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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
51 - 100 of 315 results
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Radon emanometry over some kimberlites and lamproites in Southern and Western Botswana
Authors G. McDowall and H. KoketsoRadon decay product measurements have been widely used in uranium exploration, but until now this method has not been used in the exploration for kimberlites and lamproites.
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The application of transient electromagnetics to an integrated mineral exploration program
Authors N. J. Cooper and G. MaliotisFor the past 30 years, considerable geophysical interest has been focussed at mineral exploration of the Troodos Ophiolite, Cyprus. The local mining environment broadly comprises medium to small scale sulphide lenses, hosted at intermediate levels of the extrusive section. Mineral concentrations result from post-extension secondary phase alteration along ridge-parallel and transcurrent faults. Complex vertical and lateral inhomogeneities in the ophiolite section have complicated geophysical ground studies. Conventional ground exploration techniques give both inconclusive and inconsistent results when used to delineate structural features and target anomalies. Some previous success has been achieved with the I.P. method for detailed target evaluation, but this labour- and time-intensive approach is not suitable for broader scale exploration of the Ophiolite.
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On the application of geophysics in the exploration for copper and chrome ores in Albania
Authors A. Frasheri, L. Lubonia and P. AlikajThe exploration of copper and chrome ores in Albania is carried out through a wide complex of geophysical, geochemical and geological methods, which are applied in coordination to each other, according to the geological problems to be solved and the exploration phases.
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Geophysical investigations for the troll sub-sea pipe tunnel
Authors O. Kr. Fjeld, H. Deisz and B. AagaardThe Troll Pase I Project is a part of NS Norske Shell's development plans for the Troll Gas Field located 80 km west - northwest of Bergen, the Troll field is the Europes largest offshore gas field and in full production will be capable of providing 10 % of Europes total gas requirement. The wellstream will be produced on a minimally manned production platform and lead ashore for processing at a plant 40 km north of Bergen.
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Resistivity measurements on the sea bottom
Authors O. B. Lile, K. R. Backe, H. Elvebakk and J. E. BuanIn connection with the construction of a sub-sea tunnel under the 1050m wide and 35m deep Maursund strait, the Norwegian Public Roads Administration had the foilowing preinvestigations carried out: 1. Refraction seismies on the sea bottom. 2. A cored borehole from the shore 290m along the tunnel trace. 3. Acoustic tomography between the sea bottom and the borehole. 4. Resistivity measurements on the sea bottom. 5. Resistivity tomography between the sea bottom and the borehole. 6. Resistivity logging in sounding drillholes in front of the tunnel face. This paper reports the results from the resistivity measurements on the sea bottom and between the sea bottom and the diamond drillhole.
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Very high resolution 3D reflection seismic imaging of small scale structural deformations
Authors J. -P. Henriet, M. Verschuren and W. VersteegA small clay diapir under the river Scheldt in Antwerp has been used as a target for testing the possibilities of very high resolution 3D reflection seismic imaging in geotechnical practice. Results of this test may be relevant to a number of site survey cases in water-covered areas, e.g. where it is essential to elicit and analyse the spatial distribution of structural defonnations.
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Stratigraphic processing techniques for digital narrow-beam echosounding data from deep sea sediments
Authors M. Breitzke and V. SpieszContinuous high-resolution echosounding of deep sea sediments up to a maximum penetration of 200 m images sedimentation patterns and processes related to climatic and oceanographic changes. A new technical concept was applied to transmission and reception of high-frequency signals and requires arevision and adaption of classical stratigraphic processing techniques and interpretative approaches with respect to signal length and bandwidth. Selected methods are discussed in comparison to the frequency range and signal shape used in conventional (marine) seismics.
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3D Monte Carlo residual statics
By R. E. HaleStacking of common mid-point gathers assumes that reflectors corrected for normal move-out (NMO) and dip move-out (DMO) will align across the gather. If reflections do not align due to incorrect statics the stacked traces will be degraded. Static corrections for land seismic data calculated from measurements made in the field have limited accuracy so it is standard practice to pass all land data through a residual statics process before stack. The residual statics process estimates the difference between the field statics and the "true" statics required to align the reflectors. Wiggins, Larner and Wisecup (1976) described how the residual statics could be resolved into surface consistent components and this method is now called conventional residual statics. Ronen and Claerbout (1985) showed how surface consistent components could be calculated directly from stacked correlations and allowed larger residual statics to be estimated. Rothman (1985) extended this work by describing a method of estimating residual statics using simulated annealing (Monte Carlo residual statics). All three techniques were originally designed for 2-D seismic data. The conventional residual staties method generalised to 3-D seismic data with only minor modifications. The upgrade of the Monte Carlo method to accept 3-D data was more difficult and is the subject of this presentation. Real 2-D and 3-D data examples are shown which demonstrate the effectiveness of this technique.
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Attenuation of land multiple energy case studies from the Saudi Arabian Peninsula
Authors P. G. Kelamis and E. F. ChiburisThe use of Radon transforms for multiple suppression has recently gained popularity in the processing of seismic reflection data. The increased separation of events in the model space, obtained through the forward transform, is the cornerstone for multiple elimination. The application of the forward Radon transform effectively deconvolves the smearing and sharpens the image of the events. Thus, by inverse mapping the multiple energy and subtracting it from the original CMP gather. a primartes-only gather is obtained. In this paper, we first describe a simple, cost-effective methodology for multiple elimination via Radon transforms. Then, we show two distinct, case histories from the Saudi Arabian peninsula. Our processing scheme combines partlal stacking and the application of the forward/inverse Radon transform. Partial stacking just prior to the forward transform improves S/N and regularizes the geometry of the data. Thus, the Radon operators are computed only once, i.e., for the first gather. Finally, we examine the proposed Radon multiple elimination scheme in conjunction with AVO studies. We show that amplitudes and phases may be altered after Radon multiple elimination. Thus caution is advised in the application of Radon transforms for AVO studies.
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Migration of refraction events - A combination of travel time and wavefield techniques
By E. P. BrücklArbitrary lateral changes of thickness and velocity are allowed for. First order discontinuities are assumed as dominant features. Vertical velocity gradients if present should be positive and comparatively small. Topographic irregularity and layer thickness may be of comparable magnitude. The "plane layer and constant velocity"-model is considered inadequate.
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Cooling schedules for simulated annealing in the residual static estimation problem. Construction of master schedules
By E. NormarkVarying thicknesses and velocities of the surface layers in reflection seismic surveys on land induce different delays on the seismic recordings, which make static corrections necessary. If the a priori information in terms of the field statics is insufficient, residual static corrections are needed. Normally, residual statics are estimated by traveltime picking methods. An alternative technique is to maximize the stack-power function (Ronen et al., 1985). Originally, the solution was found by linearized optimization along the axis in the parameter space, defined by the source and receiver static parameters. However, if large residual statics are required the objective function, the stack-power, has more than one maximum. Therefore, using linearized optimization the solution might easily be trapped in a local maximum.
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Feed-back stacking and applicatoin for velocity analysis
Authors O. E. Næss and L. BrulandIn the present work a new algorithm, Feed-Back Stacking (FBS), to discriminate better between primary and other interfering events, is presented. The method has been developed especially in order to improve stacking velocity determination of difficult data. For velocity analysis a new version of Constant Velocity Stack (CVS) which incorporates the FBS method is proposed. with the new type of analysis a stepwise relative reduction or change of noise events will occur. The changes itselves are used as a criterium to discriminate between the different events. The new algorithm (FBS) shares the principle of a stepwise change of events with the Superstack method (Næss, 1979), but is otherwise different. Especially, the FBS is a linear method while Superstack is not.
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Model-based stack: A method for constructing the true zero-offset section
More LessUsually it is assumed that a stacked time section represents a zero-offset section. But in many areas with complex structures or streng lateral variations it is not valid. In this case a poststack depth migration gives a wrong image of the model. We propose an 'optimal' way to produce the 'true' zero offset section. We call this section the Model-Based Stack. This method consists in calculation the actual traveltime trajectories by ray tracing for each CMP and for each time sample according to the velocity-depth model obtained from the coherency inversion, and stacking the traces within CMP gather along these trajectories. Now a poststack depth migration applied to Model-Based Stack (MBS) produces a better image of the subsurface because MBS represents a zero-offset section even in cases of complex geology. We believe that the processing procedure including Model-Based Stack followed by poststack migration is a fast and reliable alternative to prestack migration.
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Improving the performance of F-X prediction filtering at low SNR
Authors P. E. Harris and R. E. WhiteAs commonly forrnulated, the method of random noise reduction by linear prediction in the f-x domain (Canales, 1984) suffers from three problems. These are (White and Sumrners, 1989): 1) The wavenumber response of the prediction filter does not peak exactly at the wavenumbers of the signal components. 2) The amplitude of the filter response is less than one at the signal component wavenurnbers, leading to attenuation of the signal. 3) Sidelobes in the filter response cause noise at wavenumbers weIl separated from the signal wavenumbers to leak through into the filtered data.
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Planning a marine 3D survey in front of Rotterdam harbour
Authors J. J. Nooteboom, J. A. Bandell and C. J. A. CorstenDuring 1989 the acquisition department of NAM was asked to prepare a 710 km2 marine 30 seismic program. to be recorded in 1990. This marine 30 program was part of a total of 3100 km2 which had to be recorded by GECO between April and October using the twin vessel technique which was developed during previous years.
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How to fight a monster
By F. KochMONSTER" is a village near Rotterdam, which gave its name very symtomatically to a NAM-survey area, in which a complicated 3D-survey was to be conducted.
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Enhanced servovalve technology for seismic vibrators
By D. ReustA criticaI component of a servohydraulic vibrator is the servovalve. The servovalve meters the flow of hydraulic oil into and out from the actuator, producing a force which is applied by a baseplate, to the earth's surface.
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The seismic signals generated by the bit and the drillstring in a horizontal well in the Larderello geothermal field
Authors J. -J. Chameau, F. Batini and G. OmnesThe Larderello geothermal reservoir is locally situated in Paleozoic phyllites and quartzites. Permeability is controlled by fractures. The reservoir is identified with a fractured zone usually not more than 150 meters thick and located at the top of the Paleozoic basement.
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Planning and field techniques for 3D land acquisition in highly tilled and populated areas. Today's results and furure trends
Authors L. Bertelli, B. Mascarin and L. SalvadorIn recent years, 3D Seismic Techniques have acquired an increasing importance in field appraisal and development. The results obtained, in terms of greater knowledge of the geological setting, more accuracy in field development and finally in the rate of success, have spurred oil companies to move quickly towards 3D Exploration Surveys, in spite of the considerable investments involved. Once again, the outcome of 3D surveys has confirmed that the rate of success in properly locating wells on the basis of 3D seismie data has increased dramatically, especially in areas characterized by a complex geological setting.
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A moving coil dynamic accelerometer
Authors A. Obuchi and T. FujinawaThis paper describes a new type of seismic sensor that enables flat frequency response from 3 Hz to 200 Hz with any angle of tilt.
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Safety in seismic - exploring the myths
More LessSafety has long suffered from being seen as necessary but a nuisance. Reduced production, unnecessary cost, and "insurance will pay" are all reasons why safety is of ten dropped down the list for discussion.
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Porosity and pore structure from acoustic well logging
Authors G. Tao and M. S. KingWyllie's time average equation and subsequent refinements have been used for over 20 years to estimate the porosity of reservoir rocks from compressional (P)-wave velocity (or its reciprocal, transit time) recorded on the sonic log. This model, while sirnple, can no longer be convincingly explained in theory or satisfactorily used in practice.
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Wave propagation through fluid saturated porous media, energy losses (permeability) incorporated
More LessIn the oil industry there is a need for obtaining physical properties of potential reservoir rock from in-situ measurements. Acoustic measurements can be executed as borehole refraction measurements in the petrophysical case and as crosswellshooting in borehole seismics. A quantitative determination of permeability from acoustic measurements cannot be made because a suitable model is not yet available. It is suggested that, by including the permeability as an energy loss term in the mathematical model, this determination might become possible.
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Elastic anisotropy due to aligned cracks in porous rock
By L. ThomsenThe theory of the effects of cracks upon acoustic velocity in rock is founded upon the classic paper of Eshelby (1957). The effects of a dilute set of aligned ellipsoidal cracks (cf e.g., Hudson, 1980) are well established, although finite concentrations or crack-shape distributions are problematic (Thomsen, 1986b). Despite a long history, there has been essentially no experimental confirmation of this theory. The difficulty has been in fabricating samples containing cracks of controlled number, location, orientation, shape, and size. The new technique discussed by Rathore, et al. (1991) appears to have solved this problem, at least for cracks in porous rock.
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Effect of fluid micro-distribution on velocity and attenuation in porous rock
Authors A. L. Frisollo and L. ThomsenWhen two phases of fluid are present within a porous rock, there is an a priori possibility that the distribution (on the intergranular scale) of the fluid phases may affect the elastic and attenuative properties of the rock. If this were true, it could have important implications for the interpretation of seismie veloeities, especially since the microscopic distribution (as weIl as the macroscopic distribution) may vary during production of a hydrocarbon reservoir, and since the production process may be monitored remotely by seismic means.
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Critical porosity, elastic bounds, and seismic velocities in rocks
By A. NurData for cornpressional Vp and shear Vs velocities versus porosity in sands show that none of the existing models are adequate predictors of velocities over the entire range of porosity: The Voigt and Reuss average are too far apart and too far from the data. Although self-consistent approximations for spherical pores and especially needles come closer to the data, they rail to account Ior the critical porosity Фc = 38% below which grains are load-bearing and above which grains are in suspension.
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Seismic wave propagation in uncracked and cracked carrara marble
Authors S. Peacock, C. McCann and J. SothcottWe have used a simple rock, Carrara marbIe, to test theories of seismic wave propagation through media containing suites of small cracks. The theories (Hudson 1980, 1981, 1986) predict the effects of the shape, alignment, filling and number density of the cracks on seismic velocity, attenuation and anisotropy. They are used in specialist seismic interpretation to evaluate cracks and crack alignment for determining the potential of oil and geothermal reservoirs and indicating stress regimes.
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Seismic attenuation and permeability in clay rich sandstones
Authors A. I. Best, J. Sothcott and C. McCannThe attenuation and velocities of compressional and shear waves are important parameters for predicting the nature of porous rocks. Intrinsic attenuation gives us information on the nature of the pore fluid, whilst seismic velocities teIl us about the rock matrix. Surprisingly, little is known of the relationships between the seismic attenuation of sedimentary rocks and their petrophysical and geotechnical properties such as porosity, permeability and clay content. Part of the work of the Geophysics Research Group at P.R.I.S. is to study these relationships by making small scale laboratory measurements under simulated reservoir pressures.
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Joint inversion of seismic and geoelectric data recorded in an underground coal mine
Authors L. Dresen, M. Dobroka, A. Gyulai, T. Ormos and J. CsokasUntil now, the layer sequence "rock-coal-rock" and faults in coal seams in underground hard coal mines have been detected by seismic waves and geoelectric measurements. To determine the geometrical and petrophysical parameters of bedrock and coal, the data recorded using seismic and geoelectric methods have been inverted independently. In consequence, the inversion of partly inaccurate data necessarily resulted in a certain degree of ambiguity.
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Shallow fault location in coal measures using offset wenner resistivity profiling
Authors B. A. Hobbs and A. M. ReadingA region within the Westphalian Lower Coal Measures of the Midland Valley of Scotland is currently under consideration as a site for open-cast mining. Mining will only be viable to one side of a fault running through the region and drilling to determine the exact position of the fault is not possible. A technique employtng nondestructlve surface measurements will therefore have to be used for precise fault location. The coal measures are overlain by 10-20m of boulder clay above which lies peat which has been worked commercially.
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Hole-to-surface seismic reflection surveys for opencast coal exploration
Authors E. Kragh and N. R. GoultyOpencast coal mines in such a densely populated country as the UK are mostly of small size by world standards, but they are highly profitable. Annual production of opencast coal is currently about 17 million tonnes, constituting some 20% of the UK coal industry's total output. Transport costs are low as the coal is produced for domestic consumption, and consequently opencast coal is a very important national resource.
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Self-potential survey for a preliminary geothermal research in a Southern Italy coastal area
Authors R. Quarto and D. SchiavoneSystematic self-potential exploration of large areas proved to be effective in geothermal research. In this field, SP measurements directly depend on the fluid flow through thermoelectric and electrokinetic phenomena. Thus, due to its low prospecting costs, a preliminary geophysical survey with the SP method can be particularly useful in new areas of geothermal interest.
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Crustal imaging of the Larderello geothermal field, Italy, from teleseismic P waveforms
Authors J. E. Foley, M. N. Toksöz and F. BatiniThe seismic velocity structure of the Larderello Geothermal Field of central Italy is investigated by inversion of seismic travel travel-time and waveform observations made on the 26-station seismic monitoring network operating in that area. We have divided the program of imaging the velocity structure of the Larderello area into two separate stages. The first stage utilizes abundant low frequency (0.5 to 3.0 Hz) teleseismic P-waveform data. Both teleseismic travel-time residual and waveform analysis techniques are used to produce a broad picture of the crust to depths of about 40 km. The second stage utilizes higher frequency (4.0 to 12 Hz) P and S wave travel-time observations from local earthquakes, and images the shallow and mid-crust to depths of about 15 km with higher resolution. The results of the first stage techniques are presented in this paper and the results of the second stage inversion in the following report (Block et al.).
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Crustal velocity structure of the Larderello geothermal field determined from local earthquake arrival time data
Authors L. Block, M. N. Toksöz and F. BatiniA joint hypocenter-velocity inversion of local earthquake arrival time data has been performed in the Larderello Geothermal Field in Tuscany, Italy. More than two thousand local earthquakes have been recorded in the Larderello area since 1977. Most of the earthquakes occur shallower than 8 km depth, although a few occur as deep as 30 km. The corner frequency of the data is typically 25 Hz. P wave and S wave arrival times from 269 of these earthquakes were used to simultaneously determine the hypocenter parameters and the three-dimensional P wave velocity and Vp/Vs structures. Due to a variety of rock types present in the uppermost 2 km, as well as a complex tectonic history, the shallow velocity structure is very complicated. This shallow structure was determined from seismic reflection and vertical seismic profiling (VSP) data and kept fixed during the joint hypocenter-velocity inversion.
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3D Surface seismic and well seismic applied to the delineation of geothermal reservoirs in metamorphic formations in the Larderello area
More LessThe delineation of deep (>3000m) reservoirs is a primary objective in the exploration of the LardereIlo and Mt Amiata (Italy) geothermal fields. Drilling wells at such depths in relatively hard rocks is expensive, therefore risk must be minimized through optimum application of surface exploration techniques. Since 1974 ENEL has performed extensive reflection seismic surveys. They made it possible to identify well defined groups of reflectors within a metamorphic basement composed of phyllites, micoschists and gneisses (Batini et at, 1978). An exploratory well, the San Pompeo 2 well, revealed the presence of a permeable zone filled with geothermal fluids at very high temperature (>400°C) and pressure (>240 bars) and corresponding to the main reflector or K marker. Subsequent deep drilling targeted at reflections with lateral continuity over 1-2 km above the K marker demonstrated the existence of productive zones associated with these reflections. Because these reflections are less continuous, their delineation is a major objective. A 3D seismic reflection survey combined with multi-offset well seismic profiles, well logs and measurements on core samples (ERL/MIT, 1989) was therefore planned as a pilot study. It was carried out at the Monteverdi test site where an extensive drilling programme was to take place.
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Inversion of combined data of the CRE nad CEE imaging
Authors A. Berkovitch, B. Chelchinsky, S. Keydar and V. ShtivelmanThe present paper is devoted to solution of an inverse problem - construction of 2D "true model" of a studied medium, using data obtained by the Common Reflecting (CRE) and Common Evolute Element (CEE) methods. The CRE and CEE methods are components of a novel system of processing and interpretation of seismic data, which is based on fundamental topological ideas, new different types of stack and homeomorphic images of studied media. The mentioned and other components of the homeomorphic imaging system were presented on Adelaide ASEG, Dallas and San Francisco SEG, Berlin EAEG, Istanbul IASPEl and Jerusalem Mathematical Geophysics Meeting and several journal papers.
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Depth focusing analysis: The effects of elevation corrections and structural DIP
More LessPrestack depth migration, combined with depth-focusing analysis (DFA), is a useful method for imaging and velocity estimation in areas of complex structure. However, such regions often possess significant topographic relief that leads to large elevation-static corrections prior to migration. The result is a distorted wavefield that yields erroneous depth-focusing information. Of further consequence are the limits imposed by the zero-dip approximation to the wave equation used for velocity estimation.
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Migration velocity computation using plane wave decomposed data
By M. T. TanerIn general, seismic data is acquired and processed as point source illuminated reflections. CDP or CMP processing is the central point of the processing sequence. Imaging is referred to as pre- or post- stack migrated data. Conventional CDP processing requires several velocity analyses, each for a different stage of the processing sequence to produce the final subsurface imaging. These velocities do not strictly relate to the subsurface physical parameters. However, the pre-stack shot domain imaging is the only procedure where the data satisfies the wave equation with no simplifying approximations and the velocities used for image construction are directly related to the subsurface physical characteristics.
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Migration velocity analysis by migration of velocity spectra
Authors C. J. Beasley and R. KlotzMigration of seismic data is required to collapse diffractions and position reflectors in the correct spatiallocation. Today's accurate migration algorithms applied to high-resolution seismic data can, in theory, produce accurate images of nearly vertical structures. However, in practice, migration accuracy depends not only on the accuracy of the migration algorithm, but also on the accuracy of the migration-velocity field. Indeed, as dip increases, accuracy in migration becomes critically dependent on the migration velocity.
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Velocity model development for prestack depth migration at different geological environments
Authors P. Farmer, S. Eidsvig and A. HeggernesIn the presence of lateral velocity variations depth migration rather than time migration should be used to obtain a proper image of the subsurface. If the velocity model is known and if the stack is a good estimate of zero-offset section, post-stack depth migration often gives good results. Normal common-midpoint stacking relays on the assumption of hyperbolic moveout curvature. Prestack depth migration, however, is not dependent on the validity of the common-midpoint assumption, and therefore it should be used in complex areas with strong lateral velocity variations to develope the velocity model and to image the structure properly.
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The hyperbolic radon transform
Authors E. de Bazelaire, S. Desaunay and J. R. ViallixReducing to a point the moveout curve of the reflections in common midpoint is the oldest problem facing seismic processing. This is currently achieved through a dynamic correction and multicoverage stacking. The process is costly, both in computing and preparation time, and of limited efficiency for multiple attenuation. So much so that, in recent years, new methods have been sought.
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Multiple suppression in the Barents Sea using the generalised radon transform as a notch velocity filter
Authors R. G. Williams, A. Haugen and N. CooperThe generalised Radon transform has been proposed as a means of multiple attenuation on CDP records. Initial results, especially on synthetic data, suggested an improvement in both velocity resolution and amplitude preservation compared to conventional FK techniques. The generalised Radon transform discriminates between primaries and multiples by measuring their curvature with respect to offset unlike the FK transform which discriminates on the basis of gradient. The use of curvature offers the possibility of separating primaries from multiples even at very short offsets. It is also possible to separate multiples with smaller velocity differences than is possible with FK methods. However, despite the initial optimism, generalised Radon transform techniques have failed to replace FK methods as the industry standard. This is partly because of the increased cost and partly because of technica1 prob1ems.
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A t-p inversion technique including amplitude
More LessTo improve the reliability of a velocity-depth function v(z), it is desirabie to use the amplitude information throughout the entire inversion process instead of interpretating it only in the x-t-domain ( x-distance, t-tirne ), which is of ten arnbigous because of crossing traveltime curves and shadow zones.
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Angular coverage compensation in transmission tomography
Authors G. Boehm, P. Carrion, F. Pettenati and A. VesnaverIn seismic tomography, the quality of reconstruction depends on different parameters such as angular coverage, signal-to-noise-ratio, local ray geometry, imposed constraints, etc. For the basic definitions and references on geotomography we send the reader to Nolet (1987) and Carrion (1987). In this study, we pursue the following goals: to give a detailed account on recoverability versus angular coverage, to study image blurring due stochastic characteristics of noise and to show the role of constraints in those cases when angular coverage is incomplete.
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Acoustic tomography for enhanced oil recovery and reservoir description
Authors J. H. Justice, A. A. Vassiliou, M. E. Mathisen, W. H. Troyer and P. S. CunninghamAs large commercial oil fields become increasingly difficult to find, attention has turned to the extensive reserves which still remain unproduced in our existing oil fields. Many of these fields are in decline and continued economic production depends on secondary (waterflood) or tertiary enhanced oil recovery (EOR) procedures. EOR programs are generally expensive and are often carried out in complex reservoir environments where even small improvements in efficiency may carry major economic benefits.
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Imaging reflectors in a crosswell seismic experiment
Authors M. Becquey, J. O. Bernet-Rollande, J. Raulent and G. NoualCrosshole seismic surveys can give improved resolution because the source and receivers can be lowered as close to the target as possible. CrossweIl tomography is now extensively used. But, to image reservoir layers, this technique needs wells drilled rather far below the reservoir, an uncornmon configuration for production wells. The crosshole reflection method does not entail such a limitation.
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High resolution cross-hole seismic tomography at the Backhorn test facility in Illinois, USA
Authors H. Saito, T. Takahashi, T. Imayoshi and S. SakashitaHigh resolution cross-hole seismic experiments were conducted at the Backhorn Test Facility in Illinois, U.S.A., where we drilled five 270 m boreholes over the oil producing field for the purpose of various borehole geophysical experiments. According to the exploration and production histories of the field, the oil reservoir is the porous zone of Silurian Kankakee "White Lime" carbonate which is horizontally distributed at the depth of about 200 m, and has no more than 10 m thickness. In fact, two boreholes out of five we drilled produce oil from the Kankakee formation. The results of several kinds of logging measurements conducted just after drilling five boreholes indicate a good correlation between formations from borehole to borehole. However, they also suggest significant lateral changes in porosity within the Kankakee formation.
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New developments in wave equation tomography
Authors Y. Luo and G. T. SchusterA new methodology is described which allows for wave equation inversion of skeletalized parts of data records. Skeletalized data is a reduced subset of the original raw data that is mainly influenced by only one type of model parameter; e.g. first arrival traveltimes skeletalized from seismograms are mainly influenced by the velocity (not density) parameters.
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Iterative geophysical diffraction tomography
Authors L. -J. Gelius and J. J. StamnesA new iterative diffraction-tomography method, tailored for geophysical applications, is described. The main problem in geophysics is the complicated background medium. Therefore, we have derived a reconstruction algorithm which take into account that the point-spread function is space variant. Using this algorithm, we compensate for the limited-view problem in an iterative manner.
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Traveltime tomography in anisotropic media: some examples
Authors R. G. Pratt, C. H. Chapman and W. J. McGaugheyTravel-time tomography is often applied to cross-hole seismic data and images of the velocity distribution are used to provide stratigraphic and/or structural interpretations. The technique is often applied in regions where one might expect significant anisotropy (for example in complex, layered stratigraphy or in fractured rocks). Without accounting for anisotropy, the tomographic images suffer from distortions that can render the results un-interpretable.
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