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58th EAGE Conference and Exhibition
- Conference date: 03 Jun 1996 - 07 Jun 1996
- Location: Amsterdam, Netherlands
- ISBN: 978-90-73781-07-8
- Published: 04 June 1996
1 - 100 of 604 results
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The tip wave superposition method based on the refraction transform
Authors A. Aizenberg, H. B. Helle, K. D. Kiem-Musatov and J. PajchelIn seismic modelling is widely used the diffraction Kirchhoffs integral with the spherical Green function obtained from the exact Kirchhoffs integral by means of replacement of the integral boundary values with their high-frequency asymptotics. This approach is only applicable in homogeneous medium because the Green function in the inhomogeneous medium is non-spherical. In [1] we introduced the tip wave superposition method (TWS) for seismic simulation in 3-D geological models with structurally complex target but with a homogeneous overburden. In this paper we generalize the TWS to account for an inhomogeneous overburden but restricted to the scalar case.
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A Pertubation technique for travel time computation with FD-Eikonal solvers
Authors N. Ettrich and D. GajewskiThe theory of first order perturbation is well known and commonly used in classical ray tracing techniques. Desired quantities in a perturbed medium are calculated along rays which propagate through a reference medium. Reference and perturbed medium differ only slightly. First order traveltime perturbation can be used, e.g., for pre-stack Kirchhoff-type migration. In general in migration the correct subsurface model is unknown. Therefore, a simultaneous migration with several macro models should be performed. The results can be used for velocity estimation techniques. The time consuming procedure of computing traveltimes from sources to all subsurface points is performed directly only in one model. For the other slightly deviating models the traveltimes are computed efficiently by perturbation. Moreover, most of the recently developed methods for fast computation of traveltimes are designed for isotropic media. For Kirchhoff-type migration of data from anisotropic media perturbation techniques provide a fast way to compute diffraction surfaces for slightly anisotropic media where the reference medium is isotropic. Here existing tools to compute traveltimes in isotropic media can be used. For highest computational speed we implemented the perturbation formulas into Vidale's method (Vidale, 1988) and call it FD-perturbation method.
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Hard and smooth models for seismic imaging based on raytracing
More LessUnderstanding of wave propagation is essential in geophysical imaging techniques. Most of these techniques require an a priori model of the subsurface. Conventional imaging techniques usually employ a highfrequency approximation: only the wave front is considered. In the limit of infinite frequency, only discontinuous variations of the velocity produce reflections. It therefore seems an obvious choice to require the velocity model to be smooth: all reflection data are caused by discontinuities that can be considered as perturbations on the smooth background model.
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Multiple scattering of seismic waves in thinly layered media
Authors S. A. Shapiro and S. TreitelWe study the influence of higher order multiple scattering in 1-D layered structures on the transmissivity and the reflectivity of normally incident delta-pulse plane waves. For the case of the transmissivity we obtain an approximation which is more accurate than the familiar O'Doherty-Anstey formula. We derive also an approximation of the reflectivity, which clarifies the nature of this imp rtant quantity. We show how our results have implications for both forward and inverse modeling.
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Heterogeneous modelling behaviour at an interface in porous media
Authors G. Quiroga-Goode and J. M. CarcioneHeterogeneous numerical modeling satisfies implicitly the boundary conditions at physical interfaces. In models including a one-phase rheology, i.e., solids or fluids, where only one kind of boundary condition is satisfied, heterogeneous modeling has proved very useful.
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Theoretical description and numerical simulation of elastic wave propagation in random media
Authors E. Burr, N. Gold and S. A. ShapiroA wave propagating through a random medium is scattered due to its interactions with the spatial variations in material properties of the medium. We split the wave u into a scattered part u f and a coherent part < u > , the so-called Meanfield. The Meanfield is carrying information about the attenuation due to scattering and implicit about irregularities in the structure of the medium. Our aim is to extract this information.
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Waves in scaling media - The implication of non-differentiability
Authors F. Herrmann and J. StaalDifferentiability forms the predominant cornerstone on which many laws of physics are based. The wave equation, as such, does not form an exception. The main goal of this paper is to validate whether this important presupposition of differentiability is met with in the seismic context. For this purpose the rather new concept of wavelets is reviewed. Given this theoretical body it is shown that it is possible to not only estimate the local degree of regularity (differentiability) of the well - log measurement, but also the singularity spectrum, capturing the hierarchy of scaling exponents. Application of these analyzing techniques to the well data shows that their behaviour is singular within the seismic scale range and beyond. The implication of this observation is that when these profiles are submitted as coefficients in the wave equation, one is confronted with a fundamental problem. In our judgement this problem lies in the fact that the constitutent parameters, the coefficients in the wave equation, show a too strong spatial fluctuation with respect to the characteristic scale of the seismic disturbance. This may not only result in a possible instability of the wave equation but also in a new type of reflection and dispersion mechanism which has to be related, from first principles, to the local and global apparent degree of non-differentiability.
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Time-domain representation of waves in media with frequency power law of dispersion
By V. E. RokAs it was observed and theoretically proved the dispersion of sound in microinhomogeneous media may be connected with relaxation process that leads to equalization an additional thermodynamic parameter ξ that depends on wave pressure (or stress) differently in inclusions and host medium.
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Non hyperbolic reflection move out for horizontal transverse isotropy
Authors A. F. Al-Dajani and I. TsvankinRecent experimental studies (Lynn et al., 1995) have shown that P-wave signatures, including reflection moveout, are sensitive to the presence of azimuthal anisotropy. Transverse isotropy with a horizontal axis of symmetry (HTI) is the simplest azimuthally anisotropic model caused by vertical penny-shaped cracks embedded in an isotropic matrix.
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Wave polarization in transversely isotropic and orthorhombic media
Authors J. M. Carcione and K. HelbigElastic waves can, in principle, be classified according to their propagation velocity, e.g., through the values of the slowness or the group velocity surfaces along a given direction. Generally, in media of relatively high symmetry (isotropic to orthorhombic), the fastest wave is quasi-compressional and the slower wave is quasi-transverse.
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Frequency-dependent anisotropy of seismic waves in transversely isotropic multilayering
Authors U. Werner and S. A. ShapiroOur goal is to find out whether a multilayered medium can be substituted by, a frequency-dependent equivalent averaged TI-medium. To achieve this we formulate the influence of multilayering on elastic wave propagation analytically and compare our results with numerical simulations of wave propagation in multilayered media.
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Effective parameters and seismic velocites for azimuthally anisotropic media
By I. TsvankinAnalytic description of seismic signatures in azimuthally anisotropic media is of utmost importance in seismic investigations of fractured reservoirs. Since many fractured formations in sedimentary basins are believed to have orthorhombic symmetry (e.g., Wild and Crampin, 1991), azimuthally-dependent seismic wavefields are governed by nine stiffness coefficients. A new notation introduced in this work is designed to make seismic treatment of orthorhombic media more practical by reducing the number of independent parameters and simplifying the description of seismic velocities and amplitudes. For the transversely isotropic model with a horizontal symmetry axis, which can be considered as a special case of orthorhombic media, the formalism developed here makes it possible to use P-wave reflection moveout to estimate the shear-wave splitting parameter and the crack density.
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About the relation between the effective model of layered fluid-solid medium and transversely isotropic biot model
Authors L. Molotkov and A. BakulinInvestigation of wave propagation in porous and fractured media is usually carried out on basis of effective models because it is impossible to solve the wave diffraction problem on one or several pores of arbitrary shape. One of such model is effective model of layered solid-fluid medium.
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Stacking analysis in transversely isotropic media
Authors M. Le Bougeant and R. G. PrattMany seismic data processing algorithms are based on a number of simplifying assumptions. As larger offset data are now more commonly acquired, it makes sense to examine and possibly overcome these assumptions. One such assumption is that a small-offset, isotropic approximation can be used to define the moveout of reflected arrivals. At finite offsets in 1D media, especially in anisotropic media, the travel time-offset relationship (the moveout) becomes non hyperbolic. This non-hyperbolic behaviour causes degradation of standard velocity analysis, causes a degradation of the stack, and causes a bias to appear in the velocity values.
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Velocity analysis using nonhyperbolic move out in transversely isotropic media
More LessP-wave reflections from horizontal interfaces in transversely isotropic (TI) media have nonhyperbolic moveout.
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Anisotropic seismic data processing
Authors O. Uzcategui, D. Mujica and V. CelisIt is known that in areas with significant shale content, the isotropic assumption in seismic data processing is not correct, and this can lead to misfocusing of dipping events and wrong depth positioning if not properly accounted for.
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Poststack time remigration
Authors M. S. Jaya, J. Schleicher and P. HubralCommon-mid-point (CMP) stack sections can be transformed by a so-called image-wave equation into a panel of time-migrated record sections for a continuum of (constant) migration velocities. The derivation of this image-wave equation as well as its kinematic and dynamic aspects will be presented. The resulting panels of time-migrated sections will be shown for selected examples.
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Different simulations of the kinematics of time migration - A Review
More LessThere are many applications that require simulation of the kinematics of time-migration. Time-demigration, for example, is an important step in map migration and stacking velocity/traveltime inversion. Mapping of well trajectories from the depth domain into the time-migrated block is another [4]. We compare the simplistic techniques of vertical stretch and image ray mapping with more sophisticated methods that accurately simulate the time-migration operator used. We simulate Kirchhoff-type time-migration analytically and finite difference time-migration by anisotropic ray tracing in the time-migrated domain.
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Accurate 3D time migration focusing in regions of lateral velocity gradients
By C. HarrisMany 3D zero offset time migration algorithms allow lateral velocity variations. However, these methods are generally derived by employing simplifying assumptions about the medium velocity. It should not be expected that, if the derivation is based on a constant velocity or v(z) medium, the resulting algorithm will be accurate in regions with lateral gradients. It is well known that time migration mispositions events relative to their true subsurface position whenever lateral gradients are present. Here, I address only the issue of focusing. Insight into this aspect of the problem may be gained by analyzing the scalar wave equation in the natural coordinates of the time migration output space: image ray surface location and two-way traveltime.
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Accurate migration using offset-checkshot surveys
Authors J. R. Krebs, D. R. Fara and A. E. BerlinAccurate seismic migration, particularly of steeply dipping or overturned reflectors, requires an accurate velocity model (Lamer, Beasley et al . 1989). Recent research has focused on the notion that the migration velocity model must include velocity anisotropy (Alkhalifah and Lamer 1994; Larner and Cohen 1993; Lynn, Gonzalez et al. 1991). While it is true that most migration algorithms require accurate velocity models, Kirchhoff migration is an exception to this rule. Unlike most migration methods, Kirchhoff migration does not require a velocity model as input. Rather, all that is required is a table listing the seismic traveltime from subsurface imaging points to points in the surface migration aperture (Gray and May 1994). Thus, in the case of Kirchhoff migration, accurate traveltime tables would ensure a kinematically accurate image.
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Quick-look 3D prestack time migration
Authors R. Ferber, B. Sander and A. E. BerlinA major product of 3-D seismic data processing is the time migrated data cube. With conventional processing, this cube is generated by poststack time migration of the zero-offset cube, obtained by NMO-DMO stacking of multi-offset data. A major drawback of this processing sequence is, that migrated data can only be inspected at a very late stage of data processing and that there is no simple and efficient technique to derive the migration velocity field. In this presentation we show results of a recently developed 3-D prestack time migration technique to overcome this deficiency. Our method allows the generation of time migrated bin-gathers on a coarse grid within the 3-D survey, without a priori knowledge of a migration velocity field. These gathers hold an offset axis and a two-way travel-time axis, where the time is measured along the image ray. The migration velocities can then be estimated from the generated migrated data by conventional means, e.g. from semblance velocity spectra and multi velocityfunction stacks. The stack of the migrated data generates the proposed quick-look of the 3-D time migrated data set on the coarse grid. The estimated migration velocities are an additional product of this technique. They can be used to replace the smoothed dip-corrected stacking velocity field for the poststack migration of the zero-offset cube on the final grid.
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Can we afford the time for ´time´ processing?
Authors K. Hawkins, D. Evans, R. Laver, M. Vickers, N. Oliver and J. AndersonAnyone that is involved today with exploration for new gas reserves or production from fields about to come on stream in the Southern Gas Basin will be acutely aware of the recent dramatic drop in gas prices as a reward for their efforts. The situation is further aggravated by the fact that many of the remaining potential discoveries he in Carboniferous structures under complex overburdens and at considerable depth - in excess of four kilometers - which can make prospect definition uncertain and drilling cost high. In these situations the risk has to be defined as accurately as possible.
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Converted-wave (P-S) prestack migration and velocity analysis using equivalent offset migration
Authors J. C. Bancroft and S. WangA new approach for converted-wave prestack migration and velocity analysis is based on equivalent offset prestack migration using common scatter point (CSP) gathers (Bancroft et al. 1995, Wang et al. 1995). Each prestack migrated trace, gathers all input traces within the prestack migration aperture, and sorts them into offset bins based on an equivalent offset. The equivalent offset may be time varying, and is based on the offset of the scatter point from both the source and receiver.
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Steep dip Kirchoff migration for linear velocity gradients
Authors N. W. Martin, M. S. Donati and J. C. BancroftA non-hyperbolic Kirchhoff summation curve is introduced for 2-D post-stack time imaging of dips up 120° in a linear-with-depth (or constant gradient) velocity medium. It can be expressed as a 8th order polynomial equation whose coefficients are function of the half offset and velocity gradient.
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Some geophysical consequences of fractal scaling
More LessFractal models are being used to an increasing extent in the earth sciences to describe objects as diverse as cloud shapes or rock fractures (Korvin, 1992, is an excellent review of the topic.) Implicit in this is the belief that many natural phenomena have attributes that are unchanged over a wig range of scales. A photograph of a cliff face, for example, reveals a number of sedimentary layers; closer inspection reveals yet more layers. If the picture does not include the requisite rock hammer, or geology student, the scale of the object is unknown and in essence the image is scale free.
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The quantification of seismic data quality - (i) The method
Authors I. Scott, P. Armstrong and D. IresonEveryone would agree that it is essential to assess the quality of seismic data at each stage of the seismic process. Currently, this is generally based on subjective notions of quality which can only lead to a final product with unknown quality. There is obviously a need for an objective method to assess quality which can be used to improve the reliability and efficiency of acquisition and processing decisions.
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The quantification of seismic data quality - (ii) A case study
Authors L. Peardon, T. Birks and I. ScottThe Quantified Quality Assurance (QQA) methodology is a combination of statistical and deterministic analysis to quantify the quality of seismic data. This approach, described in detail in a companion paper by Scott, Armstrong and Ireson, 1996, is applicable to both acquisition and processing. In this paper, we describe an acquisition case study showing the use of the method in a 3D marine seismic survey.
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A Field case of data quality analysis
Authors B. C. Scheffers, E. C. C. Wildeboer Schut, J. A. C. Meekes and H. L. H. CoxTNO Institute of Applied Geoscience has developed a prototype Data Quality Analysis system that has been applied on several datasets. This paper presents the application on a high resolution seismic survey. The objective of this survey was to obtain a detailed image of the upper 50 to 400 m. For this, data with a wide frequency range up to 250 Hz is required. We encountered cases of very poor data quality without apparent evidence for a particular type of noise, or deficiency in the field operations. The aim of performing an analysis on quality aspects of the data is to enable us to anticipate on lateral variations at acquisition time.
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A Comparison of multi-dimensional wavelet compression methods
More LessMulti-dimensional, wavelet transform based, lossy compression of seismic data is increasingly being recognized as an effective means of substantially reducing seismic data volumes while maintaining the data integrity sufficient for many, if not all processing and interpretation applications. The benefit, or increase in compression ratio for a given quality level, gained by using a three dimensional (3D) compression method versus a two dimensional (2D) method is described quantitatively in terms of L1 and L2 norm error metrics. Conversely, the drawbacks incurred by using higher dimensional compression methods are illustrated in light of realistic operational environments.
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Analyzing and zerophazing seismic data using the wavelet transform
Authors S. Mansar and J. -M. RodriguezThe aim of any signal analysis method is to transform a signal into a new space where extraction of relevant information is possible. For instance, time structure information is obtained via the time sampling and the frequency structure information via the Fourier transform. When dealing with seismic signals and reservoir characterization, we need a description of the evolution of the frequency structure with time. The Continuous Wavelet Transform (CWT) [1] can achieve this purpose. An important condition for seismic signal analysis and reservoir characterization is the zero-phase condition. In this paper a CWT based method is derived for target ori ented zero-phasing.
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Coherence computations with eigenstructure
More LessSeismic coherence is a measure of similarity or dissimilarity between seismic traces. Varying between zero and one, zero indicates the greatest lack of similarity, while a value of one indicates total similarity, i.e., two identical traces. Coherence for more than two traces is defined similarly.
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Extraction of attributes from 3D seismic data
Authors P. Steeghs and G. G. DrijkoningenThe characterization of seismic data in terms of their spatial properties, such as horizon dip and azimuth, has greatly improved the interpretation of 3D seismic data (Hoetz and Watters 1091). In most cases the 3D seismic attributes are extracted at or between picked seismic horizons. In our paper we present an alternative method for 2D and 3D attribute extraction, that does not require the picking or tracking of horizons. We extend the relation between the 1D complex trace attributes and local spectra (Steeghs et al. 1995) to higher dimensions. Using the local 2D wavenumber-frequency spectrum we can extract dips and azimuths over time slices in 3D seismic data.
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Design, Manufacture and installation of a permanent seabed sensor network for seismic reservoir monitoring
Authors K. Roed, E. Dietrichson and D. IresonThe abstract of this presentation was made available at the Conference
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The dual-sensor ocean bottom cable method - Attributes, North Sea tests and recent advances
Authors F. J. Barr, J. Paffenholz and W. RabsonIncreasing numbers of 3-D seismic surveys are being conduct ed over producing fields to help improve hydrocarbon recovery. Existing fields are covered with oil-field equipment, including production well heads, pumps, and compressors. The amount of equipment and its areal distribution increases during the life of the field. Offshore, this equipment is installed on platforms that are dangerous obstacles for marine seismic vessels towing streamers.
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Seismic methodologies for a 3 components sea floor geophone experiment on a potential flat spot in the Vøring Basin
Authors M. Brink, P. Y. Granger, M. Manin and S. SpitzThe usefulness of shear waves to obtain physical properties in relation to lithology is well known. Indeed, shear waves are less sensitive to pore fluid changes than compressional waves. At the same frequency, S waves achieve higher vertical and lateral resolutions than P waves. The advantages of S waves methodology have been demonstrated in land seismic. In the last years the industry has shown a growing interest in acquiring S waves at the bottom of the sea. Valuable information in a blind area for P waves caused by a gas chimney have already been obtained from such a 3 components acquisition (Berg et al., 1994). In this paper we present an experiment made to confirm the interpretation of a potential flat spot detected by surface acquisition.
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P- and S-wave separation for ocean bottom seismic recordings
Authors M. S. Donati and R. R. StewartThe quest for a more detailed and confident description of the subsurface has led to the rise of three-component (3- C) seismic surveying. Considerable effort is currently being expended to make and use converted-wave (P-to-S reflection) seismic sections from these 3-C recordings. Most of the effort to date has been with land recordings. However, with the advent of the SUMC (subsea seismic) survey and its very promising early results (Berg et al., 1994), more attention is focusing on the marine 3-C case. A fundamental aspect of deciphering 3-C records from the ocean bottom is to understand how seismic waves interact with a geophone at a fluid-solid interface. This paper reviews the basic theory of the geophone response at the water bottom and proposes a new filter to separate P and S waves so recorded. The separation filter is based on the methods of Dankbaar (1985). The separated waves (pure P and S) are then further processed into their respective sections.
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An Improved method for determining water bottom reflectivities from dual-sensor ocean bottom cable data
Authors J. Paffenholz and F. J. BarrThe primary goal of the Dual-Sensor technology is the elimination of the receiver ghost response which corrupts single hydrophone ocean-bottom cable data. The Dual-Sensor data allow separation of the wavefield into up- and downward traveling components. Therefore, elimination of the surface reflected energy is straightforward and is achieved by simple summation of the pressure and velocity signals. If however a suitably scaled velocity trace is used in the summation, both the up- and the downward traveling parts of the wavefield trapped in the water layer can be eliminated (Barr and Sanders, 1989; Barr, 1989). The scaling factor is a function of the oceanbottom reflectivity. Here we describe a method to derive the ocean-bottom reflectivity from production seismic data. The method eliminates the need for a separate calibration survey and is stable in the presence of random and shot-generated noise.
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Ghost attenuation in dual-sensor cable data
Authors L. L. Canales and M. L. BellBottom cable acquisition has become in use because it permits very sophisticated geometries. Particularly it permits placing the receivers very close to platforms or other obstacles. The bottom cable presents new problems, particularly the ghost produced by reverberations in the water column above the cable. One solution to this problem has been introduced by F.J. Barr et. al. (1990). A dual sensor bottom cable is used, one sensor records pressure and the other records vertical velocity displacement. The two sensors have the ghost notches at different frequencies so that the properly scaled sum produces a high signal to noise trace that is free of the ghost.
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Multiple attenuation on multicomponent sea floor data modelling
Authors A. Osen, L. Amundsen and A. ReitanFor several years, new technologies for recording seismic data directly on the sea floor have been evolving. Bottom cables for recording 3-D surveys near obstacles in shallow water were introduced in the mid-80's. Barr and Sanders (1989) designed a water-bottom cable with pressure and vertical velocity detectors to attenuate water-column reverberations. Berg et al. (1994) developed a concept for directly recording three-component particle velocity data along with pressure data on the sea floor. Recently, seismic contractors have licensed acquisition technologies for acquiring multicomponent sea floor data.
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Integrated 3D acquisition and processing techniques in heavily obstructed areas - the Belayim Field case history
Authors G. La Bella, D. Calcagni, G. Ronchitelli and I. MohamedThe sub-salt sequence in the Gulf of Suez (Egypt) is worldwide renown as one of the most difficult to be correctly imaged on seismic sections, due to the very strong multiple effects that almost completely mask the target seismic response.
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The impact of the 3D seismic reshoot on field development in the Greater Gannet Area, Central North Sea
Authors D. J. Davies, M. E. Minning, C. Grant and R. A. BealeAn improved geological subsurface image has been obtained in the greater Gannet/Guillemot area, as a result of the 1993 3D survey reshoot. This has resulted in: > Revised subsurface models in a number of fields > Revised well locations -avoiding side-tracks in areas of high uncertainty > Better definition of direct hydrocarbon indicators (dhi's) > Improved well-ties
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Fluid imaging using densely spaced OBS-data from the Vøring Basin
Authors E. W. Berg, L. Amundsen, A. Morten and R. MjeldeIn 1992 a comprehensive three component seismic OBS survey was performed in the central and northern part of the Verring Basin, offshore Norway. The data acquisition program consisted of three parts where the two regional parts provided the large basin coverage. The third part concerned a local OBS survey with 20 OBSs dropped at 1300 meters water depth along the seismic line VB-8-89. The distance between the OBSs was approximately 200 meters. The purpose of the local survey was to acquire densely spaced P-and S reflection OBS data above an observed seismic "flat spot" anomaly in order to more accurately predict if hydrocarbons could be related to the seismic anomaly.
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3D Ocean bottom cable project over the Abu Al Bukoosh Field
Authors D. Enns, J. -L. Piazza and Ph. PratA 3D dual sensor ocean bottom cable seismic survey was conducted over the ABU AL BUKHOOSH oil field to optimize the future development program which includes production wells, injection wells and enhanced oil recovery. A complete approach was taken that included a pre-survey feasibility study, processing of a 2D test line, fast track 3D cube and finally an independent full processing sequence.
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Seismic imaging in the presence of basalts wide-angle profiling in the Faroe Basin
Authors R. Masotti, P. J. Barton, K. R. Richardson and R. CameronThe frontier areas of the North Atlantic margins are overlain and interbedded with Tertiary volcanic rock's, which form a high impedance barrier that obscures deeper structure on conventionally acquired seismic profiles. However, wide angle energy, recorded by sea-bed seismometers to offsets of ten of kilometers, may be used to characterise and model structures under these acoustic barriers. Sediments trapped within or beneath basalts form a classic low velocity zone which cannot be resolved unambiguously using simplistic one-dimensional refraction analysis. However in reality these layers are always two-/three-dimensional in shape, and dense overlapping sampling with diving rays using a well designed wide-angle seismic survey allows the low velocity zone to be delineated. Wide-angle diving rays refract along curved paths through a velocity gradient, with energy at greater offset sampling progressively deeper into the Earth. A seismic phase generated by diving waves within a layer is typically continuous in offset until it is truncated abruptly at the offset corresponding to the ray grazing the base of the layer: this observation strongly constrains the seismic model. Thus the high velocity layers are detected directly, and a low velocity layer sandwiched between them may be modelled by defining its top as coincident with the base of the high velocity layer above, its base at the top of the layer below, and its velocity in terms of the delay experienced by diving rays travelling through the layer. A programme of wide-angle acquisition coupled with intensive development of data processing, analysis and inversion techniques has produced a revolution in wide-angle imaging during the past five years, so that it can truly be regarded as an exploration tool. With the high data density achieved by using tens of receivers and hundreds of sources, laborious forward modelling has been superseded by tomographic inversion, allowing the resolution of different parts of the model to be evaluated quantitatively.
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Lithology and compaction from a seismic velocity study in the Vøring Basin, Norwegian Sea
Authors S. Fanavoll, O. Hansen, H. Løseth and Ø. SyltaAs part of an extensive geological and geophysical study in the Voring Basin, a seismic velocity study was performed on 9 seismic lines evenly spread in the study area. The purpose of the study was twofold, to obtain information on compaction and/or lithology along the profiles, and to get a velocity database for depth conversion of digitised time sections. In this paper, we will present results from the lithology interpretation on three parallel, NW-SE running profiles, covering different parts of the Voring Basin.
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Subsurface image of geopressured rocks using seismic velocity and acoustic impedance inversion
Authors N. C. Dutta and A. RayA good understanding of subsurface effective stress and fluid pressure is essential at several stages of an exploration/development program. It can be used during exploration to assess the effectiveness of a regional topseal section, to provide a map of hydrocarbon migration pathways, and to analyze 'trap' configuration and geometry of a prospective basin. In the exploration and appraisal drilling and development phase, pressure prediction is a pre-requisite for safe and economic drilling. An optimized casing and mud-program design can avoid well control problems.
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Stratigraphic interpretation below salt from 3D PreSDM AVO
Authors R. O. Lindsay and D. W. RatcliffSubsalt lithology prediction is a rather all encompassing topic. Constraints can be placed upon the concept by limiting the breadth of the field to include only; stratigraphy, seismic amplitudes extracted from 3-D prestack depth migration (3-D PreSDM) and depth domain 3-D amplitude versus offset (AVO) -- a still none too small set of tasks.
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Can we separate the effects of anisotropy and structure from P-wave travel time data?
More LessThe kinematic features of wave propagation in a general medium are affected by the presence of anisotropy as well as the deviation of any reflectors from the horizontal. There are places where these structural variations are quite common (such as salt domes). The effects of anisotropy and structure are then coupled. Current processing [Muller (1991)] and analysing methods for seismic anisotropy often assume horizontal layers and use sites where the structural influence is minimal [Lewis (1991)].
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Pore space structure and anisotropy properties of small porosity cracked media
Authors B. P. Sibiriakov, S. B. Gorshkalev and W. V. KarstenMedia, containing cracks, have very special pore space structure. They generally have large specific surface even with very small porosity. It is known that small porosity cracked media are changing their P and S velocities much more than 1-ƒ times for porosity ƒ. Evidently there are different parameters of pore space that are responsible for decreasing P and S velocities. There are models considering isolated ellipsoidals and their aspect ratio is very important parameter of pore space. These models imply strict alignment of cracks. Determinining the aspect ratio is hard since the physical reason for the large axis of the ellibsoids is indefinite.
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A Resolution analysis for anisotropic inversion / migration
Authors M. V. de Hoop and C. SpencerWe consider spatial and parameter resolution aspects of G(eneralised) R(adon) T(ransform) / A(mplitude) Versus scattering) A(ngles) inversion in anisotropic media. Particular attention is paid to the proper treatment of amplitudes within the distorted Born approximation, viz., by taking into account the radiation patterns of the contrast sources corresponding to the medium perturbations giving rise to the images. By treating the amplitudes correctly, the spatial resolution can be improved even if one leaves the parameter interpretation aside. On the other hand, the parameter resolution, through a singular value analysis, provides a hierarchy of parameter combinations with a decreasing number of symmetries.
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Far field radiation patterns in anisotropic media
Authors T. N. Teles, L. A. Diogo and I. PsencikThe effects of point source situated in anisotropic structures can be investigated by means of the radiation pattern, i.e., the magnitude of the displacement field at a unit sphere around the source. The first attempts to perform ray calculations of the radiation patterns generated by point sources in inhomogeneous anisotropic media were made only recently (Ben-Menahem et al., 1991, Gajewski, 1993).
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Ray theory Green's function in inhomogeneous anistropic media
Authors T. N. Teles and I. PsencikOne of the most efficient methods for obtaining synthetic seismograms for complicated models of laterally varying anisotropic layered media is through the use of zero order approximation of asymptotic ray theory. Although it is only approximate and with restrictions, e.g., the medium must be smooth enough and it doesn't work in the vicinity of caustic and shadow zones, it can yield results with suficient accuracy in many cases. For a long time the wave field by the ray method was computed by situating the source in an isotropic layer due to inexistence of an adequate formulation for the Green's function in general anisotropic media. Recently, various formula for its computation appeared in many ways (Cerveny, 1990; Ben-menahem and Sena, 1990; Kendall et al, 1992).
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Shear wave analysis for azimuthal anisotropy using pseudo-rotation of marine VSP
By C. MacBethResolution of seismic anisotropy for marine VSP remains poorer than for land VSP, primarily due to lack of control for converted shear-waves, and more development is required. We demonstrate that the Alford rotation can be adapted for VSP analysis provided an optimal acquisition geometry is used. This application relies upon efficient near-offset shear-wave conversion.
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Determination of the principal directions of azimuthal anisotropy from P-wave seismic data
Authors S. Mallick, K. L. Craft, L. J. Meister and R. E. ChambersIn an azimuthally anisotropic medium, the principal directions of azimuthal anisotropy are the directions along which the quasi P and the quasi S waves propagate as pure P and S modes. When the azimuthal anisotropy is induced by oriented vertical fractures, two of these principal directions correspond to the directions parallel to and perpendicular to the fractures. S-waves propagating through an azimuthally anisotropic medium are extremely sensitive to the direction of their propagation with respect to the principal directions. As a result, primary or mode converted S-wave data can be used to obtain the principal directions from seismic data. Apart from the high acquisition cost, processing and interpretation of S-wave data require a special processing technology. Moreover S-wave data quality is poor in many regions. Thus, an alternative involving only P-wave data is attractive but is limited to a few qualitative studies of the amplitude-variation-with-offset (AVO) for different azimuthal directions (Mallick and Frazer, 1991; Chang and Gardner, 1993; Allen and Peddy, 1993 ; Lefeuvre, 1994 ; Lynn et.al, 1995, Rüeger and Tsvankin, 1995), and to velocity studies of selected horizons on data for different common mid point (CMP) locations (Neidell and Cook, 1986).
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Shear wave anisotropy and permeability changes in sandstones with aligned cracks
Authors M. S. King, A. Shakeel and N. A. ChaudhryAligned fractures and cracks in porous sedimentary rocks are of considerable interest to those engaged in studies of subsurface fluid flow, such as hydrolog ists and petroleum engineers. Geophysical techniques, especially seismic, are often used to determine the physical characteristics of such aligned discontinuities.
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Measurement of ultrasonic bulk properties of water-saturated porous media
Authors O. Kelder and D. M. J. SmeuldersAcoustic signals are used extensively in the oil industry to determine the physical properties of reservoir rock. In the interpretation of these signals empirical laws play a major role. To obtain a more fundamental interpretation of the recorded wavetrains, the need for a comprehensive theory for wave propagation and attenuation in rocks is obvious. For this purpose, Blot's theory (1956) turned out to be a simple but effective two-phase theory. One fundamental feature of this theory is the existence of both a fast and 'a slow compressional wave, as well as a shear wave. Preliminary measurements of ultrasonic properties of water-saturated porous media, i.e. bulk attenuation and phase speed, show the predictive power of the Biot theory (Johnson, Plona and Kojima, 1994). In this paper we present new experimental data of bulk attenuation and phase speed for all three types of bulk waves, covering an ultrasonic frequency range from 100 kHz to 1 MHz. The results are compared with theoretical predictions of the Biot theory.
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Up-scaling for geochemical modelling - Non-symmetric rigidity tensors and their symmetric approximation
Authors W. Zijl, G. Rijpsma and I. RitsemaThe rigidity is often assumed to be a symmetric tensor. A symmetric rigidity tensors has many advantages. Therefore, it is worth while to review the arguments for a symmetric rigidity tensor, and to investigate whether, and under what conditions, non-symmetric rigidity tensors may occur.
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Acoustic velocities of North Sea shales
Authors R. M. Holt, E. F. Sønstebø and P. HorsrudSeismic waves used for petroleum reservoir exploration spend most of their time travelling through shales. More knowledge of acoustic properties of shale will improve the quality of reservoir interpretation (e.g, corrections for shale anisotropy are important for AVO) as well as of basin evaluation and predicted safety/stability during drilling. Previous studies of velocities in shales (e.g. Jones and Wang, 1981; Johnston, 1987; Homby et al., 1994; Johnston and Christensen, 1995; Sayers, 1995) have demonstrated stress dependency, interpreted mainly as a result of deformation of the contact regions between clay particles. Anisotropy has been observed and modelled primarily on the basis of stress sensitive alignment of clay particles. Previous measurements have, however, often been performed with outcrop or shallow subcrop cores, and data are usually obtained only for a limited range of sample characteristics.
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Seismic attenuation in thinly layered, fluid saturated and permeable sediments
Authors S. Gelinsky and S. A. ShapiroTypical reservoir rocks are often thinly layered, porous, fluid saturated and permeable sediments. Scattering of an incident wavefield by thin layering is an important reason for the attenuation of seismic waves in sediments.
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Permeability estimation of rocks using local porosity theory
Authors U. Fauzi, A. Hördt, F. M. Neubauer and K. VozoffPermeability is one of the very important parameters in reservoir engineering and in environmental geophysics. Direct measurement of permeability however, is very expensive. Thus, the study of pore geometry that controls permeability of porous media remains an active research field both theoretically and experimentally.
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Stress-induced coupling between anisotropic permeability and shear wave splitting
Authors S. V. Zatsepin and S. CrampinThe stress-sensitivity (poro-reactivity) of fluid saturated porous rock leads to effective anisotropy in the permeability of many laboratory experiments and in large-scale field phenomena such as the natural directivity of water-flooding caused by horizontal differential components of the regional stress field (Heffer and Koutsabeloulis 1993). One of the most interesting features of stress-sensitivity of porous rock is the effect of the coupling between stress-induced variations in anisotropic permeability and elastic shear-waves (King et al. 1994) that in principle allows changes in reservoir rock permeability to be monitored with seismic shear waves (Crampin 1994). A previous theoretical consideration of the possible stress-induced coupling between the permeability and elastic waves (Gibson and Toksoz 1990) was restricted to uniaxial stress that seems inappropriate for in situ situations where the horizontal differential stress field and horizontal permeability anisotropy is of primary importance. We present a theoretical model for the coupling between permeability and elastic waves in a triaxial differential stress field and elevated pore-pressure based on the APE model of anisotropic poro-elasticity of Zatsepin and Crampin (1995).
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Acoustic monitoring of hydraulic fracture growth
Authors J. Groeneboom and R. RomynHydraulic fracturing is a technique to improve the production of oil and gas reservoirs by creating large fractures around the wellbore. The success of fracture treatments is critically dependent on our ability to predict and influence the fracture orientation and shape. Computer models based on simplified theoretical concepts are a useful tool to study the physics of fracture growth. However, theoretical models have to be calibrated by real physical experiments to prove their value. For this purpose an experimental set-up has been built in our laboratory in which we can carry out scaled hydraulic fracture treatments in a true tri-axial pressure cell [WIJERS 1995]. We have chosen to monitor the fracture growth process with ultrasonic waves. In this manner we obtain information on the growth process also away from the wellbore [SAVIC 1995]. The scaled experiment closely resembles seismic monitoring surveys of fracture treatments in the field [MEADOWS AND WINTERSTEIN 1994, WILLS ET AL. 1992]. Since the start of the project considerable experience has been gained in using ultrasonic measurements to interpret the scaled fracture experiments. We will present the major achievements of the ultrasonic experiments during the last few years.
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Numerical simulation of ultrasonic experiments on poroelastic samples
By B. GurevichSince the theory of dynamic poroelasticity was developed by M. Biot some 40 years ago, the existence of the type II or slow compressional wave (P2-wave or SCW) remains the most controversial of its predictions. This prediction was confirmed experimentally when SCW was observed in ultrasonic experiments by Plona (1980), and later by Rasolofosaon (1988) and others. Theoretical analysis shows that the observed vaveltimes of the SCW are consistent with Biot theory. And yet, the very existence of the SCW remains a mystery to many geophysicists and acousticians alike. The purpose of this paper is to further verify the predictions of Biot theory by accurate numerical simulations of the ultrasonic experiments in which SCW was observed.
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Effects of the conductivity / permittivity ratio on the dispersion and attenuation of radar signals
Authors J. W. Schoolmeesters and E. C. SlobIn modelling and processing of Ground Penetrating Radar (GPR) data, the distortion of the radar signal as it propagates through the ground is often neglected, see e.g. Davis and Annan, 1989. In the frequency range of interest for GPR applications, 10 MHz to 1 GHz, the loss mechanism in the ground is usually described by an exponential damping factor along the path of propagation.
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Dispersion of GPR waves
Authors S. Tillard, F. Hollender and F. JulienPerformed in boreholes drilled in galleries, Ground Penetrating Radar (or GPR) is a geophysical exploration technique expected to be used to characterize geological formations for deep radioactive waste disposal. In the case of an electrically resistive host rock such as granite, this technique must be able to localize in 3D, on a 0-100 m investigation range, the altered fissured zones. The aim is to know the ground water flow around the disposal.
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GPR Forward modelling applied to archaeological and engineering problems
More LessThe applications of ground penetrating radar (GPR) as an ecological, high resolution nondestructive technique are widely documented. A good review and state of the art can be found in the last special issue of the Journal of Applied Geophysics (Owen, 1995). In particular, forward modeling in heterogeneous media is essential to validate the geological interpretations.
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Recent advances in the multifold and 3D exploration of the Aquileia Archaeological Park
Authors M. Pipan, I. Finetti, A. Del Ben and F. FerigoA GPR exploration programme was completed in the area of the archaeological park of Aquileia. The research programme pursued the following primary objectives: 1. Implementation and test of integrated 2D and 3D multifold GPR techniques for the detection of buried masonries of archaeological interest 2. Implementation and test of radar soundings analysis techniques for optimum subsurface imaging and detection of the characteristic response of specific targets 3. Investigation of a unexplored sector of the Aquileia archaeological park for the identification of targets of potential archaeological interest
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Fragmentation processes in geodynamic systems - Clustering phenomena
Authors C. Suteanu, D. Zugravescu, C. Ioana and F. MunteaunuCracks' nucleation, growth and interaction, followed by solids fragmentation, are important processes affecting many geodynamic structures, with direct impact on man's environment.
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The effect of micro-properties on P- and S-wave attenuation in rocks
Authors M. Nabi-Bidhendi and M. S. KingThe spectral ratios technique is employed to measure the values of dissipation factor Q' , on four sandstone samples as functions of confining stress in both the dry and fully saturated states.
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Shock tube experiments on air-filled porous rocks
Authors C. J. Wisse, M. E. H. van Dongen and D. M. J. SmeuldersInterpreting seismic and petrophysical exploration data requires a thorough understanding of the effects of the medium's properties on wave propagation characteristics. It was already argued by Biot (1956) that acoustic waves in porous media may at low frequencies be described by diffusion-like equations. This means that inertia terms are negligible compared with viscosity terms, and that a strong dispersion both in damping and velocity occurs. Experiments, analytical and numerical calculations on an air-saturated porous rock sample have been carried out.
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Non destructive control of non metallic pipes using direct current and georadar
Authors P. Andrieux, O. Martire, L. Frobert and F. DubreucqGeoradar is now classical for non destructive control of large sewers and water pipes. There may be difficulties for the base of the pipes because of running water. A direct current technique, intermediate between logging and surface geophysics, is thus proposed for this purpose.
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A Practical accuracy of the surface NMR measurements
More LessA measurement of a Nuclear Magnetic Resonance (NMR) signal directly from subsurface water molecules is the most important distinction of the NMR in comparison with other methods of geophysics. The existence or absence of the NMR signal with a high degree of reliability also proves the existence or absence of subsurface water. The surface NMR system "Hydroscope" was developed by ICKC in the early eighties. It was tested in many countries in a variety of geological environments. The results affirm that parameters of the NMR signal depend on parameters of the aquifer. However the weakness of the signal, whose amplitude is not more than just a few micro volts, rises a question about the accuracy of the field measurements.
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The contact mode of polarisation curve - A Technique to evaluate conducting mineral deposits
Authors C. Serwanga, J. Roy, O. Putikov and M. HaleModem mineral exploration techniques are aimed at deep deposits under complex geological conditions, but the ultimate procedure for evaluating these deposits requires costly core drilling. Geophysical and geochemical methods of borehole investigation allow a significant decrease in the number of boreholes necessary to evaluate a deposit (Ryss, 1981). Some of these techniques are efficient but sometimes the lack of a good physical property contrast does not permit to decrease the number of boreholes necessary for a reliable evaluation due to the difficulties in the interpretation of the results. For a physicochemical investigation to be an adequate substitution for some of the boreholes, the chemical elements and minerals of which the ore body is made, should participate in processes which allow them to exhibit their own characteristic properties. The research work presented, sought in part, to verify the possibility of identifying known mineral potentials in a prototype contact polarisation curve (CPC) data set. After considering the problems experienced with the technique, an evaluation of the applicability of the technique in other environments was estimated.
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Change of a geoelectrical section under effect of elastic fields
Authors B. S. Svetov, E. L. Ozerkov, V. V. Tikshaev and O. A. AgeevaIn the last years Russian scientists from Low-Volga institute of geology and geophysics (Saratov) made some experiments to study an influence of vibrating effect on electromagnetic soundings results. These works were perfumed to investigate a possibility to raise on this basis the geological efficiency of oil and gas deposits prospecting. On fig .1 the results of one of such experiments perfumed on a broken by active faults seismic structure in Near Caspian depression are presented.
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Physical modelling electromagnetic field in dispersive media and criteria of similarity
More LessPractically the theory of all electromagnetic (EM) geophysical methods is based on the quasistationary approximation, which is valid for such a frequency range, in which usual displacement currents are neglected. However most of rocks display volume induced polarisation (IP), or low frequency dispersion of conductivity (LFDC) within of the same range.
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Electromagnetic sounding of frequency dispersive media
Authors B. S. Svetov and V. V. AgeevIn the last years Russian scientists experimentally established very high resolution of transient sounding (TS) inexplicable in frameworks of quasistationary electrodynamics of continuous media.
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Multipole arrays in direct current modelling and two case histories
Authors P. Brunel, P. Poussoumandji and P. AndrieuxResistivity and polarisability are two key parameters in environmental studies, civil engineering and archeology. Multipole arrays provide a large amount of data which lead to detailed images of the geology and ultimately to acceptable models via an inversion procedure.
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Magnetotelluric tracing of the Carpathian conductivity anomaly in the Vienna Basin
Authors L. Vero, A. Madarasi, W. Seiberl and G. VargaIn the framework of the scientific co-operation between the Geologische Bundesanstalt (Austria) and the Eotvos Lorand Geophysical Institute of Hungary (ELGI) the latter performed magnetotelluric soundings along a profile of about 80 km which traverses the Vienna Basin in NW-SE direction. The line starts from the Little Hungarian Plain, part of the Pannonian Basin, at the border zone of Austria, Slovakia and Hungary, while the NW end of the profile is located in the Cretaceous-Paleogene flysch zone.
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Distinguishing of faults and determination their tracing depth by electromagnetic sounding
Authors G. B. Agakuliyev, K. M. Kerimov, A. I. Manafli and A. A. AbbasovaAs is known, deep faults exert considerable influence on formation of the regional block structure of individual areas. They are one of important factors for formation and accumulation of useful minerals including oil and gas. Besides this accurate distinguishing of faults, determination of their orientation and tracing through the depth are very important to study and prognosticate earthquakes because of that site zones of earthquakes are mainly associated with the areas of intersection of faults. Definite criteria to single out deep faults both the ones traced at the surface and buried by young deposits are characteristic to geophysical methods and particularly to seismic and gravity prospecting.
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Strategies for reconciling seismic attributes with well data
Authors C. H. Skelt, M. D. Cann, D. A. Scott and C. P. WoodAreal prediction of formation petrophysical characteristics from seismic attributes related to well data has been transformed over the past few years from an esoteric technique practised by a small coterie of specialists to one used by most self-respecting petrophysicists and geophysicists. Much of the literature published to date has concentrated on demonstrating new detailed physics or mathematics in specialized case studies. Routine use of these techniques has necessitated developing various methodologies for applying them within the context of the broader hydrocarbon exploration and development decision malting process. With the benefit of several years practical experience several key recommendations may be made.
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An integrated methodology based on 3D seismic amplitude inversion in support of reservoir characterization
Authors H. L. Triebwasser, C. H. Wagner, G. C. Robinson, M. -R. Hong and J. P. TungIn the normal course of reservoir development; large amounts of data are collected from sources as diverse as wire-line logs, core analysis, well test results, laboratory results and surface seismic; however, no one source of information can stand alone. Each suffers from its own form of ambiguity caused by such factors as washed-out hole conditions, inability to recreate reservoir conditions or lack of vertical resolution as in the case of surface seismic data. Integration of these data has always been attempted by geoscientists involved in rese rvoir characterization, but the integration of the various components could not be quantified. Recent software advances allow for quantification of this integration while retaining enough flexibility to allow for testing of alternative hypotheses which are not totally supported by one or more of the various component datasets. A reference framework which allows for the integration of the multi- disciplinary datasets is 3D seismic. The purpose of this paper is to describe an integrated methodology, with quantified-integration links, used to develop an initial geologic model for testing against 3D seismic data.
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Gas cap mapping of a carbonate reservoir in the Arabian Peninsula
Authors J. J. Lee, C. Tsingas and P. LawrenceAn integrated study has been conducted to map the mobilized gas cap of a carbonate field in the Arabian Peninsula. In this paper, we discuss some issues in detecting seismic fluids in this complex carbonate environment. Through its production history a gas cap has been induced as a result of pressure drop in the reservoir which consists primarily of dolomitic limestones and calcarenites with porosity ranging from 12% to 25%. In this study, we attempt to delineate the boundary of the gas cap by examining attributes from seismic amplitude mapping, seismic forward modeling, AVO analysis, traveltime anomalies and structural mapping. Seismic signatures of the reservoir were found to have largely come from combined effects of reservoir porosity and gas accumulation. While we found it generally difficult to separate the amplitude effects between the two components, it is possible to decouple these effects on the kinematic part of the seismic data. In this paper, we also show results from structural mapping of the depth migrated seismic volume and other borehole data to better constrain the lateral extension of the gas cap.
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Seismic monitoring and modelling of an enhanced oil recovery project at Cold Lake, Alberta, Canada
Authors A. Vafidis, E. Kalantzis and E. R. KanasewichReservoir monitoring during an enhanced oil recovery (EOR) process is critical for efficient management of the FOR process. 3-D high resolution surface seismic is used in an experiment by Mobil Oil of Canada that involves in situ thermal EOR processes in the heavy oil sands at Cold Lake, Alberta, Canada.
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Monitoring the production behaviour of the Yibal-Shuaiba reservoir using time-lapse 3D seismic data
Authors E. B. J. Kleiss, M. Klaasen and E. C. A. GeversA 3-dimensional seismic survey shot in 1988 over the Yibal field in Oman has been used to identify and reduce drilling risks, to optimize the targetting of wells and as input to detailed numerical models of the reservoir. An improved understanding of the complex water movements occurring in the Shuaiba reservoir resulted in increasing confidence in predicted high oil recovery and reserves estimates. A second 3D survey over the field was acquired in 1993 with the aim to obtain an improved structural definition of the field. Both surveys were reprocessed to form the basis for a time-lapse study. A comparison between the reprocessed data sets shows good agreement in the seismic signal at intervals above and below the reservoir and possible hydrocarbon-production-related effects at reservoir level.
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Abnormal shear wave polarizations as indicators of high pressures and over pressures
Authors S. Crampin, S. V. Zatsepin, C. Slater and L. Y. BrodovNumerical modelling of reservoir response, supported by a-case study, shows that the presence of high pore-pressures can be recognized in seismic reflection surveys or VSPs by anomalous shear-wave polarizations.
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P-P And P-S seismic interpretation of an incised valley sandstone reservoir
Authors S. L. M. Miller, E. Ö. Aydemir and G. F. MargraveCoupled P-P and P-S seismic analysis increases confidence in interpretation, provides additional measurements for imaging the subsurface, and gives rock property estimates. This paper describes the interpretation of a 4.0 km 3-component seismic survey acquired over the Blackfoot Field in south-central Alberta, Canada, in the summer of 1995. The target rocks are incised valley-fill sediments of the Glauconite Formation of the Lower Cretaceous period. Reservoir rocks are generally found in structural and stratigraphic traps where porous channel sands pinch out against non-reservoir regional deposits or low-porosity channel sands. The Glauconitic Sandstone varies from zero to over 35 m in thickness and is encountered at a depth of approximately 1550 m in the study area. The primary exploration objectives of the survey were to: 1) distinguish sandstone valley-fill sediments from regional facies, and, 2) determine sand shale ratios within the incised valley systems.
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The multi-component 3D seismic survey over the Natih Field in Oman
Authors J. H. H. M. Potters, H. J. J. Groenendaal, S. J. Oates and C. M. van der KolkSituated in the southern foothills of the Oman mountains and with a STOIIP of approximately three billion barrels of 32. API oil, the Natih field is one of the most important oil accumulations in north Oman. Situated in the concession area of Petroleum Development Oman (PDO) it was discovered in 1963 and flowed first oil in 1967. The main reservoir is the fractured limestone of the Middle Cretaceous Natih formation, with a moderate to high porosity, ranging from 12 to 28%. With its low matrix permeability, in the range of 0.5 to 10 mD, the production is almost entirely fracture-related. A well-known metaphor is that the fractures "short-circuit the flow in the field", the flow essentially being limited by the rate with which the fractures are refilled from the matrix. With over a hundred wells drilled, much of the field's initial secrets have been revealed. Still, further improvement in the characterization of the reservoir's fracture patterns is believed to be possible and should ultimately lead to better flow simulation and reservoir development schemes.
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P- Wave anisotropy - Its practical estimation and importance in processing and interpretation of seismic data
Authors K. Larner, I. Tsvankin and T. A. AlkhalifahDespite the recognition that velocity is the central parameter in processing and interpretation of seismic data, until recently its directional dependence has been ignored in seismic data processing and interpretation. Although the existence of anisotropy in P-wave data can often go unnoticed, where the subsurface is anisotropic, and evidence increasingly suggests that anisotropy is rather pervasive, processing that makes the erroneous assumption of isotropy yields erroneous images and, thus, interpretations.
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Anisotropy processing in vertically inhomogeneous media
More LessExisting work on anisotropic traveltime inversion of reflection data has been done for laterally homogeneous subsurface models. These inversions, although providing useful information on anisotropy in the subsurface, either use the weak-anisotropy approximation or require P-wave data to be supplemented by additional information (e.g., the vertical velocity from check shots or well logs).
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Depth steps for FX migration
Authors R. Bloor and S. DeregowskiExplicit F-X poststack depth migration (Blacquiere et al. 1989; Hale 1991a; Hale 1991b) has become a popular method for 3-D poststack depth migration. This is partly due to its efficiency in 3-D through the McClellan transformation. Additionally in recent years there have been considerable improvements in the accuracy of the method through improved operator design (Soubaras 1992; Gaiser 1994 ; Ruttledge & Pieprzak 1994) and improvements in the design of spatial transformations (Sollid & Arntsen 1993; Gaiser 1994; Notfors 1995). There have also been attempts to increase the efficiency via schemes such as varying the size of the spatial transform (Sollid & Arntsen 1993).
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Accurate imaging and positioning of steep salt flanks by controlled stacking
Authors D. Kessler, L. L. Canales, C. -C. Shih and P. DuncanThe accurate imaging of steep flanks of salt domes has long been a challenging problem for geophysicists. The problem continues to be relevant today as more and more salt domes are being shot with 3D seismic in order to exploit remaining reserves. Unfortunately, the seismic images of steep salt flanks are often not satisfactory, as they are either broken, or appear as smeared events. Interpretation of the salt body in these cases is difficult and somewhat confusing.
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Quasi-Monte Carlo migration of interpolated 3D seismic data
Authors C. Zhou, G. T. Schuster and B. SmithA 3-D seismic data set from West Texas is used to test the Quasi-Monte Carlo prestack migration (QM) and regular prestack migration (RM) methods; both methods use the Kirchhoff integral except the QM method applies this integral to a quasi- Monte Carlo distribution of traces. Because the field data are sparsely sampled on the regular grid points, a seismic trace interpolation technique is used to interpolate the seismic traces to the Quasi-Monte Carlo sampling positions.
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Model-based anti-aliased Kirchoff 3D PSDM
By A. PiceSpatial aliasing is a significant handicap which can significantly degrade the quality of Kirchhoff pre-stack depth migration results. The higher the aperture angle, the greater are its effects. Reducing this angle will prevent migrating steep dips. In this case, even a very simple model describing the dips of the main structural features can be used to limit the contribution of each trace to the migrated section. It results in a significant improvement in the quality of the sections. The drawback of this method is a more limited spatial resolution of the migrated image: diffraction tails will be truncated and unconformities will not be properly handled. As an alternative technique we could apply a time-variant low pass filter to the data, but the determination of its bandwidth usually depends on an implicit 1D model assumption. In this paper we show that the aliasing problem can be better solved by designing the bandwidth of the time variant filter using a 3D dip model. This new anti-aliasing procedure prevents the dipping events from being attenuated. Also the unconformities and the diffracting points are better preserved than with a model based controlled aperture only.
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3D Prestack depth migration of land data
Authors M. Kiehn, C. Haneveld and B. LahmeyerBEB wants to maintain and extend its position as the leading gas-producing company in Germany. The application of the latest technologies in 3D seismic and drilling plays a key-role to achieve BEB's business target. There are still opportunities for new discoveries, but greater depths and increasingly complex reservoir conditions make an even greater technical challenge to find an economically viable hydrocarbon accumulation. The economically important gas-fields are located in Northern Germany. The geology of Northern Germany is characterised by two dominant tectonic features: inversion tectonics and salt diapirism. Many gas- fields are situated below either of these complex structures. Thus, there is a need for sub-salt imaging and imaging below complex structures in order to identify new hydrocarbon accumulations and to allow improved reserve estimates.
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Prestack split-step fourier depth migration
Authors C. Tanis, P. L. Stoffa, M. K. Sen and J. T. FokkemaSplit-step Fourier migration method was first developed and applied to migrate zero-offset seismic-data by Stoffa et al. (1990). This method is an extension of Gazdag's phase shift migration that accommodates lateral velocity variations. It is based on decomposing the laterally varying slowness function into two components; a mean slowness component and a laterally varying component.
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Reliably picking first breaks with neural networks
By D. I. HartThe human effort required to pick first breaks for refraction-statics computations has impeded our efforts to make seismic data processing more efficient. The recent increase in the amount of 3-D data has only exacerbated this productivity bottleneck. Previous efforts to automate first-break picking have relied on signal processing techniques and geophysical models of the refraction process (Coppens, 1985 ; Spagnolini, 1991). Recently, neural networks have been brought to bear on the first-break picking problem (Murat and Rudman, 1992). To reliably pick first breaks with a neural network requires an understanding of how picking errors occur. Picking errors may arise through ambiguities in the input data or in the neural network approximation to the rule that classifies the data.
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Elimination of multiples with neural networks
Authors R. Essenreiter, S. Treitel and M. KarrenbachSeismic signal processing today often still is based on very simple linear models. Nevertheless, traditional methods such as predictive and spiking deconvolution with optimum Wiener filters are implemented very successfully in many cases. On the other hand, the theory rests on assumptions that are often not met in practice. Hence, there exist plenty of cases where traditional predictive deconvolution filters fail. For example, they can encounter great problems with strongly reverberating seismograms from certain complex offshore areas. One major assumption is, that the series of reflection coefficients, i.e. the reflectivity is a sample of a random white noise process. If the seismogram originates from a periodically stratified medium, where the reflectivity series is all but random, predictive deconvolution removes the desired primary arrivals along with the multiples.
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Residual statics estimation by optimizing a complexity-reduced stacking-power function
Authors H. L. Deng, B. Wang and K. PannThe ultimate goal of residual statics estimation in seismic data processing is to obtain highquality stacked seismic sections. One measure of the stacking quality is the stacking-power function. Therefore, statics estimation process is a mathematical optimization problem where we look for the best time-shift among traces that the stacking-power is maximum.
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Piecewise - Constant bases stacking helps to solve complex statics problem
Authors E. K. Korolyov, V. S. Kozirev, V. I. Koroleva and E. M. VoronovichevaThe ordinary technique of accounting for inhomogeneities of overburden in seismic data processing, which can be defined as "primary or field statics + automatic residual statics correction", can not always be successfully applied in regions where the velocity distribution in the overburden can not be described with simple layer models having smooth interfaces and small dips.
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Near surface modelling in Saudi Arabia
Authors A. V. Buck, A. Y. Al-Dulaijan, T. Al Yacoub and S. Al GhamdiProcessing and interpretation of seismic land data in Saudi Arabia has always been hampered by the presence of a near surface weathering layer that varies significantly in both thickness and velocity. When the uphole drilling program was cancelled in 1994, it was imperative that a 3D near surface model for the whole of Saudi Arabia be developed from existing uphole data, in order to compute surface to datum static at any chosen location. In this paper we will briefly discuss our experience with near surface model construction in Saudi Arabia, its limitations and how these limitations were addressed.
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