predict the multiple arrival times (Morley, 1982; Wiggins, 1985) had a physical basis, hence more promise for success. The rest were various forms of multi-channel filters.
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Multichannel attenuation of high amplitude peg legs multiples. Example from the North Sea
More LessAstrong impedance contrast, such as the base of the Cretaceous in the North Sea, generates a train of high amplitude water bottom peg legs which can heavily obscure the underlying less energetic reflections, often of great petroleum interest. Attenuation of these peg legs, a prerequisite for defining the structural image, is a persistent and difficult problem in processing the seismic data.
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Multiple attenuation by predictive deconvolution in the p-t domain
More LessMany multiple attenuation or suppression methods depend on discrimination of the moveouts of reflections in unstacked data. Most of these methods, such as NMO stacks and f-k dip filters, are not very effective when primary and multiple reflections have similar moveouts and stacking velocities. Predictive deconvolution in the p-tau domain does not require such velocity discrimination. Land reflection seismic data from North Africa contains many strong multiples with essentially the same moveout as weak underlying primaries. Predictive deconvolution successfully attenuated these multiples and improved the interpretation of the subsurface.
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Deconvolution by the double-streamer experiments
Authors A. Matthies, H. Huck and M. TygelIn exploration seismics the geophysicist has to face two fundamental problems during data processing, namely the recovery of the source pulse, and thereafter the solution of the inversion problem, i.e. the extraction of the reflectivity series of the medium under investigation. This research adresses to both problems in a specific measurement configuration. We demonstrate the estimation of an arbitrary (i.e. mixed-delay) source wavelet and of the medium reflectivity from point-source seismograms obtained from a vertical double-streamer experiment. We assume the medium to be acoustic and horizontally stratified bounded or not bounded by a free surface above and a homogeneous acoustic half space below.
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High resolution processing of reflection data
Authors A. L. R. Rosa, T. J. Ulrych and W. S. LeaneyThe widespread occurrence of subtle trap accumulations offshore Brazil has led to the need for the development of a high resolution processing scheme that helps the delineation of these features. To be successful, a comprehensive wavelet processing technique should take into consideration the spectral properties of both the seismic pulse and the reflectivity function. In this paper we present such a technique, and we apply it to the processing of reflection data from the Campos basin offshore Brazil.
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Tutorial interpretive evaluation of migrated data
By O. YilmazIn practice, decisions with regard to migration include choice of migration strategies (2-D/3-D, poststack/prestack, time/depth), migration algorithms (integral, finite-difference, frequency-wavenumber) , parameters for a given algorithm (aperture width, depth step, stretch factor), factors related to input data (profile length, spatial sampling, noise), and most importantly, migration velocities (the weak link between seismic and geologic sections). By using a large number of field data examples, the problem of conflicting dips with different stacking velocities and that of strong lateral velocity variations associated with complex overburden structures will be discussed. Emphasis will be made on uncertainties in migration velocities and interpretive evaluation of the results we obtain from migration.
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True-amplitude finite-offset migration
Authors P. Hurbal, M. Tygel and J. SchleicherSeismic reflection data provide traveltimes and amplitudes of seismic waves. Current processing and interpretation are, however, almost totally based on traveltime measurements. This is easy to understand as traveltimes possess the robustness and stability attributes required to the implementation of most seismic data manipulations. Amplitudes of primary reflection arrivals are strongly related to angular dependent refiection coefficients and, if properly processed, may be of great interpretational value.
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Time migration imaging for complex overburden
Authors M. O. Marcoux, A. Downie and R. WesterbergLarge lateral velocity gradient in the overburden causes structural imaging by conventionally applied time migration to fail. Depth migration can image such structures correctly but depends on a modelling procedure which is more costly than time methods and which brings interpretation into the processing sequence at a relatively early stage. In this presentation, an extension to prestack time migration is used to image the well known Marmousi synthetic to gauge the effectiveness of the extended technique relative to prestack depth migration results.
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Migration to zero-offset in variable velocity media
More LessMigration to Zero-Offset (MZO) or Prestack Partial Migration (PSPM) transforms prestack data into zero-offset data and is equivalent in a constant velocity medium to the normal moveout correction (NMO) followed by the dip moveout correction (DMO) applied as a single step process. For media with lateral velocity variation or with sharp increase of velocity in depth the migration to zero-offset process cannot be split in NMO followed by DMO, but must be applied as a single process.
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