Migration-velocity Analysis Using Image-space Generalized Wavefields

Summary Seismic exploration of geothermal resources in the German Molasse Basin (GMB) requires a proper seismic imaging of deep productive horizons. Success of this process is heavily reliant on the correct estimation of seismic velocities by migration velocity analysis (MVA). However, the MVA has a high computational cost and, depending on the quality of seismic data and approach chosen for the analysis, it might produce erroneous results. In this work, we developed an MVA workflow that enables to create a velocity model that is consistent with seismic, geological and borehole data, despite the limitations inherent to the MVA. A 2D seismic data set acquired in the GMB was used to test the MVA. The results were evaluated based on four criteria: increase in seismic resolution, availability of good quality reflectors beneath velocity components, geological plausibility and consistency with borehole data. Such an interpretive approach to the initially purely processing procedure allowed to discriminate genuine velocity components from erroneous components. Our work shows that the MVA produces an accurate velocity model of the subsurface, if the data have good quality, sufficient cable length, and geological constraints in form of lithological and velocity logs from boreholes in the vicinity of a study area.

Summary There has been a continually expanding interest in wide azimuth (WAZ) data in all its variations. We are now challenged to take full advantage of the additional information available with minimal compromise. It has been shown that the wider azimuths alone do not supply a dataset sufficient for the full range of processing that must be applied for quality imaging. The narrow azimuth (NAZ) portion of the data is also necessary for shallow imaging as well as multiple prediction and attenuation. Final imaging and both 3D Surface Related Multiple Elimination (SRME) and Wave Field Extrapolation (WFE) methods of demultiple benefit from the presence of near offset / narrow azimuth data. In this paper we present the use of narrow azimuth data from a pre-existing survey to enhance the overall azimuth and offset distribution in a wide azimuth survey with primary emphasis on better prediction of multiples with WFE method.

ABSTRACTWave‐equation migration velocity analysis is a technique designed to extract and update velocity information from migrated images. The velocity model is updated through the process of optimizing the coherence of images migrated with the known background velocity model. The capacity for handling multi‐pathing of the technique makes it appropriate in complex subsurface regions characterized by strong velocity variation. Wave‐equation migration velocity analysis operates by establishing a linear relation between a slowness perturbation and a corresponding image perturbation. The linear relationship and the corresponding linearized operator are derived from conventional extrapolation operators and the linearized operator inherits the main properties of frequency‐domain wavefield extrapolation. A key step in the implementation is to design an appropriate procedure for constructing an image perturbation relative to a reference image that represents the difference between the current image and a true, or more correct image of the subsurface geology. The target of the inversion is to minimize such an image perturbation by optimizing the velocity model. Using time‐shift common‐image gathers, one can characterize the imperfections of migrated images by defining the focusing error as the shift of the focus of reflections along the time‐shift axis. The focusing error is then transformed into an image perturbation by focusing analysis under the linear approximation. As the focusing error is caused by the incorrect velocity model, the resulting image perturbation can be considered as a mapping of the velocity model error in the image space. Such an approach for constructing the image perturbation is computationally efficient and simple to implement. The technique also provides a new alternative for using focusing information in wavefield‐based velocity model building. Synthetic examples demonstrate the successful application of our method to a layered model and a subsalt velocity update problem.

Electrical and electromagnetic methods in areas of complex geology

ABSTRACTWe present a migration velocity analysis (MVA) method based on wavefield extrapolation. Similarly to conventional MVA, our method aims at iteratively improving the quality of the migrated image, as measured by the flatness of angle‐domain common‐image gathers (ADCIGs) over the aperture‐angle axis. However, instead of inverting the depth errors measured in ADCIGs using ray‐based tomography, we invert ‘image perturbations’ using a linearized wave‐equation operator. This operator relates perturbations of the migrated image to perturbations of the migration velocity. We use prestack Stolt residual migration to define the image perturbations that maximize the focusing and flatness of ADCIGs.Our linearized operator relates slowness perturbations to image perturbations, based on a truncation of the Born scattering series to the first‐order term. To avoid divergence of the inversion procedure when the velocity perturbations are too large for Born linearization of the wave equation, we do not invert directly the image perturbations obtained by residual migration, but a linearized version of the image perturbations. The linearized image perturbations are computed by a linearized prestack residual migration operator applied to the background image. We use numerical examples to illustrate how the backprojection of the linearized image perturbations, i.e. the gradient of our objective function, is well behaved, even in cases when backprojection of the original image perturbations would mislead the inversion and take it in the wrong direction.We demonstrate with simple synthetic examples that our method converges even when the initial velocity model is far from correct. In a companion paper, we illustrate the full potential of our method for estimating velocity anomalies under complex salt bodies.

Imaging the subsurface, the ultimate goal of the seismic method, can be done in different ways and in different domains: time or depth. Although depth migration has become almost mandatory in areas of complex geology— because it accounts for traveltime nonhyperbolic moveout, it has, in fact, quite a limited purpose—to convert seismic data from one form to another for a given velocity model. Time imaging provides sufficient information for a subsurface of moderate complexity. Moreover, even for complex areas that require depth migration for correct subsurface imaging, time imaging usually constitutes a key first step that facilitates the estimation of a velocity model for depth imaging. For these reasons, improving the quality of time imaging is a focus of intensive research. A recent advance is multifocusing (MF), a method with the potential to greatly improve the quality of time imaging.

3D Seismic in Areas of Poor Data and Complex Geology of South Oman. Abstract This paper describes the problems of 3D seismic data in the complex geology of South Oman fields. Weak reflectors, strong multiples and ground roll result in poor seismic. However, 3D seismic has opened a wealth of hidden information. Amplitude maps (Marmul field) have revealed areas of the Gharif reservoir not previously recognized. In the more complex Al Khlata reservoir, seismic modeling indicates that sand/diamictite interfaces can locally give a reflector. This was successfully tested by a horizontal sidetrack and followed by three horizontal wells. In addition, gross seismic character (SW Marmul area) could be related to fluid movements in the reservoir. Introduction The geology of South Oman fields varies from the more predictable Gharif and Haima reservoirs to the more complex periglacial Al Khalta reservoirs. A simplified sketch (figure 1) shows the 3 reservoirs from a typical South Oman Field, sealed by the Nahr Umr shale and overlain by a thick layer of the cretaceous Natih carbonate. Interpretation of seismic data has been hampered by the generally poor data at reservoir level. Strong interbed multiples (figure 2) obscure primary events. Scattered ground roll confuses the interpretation of steeply dipping reflectors. The old 2D seismic was used with difficulty for structural interpretation at reservoir levels and maps were mainly based on well control. The coming of 3D seismic was a big step forward not so much in data quality but in the ability to view data in different directions. Weak events submerged in noisy data are effectively captured by steering through 3D data. 3D seismic surveys in South Oman started in 1989 and now cover nearly all the fields. SEISMIC DATA QUALITY Good continuous events define the Natih carbonate interval above the Nahr Umr shale. However, seismic data quality of the Gharif/Al Khlata reservoirs is variable. Figure 3 shows 3 data vantages across 3 South Oman fields (Thuleilat Marmul and Umr). The Thuleilat data (figure 2a) is poor partly because of the low acquisition fold (5-6 traces/ bin at the reservoir level). Acquisition at higher fold is not necessarily the solution. Imaging of Al Khlata paleovalleys illustrate this point. The Marmul paleovalley (figure 3b) is seismically visible (32 traces lbm), in contrast to the Nimr paleovalley (96 traces/bin - figure 3c). Thick layers of Al Khlata sands and diamictites fill the Marmul paleovalley reflecting strong seismic events. A geological model of the Marmul paleovalley lined with a hard (high acoustic impedance) basal diamictite (figure 4a) generates a seismic response (figure 4b) which matches the observed seismic profile (figure 3b). Absence of the basal diamictite (possibly a Nimr type paleovalley) shows a poor match with the seismic (figure 4c). ACQUISITION AND PROCESSING PROBLEMS: Near surface problems, low fold of shallow data. shooting skips over surface facilities can result in 3D data artifacts. However, the main source of interpretation pitfalls has been strong interbed multiples interfering with primary events. Normal stacking of traces will not remove interbed multiples. The velocity function (figure 2a) between the intra-Natih reflectors (i.e. the multiple generators) and the reservoir intervals, is nearly constant. Image processing has however improved multiple suppression. The method depends on pattern recognition, where multiple generating horizons (the intra-Natih reflectors) are flattened together with the rest of the data. Multiples appear as parallel events and a filter is designed to remove them. However, the method is not effective when multiple generators are divergent. Over the eastern half of the section (figure 5) multiples are successfully suppressed. However, over the western half of the section, the intra-Natih reflectors cascade divergent multiples which are not removed. A multiple was interpreted as the Top Haima event leading to an incorrect prognosis of MM-251. Capturing steep reflectors can be a problem where scattered ground roll occurs. Source and geophone arrays will not effectively attenuate ground roll scattered from shallow wadi deposits. It is also not possible to optimize source and geophone arrays under variable near surface conditions. P. 473

As we know, the notches of source ghost and receiver ghosts limit seismic frequency bandwidth, which affects seismic imaging and reservoir inversion. Deghosting is a key and difficult step in broadband processing for marine data. In order to attenuate the ghosts, a new deghosting method on variable-depth streamer data is developed based on wavefield extrapolation and iterative inversion of wave equation. In this method, a new wavefield extrapolation algorithm is developed based on initial irregular surface. Appling this extrapolation the mirror data is generated from variable-depth streamer shot gather. Then the mirror data are applied to the conjugate inversion algorithm, and the deghosted data can be obtained by an iterative inversion of wave equation. By testing on synthetic data and real seimic data from deep water area of South China Sea, it is shown that the proposed deghosting method can effectively attenuate ghosts and broaden seismic data bandwidth, which provides high-quality broadband seismic data for further seismic interpretation and inversion.

In recent years an increasing amount of new scientific data on the structure and nature of the whole continental crust has been acquired from deep seismic profiling. The Bay of Biscay profile was recorded in 1984 at sea along the Aquitaine platform for the French ECORS group in association with Hispanoil. It collected a unique set of two-ship multi-channel seismic data with constant offset and expanded spread profiles in a complex geological area. The data are of very good quality and contain clear Moho and lower crust reflections as seen under most parts of Western Europe where similar experiments have been conducted1–4. This profile provided for the first time the most convincing evidence for an anomalously thin crust below a rifted basin, the Parentis Basin, which is directly connected to the oceanic area of the Bay of Biscay. It gives new constraints for understanding the formation of extensional sedimentary basins.

Anisotropic models are recognized as a more realistic representation of the subsurface where complex geological environment exists. These models are widely needed by all kinds of migration and interpretation schemes. However, anisotropic model building is still a challenging problem in the industry. In this paper, we propose an approach to building anisotropic models from surface seismic data based on the theory of WaveEquation Migration Velocity Analysis (WEMVA). Because of the ambiguity between depth and δ , we parametrize our model space using only NMO velocity (Vnmo) and the anellipticity parameter η . We present a synthetic data example of anisotropic WEMVA applied to the shallow part of the Hess anisotropic synthetic VTI model. The results show that anisotropic WEMVA is effective in resolving the anisotropic perturbation; however, a unique solution to the inversion requires additional constraining information.

An important research topic for prospecting seismology is to provide a fast accurate velocity model from pre-stack depth migration. Aiming at such a problem, we propose a quadratic precision generalized nonlinear global optimization migration velocity inversion. First we discard the assumption that there is a linear relationship between residual depth and residual velocity and propose a velocity model correction equation with quadratic precision which enables the velocity model from each iteration to approach the real model as quickly as possible. Second, we use a generalized nonlinear inversion to get the global optimal velocity perturbation model to all traces. This method can expedite the convergence speed and also can decrease the probability of falling into a local minimum during inversion. The synthetic data and Marmousi data examples show that our method has a higher precision and needs only a few iterations and consequently enhances the practicability and accuracy of migration velocity analysis (MVA) in complex areas.

In areas of complex geology, prestack depth migration is often necessary if we are to produce an accurate image of the subsurface. Prestack depth migration requires an accurate interval velocity model. With few exceptions, the subsurface velocities are not known beforehand and should be estimated. When the velocity structure is complex, with significant lateral variations, reflection‐tomography methods are often an effective tool for improving the velocity estimate. Unfortunately, reflection tomography often converges slowly, to a model that is geologically unreasonable, or it does not converge at all. The large null space of reflection‐tomography problems often forces us to add a sparse parameterization of the model and/or regularization criteria to the estimation. Standard tomography schemes tend to create isotropic features in velocity models that are inconsistent with geology. These isotropic features result, in large part, from using symmetric regularization operators or from choosing a poor model parameterization. If we replace the symmetric operators with nonstationary operators that tend to spread information along structural dips, the tomography will produce velocity models that are geologically more reasonable. In addition, by forming the operators in helical 1D space and performing polynomial division, we apply the inverse of these space‐varying anisotropic operators. The inverse operators can be used as a preconditioner to a standard tomography problem, thereby significantly improving the speed of convergence compared with the typical regularized inversion problem. Results from 2D synthetic and 2D field data are shown. In each case, the velocity obtained improves the focusing of the migrated image.

PreviousNext No AccessSEG Technical Program Expanded Abstracts 2005Challenges and opportunities in pre‐stack depth imaging of legacy seismic data: an overthrust belt case studyAuthors: Walt RitchieMihai PopoviciMoritz FleidnerWalt Ritchie3DGeo Development Inc; Chris Saxon, ChevronTexacoSearch for more papers by this author, Mihai Popovici3DGeo Development Inc; Chris Saxon, ChevronTexacoSearch for more papers by this author, and Moritz Fleidner3DGeo Development Inc; Chris Saxon, ChevronTexacoSearch for more papers by this authorhttps://doi.org/10.1190/1.2142225 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract Over the past five years, the introduction of affordable 3‐D pre‐stack depth migration (PSDM) has changed industry perspective on how to approach seismic data in areas of complex geology. The ability to define an accurate interval velocity model and economically perform 3‐D pre‐stack imaging in depth provides the opportunity to more fully exploit the hydrocarbon potential of these difficult areas. Legacy or vintage data sets are also amenable to reexamination using these new pre‐stack depth imaging tools. However, the PSDM method is more demanding of the data requirements than conventional time techniques to deliver the degree of velocity model accuracy required for optimal spatial positioning of target reflectors. This paper discusses a seismic reprocessing program intended to assess deep gas potential in a mature producing province with very complex overthrust geology using the PSDM method. It highlights the challenges presented by vintage seismic 3‐D acquisition methodologies in an older rock/fast velocity regime and the opportunities from fully incorporating the wealth of available geologic knowledge into the modelbuilding process. A well‐constrained velocity modelbuilding solution is presented that overcame the limited sensitivity of the available 3‐D seismic data to velocity change due to acquisition geometry in this fast rock regime.Permalink: https://doi.org/10.1190/1.2142225FiguresReferencesRelatedDetailsCited byTomographic velocity analysis and wave-equation depth migration in an overthrust terrain: A case study from the Tuha Basin, ChinaBin Lyu, Qin Su, and Kurt J. Marfurt14 November 2017 | Interpretation, Vol. 6, No. 1Reprocessamento de dados sísmicos da Bacia do Solimões - Migração Prestack x PoststackBruno M. Silva, Leo R. Nascimento, Ronan Ávila, and Roberto R. Baldanza23 November 2007 SEG Technical Program Expanded Abstracts 2005ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 2005 Pages: 2668 publication data© 2005 Copyright © 2005 Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished Online: 07 Dec 2005 CITATION INFORMATION Walt Ritchie, Mihai Popovici, and Moritz Fleidner, (2005), "Challenges and opportunities in pre‐stack depth imaging of legacy seismic data: an overthrust belt case study," SEG Technical Program Expanded Abstracts : 400-403. https://doi.org/10.1190/1.2142225 Plain-Language Summary PDF DownloadLoading ...

Characterization of the Sündikendağı deposit of moganite-rich, blue chalcedony nodules, Mayıslar–Sarıcakaya (Eskişehir), Turkey

This case study shows how the use of new seismic technology allows the recording of good‐quality surface seismic data in an area of complex geology that has historically provided poor seismic data. The enabling technology was 80,000‐lb peak force vibrators using an energetic low‐frequency sweep recording into densely sampled single‐sensor accelerometers. The study area is in the Emirate of Dubai at the edge of the Oman thrust belt. It is covered by a thick layer of dry sand with relatively large sand dunes. The preferred source, based on legacy data, would have been large dynamite charges or a very high source effort with multiple long sweeps to provide broadband data with adequate low‐frequency energy. This "intelligent" acquisition provided better final sections with respect to signal quality within realistic time frames.