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Results
On tomography velocity uncertainty in relation with structural imaging
Messud, Jérémie (CGG) | Guillaume, Patrice (CGG) | Lambaré, Gilles (CGG)
ABSTRACT Evaluating structural uncertainties associated with seismic imaging and target horizons can be of critical importance for decision making related to oil and gas exploration and production. An important breakthrough for industrial applications has been made with the development of industrial approaches to velocity model building. We have developed an extension of these approaches, sampling an equiprobable contour of the tomography posterior probability density function (PDF) rather than the full PDF, and using nonlinear slope tomography. Our approach allows for assessing the quality of uncertainty-related assumptions (linearity and Gaussian hypothesis within the Bayesian theory) and estimating volumetric migration positioning uncertainties (a generalization of horizon uncertainties), in addition to the advantages in terms of computational efficiency. We derive the theoretical concepts underlying this approach and unify our derivations with those of previous publications. Because the method works in the full model space rather than in a preconditioned one, we split the analysis into resolved and unresolved tomography spaces. We argue that resolved space uncertainties are to be used in further steps leading to decision making and can be related to the output of methods that work in a preconditioned model space. Unresolved space uncertainties represent a qualitative by-product specific to our method, strongly highlighting the most uncertain gross areas, thus useful for quality control. These concepts are developed on a synthetic data set. In addition, the industrial viability of the method is determined on two different 3D field data sets. The first one consists of a merge of different seismic surveys in the North Sea and indicates the corresponding structural uncertainties. The second one consists of a marine data set and indicates the impact of structural uncertainties on gross-rock volume computation.
- Europe > United Kingdom > North Sea (0.24)
- Europe > Norway > North Sea (0.24)
- Europe > North Sea (0.24)
- (2 more...)
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling > Seismic Inversion (0.67)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Cross-well tomography (1.00)
- Management > Risk Management and Decision-Making (1.00)
- Data Science & Engineering Analytics > Information Management and Systems (1.00)
ABSTRACT Tomographic ray tracing is the critical step in ray-based tomographic approaches when we need to represent as precisely as possible the wavepaths of the migration. To be able to update velocities from picked information in the migrated domain, one must be able to back propagate the misfocusing information (e.g., the residual moveout [RMO]) into the model through the tracing of tomographic rays. The complex kinematics associated with some types of migration, ray based or not, will give us RMO curvatures that will be impossible to explain with traditional tomographic ray-tracing methods. We have evaluated a generic method for tracing these rays by fitting constraints unique to each migration approach. We found that, on synthetic and real data, using the correct set of equations in the tomographic ray-tracing engine can make a significant difference in the velocity model update.
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (0.91)
- North America > United States > Gulf of Mexico > Central GOM > East Gulf Coast Tertiary Basin > Green Canyon > Block 654 > Shenzi Field (0.99)
- North America > United States > Gulf of Mexico > Central GOM > East Gulf Coast Tertiary Basin > Green Canyon > Block 653 > Shenzi Field (0.99)
- North America > United States > Gulf of Mexico > Central GOM > East Gulf Coast Tertiary Basin > Green Canyon > Block 610 > Shenzi Field (0.99)
- North America > United States > Gulf of Mexico > Central GOM > East Gulf Coast Tertiary Basin > Green Canyon > Block 609 > Shenzi Field (0.99)
Kinematics of common-image gathers — Part 1: Theory
Montel, Jean-Philippe (TOTAL) | Lambaré, Gilles (CGG)
ABSTRACT Common-image gathers are a useful output of the migration process. Their kinematic behavior (i.e., the way they curve up or down) is an indicator of the quality of the velocity model used for migration. Traditionally, when used for migration velocity analysis, we pick structural dips in the common attribute panels (offset, angle, etc.) and residual moveout (RMO) in the gathers. The measured RMO will then tell us how much we need to update the velocity model to improve the gather’s flatness. Understanding the kinematics of the picked events is the key to an accurate model update. This point has been widely underestimated in many cases. For example, when dealing with angle gathers, there is a general assumption that the associated tomographic rays are fully defined by the picked structural dips and the gather opening and azimuth angle, and that if the velocity model is correctly updated down to a given horizon, it is not necessary to shoot the tomographic rays upward through this horizon. We find through an original theoretical analysis that both of these assumptions have to be modified when the gathers exhibit RMO. Using a kinematic analysis, we determine that knowledge of the RMO slopes is necessary to compute the tomographic rays.
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (1.00)
Wide-azimuth seismic data sets are generally acquired more sparsely than narrow-azimuth seismic data sets. This brings new challenges to seismic data regularization algorithms, which aim to reconstruct seismic data for regularly sampled acquisition geometries from seismic data recorded from irregularly sampled acquisition geometries. The Fourier-based seismic data regularization algorithm first estimates the spatial frequency content on an irregularly sampled input grid. Then, it reconstructs the seismic data on any desired grid. Three main difficulties arise in this process: the “spectral leakage” problem, the accurate estimation of Fourier components, and the effective antialiasing scheme used inside the algorithm. The antileakage Fourier transform algorithm can overcome the spectral leakage problem and handles aliased data. To generalize it to higher dimensions, we propose an area weighting scheme to accurately estimate the Fourier components. However, the computational cost dramatically increases with the sampling dimensions. A windowed Fourier transform reduces the computational cost in high-dimension applications but causes undersampling in wavenumber domain and introduces some artifacts, known as Gibbs phenomena. As a solution, we propose a wavenumber domain oversampling inversion scheme. The robustness and effectiveness of the proposed algorithm are demonstrated with some applications to both synthetic and real data examples.
Stereotomography was proposed ago for estimating velocity macromodels from seismic reflection data. Initially, the goal was to retain the advantages of standard traveltime tomography while providing an alternative to difficult interpretive traveltime picking. Stereotomography relies on the concept of locally coherent events characterized by their local slopes in the prestack data cube. Currently, stereotomography has been developed in two and three dimensions, and precious experience has been gained. The expected advantages have been demonstrated fully (in particular, the efficiency and reliability of the semiautomatic stereotomographic picking strategies), and further studies have increased the method’s potential and flexibility. For example, stereotomographic picking can now be done in either the prestack or poststack domain, in either the time (migrated or unmigrated) or depth domain. It appears that the theoretical frame of stereotomography can reconcile, very satisfactorily and efficiently, most methods proposed for velocity-macromodel estimation for depth imaging. Moreover, an extension of the method to full-waveform inversion already exists and opens the way for very interesting developments.
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling > Seismic Inversion (0.48)
Seismic data regularization, which spatially transforms irregularly sampled acquired data to regularly sampled data, is a long-standing problem in seismic data processing. Data regularization can be implemented using Fourier theory by using a method that estimates the spatial frequency content on an irregularly sampled grid. The data can then be reconstructed on any desired grid. Difficulties arise from the nonorthogonality of the global Fourier basis functions on an irregular grid, which results in the problem of “spectral leakage”: energy from one Fourier coefficient leaks onto others. We investigate the nonorthogonality of the Fourier basis on an irregularly sampled grid and propose a technique called “antileakage Fourier transform” to overcome the spectral leakage. In the antileakage Fourier transform, we first solve for the most energetic Fourier coefficient, assuming that it causes the most severe leakage. To attenuate all aliases and the leakage of this component onto other Fourier coefficients, the data component corresponding to this most energetic Fourier coefficient is subtracted from the original input on the irregular grid. We then use this new input to solve for the next Fourier coefficient, repeating the procedure until all Fourier coefficients are estimated. This procedure is equivalent to “reorthogonalizing” the global Fourier basis on an irregularly sampled grid. We demonstrate the robustness and effectiveness of this technique with successful applications to both synthetic and real data examples.
- Geophysics > Seismic Surveying > Surface Seismic Acquisition (1.00)
- Geophysics > Seismic Surveying > Seismic Processing (1.00)
Comparison of different strategies for velocity model building and imaging of PP and PS real data
Herrenschmidt, Ariane, Granger, Pierre-Yves, Audebert, François, Gerea, Constantin, Etienne, Gisèle, Stopin, Alexandre, Alerini, Mathias, Lebegat, Soazic, Lambaré, Gilles, Berthet, Philippe, Nebieridze, Serge, Boelle, Jean-Luc
In 1999, a 2-D/4-C data set acquired with ocean-bottom cable on Mahogany Field, Gulf of Mexico, was distributed at the SEG-EAGE Research Workshop to test the feasibility of using converted waves to image under a salt diapir. We show and discuss results obtained by several imaging methods—some in the time domain, others in the depth domain—applied to both PP and converted-wave fields (PS, also referred to as the C-wave).
- North America > United States > Gulf of Mexico > Central GOM (0.34)
- North America > Trinidad and Tobago > Trinidad > North Atlantic Ocean (0.34)
- Africa > Ghana > Gulf of Guinea (0.34)
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (1.00)
- North America > United States > Gulf of Mexico > Central GOM > East Gulf Coast Tertiary Basin > Ship Shoal South Addition > Block 359 > Mahogany Field (0.99)
- North America > United States > Gulf of Mexico > Central GOM > East Gulf Coast Tertiary Basin > Ship Shoal South Addition > Block 349 > Mahogany Field (0.99)