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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
61 - 80 of 315 results
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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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