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55th EAEG Meeting
- Conference date: 07 Jun 1993 - 11 Jun 1993
- Location: Stavanger, Norway
- ISBN: 978-94-6282-135-4
- Published: 08 June 1993
1 - 100 of 500 results
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3D Concentric circle survey - The art of going in circles
Authors G. A. Hird, J. Karwatowski, M. R. Jenkerson and A. EyresExploration for hydrocarbons in the North Sea has been successful for targets below Zechstein salt since the very first few Wells were drilled in the Southern North Sea in the mid -1960's (as described by Brennand & van Hoorn, 1984). Exploration targets above the same salt layers has also been successful. However, exploration in the UK sector of the North Sea has not been as successful for targets which are immediately adjacent to Zechstein salt. It is only within the recent past that seismic acquisition and imaging techniques have advanced to the stage that more wells have been successfully drilled (Rockliffe et al, 1992) . The most critical of these techniques has been the 3D survey. This paper will describe the acquisition of a 3D survey over a salt diapir and it will explain why the concentric circle* method was chosen over the more conventional linear technique. This was the first concentric circle survey to be acquired in Europe.
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3D Concentric circle survey - Processing for steep dip imaging
Authors J. M. Reilly and L. B. ComeauxOptimal imaging of reflection energy in steep dip structural environments requires that the data be acquired and processed in a way to retain, and position (migrate) the seismic reflection information. In certain structural situations, such as salt diapirs, special acquisition geometries can be employed which substantially contribute to the overall goal of enhanced imaging of the subsurface . However, these acquisition geometries can cause unique problems in the subsequent processing of the data. Overcoming these difficulties and producing a high quality product requires retention of steeply dipping energy throughout the processing sequence, specially adapted seismic processing techniques, integration of prior knowledge and a degree of creativity.
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Developments in 3D seismic technology
More LessWe believe that 3D seismic technology is revolutionizing the geophysical industry with far-reaching effect on the exploration and production business worldwide. This belief is corroborated by published statements representing the views of a broad spectrum of the industry ranging from the giants, Shell and Exxon, through companies of intermediate size, to small independents. Although the details vary, the message is always the same.
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Client oriented marine seismic acquisition QC
By M. BrinkIn the last few years great effort has been put into the development of quality control processes, which are applied in connection with marine seismic data collection. This is made possible by both a better understanding of the acquisition proces and the introduction of more powerful and cost-effective computer technology. An increasing amount of quality control data is available to the seismic crew and the client representative in order to made decisions about the further course of the seismic operation. Saga Petroleum has been pioneering novel quality control techniques, which have been presented at previous meetings. However, in order to handle all this information its staff of representatiees onboard the seismic vessel had to be increased .
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Measurements of degradation related to 3D acquisition
By P. MeldahlWe present measurements of degradation in 3-D seismic related to line separation, shot point interval and bin size. The accuracy of a survey planning method will be demonstrated by comparisons of predicted and measured degradation .
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Seismic degradation and mispositioning as a function of random and systematic navigation errors
Authors J. Paffenholz and D. J. MonkA quantitative analysis has been performed to show the effectof navigation errors on marine seismic data . It is found that in some cases large systematic navigation errors can lead to negligible degradation or mispositioning of the seismic data, though this is case dependent. The analysis accounts for the type and magnitude of the mispositioning error, streamer feather, acquisition parameters and subsurface geology. The effects on the seismic data are reported in terms of stack degradation, error in NMO velocity and misposition of the seismic data.
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3D Seismic survey - How big do we need to make the acquisition area
Authors T. L. Armstrong and J. PionIn order to make 3D seismic surveys cost effective, it is essential that our seismic acquisition programme enables us to fully meet our interpretation objectives. Having defined interpretation objectives and the subsurface target to be properly imaged, we estimate the surface area of seismic acquisition which will achieve this.
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Planning 3D seismic surveys using ray tracing on structural models
Authors W. H. Neff and H. K. RidgonThe explorationist must select 3D acquisition parameters that will best image the subsurface target objectives for each potential prospect. The usual starting point in parameter selection is to derive acquisition geometries based on a planer earth model with no dip or with constant dip. It has been found, through ray trace modeling, that acquisition parameters derived this way will not always meet the needed exploration objectives when dealing with a complex geologic structure. In examining numerous cases, complex geological structures scatter the primary reflections in such a manner that what is thought to be imaged and what is "truly" imaged are usually quite different. The explorationist must include, as part of the parameter selection list, the location, orientation, and size of survey that will best image the subsurface objectives in complex geologic settings.
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Seimic monitoring on the Frigg Gasfield (Norway) using AVO attributes and inversion
Authors A. Bossert, J. P. Blanche, P. Capelle, J. Marrauld and E. TorheimOperational objectives: in order to monitor the water-rise and control the reservoir model on the Frigg gas-field (Norway), a 2D seismic survey has been shot in 1991 to be compared with the seismic lines acquired in 1973 before the beginning of the production. This study includes several techniques geophysics, geology, petrophysics and reservoir engineering.
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Troll West - Seismic attributes and optimal locations of horizontal wells in a thin oil leg
Authors K. T. Thon and P. -A. ReksnesThe Troll field, situated in the northern part of the Viking Graben offshore Norway, is primarily a giant gas field. However, underneath one sub-area ,the Troll West gas Province (TWGP), there is a 12m thick oil column, constituting approximately 450 x 10 6 m³ oil in place! In order to maximize recovery from this oil volume, horizontal wells satisfying specific geological requirements are needed.
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The use of 3D seismic data for evaluation of drilling hazards at Heidrun, Haltenbanken
Authors T. J. F. Austin and H. M. HaugvaldstadThe identification of potential hazards for any drilling programme has traditionally ben carried out using 2D High Resolution (or Ultra-High Resolution) Seismic Site Surveys, with lines acquired as a grid over the proposed drilling location. Recently this is changing and it is becoming apparent that many potential drilling hazards, primarily shallow gas, can be identified on 3D exploration seismic data (Gallagher 1991 ; Newman 1990). However, there are some limitations to the use of the low frequency seismic data for the study of the shallow sequence, as it is acquired principally with the aim of resolving deeper reservoir related structures. These will be discussed using examples of both 2D Site survey data and 3D Exploration seismic data which has been collected at the Heidrun Field.
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Prediction of gas-oil and oil-water contacts in Norwegian block 30/8 by means of seismic attribute mapping - A new tool for the interpreters´ arsenal
Authors M. Donelick, M. Bahorich and S. FarmerAmoco recently developed proprietary techniques to quickly extract horizon or interval based response attributes from seismic data residing on interpretative workstations. Following their extraction, maps of the various attributes are generated using vendor software. This new technology enables quick analysis of large volumes of data to ascertain which attributes show significant variation. Forward modelling of well data and physical property data provide a means of "groundtruthing" the attribute variations to lithologic changes, changes in reservoir phase, or to processing artifacts.
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Troll West Gas Province - A reservoir depth conversion study
By M. B. GundemIn this paper a depth conversion study of the top and base of the reservoir in an area in the Troll West Gas Province (TWGP) will be presented. The main objective of the study has been to use different depth conversion methods and vetocity models in order to get an uncertainty estimate of the reservoir rock volume present. The work includes the use of standard 2D- and 3D depth conversion methods as well as more sophisticated techniques. In particular, the GOC-method based on the mapping of the gas-oil contact (GOC) in the area, has been applied as the "base-case" method. The excellent seismic definition of the GOC in the area and the uncertainties associated with the correct time-picking of the GOC has been investigated in detail. The velocity fields used in the depth conversion are generated from different sources of velocities available. This includes well velocities, calibrated stacking velocities (3D) and velocities obtained from both pre-stack and post-stack vetocity modelling on selected 2D-lines.
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How AVO attributes can improve seismic calibration and interpretation on the Heimdal Gasfield (Norway)
Authors Th. Vidaillac, J. P. Blanche, T. Nguyen, J. Marrauld, P. Capelle and P. GrindeInvestigation of the migrated seismic data acquired on the Heimdal Field shows that the seismic correlations between two wells in the gas field are not obvious and the well-to-seismic ties are dubious, mainly on the top of gas hearing sands. In order to solve these misties and picking problems, detailed Amplitude Versus Offset (AVO) processing and analysis are performed on several seismic lines.
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Porosity prediction for a Zechstein-2 carbonate prospect, onshore Netherlands
Authors J. M. M. van de Sande, H. Rebel and N. CassonThe studied Collendoornerveen prospect lies in NE Netherlands. The primary exploration targets are carbonates of Permian (Zechstein) age developed on a platform along the southern margin of the NW European Permian evaporitic basin. Wells drilled on this platform were evaluated as gas-hearing with variable average porosities (4-15%) for the carbonate reservoir. The main objective of this study was to map the porosity distribution over the prospect and adjacent platform areas in order to identify a location for an exploration well and to enable accurate reserves calculations.
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Outline of the Central North Sea overpressured zone based on chalk velocities
By P. JapsenDue to overpressure below the Upper Miocene in the North Sea, seismic velocities below this level are low relative to depth in the central North Sea. The pronounced increase of chalk velocities with depth makes it possible to use deviations in velocities from a normal velocity-depth trend to outline the areal extent of overpressure in the tentral North Sea.
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A Comperative study of post-stack inversion techniques
Authors A. Skalnes, I. Sando, H. Braenshoi and G. MangeroyDuring the past years several post-stack inversion techniques have been published and also made available as commercial software packages. In this study we have made a comparison of different methods with respect to the theoretical formulations and the results on both real and synthetic data.
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A Robust optimisation strategy for parametric inversion of seismic data
Authors R. Bale, P. Farmer, O. Yrke and L. SonnelandStandard seismic processing provides an image of the subsurface which is band-limited. Although the migrated stack is an important tool for structural evaluation, limited resolution creates difficulties in its use for small scale stratigraphic interpretation. Resolution is conventionally defined in terms of Rayleigh's criterion, requiring that two equal amplitude pulses are separated sufficiently to have their centres coincident with the others' first sidelobes of opposing sign. However, it has been noted by many authors (Kallweit and Wood,1982; van Riel and Berkhout, 1985) that the actual discrimination is much better, provided there are known to be a small number of reflectors in the model when compared with the number of data samples.
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Automatic determination of background velocities with lateral variation by multiple migration fitting
Authors C. Jacewitz and G. ChaventAfter structural imaging, inversion for various seismic/rock properties is one of the major desired uses for seismic data. The former has been very successful. Here we are concerned with the latter.
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Complex seismic decomposition - Theoretical aspects
Authors G. M. Mitrofanov, H. B. Helle, V. P. Kovaliev and A. G. MadatovA new spectral-statistical approach to seismic processing and inversion has been developed based on multi-level decomposition of the wave field. Its main application is to obtain details of geological/geophysical characteristics of the medium within a local target zone. Pore pressure and lithological prediction is one of several successful applications of the CSD method. In this paper we will review the theoretical aspects of the method.
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Complex seismic decomposition - Application to pore pressure prediction
Authors H. B. Helle, O. H. Inderhaug, V. P. Kovaliev, A. Madatov and G. M. MitrofanovThe commom approach to pore pressure prediction is made in two main steps: (1)-determine the depth intervals wich contain mainly shale, (2)-evaluate the deviation in the petrophysical properties of the shale from the trend of normal compaction (e.g. Tertzaghi & Peck, 1948). Several alternative methods of velocity estimation from surface seismic data are usually employed at the second step (E.Penibaker, 1968). Some of the methods such as RMS velocity analysis, have low resolution. Other methods such as Pseudo Acoustic lmpedance transformation, do not provide the low frequency components which implies that the overall compaction trend for the shale sequences cannot be determined with sufficient accuracy. Furthermore, velocity based methods do not involve lithological subdivisions (i.e. step 1)
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Recent progress in soft inversion of seismic data for reservoir description
More LessIn recent years there has been considerable interest in the possibility of utilizing seismic data to augment sparse well control information in reservoir characterization problems, while also providing for the robust quantification of uncertainty. This problem poses significant challenges: notably that seismic data has limited bandwidth and resolution, and contains only indirect information about reservoir properties (such as acoustic impedance,porosity,thickness...). The limited information content of the data has fueled interest in Stochastic Imaging approaches for seismic inversion, particularly in view of their potential for (non-parametric) risk assessment.
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Linearized elastic inversion of synthetic data from a gas-bearing sand model
Authors V. Richard, A. Bourgeois, D. Mace and G. EtienneLinearization is a goud approximation if the reference medium is sufficiently close to the actual one. The method we propose deals with a 1D reference medium and has been presented in Macé et al (1992). It is adapted to determine a 2D elastic target zone embedded in an almost laterally invariant elastic background. The method is based on the computation of the Jacobian matrix and uses a conjugate gradient algorithm to solve the optimization problem. In this paper, the linearized inversion method is valuated on synthetic prestack marine data from a gas-bearing sand model.
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Seismic waveform inversion of gas hydrate bottom simulating reflectors
Authors T. A. Minshull and S. C. SinghSeismic reflection profiles across many continent al margins have imaged "bottom simulating reflectors" (BSRs), which are interpreted as marking the base of a methane hydrate stability field. BSRs often have very high amplitude and can be continuous for many kilometres, so they provide an ideal target for a waveform inversion scheme. Knowledge of the detailed velocity structure in the vicinity of the BSR allows us to determine whether it is underlain by a free gas zone, and estimate the thickness of such a zone where present.
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Model driven picking in multilayer refraction statics
More LessIn 1990, Zanzi presented an iterative technique for the computation of refraction statics. The method performs a least squares estimation of the near-surface model in the wavenumber-offset domain according to a linear decomposition of headwave traveltimes. Details on ill conditioning problems and noise sensitivity of the linear model are available in Zanzi and Carlini (1991).
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Determination of shallow-refractor properties by 3D-CMP refraction seismic techniques
By T. RuhlIn 3D land seismic surveys refracted waves from shallow refractors (e.g. base of weathering layer, crystalline basement) are often recorded as first breaks. Automatic picking programs can easily determine first break traveltimes which are used for the computation of static corrections. In 2D reflection seismic surveys refracted arrivals can be evaluated by standard methods based on traveltime curves of shots and reverse shots. In the 3D configuration with an irregular distribution of shots and geophones this approach is impossible or very inconvenient. Tomographic techniques must be used for the inversion of refracted waves in order to obtain the near surface velocity structure.
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The generalized reciprocal method (GRM) - A new approach without first break picking
By O. SelviIs it possible to apply the generalized reciprocal method (GRM) for interpreting seismic refraction data without first break picking?
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A Hybrid genetic algorithm/steepest ascent approach has success with difficult statics
More LessSteepest ascent gradient inversion methods are fast but suffer from finding local maxima and not the desired global maxima. Problems which have complex objective functions with many local minima require the use of a global search mechanism. However, global search mechanisms such as genetic algorithms or simulated annealing suffer from excessive computation cost when they start with a collection of random models that have a large number of parameters. We combine a steepest ascent inversion approach with genetic algorithms into a hybrid method that avoids the pitfalls of these methods. The steepest ascent iterations provides a high graded population to the genetic algorithm and speeds the inversion. The genetic algorithm keeps the steepest ascent methods from getting stuck in a local minima.
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Adaptive dip filtering of seismic data
By S. SpitzCorrect removal of events with dips contained in an assigned interval is an old problem in seismic data processing. Because the simple f-k domain fan filter shows poor results when events in the data set are spatially aliased, the filters used nowadays in the industry are modelbased, as their design requires a range of dips that, hopefully, contain the dips actually present in the data.
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Monoplane filter and applications
More LessSeismic data routinely exhibits conflicting dips where as an ideal seismic image has only one dip at each location. Here I introduce an objective measure of the presence of more than a single dip, This measure is the output of a local filter with enough degrees of freedom that it is able to extinguish a single plane wave but not enough degrees of freedom to extinguish more than one plano. This filter is called a LOcal MOnoPLane ANnihilator (LOMOPLAN). Figure 1 shows a synthetic model and the outputs of an array of LOMOPLANs applied to it. Outputs are patched together to smooth over the seams defining the many local regions.
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Trace interpolation with aliased data
By M. K. ChaseThis paper describes a set of techniques for the spatial interpolation of aliased seismic data. The approach is designed to produce accurate interpolations when more than one dip is present simultaneously, even when weak events are hidden behind strong ones, for example when diffraction energy crosses a set of primaries. The method is simple and robust enough for routine use in data processing.
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A Review of trace interpolation for cost-effective 3D
Authors S. Lancaster and P. J. Whitfield3D data acquisition is now established as a key element in risk reduction ahead of the drill -bit. However, with the present trend towards use of 3D as early as the exploration phase, there is a need to reduce costs and improve turnaround. One parameter that affects both these requirements is the cross-line spacing. If we can acquire with a wider cross-line spacing, then fewer boat traverses are required, achieving a proportionally cheaper and faster survey. The downside is that the cross-line wave field is undersampled, and so trace interpolation is required to counteract spatul aliasing. We need therefore to examine the costlquality trade-off between grid cell dimensions and seismic resolution and interpretability. Our study reviews the technical case for trace interpolation, examines the relative merits of different types of interpolator as currently available from the seismic contractors, and benchmarks algorithms on real data.
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Measurement of phase and group velocities at low frequencies
Authors N. Thirion, J. L. Mari, J. Mars and F. GlangeaudThe dispersive properties of surface waves can be used in a case of exploration technology for weathering calculations. In full waveform acoustic Jogging, the dispersive waves are the Pseudo- Rayleigh waves in fast formations only and the Stoneley modes. The phase velocity of these dispersive waves can be used to evaluate the shear velocity of a formation. The use of dipole tools (Zemanek et al 1991) enables a measurement of the shear velocity in slow formations by studying the dispersion curves of flexural modes (figure 1). The object of this paper is to compare three methods which can be used to evaluate phase and group velocities of dispersive waves. The behavior and the accuracy of the proposed methods are checked on synthetic data. Synthetic data are an acoustic common shot point gather (figure 2).
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Results on low frequency acoustic prediction ahead of the bit by geophone dynamic accelerometers
Authors C. F. Kaspareck, F. Ogliani, F. Abramo and F. MirandaBorehole seismic plays an important role in exploration and development. Standard applications, besides measuring the acoustic velocity fields between the source and the well geophone, aim to reconstruct a subsurface acoustic image around and below the borehole (vertical case, VSP), and away from the borehole (offset and walkaway case, OSP and WSP). The fidelity of this image is related to the correct retrieval and definition of the reflectivity sequence.
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τ-p Domain processing of walkaway VSP data
More LessTraditionally 3-component Walkaway VSP data U(x,t)z (acquired at different geophone levels (z) and with a line of source positions (x) crossing near the rig) are processed by first ordering the traces into successive Fixed Source gathers (U(x,t)z -> U(z,t)x). The gathers are next processed individually in order to enhance the primary reflected P wavefield followed by a recordering to the (x,t)z domain before migration. The main drawback in the processing scheme is that neither periodicity in the up- and downgoing multiples nor the possibility of efficient removal of unwanted shear events are present in the (z,t)x domain. In consequente the usefulness of Walkaway VSP data is regarded with some scepticism.
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Application of the intercept time method to full waveform acoustic data
Authors F. Coppens and J. L. MariThe intercept time method was introduced in the early sixties for the computation of static corrections, when multiple coverage became a usual recording technique. This method gives the velocity of the refractor and the delay times at the shot and receiver positions. These delay times are related to the thickness and velocity of the altered zone. This paper presents a stand alone algorithm for the computation of the formation acoustic slowness and of the delay times from full waveform acoustic data recorded with a multi-receiver tool.
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Optimal array focusing deconvolution for VSP
Authors J. Haldorsen, J. Walsh, D. Miller and H. -J. ZochWe describe a technique for performing deconvolution of VSP data. The method is a two-step process that involves (1) estimating the source signature, and (2) applying a least squares optimum deconvolution operator that minimizes the noise not coherent with the source signature estimate. The optimization is done in the frequency domain, which introduces the frequency domain semblance as an extra factor in the deconvolution operator. Comparison to conventional processing of the data shows that the introduction of the semblance weights can give significantly improved results.
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Processing crosswell seismic data for reflection imaging
Authors W. Cai and G. T. SchusterCrosswell imaging with reflections has been proposed and successfully applied to field data (Lazaratos et al., 1992). Crosswell reflections can provide high resolution images of horizons at and below the well bottom, which is a key advantage of reflection imaging relative to transmission tomography. Crosswell re- flection imaging techniques include stacking, migration, wave equation traveltime plus waveforrn inversion (WTW, Luo and Schuster, 1990) etc., where each method may need different processing procedures.
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Characteristics of crosswell seismic data for reflection imaging
Authors B. A. Smith, J. G. Gallagher, G. M. Hoover and J. M. HuffordPhillips Petroleum Company acquired three tomographic crosswell datasets in early 1992 at the British Petroleum Devine test site, Frio County, TX, using Stanford University's piezoelectric Bender source and hydrophone receivers. These three datasets, recorded with source well to receiver well offsets of 100m, 200m, and 300m, show that the wavefield changes as a function of offset, even though there is little lateral change in the lithology. Onderstanding the nature of the wave propagation at the different offsets is important in determining how to process the data for traveltime tomography and reflection imaging. We find that coherent source related noise (tube waves propagating in both the source and receiver wells is a dominant waveform at all three offsets, especially at later times in the records. At farther offsets, most of the recorded wave field energy appears both as tube waves in the source and receiver wells, and as post-critical reflections from the major lithological interfaces.
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Crosswell reflection migration by a constrained Kirchhoff integral method
Authors F. Qin and G. T. SchusterMigrated crosswell reflections can produce high resolution images of the earth structure. To achieve good imaging several problems must be overcome. First, crosswell reflections can be obscured by a mixture of interfering wave types, such as direct waves, S-waves, converted waves, multiples and tube waves from both wells. Secondly, any crosswell reflection survey has only limited aperture. Thirdly the ray coverage is very uneven in terms of both the number of visiting rays and the ray incident angles. Rays with large incident angles will stretch the source wavelet, and cause loss of resolution in the migrated section.
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Interpreting non-orthogonal split shear waves in multicomponent VSPs
Authors X. -Y. Li, S. Crampin and C. MacBethMany techniques for processing shear-wave splitting in VSP data assume orthogonality of the split shear-waves, and are consequently restricted to zero or near-offset VSPs and symmetry planes (Wintersteen and Meadows 1991; Zeng and MacBeth 1993; among others). Here, we derive equations for multicomponent seismic data (two or three sources recorded by 3C receivers) for non-orthogonal split shear-waves. The equations can be solved by the linear-transform technique (LTT) of Li and Crampin (1993) to determine the shear-wave polarizations in both zero- and offset VSPs . This method is particularly useful where orthogonal techniques often fail when deviation from orthogonality exceeds 5°, or the incidence angle exceeds 1.5°.
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Analysis of split-shear waves from near-offset VSP data using a neural network
Authors D. Hengchang and C. MacbethNeural networks provide a way of recognizing particular patterns and extracting their characteristic parameters. Here they are applied to the different patterns of shear-wave particle motion recorded by orthogonal horizontal geophones. It is postulated that the major component of this particle motion is from shear-wave splitting, and that it can be parameterized as the polarization direction of the leading shear-wave and the time-delay between split shear-waves. The network is trained initially to recognize patterns of shear-wave splitting with known parameters from synthetic seismograms. Once training has been accomplished, the network can now be used to recognize patterns which have unknown characteristics, but which still lie within or not far outside the experience of the network. It is hoped to use the network to rapidly quantity any patterns of shear-wave splitting and to analyse large sets of seismic data.
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A Comparison of two methods for measuring shear wave birefringence from VSP data - Parametric inversion and layer stripping
By J. J. WalshShear wave birefringence has been the subject of considerable study in recent years. A principal application is the determination of fracture orientation from shear wave splitting in multicomponent VSPs. This is accomplished by studying the polarization direction of the lesding shear wave -and the travel time delay between the two split shear waves, which may give a direct indication of the orientation of the anisotropy. Here, we examine two methods for detection and description of fractured zones from shear wave splitting: layer stripping and parametric inversion.
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Processing of single-source split shear waves for imaging
Authors D. R. Watts and M. BoulfoulShear wave splitting may be regarded as a nuisance in the production of a shear wave image. Splitting or diffraction effects in the near surface or the weathering layer will degrade stacks especially if there is a rapid variation in the change of polarization along the survey line. We present a technique that may utilize a single source of indeterminate polarization for shear wave imaging even for the case of splitting in the weathering layer.
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True-amplitude processing of P- and PS- (converted) - Wave data for AVO-analysis
Authors P. Krajewski, C. Krajewski and S. GrummtThe analysis of seismic reflection amplitudes for reservoir characterisation or, more general, for the lithological interpretation of reflection data has become a standard approach in exploration geophysics. However the nonuniqueness of the interpretation of amplitude anomalies in P-wave sections has shown the need for shear-wave velocity information to permit a reliable lithology discrimination. In the paper Processing of PS-reflection data applying a common conversion-point stacking technique by Tessmer et al. 1990 a field data set has been presented which has been generated using three conventional P-wave vibrators as source and in-line horizontal geophones as receivers. This technique of shear wave data acquisition is relatively simple and inexpensive compared to techniques using shear-wave vibrators, because it can be done simultaneously with the routine P-wave recording.
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AVO Versus shearwaves by mode conversion
Authors F. Nieuwland and R. MarschallThis paper describes the results of a field experiment which was conducted in the central part of Friesland - The Netherlands.
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Three component seismic experiment in Saudi Arabia
Authors D. J. Fyfe, B. E. Dent, P. G. Kelamis, K. H. Al-Mashouq and D. A. NietupskiIn 1991, Saudi Aramco conducted an experimental three-component seismic program, over a known oil field, using conventional P-wave vibrators and three component geophones. One of the main objectives of the experiment was to use the converted (P-S) energy to improve the structural and stratigraphic picture of the reservoir sands. We recorded usable P-S energy as expected, but the surprisingly strong, coherent, shear energy generated at or near the vibrators (S-S) gave us a clearer image of the reservoir interval than either the vertical component (P-P) stack or the P-S stack.
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Recording and processing PSSP waves in the Barents Sea
Authors E. de Bazelaire, I. Gausland, O. E. Naess, H. Morizot and J. RavatSeismic profiles shot in Barents Sea cross a number of potential reef features, located between two reliable reflections at the top of Paleozoic and within the Paleozoic, 200 ms deeper. Conventional seismic work using P-waves is unable to separate porous from dry reefs, and all the more to assess the fluid contents.
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Rock properties - The missing link in exploitation and exploration
Authors C. H. Sondergeld and C. S. RaiRelationships between different physical properties of rock permit one to interpret log, seismic, magnetic and gravity data. One relies upon these correlations to infer subsurface properties. To map lithology, porosity, or permeability, for example, one must employ a technique which has sensitivity to the property of interest. These techniques generally produce a one, two, or three dimensional image of some attribute say vetocity or amplitude. Beyond a knowledge of the, physics behind a technique, one must establish the correlation between the attribute mapped and the property desired. In many circumstances these properties can not be linked to the attribute in an ab inito sense. Thus healthy empiricism is required to form the bridge. A case in point, the classical theories of Biot (1956a,b) and Gassmann (1951) require the definition of a porous frame modulus, this for obvious reasons can not be derived from first principles without a priori knowledge. However, once the modulus is defined, the theory permits one to conveniently calculate the influence of various saturants on velocity. Such information is extremely useful in modeling an AVO response, for example. The problems hampering such empiricism to date have been twofold: first, the cost on a per sample basis has been prohibitive and second, the core to be characterized has suffered irreversible degradation. GEM (Geophysical Evaluation Modules) has eliminated both these restrictions. Because GEM is portable, it can be brought to a well site or Gore facility and fresh samples can be characterized. Secondly, because of the high degree of automation and parallel processing concepts, the cost for a rather extensive suite of measurements has been reduced by an order of magnitude.
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The role of core measurements in seismic lithology prediction
By J. BartonCore measurements play an important role in lithology prediction from seismic data by: - Providing an accurate set of geophysical and petrophysical measurements about reservoir and caprock lithologies. - Allowing correlations between petrophysical and elastic parameters to be established. - Enabling key elastic parameters such as shear velocity, shear and bulk moduli to be determined for geophysical modelling. This paper describes how core measurements have helped evaluate and predict reservoir and caprock lithologies in the Paleocene section of Amoco operated fields in the UK Central Graben.
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Ultrasonic monitoring of hydraulic fracturing experiments
Authors M. Savic, M. J. Cockram and A. M. ZiolkowskiThis paper describes a state-of-the-art data acquisition system built at Delft University of Technology which allows rapid acquisition of reflection, pulse-echo and transmission records during hydraulic fracturing experiments on cement or rock samples. Recent experiments show that it is possible to observe clean acoustic signals from a hydraulic fracture that is very thin compared to the wavelength of the illuminating elastic waves. Imaging with reflected compressional signals produces a clean image of the fracture. The signals diffracted from the fracture tip carry important information about the position of the fracture.
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Water saturation - A critical review of computational procedures with a suggested approach
By M. H. KamelOne of the major petrophysical parameters that is of primary concern in the formation evaluation proces, particularly in shaly pay sand reservoirs, is the water saturation (SW). The purpose of this paper is first; to explain some of the concerned in-situ problems a log analysit comes across while choosing the most suitable equation to determine such parameter. Secondly; to propose a new approach to estimate the water saturation. Thirdly; to compare our new results with those obtained by other investigators. Finally, a field example taken from the northern part of the Gulf of Suez of Egypt will be used to illustrate the new approach .
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A global algorithm for pore pressure prediction
Authors D. Scott and L. ThomsenThe issue of subsurface pore-pressure prediction has a long history, and a long future. Despite a record of sonic success in prediction (in restricted contexts) via the "classical estimation algorithms", we regard the problem to be unsolved to date.
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A Recipe for predicting the effective elastic properties of shales
Authors B. E. Hornby, L. M. Schwartz and J. A. HudsonSuccessful imaging of subsurface features in the earth depends on a knowledge of the behavior of the wave propagation through overlying strata. Shales make up about 75 percent of most sedimentary basins and most often overlie hydrocarbon hearing reservoirs. The elastic properties of shales are anisotropic (e.g. Jones and Wang, 1981 ; White et al., 1983; Bareik, 1984; Miller and Chapman, 1991). This anisotropy of shales must be taken into account, both for structural imaging of subsurface features and for more advanced inversions such as amplitude variation with offset (AVO) analysis of hydrocarbon-hearing reservoirs sealed by shale cap rocks.
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Slow wave in oil-saturated water
Authors G. M. Goloshubin, P. V. Krauklis, L. A. Molotkov and H. B. HelleBiot's theory (1956) predicts that wave propagation in a saturated porous medium involves three types of waves: two compressional waves and one shear wave. The two compressional waves are known as the fast P wave and the slow P wave. The slow P wave is the consequence of the fluid-solid coupling. Plona (1980) observed a compressional slow wave in a porous medium at ultrasonic frequencies. According to Biot's theory this wave attenuates very quickly and the attenuation is higher for low frequencies than for high frequencies. Thus, Biot's theory predicts that the low frequency slow waves cannot be observed at some distance from its origin. We discus the possibility of slow wave observation in an oilsaturated layer at seismic frequencies using experimental data and the theoretical solution.
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Effect of local inclusions on the attenuation of seismic waves in fluid-saturated rocks
Authors B. Gurevich, A. P. Sadovnichaja, S. A. Shapiro and S. L. LopatnikovInvestigations of a complex reservoir structure by VSP and cross-borehole soundings require the construction of a theory that can make predictions of seismic parameters for complex reservoir rock. In this paper we construct such a theory for a reservoir rock which can be considered as a fluid-saturated porous continuum containing randomly distributed inclusions. The host medium is assumed to behave in accordance with the low-frequency version of the Biot theory, and the effect of inhomogeneities is studied using the scattering theory.
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Unification of traveltime tomography methods
More LessTraveltime tomography methods can be divided info two general classes: 1) methods which back-project travel time residuals along raypaths, and 2) methods which back-project traveltime residuals along wavepaths. In this paper 1 present a general formula for the back-projection of travel time residuals and, for special cases, show that it reduces to the back-projection term in wave-equation traveltime inversion (or WT in Luo and Schaster, 1991), wavepath eikonal traveltime inversion (WET in Schaster, 1991), ray tracing tomography (RT), and the Woodward-Rocca (WR in Woodward and Rocca. 1988) method. This unification provides for an understanding of the differences and similarities among these traveltime tomography methods.
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The Rytov approximation aids the resolution of the broad velocity components in non-linear waveform inversion
By C. StorkThe bare of non-linear waveform inversion based on the Bom scattering model has been the need of a close starting model -- particularly for the broad components of the velocity field. Without a close starting model the method is susceptible to failure by getting stuck in a local minima (Tarantola, 1990 & Gauthier et al, 1986).
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2.5D Acoustic full-wave frequency-domain inversion
Authors Z. M. Song, P. R. Williamson and M. H. WorthingtonSeveral approaches to frequency domain inversion and imaging have been developed over the last decade (e.g. Devaney, 1984; Pratt, 1989). These methods have typically been developed for 2D geometries and are accordingly limited by the implicit assumption of line sources. Song and Williamson(1992) presented a method of extending the work of Pratt(1989) to 2.5D, in which sources can be represented in 3D while medium properties vary in 2D. This in principle allows direct comparison of calculated and real data gathered in effectively 2D media.
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Hybrid traveltime and waveform inversion of real crosswell seismic data
Authors C. Zhou, W. Cai, Y. Lou, G. T. Schuster and S. HassanzadehLuo and Schuster (1990) presented a hybrid wave equation traveltime+ waveform inversion (WTW) method to reconstruct 2-D velocity structure from crosswell seismic data. The key idea is to reconstruct the smooth component of the velocity field by first inverting first arrival traveltimes (Luo and Schuster, 1991), and then reconstivct the short wavelength components by waveform inversion. Using synthetic tests, they showed that WTW inversion was as robust as traveltime tomography and provided the high resolution of waveform inversion. In this paper, we apply the WTW method to a synthetic fault model, and also present the first application of WTW inversion to real crosswell seismic data.
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Ray perturbation theory in inverse problems
By B. J. MooreA major problem encountered in trying to form tomographic reconstruction of heterogeneous media is that the ray paths required in the integrations depend on the heterogeneities themselves and thus are unknowns of the problem. However Moore (1993) has shown how it is possible using perturbation theory to reduce the first-order solution for the two-point boundary value problem to the solution of a single initial-value problem. Basically, one finds the change in the initial direction of the unperturbed ray which is required to ensure that the perturbed ray passes through the specified endpoint to first-order in small quantities. This correction to the initial direction is then propagated along both perturbed and unperturbed rays using paraxial ray tracing techniques. This means that the total Green's function for two-point boundary value problems can be related to the propagator for initial value problems. Furthermore, it means that in problems where one needs to determine small perturbations to be superimposed on a known "average" one dimensional velocity model, one can use the ray paths joining source and receiver in the unperturbed medium as the path of integration in the reconstruction problem.
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Reflection tomography in complex structures
Authors G. Bohm, P. Carrion, A. Marchetti, F. Pettanati and A. VesnaverThe traveltime functional which relates the measured traveltime t to unknown parameters can be expressed as follows: t = Ψ (vo, α, ß, z) (1) where vo is the background velocity α and ß are the horizontal and vertical gradients of velocity and z is the depth to a reflection point. One can observe that (1) is one equation with three unknowns: α, ß and z. This causes non-uniqueness: in fact, there can be an infinite number of combinations with unknown parameters which fit the same data. In order to curb the non-uniqueness, vertical variations of velocity are substituted by their averages: it is possible to prove (a rigorous prove is put off for another occasion) that only α and z can be uniquely recovered from surface seismic data provided that an adequate angular recording coverage α of the model space is available.
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Linearized elastic inversion in the w-k domain
Authors F. Rocca and A. de NicolaoAn algorithm for the elastic inversion in the w-k domain is considered. Diffraction tomographic techniques are used in the elastic case; 3D P-waves sources and vertical receivers are located on the surface of a medium characterized by small deviations of the elastic parameters from an uniform background. Born approximation is invoked and a singular value decomposition of the parameters - data transfer function is derived. Least square estimation along the eigenvectors is used to perform multiparameter inversion on synthetic data obtained in the space time domain. Numerical inaccuracies and interpolation errors can severely degrade the inversion of S impedance and density. P impedance, as usual, turns out to be well conditioned.
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Filtration - Capacitive properties of the collectors in cross-borehole space
Authors G. E. Rudenko, O. A. Potapov and O. A. LopatnikovaFiltration-capacitive proporties (FCP) of the collectors may be described in the terras of porosity, clay content and effective depth. Proposed is the software and methodical complex, which lets to study FCP distribution in the cross-borehole space. Applied for this task PARM program complex lets to predict the elastic impedances in the cross-borehole space by log and seismics complex.
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Improvement of seismic 2D data - A case study from block 24/12
Authors N. Sorenes, A. Haugen and B. FotlandThis paper presents some experimental results from a test line accquired by the Norwegian Petroleum Directorate in block 24/12 in the Norwegian part of the North Sea. This block is caracterized by, in general, poor seismic data quality. Reasons for the seismic quality problem is thought to be soft tertiary clays, some shallow gas-accumulations and the depth of some of the targets. Main targets have been jurassic and tertiary sands. In the Northern Barents Sea, the Norwegian Petroleum Directorate has for several years experienced that summing two parallell horizontally separated streamers give a noticeable signal to noice enhancement. This has also been experienced by HGS and Saga Petroleum as documented by Brink et. al. (1991). It was decided to shoot some profiles with this accquisition configuration in order to investigate if it could be helpful in this block, especially for the jurassic targets. As these targets are quite deep, it was decided to use two long streamers (4500 m). This enabled us to test both the effect of two streamers and the effect of different streamerlengths. The results of these tests will be presented in this paper.
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Kanumas project
By L. BekgaardThe KANUMAS project is a regional seismic programme in the icefilled arctic waters offshore North-West, East and North-East Greenland. The area offshore North-East Greenland is one of the most difficult areas for seismic operations due to constantly drifting polar pack ice.
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Quality assurance of spatial sampling for DMO
More LessThe effects of irregular spatial sampling on prestack wave-equation processes such as DMO and migration have been noted by several authors for both 2-D and 3-D data. Phase and amplitude distortions arise as a result of applying DMO algorithms that assume regular spatial sampling to data not regularly sampled in offset, common midpoint (CMP), and azimuth. While algorithms now exist to compensate or equalize the DMO operator during data processing for such spatial irregularities, it is possible that random or systematic patterns in the data acquisition geometry leave deficiencies in the "DMO coverage" that cannot easily be remedied (Beasley and Klotz, 1992) . Traditional criteria for spatial sampling such as CMP fold and offset distribution are often set by rule of thumb and generally do not measure the quality of spatial sampling for DMO processing. In this paper, I demonstrate new methods of assessing the quality of data sampling for DMO that are based only on acquisition geometry and are independent of a geologic model or the actual seismic data.
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In-Field data quality control
Authors J. A. C. Meekes, J. J. G. Beckers and S. P. WijnIs doing in field data quality control (DQC) looking at the spectrum of a trace or of a mean spectrum of one shot? Or is it looking at brute stacks hampered by unknown velocities and statics? No it is not. Then what should DQC be? To answer this question we have to consider current trends in data processing and interpretation. Two aspects are important: - the increasing size of the data - the growing use of prestack real amplitude processing of seismic reflection data.
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Impact of offshore seismic on marine life
By I. GauslandPossible conflict between seismic surveys and commercial fishing has been a concern for the oil industry for many years. During the last 3 years, a significant research program on these problems have been undertaken in Norway, and the final results will be published during the spring of 1993. This paper will address the results of these research projects.
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Integrated 3D marine data acquisition and real-time quality control
Authors P. Summerfield and D. BurtonThe current trends in marine 3D data acquisition towards improved productivity, reduced cycle-time and enhanced quality control are driving seismic service companies to pursue more sophisticated techniques than have previously been used. The three goals identified above have principally been achieved by deploying multiple source and streamer configurations. In addition, enhanced positioning techniques, such as underwater acoustic systems, have been introduced together with stricter quality control over a significantly increased volume of position measurements.
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Verification of GPS carrier phase technology in tailbuoy positioning
Authors O. Orpen and J. K. HovdeDuring recent years advances in software development has made it feasible to achieve decimeter accuracy in real-time in differential GPS (DGPS) systems on short ranges offshore under dynamic conditions. The technique applied is valled carrier phase ambiguity resolution on-the-fly (OTF). In May 1992 a test was performed on the seismic vessel New Venture of GEOTEAM Exploration Ltd A/S on a survey for Saga Petroleum a.s.
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Integrated interpretation of North Sea carbonates using automated seismic modelling
Authors D. B. Neff and K. L. WestThe quality, efficiency, and level of integration for seismic interpretations of several North Sea carbonaten has been significantly improved by using an automated forwardmodeling process. The models are generated from petrophysical logs such as porosity, water saturation, and shale volume and from reservoir fluid and rock matrix density and velocity data. Volume fraction equations are then used to automatically generate hundreds of one-dimensional synthetic seismograms for a range of estimated reservoir gross pay thickness and porosity. Model waveform data and related crossplot analyses of each model's petrophysical variables are then used to address such problems as seismic amplitude versus porosity and thickness trends, pore fluid content, seismic amplitude changes expected with increased reservoir pay, seismic variables that best describe lithology changes, coat effective lithology inversion methode; errors in estimating pay thickness, and expected waveform expression of similar exploration plays.
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Inverse modelling vs. attribute analysis - 3D seismic reservoir description of Rotliegend sandstones
Authors M. Antonini and C. SchweitzerThe resolution power of stratigraphic seismic inversion has been compared to a reservoir description based on 3D seismic attributes in a real case study of Rotliegend sandstones in the NW German gas province. The challenge of seismic interpretation in the study area consists of inter-well seismic reservoir prediction of thin sandstone reservoirs (thicknesses below 25 m) at depths between 4.5 and 5 km. Facies and diagenesis are the main limiting factors of prospects. A high degree of lateral heterogeneity has to be expected for several stacked reservoir zones. To unravel this complexity, 3D seismic has been used to a large extent (Burri et al. 1992). Some 3D wells are available to calibrate the seismic signature of different reservoir quality units.
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Seismic reflections in crystalline rocks
Authors H. Wiederhold, G. Hirschmann, P. Sadowiak, C. Reichert and H. J. DurbaumIn 1989 comprehensive seismic studies were carried out at the German deep drilling location (KTB) in the Oberpfalz (NE-Bavaria) by the DEKORP group. The survey is known as Integrated Seismics Oberpfalz 1989 (IS089). The aims were to predict events ahead of the drill bit, to investigate the structures and to connect the more or les small-scale results from the KTB-drillhole with the large-scale geological/tectonical environment, to investigate parameters as seismic velocities and anisotropy and to learn about the nature of seismic-reflections in crystalline area.
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Structure identification in a deformed belt - A wrench system in Pakistan
Amoco's exploration team for the Kohat Plateau in the Northern Pakistan Foreland Region acquired and integrated a comprehensive data package which identified structures formed by oblique compression (transpression). Our data shows that basement is involved with the development of structures in this area and that each structure has very high vertical relief with thrust faults and reverse faults radiating outward. Our interpretation suggests that transpressional deformation resulted in forming en echelon tears which caused basement and the sediment cover to buckle to foren shishtarays (Pivnik & Sercombe, 1992), or structures which fit the 'Kohat Anticline Model' (Sercombe & etal, in prep.).
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Seismic modelling for gasfield development in offshore Niger delta - A case history
By A. O. IdowuIn practice, seismic data has been used to map the geometry of events in the subsurface, mainly from reflection continuity and the character of reflection packages. Seismic modelling techniques are attempts to mathematically and geometrically represent the geology of the subsurface and to depict the interaction of that geology with a propagating seismic wavefront. In the Niger Delta, recent development in stratigraphic exploration has induced the examination of more substle features of reflection mainly polarity, amplitude and waveform to define the limits of seismic resolution, and hence predict the geometry of the subsurface fluid and solid interfaces.
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Recent advances in the geophysical study of the Ross Sea (Antarctica)
Authors M. Pipan, A. del Ben, I. Finetti, F. Ping, R. Geletti and C. SauliA large multichannel seismic reflection dataset, recorded along the pacific sector of the periantarctic shelf by the OGS-Trieste during the first geophysical surveys (1987-89) of the Italian Antarctic Reserch Program, has been processed and interpreted with specific techniques in order to draw a comprehensive scheme of the structure, the seismostratigraphy and the evolution of the Ross Sea. The exploration of this basin is of considerable scientific interest for reconstructing the main geodynamic phases which have produced the present crustal conditions of the Antarctic continent with particular regard to the following topics: - The uplift of the Transantarctic Mountains - The relationships between East and West Antarctica - The relationships between the rift processes along the pacific margin of Antarctica and the Gondwana breakup The present study illustrates methods and results of the processing and the interpretation of the antarctic seismic data with reference to the following research themes: - Reconstruction of the structural and seismostratigraphic characteristics of the sedimentary sequence - Outline of the deep crustal conditions in areas with favourable signal to norse ratio - Reconstruction of 3D structural models of the interpreted horizons - Preliminary time and space analysis of the deformation
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Interactions between 3D seismic interpretation and sequence stratigraphy analysis on logs
Authors B. A. Reymond and G. M. StampfliA 3 Dimensional Spec. survey is analyzed using a CHARISMA (Geco-Prakla Schlumberger) seismic interpretation system. The study concerns sediments dating from the early Miocene to present day deposited on the detritic shelf of the northern Gulf of Mexico, 25 Km offshore Louisiana (West-Cameron region). The local tectonic style is an interaction of elongated growth faults parallel to the coast line towards the West (Texas style), with smaller arcuate growth faults directly related to more abundant salt domes more to the East (Louisiana style) (Worral and Snelson, 1989).
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Resource management and enhancement including a new strike-slip structural model - Poui Field, Trinidad
Authors J. E. Wolfe and F. T. SobolPoui Field, Trinidad is an example of continued resource development and reserve growth through multi-discipline team efforts and technology application. The Field is structurally complex and extensively drilled requiring careful and detailed planning of development wells. The multi-discipline team perspective includes reservoir and operational engineering, geology, geophysics and computer mapping experts. Methods include delineation of complex structural fault blocks, field studies, reservoir management planning and 3-D seismic enhancements. Recoverable reserve estimates for Poui Field have grown from 68 million barrels of oil in 1973 to 226 million barrels of oil, presently.
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Kinematics of seismic waves in a large stack of layers - Whole frequency domain
Authors S. A. Shapiro and H. ZienOur study is focused on the influence of multiple scattering in a layered medium on the kinematics of a transmitted harmonic wavefield. We consider theoretically and numerically the angle dependent transmission of a pressure plane wave. Our description is valid in whole frequency domain. The limitations on the medium are: the medium is acoustic; dependenties of the velocity and density on the depth are random stationary processes; their fluctuations are relatively small compared to their constant mean values (of the order of 30% or smaller).
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Study of traveltime using 3D ray tracing through triangulated surfaces
By P. ThisseThe modelling of traveltimes picked on seismic sections can be performed using a ray tracing method. In the case of a complex medium, traditional ray tracing algorithms (e.g., a shooting method) do not work well. The Sistre project developed on top of the GOCAD's modeller (Guiziou et al., 1992) is able to handle complex 3D geological structures. It is based on the discretization of the surfaces via triangles Forward modelling in Sistre is based on a three-step ray tracing algorithm which takes full advantage of the decomposition of interfaces via triangles (Guiziou et al., 1991). The first step constrains the ray to pass at triangle vertices only, while the next two steps locate the impact points inside the triangles.
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Estimation of multivalued arrivals in 3D models using wavefront construction
Authors V. Vinje, E. Iversen, H. Gjoystdal and K. AstebolWe have developed a ray tracing based wavefront (WF) construction method that finds arrivals in general 3D models consisting of blocks with smooth velocity and density variation separated by interfaces where the velocity and density change discontinuously. The method is based on the ideas in Vinje et. al. (1992) and it mimics the true wave propagation in the lense that entire WFs are propagated time step by time step creating a moving surface travelling through the model. The WF may stretch, twist and fold in any way as it moves through the medium and the spatial sampling rate of the WF is kept above a pre-defined limit. The receivers are given value as the WFs pass them, one arrival for each sheet of WF that passes. Multi-arrival travel times, amplitudes, geometric spreading etc. are thus found in an efficient way in all areas of the model that are reachable by conventional rays.
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3D Interpolation of geophysical parameters - The GOCAD approach
Authors J. L. Mallet, R. Cognot and S. CasesThere are many applications in geology which require to know the distribution of physical parameters (velocities, porosities,...) in a region of the 3D space corresponding to the subsurface. Most of the time, these applications require to know the values of these parameters at the nodes of a regular 3D grid which must be interpolated from scattered data points. Classical methods, like for example Krigging, have the following drawbacks - they are numerically unstable as soon as data are clustered, and this is often the case in geophysics - they are mathematically unable to account for discontinuities generated by complex surfaces corresponding to horizons or faults, and it is necessary to use "programming tricks" ; - they are often too slow for initializing huge grids having for example a size of 500³ nodes like it is often the case in seismic migration methods!
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3D Geologic modelling based on topological objects
Authors W. Wiggins, U. Albertini and G. StankovicThe lack of adequate methods for representing velocity fields and other spatially indexed geologic parameters has limited the sophistication, economic viability, and accuracy of many seismic processes, especially 3-D seismic depth imaging. Recently, techniques borrowed from research in computeraided geometric design have been reported by Sword (1991), Ritsema, Nobili, and Mallet (1991) and Wyatt, et al (1992). We describe a similar computer representation based on topological data structures as developed by Weiler (1988).
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A 3D cellular, smooth boundary representation modelling system for geological structures
Authors L. A. Froyland, A. Laksa, K. Strom and J. PajchelThe modelling system, Galled GeoCell, is a 3D boundary representation modelling system to build and manipulate a geometric model of geological structures. Physical properties of the geology such as density and porosity are represented in the model by functions defined over the region of interest. These property functions are smooth within each geological block or cell in the geological structure. The purpose of the modelling system is to perform or prepare for: - Visualization of the geometry and physical properties of geological structures - Seismic modelling using e.g. ray-tracing and finite differences, - Geological simulation done by incremental change of geometry, - Reservoir simulation The topological representation of GeoCell is of a non-manifold, cellular form. Therefore the system can model the anomalies of a reservoir model such as faults. Also the interfaces between geological cells are modeled as smooth faces. This is in accordance with the low frequent seismic image of geological structures, and is necessary in order to use ray tracing techniques in seismic simulations.
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Source signatures in finite difference schemes
More LessFor a marine seismic air gun array each gun location does not necessarily coincide with the nodes in a finite difference grid. Especially for coarse grid finite difference modeling this problem must be handled with care since there might be up to three or four air guns between two grid points. Most examples of coarse grid finite differente modeling of seismic data have up to now used single point sources. A conventional marine source array consists of a number (from 10 to 70) of individual air guns, typically separated by about 2 - 4 meters. For coarse grid finite difference schemes, however, the distante between each grid point is typically 10m or more. This means that effective point sources (i.e. notional sources) estimated from measurements can not directly be implemented in a coarse grid finite difference scheme. We will circumvent this problem by introducing several different source functions at the same node in the finite difference grid. The set of grid source functions at each node is a multipole expansion, and we will estimate how many terms in the multipole expansion, that are sufficient in order to achieve a reasonable representation of the directive source wavefield.
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Viscoelastic rheologies via eigenstrains
Authors J. Carcione and F. CavalliniAs Lord Kelvin quoted in his Encyclopedia Britannica article on Elasticity (Thomson 1878, p. 823): A single system of six mutually orthogonal (strains) types may be determined for any homogeneous elastic solid, so that its potential energy when homogeneously strained in any way is expressed by the sum of the products of the squares of the components of the strain, according to those types, respectively multiplied by six determinate coefficients. The six strain-types thus determined are called the Six Principal Strain-types of the body
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Integrated approach to multiple elimination and wavelet estimation
Authors D. J. Verschuur and A. J. BerkhoutSurface-related multiple elimination is a method that remover all surface-related multiplee from the seismic data without using any knowledge of the subsurface. It is assumed that all information about the surface reflectivity and the source and receiver behavior is known. However, by applying the method adaptively, the surface and source/receiver properties can be estimated from the data itself. Simultaneously, the multiples are eliminated. So far, we considered seismic data as point source responses, with an equal source signature for all shot records, see Verschuur et al (1992). In this paper we show how we can improve on source signature estimations, especially with respect to the shot to shot variations.
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Efficient multiple elimination using generalized radon transform with pseudo-hyperbolic stacking surface
More LessThis paper presents a new procedure for suppressing multiples by using a pseudo-hyperbolic transform, which is equivalent to the parabolic transform with an analytical solution of time variable p-parameter. Examples show that the new procedure is more efficient in the multiple elimination than conventional methods, so that it benefits velocity analysis (VA) and amplitude versus offset (AVO).
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Surface multiple attenuation - Theory, practical Issues, Examples
More LessSurface multiple attenuation (SMA) is a prestack inversion of a surface-recorded, 2-D; wavefield that removes all orders of all surface multiples present within the wavefield. In addition, the process determines the average acquisition wavelet imbedded in the wavefield. Surface multiple attenuation requires no assumptions or modeling regarding the positions, shapes, or reflection coefficients of the multiple-causing reflectors. Instead, SMA relies on the internat physical consistency between primary and multiple events that must be present in any properly recorded marine data set.
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North Sinai of Egypt - Re-Processing leads to a prospectivity re-appraisal
Authors A. El-Emam and M. NessimThe El-Arish area of North Sinai is characterized by NE-SW elongated anticlines which are partially breached at surface by erosion. Exploration activity over the last decade, including several Wells, has been without success, and prospectivity had been downgraded. An attempt to understand the reasons for this lack of succes through reprocessing of seismic data, together with a review of well results, has shown a serious and previously unidentified seismic multiple problem. Careful reprocessing with f-x deconvolution, together with multiple modelling & elimination has allowed a solution, and the results of the follow-up well are encouraging.
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Pre-stack multiple suppression analysis of marine seismic data - Norwegian North Sea
Authors R. V. Schneider, K. J. Marfurt and P. G. GarossinoSeismic data from the North Sea, oftentimes contain poor S/N values. Sources of noise range from random bursts to coherent patterns observed throughout the dataset. In particular, coherent noise in the North Sea is produced by geologic features including a hard water bottom, chalk, out of plane faults, and in the dataset to be shown, scattering of the seismic signal due to dewatering of the Hordaland shale.
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A Methodological comparison of several multiple suppression techniques in two real cases
Authors P. Julien, P. Desegaulx and P. M. HallThe aim of this paper is to compare several long period multiple suppression techniques and to draw practical conclusions from their results on real data. As the results from such algorithms tend to be very data - dependant, two sets of seismic data are presented: the first is from an area of quite poor data quality, the second from an area of quite good data quality.
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Industrial and seismic noise removal in marine processing
Authors M. Manin and J. N. Bonnot3D marine surveys often take place in regions cluttered with oil rigs and sometimes other seismic vessels. The acoustic environment is therefore not very conducive to the acquisition of accurate seismic data. The oil rig may be in the process of drilling and there fore creating a great deal of vibration. It may have engines or noisy pumps. It may even read to the seismic arrival by diffracting part of the energy. The other seismic vessels may also be shooting. Numerous attempts have been made to demonstrate that these kinds of noise can be attenuated efficiently by methods based on the levels of amplitudes or on randomization of this noise within certain groups of traces. Apparently the concerns of the industry have not been allayed by these method sand, as a result, drilling and seismic acquisition must be planned on a regional scale (time-sparing), so that each activity respects the silence required by the other. In certain cases, this time-sparing is uneconomic and therefore impracticable.
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Biangular decomposition of marine seismic data
Authors P. A. Vercruijsse, P. J. K. ter Doest and J. T. FokkemaThe goal of seismic imaging is to produce the reflection operator at target depth. This reflection operator is independent of the acquisition parameters and is an intrinsic property of the medium parameters (Berkhout, 1982). In the so-called Angle-versus-Angle (AVA) analysis one is concerned with the behaviour of the reflection operator as a function of the strike angle of the incident wavefield at a particular interface. In order to enhance the AVA analysis we propose a biangular decomposition of the seismic data. In the resulting domain the axes are related to the strike angle of the incident wavefield with respect to the normal at the interface and the local dip angle of that interface. Operating in this way we go from from acquisition dependent parameters to intrinsic medium parameters.
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The application of common offset and common angle pre-stack depth migration in the North Sea
Authors N. D. Whitmore Jr., W. F. Felinski, G. E. Murphy and S. H. GrayPre-stack depth migration procedures have been used in many areas for seismic imaging, interval velocity analysis, and depth conversion. Most migration procedures assume isotropic wave propagation models. However, in areas like the North Sea, the isotropic assumption is not correct, and this can produce incorrect depth imaging if not properly accounted for.
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Pre-stack turning ray imaging with a 60° operator
More LessSeismic energy traveling in an inhomogeneous medium may move along a curving ray path causing wavefronts which were originally going downward to turn through 90 degrees and move back toward the surface. Special algorithms are required to image reflectors which are illuminated by turning rays if the imaging is done poststack, since the only path between the surface and the reflector seen by the migration is a turning ray path.
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Residual depth moveout after common offset depth migration
Authors R. G. Williams and J. CowleyCowley et al (1992) have shows that common offset depth migration can be used in a layerstripping mode to perform depth migration velocity analysis (DMVA). The DMVA procedure allows direct picking of interval veldcity and depth, thus enabling a velocity depth model to be built and used for common offset depth migration. After the migration, the data are sorted info true common depth point gathers and stacked. It is common to find small residual moveout on these CDP gathers. This may happen either at CDP locations between the original DMVA locations or at intermediate depths between picks. In the first case, the lateral variation of the model has not been sufficiently defined whilst in the second case the vertical variation has not been sufficiently defined.
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Imaging salt with pre-stack depth migration
By J. R. GranliMost seismic data are processed using conventional methods onder the assumption that the earth is close to horizontal layering. This assumption is rarely fulfilled. Nevertheless, the seismic image quality obtained using these methods, is reasonably good, even in areas with structures of moderate complexity. Areas with saltdomes are beyond moderate complexity. By experience it is very difficult to obtain reliable seismic images using conventional methode here. Altemative processing methode are therefore needed. The purpose of our study is to investigate whether Pre-Stack Depth Migration can improve the seismic image when large saltdomes are present.
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Subsalt imaging via target-oriented 3D pre-stack migration
Authors C. A. Jacewitz, D. W. Ratcliff and S. H. GraySubsalt imaging and subsalt trap identification have been recognized for years as important but difficult exploration objectives. By using subsalt imaging technology the explorationist hopes to determine the depth of the subsalt structures and the presence or absence of structural closure associated with the subsalt prospect, thereby obtaining a realistic assessment of the risks involved with drilling a subsalt well. Target-oriented 3-D prestack depth migration is the most powerfuI tool available to assist the explorationist in the quest for hydrocarbon exploration below salt. In this paper we present a geophysical case study describing the steps taken to produce a nseful subsalt image.
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