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68th EAGE Conference and Exhibition incorporating SPE EUROPEC 2006
- Conference date: 12 Jun 2006 - 15 Jun 2006
- Location: Vienna, Austria
- ISBN: 978-90-73781-00-9
- Published: 12 June 2006
301 - 400 of 462 results
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Approach to Acoustic Simulation of Inhomogenous Cemented Shaly Sands
Authors J. V. Protkova and R. L. Pevznermineralogic composition and elastic modules of mineral consistituents Gassmann’s relation. Herein, fluid modeling is performed for a layer, assuming that the layer is homogenous. However, frequently this is not the case. Within a reservoir, that is to be considered as one hydrodynamic system, some lithological parameters, e.g. clay content or cementation, can significantly vary. This paper propose an approach to acoustic simulation of inhomogeneous reservoirs.
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Effect of Microheterogeneity on the Effective Stress Coefficient for Elastic Properties of Rocks
Authors R. Ciz, B. Gurevich, A. F. Siggins and J. DvorkinLaboratory measurements show that the seismic velocities as a function of effective stress yield effective stress coefficient less than 1 and usually varying between 0.5 and 1. At the same time, theoretical analysis shows that for an idealised mono-mineral rock the effective stress coefficient for seismic velocities (or, more precisely, for elastic moduli) must always be equal to 1. The presence of heterogeneity as a possible cause of this variation is investigated in the double shell spherical model. The results show that only a small amount (less than 1%) of soft component is sufficient to cause this effect. Such soft material may be present in grain contact areas of many rocks and may explain the variation observed experimentally.
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Complete Pressure and Fluid Dependence of Seismic Properties of Reservoir Rocks
Authors P. N. J. Rasolofosaon and B. E. ZinsznerA good knowledge of the simultaneous effect of fluid substitution and pressures on seismic properties is a key point in exploration for overpressure prediction from seismic data, and in exploitation for the seismic monitoring of reservoirs.The aim of this presentation is to find the simplest theoretical model that can correctly describe the complete pressure and fluid dependence of elastic velocities of rocks measured in the laboratory. We find that, for a given rock, a three-parameter model for the pressure dependence of bulk and shear moduli of the rock skeleton injected in the poroelastic equations of Gassmann (1951) type is sufficient to completely and accurately describe the pressure and fluid dependence of the fluid saturated rock, with various types of fluids. The pressure dependence of the dry rock frame moduli is either of the type M(Pdiff) = A - C exp(-D Pdiff) or of the type M(Pdiff) = A/[1 +C exp(-D Pdiff)] (e.g. Zimmermann, 1984), where Pdiff = Pconf - Ppore is the difference between confining pressure Pconf and pore pressure Ppore. As previously noted (Rasolofosaon and Zinszner, 2005) taking also into account the pressure dependence of the saturating fluid substantially improve the agreement between experimental results and theoretical predictions.
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Numerical Study of Transmission Signatures of Gas Hydrate-Bearing Microstructures
Authors O. S. Krüger, E. H. Saenger, S. R. Zanoth and S. A. ShapiroThis work is inspired by the observation, that gas hydrate bearing sediments have a high velocity in combination with high attenuation.
We study numerically the influence of different gas hydrate locations within the pore space on transmitted p-waves. From the wave propagation simulations on the micro--scale it can be seen, that different positions of the gas hydrate in the pore space results in almost the same effective velocities and attenuation, as long as the gas hydrate had contact to the sediment grains. This changes in the case of a suspension, here the attenuation increases and the effective velocity decreases. The resulting p-wave versus gas hydrate saturation plot is in a qualitatively good agreement with experimental results obtained for the Mallik 2L-38 well.
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Simple Model for Lithological Anisotropy of Shale
Authors A. Bakk, E. Fjær and R. M. HoltWe propose a simple model for lithological anisotropy that predicts the five distinct elastic constants for a transversely isotropic medium on the basis of three parameters. The predictions of the model show good agreement with experimental data on several shales.
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Reduction of Acquisition Time Sharing in the North Sea by Seismic-Interference Attenuation
Authors T. Manning, J. H. Kommedal, R. Wombell, T. Noss and T. PokrovskaiaIn 2003, Seismic Interference (SI) was recorded during the simultaneous acquisition of BP’s Valhall OBC data and towed streamer data in Quad 30 (Figure 1). BP and Veritas jointly initiated a project to investigate SI attenuation methods, in order to define new noise thresholds for acquisition in the presence of SI and so reduce the need for time sharing. A range of standard and non-standard processing algorithms and flows were evaluated on the contaminated data. It was concluded that the recorded SI could be successfully removed pre-stack from both datasets with accurate signal amplitude preservation. This extends the findings of Jack and Lancaster 1989 and Lynn et al 1987. In 2004 streamer test data were recorded containing SI from the Valhall source. These were used to build an interference noise library to model SI arrivals from a range of bearings and strengths. The results of subsequent processing and analysis enabled both companies to agree to acquire surveys in this area during 2005 with reduced time-sharing.
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Water-Borne Noise Levels from Airgun Arrays - Examples from Surprisingly Quiet Chicxulub Survey
Authors P. J. Barton, J. B. Diebold and S. P. S. GulickIn 2005 the US research vessel Maurice Ewing conducted an academic seismic survey of the Chicxulub impact crater in shallow water offshore Yucatan, Mexico. A combination of US legal requirements and Mexican government restrictions resulted in highly restricted time windows in which the work could be conducted. A casual observation by a diver in 20m-deep water just outside the predicted 180 dB (RMS) safety radius at 3.5 km that he could hardly hear the airguns led us to examine the data collected by the seismic streamer and by ocean bottom hydrophones. Calibrated plots of peak seismic arrival amplitude against offset show that in this environment, levels of deep, post-critically reflected and refracted arrivals outweigh those of water-borne arrivals at offsets greater than 1 km. Since the predicted 3.5 km radius was based on measurements of the same array in 60m-deep water we conclude that through a combination of very shallow water depth and geologic structure, seismic source energy was partitioned so as to reduce the intensity of water column reverberations. These results suggest that monitoring of hydrophone array data may provide a method of determining the safety radius in real time.
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Evaluation of 3C Sensor Coupling Using Ambient Noise Measurements
Authors H. J. Watt, J. B. Gibson, B. Mattocks, M. Cartwright, R. Burnett and S. RonenGood vector fidelity is crucial for various multicomponent applications and can be adversely impacted by poor sensor coupling during data acquisition. We present a method to evaluate the ground coupling quality of three-component (3C) sensors during field deployment using ambient seismic noise data. The method utilizes the directional nature of the noise on the three sensor components for coupling assessment. This is believed to be the first method developed to evaluate in situ sensor coupling from ambient noise measurements.
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Numerical Simulation of the Use of Continuous Repetitive Noise Signals as Seismic Sources
More LessThis paper describes the numerical simulation of the use of continuous noise sources in seismic exploration where each source is orthogonal with all others if summed over a number of repeats of the signals. The orthogonal property allows a number of sources to operate simultaneously and continuously with the signal from each source being identified in the recorded signal at each receiver. The technique has been used to generate seismic recorda equivalent to those from short duration or impulsive sources. Continuous sources should have application in time lapse studies of reservoirs where small changes in the signal need to be detected.
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Optimal Seismic Survey Design Based on Seismic Wave Illumination
Authors P. M. Li and L. G. DongIn central-west China, over-thrust structures widely exist. Seismic survey design is a key issue for seismic exploration in such complex areas with complicated structures. This paper proposes a seismic design method based on CRP and seismic wave illumination. According to the wave illumination equation, the optimal source positions can be determined through a statistical illumination method, one-way or two-way equation modeling methods, in which we know where to increase the density of shot points in order to improve the image quality of the objective surfaces. Both ray tracing and seismic wave equation simulation can be used together to calculate the folds and energy distribution for different offsets along the CRPs of a certain surface. Then the optimal receiver array pattern and receiver array length can be determined from these distribution curves. The survey design using this target-oriented illumination analysis offers much better images for the objective surfaces.
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Overview of Efficient Vibroseis Acquisition Methods
By C. BagainiMany methods have been developed in the past 25 years to speed up Vibroseis acquisition. I give an overview of the most promising ones and propose a classification of them in three categories: simultaneous shooting, cascaded sweeps and slip sweeps. The main features of these methods are summarized and some criteria for the selection of the most suitable are introduced.
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Adaptive Beamformers and Time Corrections
Authors I. Panea and G. G. DrijkoningenReceiver arrays are used in seismic acquisition for data reduction. The array response can be computed based on single-sensor recordings, preferable after a data pre-processing performed in order to eliminate the static and amplitude effects. An adaptive beamforming algorithm is presented in this paper with application on synthetic and field seismic data; we compare these results with those obtained using standard arrays. in addition, we study the effectiveness of time corrections applied in order to correct the effect of low apparent velocities of the reflected waves on beamformer response.
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Single-Sensor Recording - A Case Study from the Partitioned Neutral Zone
Authors J. M. Garrity, M. Rajab, T. Perrin, P. M. van Baaren, H. Bayri, A. Shabrawi, A. S. Smart, P. Gallaway and I. Al-HakimThis paper shows how changing the paradigms used for 3D seismic acquisition and processing can extend the usefulness of 3D surface seismic methods.
To demonstrate this, a case study from the Partitioned Neutral Zone between the Kingdom of Saudi Arabia and Kuwait, which is jointly operated by the Kuwait Oil Company (KOC) and Saudi Arabian Texaco (SAT), known collectively as the Joint Operating Company (JOC), will be used.
As demands for the information provided by seismic data have increased from delivering a structural picture to generating reservoir information in the inter-well space, conventional 3D seismic survey fold has increased to very high levels in an attempt to attenuate residual coherent noise while trying to deliver the high-fidelity seismic datasets required for such analysis, all of which comes with additional cost and uncertainty. In the Middle East, where strong coherent surface-generated noise is prevalent, a plateau has been reached regarding how much information conventional seismic data recorded with surface arrays can provide. This paper show that single-sensor recording allows the recording of higher-fidelity seismic data with both a broader bandwidth and lower noise floor, extending the usefulness of the surface seismic data.
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Land Data Regularization and Interpolation Using Azimuth Moveout (AMO)
Authors A. M. Popovici, S. Crawley, D. Bevc and D. NegutAzimuth Moveout (AMO) is a wave-equation correct binning algorithm that accurately handles dipping geological strata and variable velocities. AMO has applications for both marine and land data, though this paper focuses mainly on the application of AMO to 3-D and 2-D land surveys which have irregular and sparse geometry that often leads to spatial aliasing. AMO can be successfully used to regularize and interpolate land data, while preserving steep dips and diffractions better than standard binning.
Traditionally, AMO was designed to address the marine acquisition shortcomings, and regularize the data. Recent focus by exploration and production companies to exploit shallow resource plays has demanded new research into utilizing existing 3D data for mapping the shallow reservoirs. The land examples provided in this paper demonstrate the success of applying AMO to conventional land 3-D data.
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A Linear Motor as Seismic Horizontal Vibrator
Authors G. G. Drijkoningen, A. Veltman, W. Hendrix, K. Faber, J. Brouwer and G. A. HemstedeIn this paper we propose to use the concept of linear synchronous motors to act as a seismic shear-wave vibratory source. We show that a linear motor, even with a design that is not focussed on application of seismic surveying, gives seismic records that are convincing and comparable with an established type of horizontal vibrator. Therefore linear motors are very promising as a vibratory source for seismic exploration and monitoring of the subsurface.
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Determining Infill Specifications Based on Geophysical Criteria
More LessDuring marine seismic acquisition, areas of deficient coverage can arise. If these deficient areas are sufficiently large to be considered outside the specifications, additional infill lines must be acquired. It is desirable that this infill acquisition be targeted such that the additional acquisition requirements are minimised whilst maintaining adequate data quality.
This study provides a method for predicting the effects of coverage holes on the migrated image on a case-by-case basis. Synthetic modelling followed by pre-stack time migration is used to quantify these effects for the particular model and targets of interest. These results may then be used to assess infill requirements based on a permitted level of data degradation. Such modelling can be performed at the pre-survey planning stage, thus allowing infill considerations to be taken into account during the survey design process.
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Sensor Design Parametric Study and Impact on Seismic Imaging
Authors C. A. M. Faber, D. Yu and J. CrissWe seek to reveal the impact of sensor parameters drift on image quality. Drift may derive from manufacturing tolerance or environmental temperature variation etc. The study composes of two parts: I) classical closed form discussion, including limiting benefits of inverse transfer function deconvolution. II) numerical synthesis of signals convoluting with sensor transfer function and the difference of input and output. Particular interest is placed on the wider range of damping value changes and the need of a tighter control of the specification. Both part I and II demonstrated significant impact on the signal integrity by parameter drifting as much as -10% to +30%, frequency content depending.
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An Anisotropic Acoustic Wave Equation for VTI Media
More LessHere we propose a new anisotropic acoustic wave equation based on the same dispersion relation as Alkhalifah’s (2000), but introducing an auxiliary function which allows the original fourth-order differential equation to become a coupled system of lower-order differential equations. Of these two equations, one equation can be considered as a hyperbolic wave equation for elliptical anisotropy, but with a correction term that compensates for the loss of anisotropy for VTI media, while the other can be considered as the additional expansion or contraction of the wavefront in the lateral directions. This two-way anisotropic wave equation can be used for both modeling and reverse-time migration.
The new anisotropic acoustic equation has the obvious physical meaning and is much easier to implement. Impulse responses for both modeling and migration have been shown to validate the proposed anisotropic acoustic equation.
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Traveltime Calculation in 3D TTI Media by the Fast Marching Method
By M. LouI developed an efficient, stable, and accurate algorithm to calculate first arrival traveltimes in 3D transverse isotropic media having tilted symmetry axis (TTI). This algorithm is based on a fast marching scheme to solve the eikonal equation using a higher order finite difference (FD) method generally used to calculate first arrival traveltimes in 3D isotropic media. Numerical examples demonstrate that the newly developed algorithm is able to construct accurate first arrival traveltimes in any complicated 3D TTI model that contains a large velocity gradient and an arbitrary orientation of its axis of symmetry. This algorithm can be applied to 3D Kirchhoff pre-stack depth migration and anisotropic parameter inversion in 3D TTI media.
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An Automated 3D Method for Azimuthal Anisotropy Analysis in Marine Seismic Data
Authors B. Hung, F. M. Zhang, J. Sun, M. Stanley Pty Ltd and A. Osadchuk Pty LtdWe present an automated and efficient method for estimating the azimuthal anisotropy parameters for large volumes of 3D marine data. The method is automated because it is data driven and it does not involve any picking of velocity or semblance. It involves mainly two steps: first, flattening the CMP gathers that have been NMO corrected to generate time-variant timeshifts; second, fitting these timeshifts to the azimuthal anisotropic traveltime equation to generate the velocity ellipticity and its direction. Using a recent survey that was acquired off Western Australia, we demonstrate that this method is able to obtain dense anisotropies and directions in large-scale areas in an automated way.
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Angle-Domain Common Image Gathers and RMO Analysis for Anisotropic Migration
By B. L. BiondiI present a general methodology for computing and analyzing Angle-Domain Common Image Gathers (ADCIGs) in conjunction with anisotropic
wavefield-continuation migration. I demonstrate that the aperture angles estimated by transforming prestack images using slant stacks along the
subsurface-offset axisi are good approximations of the phase aperture angles.
I analyze the perturbation in the angle-domain image caused by perturbations in anisotropic parameters, and present a general expression for computing Residual Moveout (RMO) curves.
Several synthetic-data and real-data examples demonstrate the accuracy of the RMO function predicted by my kinematic analysis.
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PP/PS Wavefield Separation by Independent Component Analysis
More LessIndependent component analysis (ICA) is a rapidly emerging technology in the field of advanced signal processing. It separates a set of observed signals into the statistically most independent components by appealing to higher-order statistics. ICA retrieves the original source signals if the observations are actually a mixture of superposed signals that are statistical independent. No a priori information is required about waveforms or polarisations of the desired source signals.
ICA can be used to separate P- and S-waves in 3-component seismic reflection data without knowledge of P- and S-wave near-surface velocities or density. Wavefield separation is achieved by exploiting statistical differences between P- and S-waves only. The ICA wavefield separation technique is exact in a laterally inhomogeneous anisotropic Earth with a homogeneous anisotropic near-surface layer if only upgoing waves are present.
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Data-Driven P-S Separation and Multiple Suppression in OBC Data
Authors P. Edme, M. T. A. Soudani, J. L. Boelle and S. C. SinghOBC technology provides information on both pressure and 3 components particle velocity recorded at the seafloor level, allowing complementary PP and PS imaging. In the following paper, we describe a new elastic decomposition scheme in the tau-p domain that separates upgoing-downgoing P and S wavefields and eliminates all water layer related multiples. We introduce frequency dependant operators to calibrate the geophones and to characterize the P wave reflection coefficient at the seafloor. The process does not require any a priori knowledge of seafloor parameters. We start by describing theoretical aspects for complete decomposition. Then, we detail a multi-step procedure for estimating the shear wave velocity at the seafloor, for extracting the upgoing S wavefield and finally for removing peg-leg arrivals. We show successful tests on a real dataset. The results suggest promising avenues for further processing.
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Minimization of the 4C Alford Data Matrix for Non-Orthogonal PS-Wave Reflection Modes
By J. E. GaiserJoint inversion of P-waves (compressional waves) and PS-waves (converted P to S-waves) is becoming more important for pore pressure prediction, time-lapse effects and characterizing the stress and fracture state of reservoirs. However accurate results will depend on how well the principal S-wave (shear-wave) directions can be determined in anisotropic media. PS-wave reflection coefficients in azimuthally anisotropic media produce a set of non-orthogonal amplitude modes that can complicate the estimation of principal S-wave directions.
The purpose of this study is to show that a conventional Alford rotation provides the correct principal direction for non-orthogonal PS-wave modes in azimuthally anisotropic media when reflection coefficients vary azimuthally. Fast and slow S-wave components of 4C (four component) Alford data are examined in terms of azimuthally anisotropic PS-waves reflection coefficients. Results for orthorhombic media illustrate that the Alford data matrix is diagonally dominant when properly aligned with the principal directions. A synthetic data set consisting of a 5-layer model representing vertical fractures of different orientations demonstrates that conventional Alford rotation and layer stripping will give the correct principal directions when the off-diagonal traces are minimized.
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Velocity-Independent Layer Stripping of PP and PS Reflection Traveltimes
Authors I. Tsvankin and P. DewanganWe adapt the so-called ``PP+PS=SS'' method to devise an exact technique for constructing the interval traveltime-offset function in a target zone beneath a horizontally layered overburden. The algorithm is designed for arbitrarily anisotropic target layers, but the overburden is assumed to have a horizontal symmetry plane (i.e., up-down symmetry). Important advantages of this layer-stripping technique compared to the existing Dix-type equations include the ability to handle the asymmetric moveout of mode-converted waves and laterally heterogeneous target layers with multiple curved reflectors. Also, our method is entirely data-driven and does not require knowledge of the velocity field anywhere in the medium.
The computed interval moveouts of PP- and PS-waves can be used to estimate the interval parameters of transversely isotropic layers with a tilted symmetry axis (TTI), which is essential for accurate imaging in fold-and-thrust belts
(e.g., the Canadian Foothills) and near flanks of salt domes. Other applications include dip-moveout inversion for VTI media and stable computation of interval long-spread (nonhyperbolic) moveout for purposes of anisotropic velocity analysis.
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Inversion of Traveltime Error for Anisotropy Parameters - Foothills Thrust Model
Authors C. Kumar, M. K. Sen and R. J. FergusonIsotropic pre-stack migration of the Foothills Synthetic Data Set results in erroneous positioning of reflections below the thrust sheet. Using CFP analysis, we invert imaging error as measured in the time/offset domain by least squares to deduce anisotropy parameters ε and δ under a number of assumptions: homogeneous, weak TI, known depth, known thickness, and known P-wave velocity (slow direction). We find for this model that ε and δ may be estimated to within ten percent of their true values. We speculate that the assumptions necessary for convergence may be relaxed when S-wave data is incorporated.
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Relevance of Matrix Isotropy Assumption to Elasticity in Porous Anisotropic Rocks
More LessThe poroelastic theory of Gassmann (1951) is widely used in fluid substitution problems of seismic monitoring in media considered as isotropic. It is not commonly appreciated that this author also dealt with anisotropic porous media, but of a special type. In his simplified theory the grain constituent is assumed isotropic, and only the rock skeleton is anisotropic. Because anisotropy in sedimentary rocks is very common , the purpose of this presentation is to compare the relative contributions of the anisotropies of the rock skeleton and of the constituent matrix to the overall anisotropy of a fluid saturated rock. The results clearly show the essential contribution of the skeleton anisotropy. As a consequence assuming an isotropic matrix is quite reasonable. In practice, in fluid substitution problems in anisotropic rocks this considerably reduces the number of characteristic elastic parameters to be estimated for the grain constituent, namely from 6 in the general theory to 1 in the simplified anisotropic theory. Finally this work a posteriori justifies the matrix isotropy assumption in the simplified anisotropic model of Gassmann.
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Errors Due to the Anisotropic-Common-Ray Approximation of the Coupling Ray Theory
More LessThe coupling ray theory provides a continuous transition between the isotropic and anisotropic ray theories, and is particularly important at degrees of anisotropy and frequencies typical in seismic exploration and structural seismology on all scales.
In the common ray approximation of the coupling ray theory, only one reference ray is traced for both anisotropic-ray-theory S waves, and both S-wave anisotropic-ray-theory travel times are approximated by the first-order perturbation expansion from the common reference ray.
The common ray approximation thus considerably simplifies the coding of the coupling ray theory and numerical calculations, but may introduce errors in travel times due to the first-order perturbation.
Fortunately, the accuracy of the common ray approximations can be estimated by quadratures along the reference rays.
The accuracy of the common ray approximation depends considerably on the selection of the reference rays, and it is much worse for the isotropic common rays calculated in the isotropic reference model, than for the anisotropic common rays, traced in the anisotropic model using the averaged Hamiltonian of both anisotropic-ray theory S waves.
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3D Eikonal Solver in Tilted TI Media
More LessIn this paper, I developed an eikonal equation 3D-solver for a tilted TI media. The method is based on an iterative approximation of the vertical slowness. The approximation technique is inserted into a second-order expanding box finite-difference upwind ENO algorithm. Numerical examples of qP-wave traveltime computation for tilted TI media with large anellipticity illustrate the accuracy and efficiency of the proposed eikonal solver.
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TTI Anisotropic Depth Migration - Which Tilt Estimate Should We Use?
Authors F. S. Audebert, A. Pettenati and V. DirksWe perform a series of numerical modelling and migration experiment with different homogeneous TTI anisotropic media, characterized by tilt axis, polar velocity and anisotropy parameters. In the case of structurally conformable media, where the tilt of the medium coincides with the dip of the structure, great simplifications arise in the decoupling of the anisotropy parameters. In particular, positioning and short spread focusing become decoupled from long-spread behaviour. We show that in this case, the tilt of the medium can be observed with sufficient accuracy on an image obtained by isotropic or VTI elliptic migration with an educated estimate of the Thomsen parameter delta.
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Wide-Angle Phase-Slowness Approximations in VTI Media
Authors O. Pedersen, B. Ursin and A. StovasSeveral phase-shift migration methods depend on the vertical slowness, which in general can be represented as a nonlinear function of the horizontal slowness. In a VTI media, the dispersion relations relating the vertical and horizontal slowness, are complex expressions. Simple and accurate approximations of the exact slowness for both qP and qSV waves are desired for computationally fast and accurate migration algorithms. We describe new wide-angle phase slowness approximations for a VTI media.
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Semblance-Based Anisotropy Parameter-Estimation in Layered VTI Media Using Rational Interpolation
Authors H. Douma and M. van der BaanThe tau-p domain is the natural domain for anisotropy-parameter estimation in horizontally layered media. However, the need to transform the data to the \\tau-p domain or to pick traveltimes in the t-x domain, is a practical disadvantage. To overcome this, we combine a \\tau-p domain inversion technique with rational interpolation of traveltimes in the t-x domain. This combination results in a highly accurate and efficient semblance-based method for anisotropy parameter-estimation from the moveout of P-waves in layered transversely isotropic (TI) media with a vertical symmetry axis (VTI).
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A Robust Algorithm and Associated QC for Finding the Anisotropy Directions for Converted Wave Data
By B. Gratacoss the phenomenon by which an incident S wave travels at two different velocities depending on the angle between the wave polarization and the so-called natural directions of the anisotropic medium; When the polarization is parallel to the fast (or slow) direction a single arrival is observed. However, for intermediate polarization angles, two arrivals are observed with their amplitude and polarity determined by the angle of the wave’s polarization with the natural directions. This well known property gives a direct observation of the fracture directions, even though wide-azimuth P-wave data can also give such an estimate through a rather sophisticated processing sequence. In this paper we propose a robust method for estimating the natural directions based on the direct least-squares fit of the observation to a 1D, HTI model of the converted waves. The result of this algorithm is illustrated using synthetic and real data.
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Efficiency of Linux Clusters in Multi-Component Elastic Wave Field Modeling in Anisotropic Media
Authors T. Danek and A. L. LesniakIn the paper the effectiveness analysis of the seismic waves modeling in anisotropic media are discussed. The computations were performed on 31 nodes PC cluster working under Linux Fedora operating system. The efficiency analysis of the computation time for a typical seismic set-up consisted of 10 shoot points was made. The considerations are made for a weak anisotropy geological medium so the Thomsen parameters could be used. The results of modeling time for acoustic, elastic, elastic with dumping, anisotropic and anisotropic with flux corrected transport algorithms are presented. Not only different algorithms abut also different cluster nodes configurations, different switches and even different Linux kernels ware compared in the presented analysis. The results clearly shows that inexpensive PC clusters are efficient tools for wave propagation modeling in the scale necessary for engineering applications.
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Estimation of Parameters of VTI Model via Combining Reflection and Diving P-Waves
Authors K. V. Bykov, Y. V. Kiselev, V. N. Troyan and B. M. KashtanA correct anisotropic velocity model is critical for seismic imaging. We investigate here the possible advantages of inverting simultaneously for reflected and diving waves using P-waves recorded at the surface.
Simple tests in 1-D and 2-D VTI (Vertical Transversely Isotropic) models with spatially varying Vp and e show that reflected energy alone only provides partial information on the sub-surface properties after travel time tomography. However, both the velocity profile Vp, the depth interfaces and e can be well inverted by combining the two types of waves in the 1-D and 2-D examples investigated here.
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Attenuation of Coda Waves in the Central Alborz, Iran
Authors H. Rahimi, N. Kamalian and H. HamzelloTraditionally, estimates of inverse quality factor Q-1 as a measure of intrinsic attenuation are extracted from the wavelet w(t) amplitude spectrum assuming preferential absorption of high frequency component of the spectrum. In contrast, the energy losses associated with reflections from interfaces between layers with different absorption were considered by now mainly theoretically. We show that time windows implemented in these two techniques shall be localized differently. Also, we present a new set of frequency-dependent attributes to be used in predicting storage capacity, permeability, and oil production rate from seismic reflections originated within the reservoir. At a series of oil fields, the increase of porosity, permeability and production rate correlates with the increase of relative energy of high frequencies. At several other oil fields, the reverse trend is observed. Assumingly, this diversity is caused by the difference in the pore space microgeometry: the increase of high frequency amplitudes with the reservoir properties enhancement is inherent in reservoir rocks with dual porosity and dual permeability.
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Frequency Dependent Reflectivity Discloses Hydrocarbon Potential of Reservoir Rocks
Authors E. A. Kozlov, E. A. Davydova, N. L. Baransky and A. N. InozemtsevWe analyze compressional wave attenuation in fluid-saturated porous material with porous inclusions having different compressibility and very different spatial scale in comparison with the background. Such a medium exhibits attenuation due to wave-induced fluid flow across the interface between inclusion and background. We show that overall wave attenuation is governed by the superposition of two coupled fluid-diffusion processes. Associated with two characteristic spatial scales, we compute two cross-over frequencies that separate three different frequency regimes. We give a physical explanation for an intermediate range of frequencies, where attenuation scales with square-root of frequency.
The potential application of this model is in estimation of the background permeability as well as inclusion scale (thickness) by identifying these frequencies from the attenuation measurement.
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Fracture Related Cross-Over Frequencies of Seismic Attenuation in Porous Rocks
Authors M. Brajanovski, T. M. Mueller, B. Gurevich and M. BrajanovskiIt is well-known that in inhomogeneous porous media the effect of wave-induced fluid flow causes significant attenuation and dispersion of seismic waves. In connection to this phenomenon we study the impact of permeability fluctuations on the dynamic behaviour of porous materials. Based on the method of statistical smoothing applied to Biot's equations of poroelasticity we derive a model for a dynamic-equivalent permeability in 1-D and 3-D randomly inhomogeneous media; the low-frequency limit of this permeability corresponds to the hydraulic permeability governing fluid flow in porous media. We incorporate the dynamic-equivalent permeability model into the expressions for attenuation of P-waves also obtained by the method of smoothing. The results suggest that the effect of wave-induced fluid flow can be observed in a broader frequency range than previously thought. We confirm this through comparison with numerically modelled poroelastic multilayered media. We conclude that estimation of hydraulic permeability from seismic signatures like attenuation is only possible if permeability fluctuations are properly accounted for.
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Dynamic Permeability of Thin Layered Media - Theory vs Numerical Simulations
Authors T. M. Müller, G. Lambert and B. GurevichAn increasing interest in gas hydrates as a potential energy source gave reason for numerous field studies, laboratory and numerical experiments, that have revealed some interesting aspects of sediments containing gas hydrates. While there exist several models explaining observed increased seismic velocities, the mechanism of formation of gas hydrates and the reasons for observed strong attenuation are not fully understood. Two rock physical models are controversly debated: one attributes occurrence of hydrates to the properties of the rock's matrix, the other relates presence of hydrates to the properties of the pore fluid.
In our approach we assume, that an occurrence of hydrates affects the properties of the fluid, frame and grain of the host sediment. A poroelastic generalization of the O'Doherty-Anstey theory indicates that this would result in increased values for attenuation. To work with realistic models of multilayered, poroelastic media and to account for observed strong fluctuations in hydrate-bearing sedimentary layers we investigate exponentially correlated, random media. Numerical and analytical results confirm, that correlated fluctuations in properties of the frame, grain and fluid cause significant attenuation values. Especially in the lower seismic frequency range they are comparable to those observed in field measurements.
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Attenuation of Seismic P-Waves in Multilayered Gas Hydrate-Bearing Sediments
Authors A. Gerner, E. H. Saenger and S. A. ShapiroWe present an analysis of a time lapse walkaround, multilevel vertical seismic profile (VSP) data set from a carbonate field in Oman which has a long history of steam injection and production. The aim of this study is to compare changes in traveltime and attenuation variations in response to fracture orientation. The method used for calculating the attenuation factor Q, is based on the instantaneous frequency technique. Velocities and attenuation are sensitive to the fractures, presenting shorter travel times and lower attenuation in the direction of the fracture plane. We show through the case study the effectiveness of the method and the potential of azimuthal attenuation and travel-time studies of P-waves for fracture characterization.
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Anisotropic Azimuthal Attenuation as an Indicator of Fracture Properties, a Case Study on Time-Lapse Walkaround VSP Data
Authors I. Varela, M. Nasser, M. Chapman and E. LiuWe derived a formula for azimuthally varying attenuation and provided a method QVOA (seismic quality factor Q versus offset and azimuth) for fracture-direction estimation (o mapping horizontal earth stress) from azimuthally varying attenuation in PP-wave reflection data. The method is illustrated in synthetic data. We introduced a new seismic attribute, the QVO gradient, which can be extracted from 3D wide-azimuth reflection data. QVO-gradient inversion indicates fracture orientation. The magnitude of the QVO-gradient expresses the degree of Q-anisotropy. Q-anisotropy weakly depends on fracture parameters whereas strongly depends on the host-rock Vs-to-Vp-parameter. The QVO-gradient inversion for the fracture parameters is complicated. We believe, that azimuthal QVO analysis (QVOA) can resolve an uncertainty in fracture-direction estimation. Q-anisotropy may be more distinctive than reflection anisotropy, on which azimuthal AVO analysis is based. The QVOA method may have a potential advantage over other approaches because it uses relative characteristics of attenuation and not the absolute ones that are known to be deficient in accuracy.
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QVOA-Method or Q-Anisotropy for Fracture-Reservoir Characterization
Authors T. Chichinina, V. Sabinin and G. Ronquillo-JarilloNew seismic acquisition technology enables us to record wider bandwidth with higher signal-to-noise ratio. With this benefit we are able to improve seismic resolution and inversion quality. However, as higher frequencies are being propagated and recorded, the seismic signal is more susceptible to attenuation and energy losses. Because attenuation is proportional to the wavelength and not to the actual propagation distance, illuminating the subsurface with high frequencies enhance attenuation effects on seismic data. As attenuation, like any other rock property, is spatially varying high frequency seismic data will carry the attenuation footprint. Besides obvious effects such as reduced bandwidth and amplitudes, which will be present in the seismic data, attenuation also deteriorates the seismic wavelet. Hence, seismic inversion algorithms, which are based on recovering the reflectivity series from a given wavelet, will carry the spatial signature of the subsurface attenuation.
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Attenuation Estimation and Compensation as Applied to Seismic Inversion
Authors R. Bachrach, A. Salama, N. Banik, H. Xu, R. Ferber, R. Utech, D. Keller and M. BengtsonMature hydrocarbon provinces are increasingly being re-explored to extend the productive lives of existing fields by testing outlier or deeper objectives for additional reserves. In recent years the introduction of affordable 3-D pre-stack depth migration (PSDM) has allowed very complex geology to be properly imaged to more fully assess hydrocarbon potential in many of these mature areas. This paper discusses a 3-D seismic depth reprocessing program undertaken in a complex U.S. Overthrust area to assess the validity of a deep structure (approx. 5500m) identified on 3-D time-processed data in this mature prolific gas producing province. The 1998 vintage seismic 3-D acquisition method did not allow a seismically derived velocity model capable of accurately representing the subsurface to be defined in this older/fast rock regime. A 3-D pre-stack depth migration velocity model-building solution is presented that overcame the limited sensitivity to velocity of these 1998 3-D seismic data. By collaborating with client geologic expertise to integrate key geologic insights into the 3-D velocity model-building process and tightly constraining the seismically derived velocity with available well control, a subsurface velocity depth model was obtained that provided an accurate depth image of the deep target zone in this geologically complex area.
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Geological Interpretation of Acoustic Inversion for the Complex Frasnian Carbonates (Inzyreiskoye Field of North Russia)
Authors N. Tsyganova, I. Khromova and A. GlebovThe Inzyreiskoye field is located in the Timano-Pechorskaya oil-and-gas province of Russia. The productive intervals are the Middle Devonian terrigenous and Upper Devonian carbonate complexes characterized by the complicated geological structure with sharp facies variations by lateral, pinch-out zones presence, large number of tectonic faults and non-anticline traps. The field is under investigation for a long time and the traditional results of the seismic and logging data sequential interpretation were sets of structural and predicted maps provided by 2D attribute analysis.
The geological structure complication often provides ambiguousness of the results interpretation. In order to increase the results reliability, specify the limits of lithofacies changes and receive 3D models of the rocks properties in 2005 in the southern part of the field acoustic inversion was carried out in its common variant (full stack inversion) for the first time. The most interesting results were received for the thick Upper Devonian reef complex. We managed to reliably perform the facial classification and particularly define the high cavernosity and karsting zones which can provide increased production and in extremis even emergency situations in wells. There is the possibility of the elevated risk zones insulation via the impedances values.
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Sequence Stratigraphic Interpretation in the Wheeler Transformed (Flattened) Seismic Domain
Authors H. J. Ligtenberg, G. de Bruin, N. Hemstra and C. GeelA seismic interpretaion technique will be presented based on the Wheeler transform, in which stratigraphic events are plotted versus (psuedo-)geological time. We perform a chrono-stratigraphic flattening of seismic data. In the the transformation geological hiatuses and truncations are honoured. The Wheeler transformed data allows us to study seismic events in space and geological time, which is one of the key aspects of seismic sequence stratigraphic interpretation. The method to create a Wheeler transform will be explained, and examples will be provided.
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Fault Detection and Mapping Using 3D Diffraction Imaging and Coherency Analysis
Authors J. Vermeulen, B. Gurevich, M. Urosevic and E. LandaAutomatic detection of geological discontinuities such as small throw faults, and pinch-outs is an important problem in the interpretation of 3D seismic data. This is commonly done using coherency analysis. However coherency may be affected by noise, which may create false anomalies. We propose a new interpretation workflow for the detection and mapping of faults, which enhances the coherency-type analysis with identification and detection of diffractions produced by the discontinuities. The algorithm utilizes migrated and unmigrated stacked seismic volumes and the cube of stacking (NMO) velocities. Tests on a simple 2.5 D model show that the method is capable in detecting and mapping of faults below seismic resolution.
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Parallel Implementation of Image Segmentation for Tracking 3D Salt Boundaries
More LessNormalized cuts image segmentation can be used to track salt boundaries when the salt boundary amplitude is greater than any other local reflections. We apply a modified version of this image segmentation method to partition seismic images along salt boundaries. In principle, our method is capable of tracking boundaries that are not continuous: boundaries where conventional horizon tracking algorithms may fail. Normalized cuts image segmentation creates a weight matrix of weights connecting each pixel in the image to other pixels in a local neighborhood. The weight is made weak where the amplitude between two pixels is high. The weight matrix for 3D seismic data cubes can be quite large thus requiring memory and computational-time saving measures to be implemented. By imposing bounds, we significantly reduce the size of the problem and, as a result, increase efficiency and robustness. Also, errors created by segmenting thin images can be rectified with novel boundary conditions described here. To reduce the extensive computation time of this algorithm, we distribute the calculation of the weight matrix on a parallel network. This method is demonstrated to be effective on a 2D seismic section. Preliminary 3D seismic results are also included.
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High Frequency Imaging in an Exploitation Production Environment - The Murzuq Basin, Case History
Authors N. Hamarbatan, A. Ajaily and A. HassanInterpretation of high-resolution 3-D seismic imaging identifies subtle stratigraphic and structural changes that help in the placement of water injection wells and the development of new drilling opportunities in the Murzuq Basin, onshore Libya Case Study.
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Detecting Subtle Stratigraphic and Structural Features in Deep Water Reservoirs Using 3D Image Processing Techniques
Authors M. Spinola, R. Santos and R. AlvesThis paper discusses some important aspects of 3D-seismic reservoir characterization for deep-water facies in southwestern Atlantic margin, using many of the latest industrial algorithms for detection of subtle stratigraphic and structural features, applied to seismic volumes. Features that are direct related to local geologic factors on the controlling of turbidite depositions in a deep-water setting.
A basic workflow for a 3D-seismic attributes selection during a seismic reservoir characterization is suggested, in which the choice of the seismic attribute to be used will be intrinsically dependent of the quality of the data; the kind of geological feature to be detected and the knowledge of the attribute itself, to be used for enhancement and detection of the sedimentary targets.
3D-image processing techniques applied on the post stack data includes, structural oriented noise cancellation filter, calculations of dip and azimuth cubes, fault detection algorithm and elementary cube mathematics. Different seismic attributes were selected and combined to extract qualitative information on subtle faults.
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Pre-Stack Gather Conditioning for Post-Stack Interpretation - A Study of Custom Processing for Interpreters
Authors S. J. Clifford and J. D. KerrRecent software advances have changed the way interpreters look at and work with pre-stack data. Not only are they now able to store multiple versions of the post-stack data, but they can also view and process pre-stack data too. Such a change in emphasis in the interpreter’s influence must impact the way gather processing is performed. Careful gather conditioning techniques can provide data optimised for different interpretation tasks, such as AVO analysis, fault resolution or tying to wells.
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Using Long Offsets Information for Improved Pre-SDM Imaging - A Successful Application for the Sub-Salt Plays in Northern Germany
Authors S. Zimine, J. Thiessen, U. Weber, P. Mitouard, M. Reinier, L. Lopes and H. PrigentSalt intrusions are known as being the main cause of velocity contrasts in Northern Germany. These salt accumulations from the Zechstein formation often mask reflections from the under laying target zones at circa 4.5 km depth. In addition, significant velocity contrasts may occur in the Cretaceous series. For this reason PreSDM has gained popularity over PreSTM in this area, thanks to its ability of better dealing with significant velocity contrasts in the overburden. Nevertheless, even using advanced migration algorithms a reliable interpretation of target areas is not always possible. The usual offset range used for velocity model building (VMB) and migration is generally limited to 3 km for land acquisitions, which does not allow to fully explain ray path distortions and which makes anisotropy estimation difficult. Furthermore, the nominal fold is usually low and the data is therefore generally very noisy. Specific acquisition schemes have been designed to record longer offsets data up to 17 km in order to successfully penetrate the high velocity discontinuities or to undershoot them altogether. One challenge that arises with such long offset data is how to correctly process and integrate them into regular offset data before migration. Illumination studies are frequently used to assess the potential of long offset data and their possible contribution to the final image, but such studies are limited by the quality of the available initial depth model. In our paper we use two different long offset datasets from the northern part of Germany to discuss the challenges of integrating them into the standard PreSDM methodology, using both Kirchhoff and Wave Equation imaging techniques. We will demonstrate the benefit of including LO data in achieving a much improved image quality.
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Late Miocene Pockmarks in the Danish Central Graben - Origin and Significance
Authors K. Andresen, O. R. Clausen and M. HuuseThe present focusses are on the Balakhany and Pereriva Suites of the Productive Series over the Umid Babek structural trend within Baku Archipelago, since these are considered to be the most prospective in terms of hydrocarbon reserves potential.
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Hydrocarbon Potential of Umid-Babek Areas, South Caspian Basin
More LessThe presentation attempts to show an application of spectral decomposition as a qualitative tool for fast delineation of different origin carbonate bodies. Two cases coming from Polish Permian Basin are discussed. Both of them are connected with objects hardly visible on regular amplitude seismic sections. In the first case spectral analysis was focused on slumps situated at the foot of carbonate platforms where in recent years hydrocarbon occurrence was confirmed in some places. The second case concerns reefs associated with older Zechstein Limestone deposits. The common feature of both structures is wedge shape external slope make perfect condition for spectral decomposition.
Comparing pictures obtained from spectral analysis with results of amplitude interpretation and inversion modeling revealed striking similarity. It is clearly visible that working with discrete frequency images can exposed a structural framework on a very initial stage of seismic interpretation. Furthermore these images not only confirmed the amplitude anomaly seen on the traditional amplitude map but also illuminated structures seemingly impossible to detect because of the tuning phenomenon. Due to the different object thickness digitally blending of chosen frequency slices can enhance geological contents on final pictures.
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The Fast Identification of the Porous Carbonate Build-Ups in Polish Permian Basin Using Spectral Decomposition Method
Authors D. Gierszewska and P. PomianowskiThe evolution of a sedimentary basin is mostly affected by deformation. Large-scale, subsurface deformation is typically identified by the interpretation of seismic data and evaluated by palinspastic reconstruction. These studies also show, however, that sub-seismic small-scale fractures play a significant role: they may accommodate a significant proportion of the total strain (up to 50%) during basin evolution, lateral variation may cause compartmentalised deposits and reservoirs, and fracture networks may act as conduits for fluids. In the North German Basin (NGB) these aspects require an analysis of location, orientation and length distribution of the fracture systems with different methods for determining (a) the magnitude of deformation, (b) the strain accumulation in space and time, and (c) the processes that control (a) and (b) during basin evolution under varying kinematic constraints. To address these topics, our study aims to 1) quantify and predict areas of high- or low-strain accumulation in the NGB with seismic interpretation and 3-D retro-deformation, 2) validate these with drill hole and sample data, and 3) compare observed and predicted strain patterns with those from scaled analogue experiments to unravel the relationship at different scale lengths within structures representative of units in the NGB.
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Structural Architecture and Deformation Styles Derived from 3D Reflection Seismic Data in the North German Basin
Authors C. M. Krawczyk, T. Lohr, D. C. Tanner, H. Endres, R. Samiee, H. Trappe, O. Oncken and P. A. KuklaThe Upper Cretaceous Ilhabela sandstones, which represent the main reservoir unit in the Santos Basin, were deposited in a fluvial to shallow marine environment during a period of active basaltic volcanism. The volcanics have been partly eroded, providing the reservoir with volcanic rock fragments and causing a complex diagenetic history. This has resulted in the local occurrence of sandstones with complex lithology and elastic properties differing from the basin trend (Klarner et al, 2005). For better prediction and interpretation of the amplitudes and AVO behaviour of the Ilhabela reservoir it is essential to map the occurrence of the volcanics as well as the transport direction of their erosional products. In this paper, typical seismic features are presented which may help to identify volcanics in the Upper Cretaceous sequence.
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Seismic Signature of Upper Cretaceous Volcanics; Santos Basin, Brazil
Authors S. Klarner, B. Ujetz, R. Fontana and J. AltenkirchSubject of this poster is to present the result of a revised geological –geophysical interpretation of the East Slovakian Basin which is a mature basin in terms of both exploration and production activities. The revised tectonic pattern is essentially from 3D seismic interpretation. Once unravelled, the new pattern also could be verified on 2D seismic, i.e. at larger scale. Analysis of the seismic data indicates the development of distinct flower structures related with the dominant fault system, reflecting sinistral strike slip movements. The areal extent of the fault system strongly suggests the interpretation of a horse tail pattern in the reviewed part of the East Slovakian Basin.
The new 3D seismic data also rendered the chance to apply a seismostratigraphy interpretation approach. Due to heavy faulting of the Miocene section and due to Badenian and Sarmatian volcanism, however, application of seismostratigraphy, was limited mainly to Sarmatian sediments.
Good quality seismic complemented by reinterpretation of existing well logs and well cores served as a basis for sequence-stratigraphic approach and proved its value upon revisiting older gas fields.
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New Results in the Mature East Slovakian Basin Based on 3D and 2D Seismic Data Interpretation and Sequence Stratigraphy
Authors B. Šály, J. Janočko, S. Jacko, V. Jureňa and I. HlavatyDuring June 2004 well A-1 was drilled in the onshore Coastal Swamp area of Nigeria (Figure 1). The target of the drilling had been planned on seismic, which had rudimentary AVO processing applied. Other wells in the area had shown good correlation between the seismic and hydrocarbon finds. However on well A-1 although hydrocarbons were detected they did not tie with the AVO anomalies seen in the seismic data. Two more wells were about to be drilled from the same location into adjacent fault blocks so it was necessary to be sure that the seismic was telling the full picture. A volume of data around the well site was extracted and reprocessed through a sequence that included high resolution velocity analysis and surface consistent scaling. Intercept and Gradient data were extracted to calculate a product gradient volume. Although anomalies could be seen on the product gradient it was not until the attributes were inverted and the calibrated Lambda-Rho volumes calculated that the full potential of the seismic was revealed.
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Finsler Geometry Applied on Ray-Tracing for Seismic Tomography
More LessWe present an alternative formalisation of the ray-tracing equations, using Finsler geometry elements. As the employement of classical geometry in seismic rays has faced not few problems in the past, we generalize the ray-tracing equations on the basis of the geometry of a total bundle. This new approach may lead to a more effective way to handle tomographic problems in anisotropic media.
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A Robust Three-Dimensional Iterative Solver for the Time-Harmonic Wave Equation
More LessA robust three-dimensional iterative solver for the frequency-domain constant-density acoustic wave equation have been implemented. The linear system arising from the discretisation of the wave equation is solved with a preconditioned conjugate gradient method for non-definite matrices (bicgstab). The preconditioner system is based on a heavily damped wave equation and its inverse is built with one multigrid cycle. The multigrid method fails on the undamped wave equation. 2D and 3D numerical results show that the number of bicgstab iterations is linearly dependent on the frequency.
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The EAGE 3D Anisotropic Elastic Modelling Project
Authors W. A. Mulder, L. Nicoletis and T. A. AlkhalifahThree-dimensional anisotropic elastic data computed by a finite-difference code on a two-dimensional section of the SEG/EAGE Overthrust model will enable benchmarking of processing and imaging algorithms and other three-dimensional modelling tools. The use of a two-dimensional model reduces the number of shots and the data volume.
We present a description of the model, software and hardware, acquisition layout, and some initial results obtained on the IBM Blue Gene computer in Groningen.
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Wave Propagation in a Heteromodular Elastic Medium
More LessRocks, soils, and oil and tar sands are complex materials containing pores, cracks, and other defects. If this is the case, their constitutive behaviour can be nonlinear and stress-dependent, which implies that loading changes the properties of the material. If materials react differently to compression and tension, this can have a strong influence on the propagation of seismic waves. A possible approach to describe such materials is heteromodular elastic theory: a piece-wise linear theory with different elastic moduli depending on the stress state. The formulation of such a theory for the 2D and 3D cases is a difficult task. Even for the 1D case, there are only a few dynamical problems solved. One needs a simple model problem to imagine how the signal behaves when passing through such a medium. We consider a 1D problem with a small heteromodularity, obtain its analytical (asymptotical) solution for the case of a suddenly applied harmonic load, and compare it with numerical results. This gives us a general idea of the character of the wave propagation we may expect in such media, and the technique to apply in more complex cases.
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Resonance Waves in the Media with Weakened Interfaces
Authors A. P. Krauklis, P. V. Krauklis and A. G. FatyanovIt is shown that in the layered media with weakened interfaces the resonance waves (Kr-waves) are arose. Such waves have the very characteristic frequencies and a lot of other cinematic and dynamic features distinguish them from the well-known Love and Rayleigh type waves. The comparison of the theoretical results with experimental seismic data indicates that Kr-waves can be formed in the real geological section. We show theoretically and numerically that in a sedimentary basin of finite lateral extent horizontal Kr-wave can be trapped and resonate. This wave has a number of interesting properties: wave spectrum has resonance frequency; wave group velocity is equal to an intermediate value between shear and longitudinal velocities; attenuation of wave increases when the frequency varies from the resonance value.
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Tube-Wave Interaction with a Fluid-Filled Circular Fracture of a Finite Radius
Authors S. R. Ziatdinov, A. V. Bakulin and B. M. KashtanTube waves in boreholes are used for characterizing formation properties and hydraulic properties of fluid-filled fractures and permeable zones intersecting wellbores. At low frequencies there is a well-known approximate formalism describing reflection/transmision of tube waves on layer boundaries, infinite fluid-filled fractures and small-diameter washouts. However for fractures or washouts of finite size one can only use numerical methods such as finite difference that are time-consuming and do not provide physical insights.
Here, we extend existing formalism to analyze reflection/transmission of tube waves on a circular fluid-filled fracture/washout of an arbitrary finite size. We break the problem into three tasks. First, conversion of tube waves into guided waves is modeled using method of Kostek et al. (1998). Second, we derive new analytical solution for reflection of diverging guided slow wave from a fracture tip. Finally, we derive conversion coefficient describing transformation of imploding guided into tube waves in a borehole. Combining three solutions, we obtain simple analytical representation of a total wavefield in the borehole as a superposition of upgoing and downgoing tube waves generated at the fracture intersection and borne by incoming guided waves and their multiples. New solution is in good agreement with finite difference computations.
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To the problem of reservoir mapping using seismic observation.
Authors P. V. Krauklis and A. P. KrauklisWe investigated the anomalous low-frequency tremors-like response of hydrocarbon traps from seismic noise. Two types of oil-filled reservoirs descriptions were studied: cracked layer and poroacoustic Biot’s layer. The problem of conversion of slow wave inside the layer into the tube wave inside well was considered.
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Cluster Nature of Scattering Energy Response of Groups of Caverns and Fractures
Authors V. B. Leviant, F. B. Chelnokov and I. Y. AntonovaThe paper treats the nature of a reflected wavefield produced by scattering of elastic energy by zones of diffuse fracturing and/or cavernosity in massive rocks by numerical modeling of elastic waves in randomly inhomogeneous media.
It is shown that the intensity of the of scattering response of the entire deconsolidated macrozone increases considerably with nonuniformity of distribution of microinhomogeneities in such zones and their concentration (formation of clusters). Energy scattering does not happen when the distribution of microinhomogeneities is uniform. Reflections are generated by the upper and lower interfaces of the macrozone only.
The scattering response of a fractured zone at the same structure of the wavefield differs greatly from that of a cavernous zone in stronger attenuation of its total energy and change of relative intensity of compressional and converted waves in favor of the latter.
The effect of nonuniformity of microinhomogeneities is the key factor of generation of a response of scattered energy, that is why we can speak about its "cluster" nature. Methods of quantitative estimation of scattering response energy and the degree of nonuniformity of microinhomogeneities in fractured and/or cavernous macrozones are suggested in the paper to analyze the results of numerical modeling.
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Interaction of Low-Frequency Tube Waves with Poroelastic Reservoirs Containing Perforations
Authors S. R. Ziatdinov, A. V. Bakulin, B. Gurevich and R. CizTube or Stoneley wave is known to strongly interact at low frequencies with poroelastic formations provided that flow is not restricted at the borehole-formation interface. Increased permeability leads to increased attenuation and decreased velocity of the tube wave. In this study we focus on reflection of low-frequency tube waves from various finite-size poroelastic structures. First, we examine a model of a thin reservoir and demonstrate good applicability of the approximate 1D effective wavenumber approach to describe interaction of tube waves with porous formations. We confirm that higher permeability leads to higher reflection coefficient. Then we analyze model of an idealized (disk-shaped) perforation inside a poroelastic layer and show that it has higher reflectivity compared to washout zone of the same geometry but with no-flow conditions at the face.
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Radiation Characteristics of Seismic Waves in Poroelastic Structures
Authors F. Karpfinger, T. M. Müller and B. GurevichExcitation and radiation of seismic waves in poroelastic media are analyzed. Based on the Fourier transform method we derive the fundamental solutions (Green’s functions) of dynamic poroelasticity. In analogy to the elastodynamic problem, the representation of the results allows identification of near- and farfield terms.
It is shown that seismic sources in poroelastic media can be consistently described with moment tensors.
The results are illustrated with help of radiation patterns for various source types. Synthetic seismograms of the slow P-wave mode demonstrate the importance of viscous dissipation in seismic modeling of poroelastic structures.
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Reflection Signal Shape Analysis as a Tool for Tuning Effects Removal from Amplitude DHI Analysis
Authors P. Marzec, K. M. Pietsch, A. Tatarata and M. KobylarskiHydrocarbon reservoir prospecting within thin-beded strata with variable geometry is difficult. Direct hydrocarbon indicator (DHI) analysis can lead to false conclusions. Seismic image depends not only on lithology of reservoir layers, porosity and hydrocarbon saturation but also on their geometry. Interference of reflected signal from top and bottom of a layer can result in increase or decrease of amplitude and causes change of wavelet shape.
Analysed seismic data include pinching-out Cenomanian sandstones (Cretaceous) composing the Mesozoic complex which creates base of the central part of the Carpathian Foredeep toward the east of Cracow (Poland).
The paper presents procedure that removes gains caused by pinching-out layers using the phenomena of wavelet shape variation within tuning zones. The procedure is based on construction of filters removing tuning effects form amplitude maps. In first stage the procedure was applied to synthetic seismic data (synthetic image of Rylowa gas reservoir). Positive results were obtained what allowed to apply filters to amplitude maps derived from recorded data.
Procedure application resulted in anomalies diminution what allowed to draw conclusions that some amplitude anomalies within studied area are caused by geometry of layers.
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3D Seismic Diffraction Modeling in Multilayered Media in Terms of Surface Integrals
Authors M. Ayzenberg, A. M. Aizenberg, H. B. Helle, K. D. Klem-Musatov, J. Pajchel and B. UrsinWe present an improved multiple tip wave superposition method (MTWSM) for the 3-D seismic modeling in multilayered media in terms of the Kirchhoff type integrals. The main improvement consists in correct description of their boundary values by the reflection/transmission operators. The high-frequency approximation of the operators (effective coefficients) generalizes the plane-wave coefficients used in the conventional seismics and allows adequate reproduction of multiple reflections and transmissions accounting for the nearcritical effects. To show the potential of the MTWSM we give results of the Green’s function modeling for a strong-contrast curvilinear interface in two-layered acoustic medium.
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Model-Based Multiple Prediction Using Pre-Computed Ray Field Maps
Authors D. A. Kraaijpoel and T. J. MoserExisting algorithms for ray modeling in layered media become exceedingly complex and time- and memory-consuming for models with many interfaces and complicated ray codes (e.g., multiples).
Moreover, for multi-shot/multi-receiver modeling calculations become highly redundant because the same parts of the medium are crossed over and over again. We propose to overcome these problems by pre-computing and storing the four-parametric ray fields between adjacent interfaces in the form of two-way ray field maps in position/angle coordinates. Subsequently, this information is used to construct arbitrary ray codes. The maps provide the ray segments between the interfaces, while Snell's law is used to connect the segments. Arbitrary ray codes can thus be constructed quickly and without redundancy. An example of multiple prediction in a synthetic layered medium is shown to be in agreement with finite difference results.
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2.5D Elastic Finite-Difference Modeling
Authors J. C. Costa, F. A. Silva Neto and A. NovaisFinite difference modeling of elastic wavefields in 2.5D is described in the velocity-stress formulation for anisotropic media. The 2.5D modeling computes the 3D elastic wavefield in a medium which is translation invariant in one coordinate direction. The approach is appealing due reduced storage and computing time when compared to full 3D finite difference elastic modeling. The scheme handles inhomogeneities in mass density and elastic moduli, includes free-surface and perfect matched layers as absorbing boundaries. High order finite difference operator allows the use of a coarse mesh, reducing the storage even more without producing numerical dispersion and numerical anisotropy. Numerical experiments show the accuracy of the scheme, its computational efficiency and the importance of 2.5D modeling in complex elastic media.
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2D Seismic Wave Propagation by a Finite-Volume Method
Authors N. Glinsky-Olivier, M. Ben Jemaa, J. M. Virieux and S. PipernoFor seismic wave propagation, we propose a complete reanalysis of the finite-volume approach based on unstructured triangular meshes. Triangular control volumes are particularly well adapted to the propagation of elastic waves in heterogeneous media. We consider a non-staggered pseudo-conservative formulation as time variation is controlled by fluxes on edges of the element and we implement in a 2D geometry both source excitation and absorbing boundary conditions as PML zones. Simple illustrations show that this method could be a competitor of more traditional finite-difference methods.
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Towards More Geologically-Consistent Seismic Amplitude Modelling
Authors L. T. Brown, V. L. Ball, R. J. Bosley, G. D. Myers, F. D. Lane and M. I. ChallisHow can the potential value of seismic attributes for predicting reservoir properties in a vertically and horizontally heterogeneous reservoir be assessed? We answer this question by combining statistical modeling of the subsurface with theoretical modeling of its seismic response. Starting with a geocellular model with depofacies classes, lithology layers are simulated using a conditional-hierarchical approach. Elastic properties are subsequently populated using lithology trends to which variability is added at multiple, geology-based scales. Offset synthetics are generated and interrogated for the predictive relationship between net-pore-feet of reservoir and lambda-rho. The improvement in reservoir prediction with improved seismic bandwidth is quantified - a key piece of information in deciding whether to reprocess or reacquire seismic data.
This paper presents an integrated workflow for reservoir characterization. A realization of a synthetic seismic volume is generated, based on a coarse, reservoir-simulation-scale model and fine-scale lithology and elastic property information from well logs. Elastic properties are modeled in a geologically-consistent manner, and the correlation between reservoir properties and seismic attributes can be calculated for different levels of data quality. The synthetic volume can be compared with actual seismic data for validation of the reservoir model.
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Phase Velocity Approximation in a Finely Layered Medium
By A. M. StovasThe effect of multiple scattering in finely layered sediments is of importance for stratigraphic interpretation, matching of well log-data with seismic data and seismic modelling. In our paper we derive a new approximation for the phase velocity in an effective medium show how it depends on the strength of the reflection coefficients. Approximation is tested on the real well log data example.
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DeliveryMassager - Propagating Seismic Inversion Information into Reservoir Flow Models
Authors J. Gunning and M. E. GlinskyWe present a new open-source program for the integration of stochastic seismic inversion information generated from the freeware Bayesian seismic inversion tool Delivery (Gunning and Glinsky 2004) with reservoir simulation models commonly used in modeling and flow-simulation packages. This modeling tool is able to integrate the full range of complex knowledge inferable from probabilistic seismic inversion with auxiliary geological and petrophysical information to produce an integrated model; a process we call massaging. The major innovative achievement of this code is the synthesis of multi-layer multi-property correlations inferable by the inversion with the transverse correlations induced by geological processes. The massaged model(s) are then directly suitable for uncertainty studies of volumetrics, scenarios explorations, or fluid recovery risking.
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Multi-Scale Heterogeneity Modelling - A Method for Reservoir Data Integration
By R. WenAn ongoing challenge in reservoir modelling workflows is to integrate multi-scale heterogeneity into the models for the purpose of evaluating full-field reservoir performance. We present a modelling and upscaling approach to integrate reservoir data at multiple grid scales. The approach focuses on generating 3D sub-seismic-scale (< 10 m) stratigraphic heterogeneity models. The workflow begins with a conceptual interpretation of reservoir geology based on core and outcrop observations, and seismic interpretation. A high-resolution grid is generated based on sedimentary processes, and conditioned by a seismic interpretation framework grid. The grid is then populated with petrophysical data and upscaled by numerical simulation methods. The upscaled models reflect the effective petrophysical properties of the rocks and can be calibrated to well logs and seismic data. Using the same approach for simulating cm-scale bedding structures, we have developed algorithms to generate detailed geometrical grids representing internal stratification and large-scale bedding in point bars, tidal bars, and lobes. Direct upscaling of reservoir properties from sub-seismic stratigraphic models allows geoscientists to evaluate flow properties from multiple geological scenarios. By considering multi-scale heterogeneity, reservoir asset teams can effectively integrate reservoir data and improve the prediction of reserve volumes and production profiles.
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The Use of Modern Analogues in Shared Earth Modelling - Case Study of the Cook Formation, Oseberg Field, Offshore Norway
Authors R. A. J. Thomassen, R. A. F. Dalman, M. E. Donselaar, T. Dreijer, S. M. Luthi and G. ToxopeusShared earth modeling aims to use subsurface data sets with a wide range of scales for the modelling and visualization of reservoir bodies in the subsurface. In the present paper the shared earth modeling of potential reservoir bodies in a complex sedimentary environment (estuarine Cook Formation, offshore Norway) is extended with the aim to identify subtle stratigraphic shapes in real migrated seismic data. First, core, wireline log and seismic data were integrated to interpret the estuarine depositional environment and general spatial distribution of the reservoir bodies therein. Next, quantitative size and shape data were extracted from sediment bodies in modern reservoir analogues in the Gironde and Western Scheldt Estuaries. These data were then compared with the shapes and spatial extent of inclined reflectors in seismic cross sections and plan views. This approach yielded the detailed interpretation of reservoir bodies in the Cook Formation as sand bars and adjacent channels in an estuarine environment. Sizes, shapes and orientation of the reservoir bodies very well compare with the modern analogues. The extension of the shared earth modeling with quantitative data from modern analogues resulted in an improved visualization of the reservoir architecture.
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Seismic Inversion for Lithology and Petrophysics
Authors T. Coleou, R. Bornard, F. Allo, J. G. Hamman and D. H. CaldwellIn this paper, we present a method for the direct stratigraphic inversion of lithology from pre-stack seismic data. This approach is a generalization to discrete variables of a methodology for petrophysical seismic inversion of continuous attributes.
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Diffractor Scan (DSCAN) for Attenuating Scattered Energy
Authors N. Gulunay, M. Magesan and J. ConnorWe present a method of attenuating diffracted noise in marine seismic surveys. Such noise originates from heterogeneities near shallow sea beds. We locate position of such obstructions using semblance scan analysis, and then extract the noise from the traces at the calculated travel times for each diffraction trajectory.
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Predicted Scattered Guided-Wave Removal from Shallow Marine Data
Authors J. Beishuizen and G. C. HermanWe discuss a deterministic technique to predict and remove scattered coherent noise from shallow marine data. It is based on a mathematical model of guided-wave wave propagation and scattering, and is almost completely data-driven. Application of the method to a shallow-marine data set demonstrates that it effectively reduces scattered noise without seriously affecting reflection energy.
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Structurally Consistent Filtering
Authors H. C. Hoeber, S. Brandwood and D. N. WhitcombeIn this paper we review some commonly used image filters with respect to their structural consistency, suggest modifications for improved edge and detail preserving filtering and introduce two new filters. We show the strengths of these filtering techniques both with and without local dip consistency.
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3D Seismic Amplitude Filtering Using Dip Steered Geostatistics
Authors C. Magneron, L. Sandjivy and M. VasseurIt is now commonly acknowledged that spatial filters using geostatistics (factorial kriging) may prove useful when applied to seismic cubes in several operational contexts such as spatial quality check, noise reduction, 4D equalization and spectral decomposition. But even when based upon a reliable co-regionalization model defined along the main inline, crossline and time or depth axes, these spatial filters are opened to criticism as no structural dipping information is taken into account. A dip steered factorial kriging solution is presented which consists in introducing local dip and azimuth information into the kriging equations system, thus leading to more geologically consistent results.
A real data case study illustrates the contribution of the dip steered factorial kriging when applied to a post-stack amplitude cube for noise reduction purpose: The coherency of the geological seismic events is enhanced when compared to results obtained by factorial kriging disregarding structural dipping.
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A Data-Mapping Approach to Redatuming
Authors S. Tegtmeier, D. J. Verschuur and A. GisolfBy incorporating a redatuming step in the seismic processing stream, the quality of the seismic image will be improved. An important issue with respect to redatuming is the application to 3D data acquired by conventionally used acquisition designs. Because of the sparseness of these data, it is impossible to apply the conventional wave-equation datuming method.
In this paper, we present a new redatuming methodology, which follows the concept of Kirchhoff data mapping. The proposed approach is tested on 2D and 3D synthetic data sets and shows satisfying results.
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Robust Estimation of Large Surface-Consistent Residual Statics
More LessSurface-consistent residual static correction is applied to land data on a routine basis. However, if the statics are large, conventional methods often break down due to, among other reasons, the difficulty to construct a good pilot trace. We developed a large static correction method that constructs the pilot trace by a local robust L1-norm inversion at each CDP location. The local inversion calculates the relative time shifts of the traces within a CDP gather by minimizing the trace-to-trace time difference. A pilot trace is obtained by stacking the CDP gather after aligning the reflectors by applying the time shifts to the traces. Finally, a global L1-norm inversion resolves the source and receiver statics from the time shifts of traces of all CDP gathers relative to their pilot traces. The method is demonstrated with synthetic and real data examples.
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Seismic Trace Matching to Well Logs in a Weakly Non-Linear Earth
By J. DuBoseOne of the well established techniques for matching seismic data to well logs is to calculate and apply simple matching filters. When following this approach, one minimizes the residual error between the filtered trace and a nearby reflectivity well log. This is done in the least squares sense by adjusting coefficients of the filter.
That is, minimize Formula 1,
where
F is the filter
S is the seismic trace
R is the reflectivity log
L and M define the span of the filter
Setting partial derivatives of this functional to zero, leads to an easily solved set of linear equations. After the filter is determined, it will typically be applied to many traces in the vicinity of the well.
A key physical assumption is that stress is strictly proportional to strain as seismic waves propagate through the earth.
Proposed in this paper is a method to compensate for non-linear effects and attain a much better match with well logs.
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Time-Frequency Attributes
Authors E. Zabihi Naeini and H. R. SiahkoohiLong offset analysis of seismic data has gained interest during the past years, both for conventional AVO-analysis and time-lapse seismic data. A common problem for such analysis on marine seismic data is the presence of overburden generated refraction noise, similar to the ground roll noise problem of land data. A synthetic study shows that it is possible to diminish this overburden noise by beam-steering the source array. In marine seismic acquisition this is a well known technique, and we find that for a synthetic modeling example the beam-steering technique might attenuate this type of noise by a factor of 2-3, possibly enabling a better utilization of long offset seismic data.
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Attenuation of Long Offset Ground Roll Noise by Source Beam-Steering
Authors H. Mehdizadeh and M. LandrøThis paper demonstrates the use of plane-wave annihilator (PWA) filters, a powerful tool for estimating dips in seismic data. Compared to conventional dip scanning methods, the PWA technique provides a more accurate and robust dip field estimate without any dip limitation. It also provides a coherency measure, useful for QC, at each analyzed point. We compare the accuracy and efficiency of two PWA dip estimation algorithms with a conventional dip scanning method on the BP EAGE 2004 velocity benchmark dataset. The estimated dip fields produced with PWA filters are very high resolution and correlate well with the seismic data.
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High-Resolution Dip Estimation Using Plane-Wave Annihilator Filters
Authors W. Mao and R. FletcherA new parametric method for phase velocity estimation with an array of sensor is presented. It does not need a preliminary group velocity calculation and it estimates the function as a whole, providing a continuous and analytic expression. The novelty of the method stands in the velocity model and the initialization of the estimation. The model is based on a spline. This makes the functions smooth enough to guarantee the method's robustness to noise and undesired waves. The search of the best velocity function is initialized with a rough estimate provided by a frequency-wavenumber transform of the data, which highly helps the convergence. This method has been applied to synthetic and real surface wave signals. In both cases the signals corrected with the estimated velocity have a good alignment, in nearly all the spectrum of the wave.
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Parametric Phase Velocity Estimation with an Array of Sensors - Application to Dispersive Waves
Authors C. Kotenkoff, J. L. Lacoume and J. MarsSVD (singular value decomposition) is a coherency-based technique that provides both signal retrieval and noise suppression. It has been implemented in a variety of seismic applications - mostly on a global scale only. We use SVD to improve the signal-to-noise ratio of prestack seismic gathers, but apply it locally to cope with
signals that vary both with time and offset.
SVD is based entirely on second order statistic (i.e., the covariance matrix) which are optimal only if the data is white and Gaussian. Independent component analysis (ICA) can overcome these restrictive assumptions and takes advantage of higher order statistics (beyond 2nd order).
Local SVD/ICA techniques are compared with f-x deconvolution for improving the signal to noise ratio of prestack NMO-corrected CMP gathers. The local SVD/ICA methods are better than f-x deconvolution in removing background noise but they perform less well in enhancing the lateral coherency of weak events and/or events with conflicting dips. Combining f-x deconvolution with SVD/ICA signal enhancement overcomes the main weaknesses associated with each individual method and leads to the best results.
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Local SVD/ICA for Signal Enhancement of Pre-Stack Seismic Data
Authors M. Bekara and M. van der BaanWe propose a data reconstruction based on a combination of migration Kirchhoff-style algorithm that migrates coefficients obtained by curvelet decomposition of seismic traces over time and a demigration scheme that generate data to the desire location. Curvelet data reconstruction involves four steps: Curvelet decomposition of the seismic data, thresholding of the resulting curvelet coefficients, multiscale Kirchhoff like migration, and demigration or data reconstruction from the multiscale images.
The migration procedure applied to each curvelet scale is the same as conventional Kirchhoff migration but operates on curvelet coefficients. Since only the curvelet coefficients are migrated, the cost of curvelet-based migration is reduced compared to that of conventional Kirchhoff migration.
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Seismic Data Reconstruction Using Curvelets
More LessWhen smoothing a function with high-frequency noise by means of optimal cubic splines, it is often not clear how to choose the number of nodes. The more nodes are used, the closer the smoothed function will follow the noisy one. In this work, we demonstrate that more nodes mean a better approximation of Fourier coefficients for higher frequencies. Thus, the number of nodes can be determined by specifying a frequency up to which all Fourier coefficients must be preserved. A comparison of the corresponding smoothing results with those obtained by filtering using a moving average of corresponding length and a lowpass with corresponding high-cut frequency show that optimal cubic splines yield better results as they preserve not only the desired low-frequency band but also important high-frequency characteristics.
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A Frequency Criterion for Smoothing with Optimal Cubic Splines
Authors J. Schleicher and R. BilotiMarine surveys acquired in deep-water areas often exhibit static variations between sail-lines that are the result of changes in sea level elevation (due to tides) or/and in water velocity. They manifest themselves as lateral discontinuities (or jitters) on cross-line sections or on 3D CDP gathers and can lead to stack deterioration. In this paper, we review the methodologies available to correct for these two types of variations. We will first show how GPS data can be used to compensate accurately for tidal statics. We then propose to correct for water velocity variations using a direct measurement on seismic gathers. The objective is to replace the measured water velocity (varying from sail-line to sail-line) by a constant water velocity that is consistent across the whole survey. This is the key point of the methodology as all the sail-lines (and all the vintages in 4D) will be re-aligned to this spatially constant water velocity. The main advantage of this method is that it does not rely only on a water bottom time measurement, which could be biased by residual tidal statics.
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Correction for Water Velocity Variations and Tidal Statics
Authors C. Lacombe, J. Schultzen, S. Butt and D. LecerfLate Miocene pockmarks have been identified and mapped in 3D seismic surveys covering large parts of the Danish Central Graben. The upper Miocene succession comprises a thick westward progradational wedge of clay, silt and sand grade siliciclastic sediments. The majority of the pockmarks are located in the clay prone toesets. More than 250 pockmarks were identified by changes in reflection amplitude and continuity on time-slices and amplitude maps. The length, width, depth, and bottom geometry of each of the observed pockmarks was quantified and recorded in order to facilitate a statistical analysis of their relation to depositional and structural elements that may have affected their distribution and morphology. The pockmarks are generally elliptical in plan view with ellipticity between 1 and 11, the majority having ratios between 2 and 6. In cross section the pockmarks have U- V- and box-like shapes, and could be mistaken for channel incisions. The orientation of the elliptical pockmarks is clustered around 150° (NNW-SSE), parallel to the general strike of the Miocene clinoforms. The marked ellipticity suggests an influence of contour-parallel bottom currents whilst the toeset location suggests that pockmarks only formed in the fine-grained (sealing) parts of the depositional system.
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Extended Elastic Impedance and Its Relation to AVO Crossplotting and Vp/Vs
Authors G. J. Hicks and A. M. FrancisExtended Elastic Impedance (EEI) has proved to be a highly convenient framework for seismic AVO studies. EEI logs can be directly related to the petrophysical properties of interest and seismic EEI reflectivity volumes can be obtained directly from the prestack data via linear projection in sin2theta. The directness of the EEI method is one of its strengths, making it unnecessary to perform intermediate steps such as examining conventional AVO attributes (e.g., intercepts and gradients). Consequently it can be difficult for those more familiar with the conventional AVO analysis to understand the parallels between the two methods. Once these parallels are understood we show how they can be exploited to provide new AVO attributes for background Vs/Vp ratio and AVO class based on the EEI seismic projection method.
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Seismic Acoustic Impedance Inversion in the Pre-Stack Domain
More LessRay impedance concept, proposed by Wang (2003), is a physical property measurement, based on pre-stack seismic data, and is an extension of the conventional acoustic impedance computation. After seismic data transformation from the time-offset distance plane to the intercept time-slowness plane, the acoustic ray impedance is implemented along the constant p trace, in the same way as in conventional acoustic impedance computation along a stack trace. The latter is often called the pseudo-logging method.
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AVO-Inversion for Postcritical Reflections
Authors L. V. Skopintseva and T. V. NefedkinaWe propose a technique for determining elastic properties of a medium by the AVO-inversion for postcritical reflections. The inverse problem is solved by Nelder-Mid optimization method. Numerical experiments were used for testing the accuracy and stability of the algorithm for different offset ranges and initial models. It is shown that the velocity estimations can be obtained with the error <1% for all the offset ranges considered, if noise is absent. In the presence of noise the accuracy of the S-wave velocity estimation is lower at the precritical offsets. Using the nearcritical and postcritical offsets we obtain the errors <1% in the P-wave velocity and <6% in the S-wave velocity , when signal/noise ratio is >5
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A New Technique for Pressure - Saturation Separation from Time-Lapse Seismic - Schiehallion Case Study
Authors M. Floricich, C. MacBeth, J. Stammeijer, R. Staples, A. Evans and C. DijksmanA technique is shown for estimating pressure and three phase saturation changes from 4D seismic. The technique is a further development of a recently published method which does not require defined rock and fluid physics relationships. A multi-attribute approach is taken, in which 4D seismic attributes are linked (by relationships with a physically reasonable form) to production data, and an optimal combination of seismic attributes are then selected to perform a bayesian inversion separating pressure and saturations. The methodology is applied to the Schiehallion field in the West of Shetlands with encouraging results. It has highlighted saturation changes, previously hidden by the effect of pore pressure on the rock frame, which can be validated against production and tracer data. Additionally, it has shown areas where gas is coming out of solution that have been validated against information acquired in a recently drilled well.
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Reducing Risk and Monitoring Water Injection Using Time-Lapse (4D) Seismic at the Ekofisk Field
Authors G. Byerley, K. O. Roervik, J. Pedersen and K. RanaweeraOver the past decade, time-lapse (4D) seismic has evolved into a valuable reservoir-monitoring tool used on numerous fields throughout the world. 4D seismic has been used at the Ekofisk field to monitor water injection and reservoir depletion. The 4D seismic signal is complex and can be broken down into two main components. The first and most apparent of these being 4D travel time differences caused by the geomechanical effects of compaction and stress changes occurring in the reservoir and overburden. The second and subtler component of the 4D signal is an amplitude difference caused by impedance changes occurring as the reservoir responds to water injection and pressure depletion. The combined response from these two 4D attributes has been used to optimize well placement and reduce the risk of drilling into water swept areas of the reservoir.
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Revealing the Secrets of the Troll 4D Seismic Response - Case Study Using Compound Modelling
Authors R. Ona, N. Skjei and S. Leiknes4D seismic data is extensively used in both planning and in geo-steering horizontal wells in the thin oil leg on Troll West. The 4D signal is complex due to a thin oil column, a mix of high and low permeable sands and gas saturation changes caused by pressure drop from production. Several seismic modelling studies have been conducted to understand the 4D response on the Troll field. On the 2003 monitor survey a deep 4D response was observed on the difference cubes, which has not been evaluated in earlier 4D modelling studies. To improve the interpretation of the time-laps seismic and to understand the reservoir dynamics, a more advanced seismic modelling study was initiated to take account for the increasing complexity of the 4D signal. The results from this study show that both low and high permeable sands are drained during production and that the deep 4D event is caused by gas out of solution due to pressure drop in the residual oil zone below the interpreted OWC. The modelling results have implications for both the geological and the reservoir model.
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4D Probabilistic Inversion to Detect Remaining Oil in Troll West Oil Rim
Authors M. Kleemeyer, J. W. M. Dankbaar, R. Staples and J. G. F. StammeijerThe 4D ROVER (Remaining Oil Volume Estimation And Redistribution) project aimed to provide a quantitative measure of remaining oil in the oil rim from the Troll West field, by deploying the Shell proprietary probabilistic layer-based inversion tool to the license partners. Due to structural complexity of the static model, the three reservoir fluids and a complicated 4D signal, a stepwise inversion approach proved most successful, solving for the most uncertain reservoir parameters first.
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