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79th EAGE Conference and Exhibition 2017 - Workshops
- Conference date: 12 Jun 2017 - 15 Jun 2017
- Location: Paris, France
- ISBN: 978-94-6282-219-1
- Published: 12 June 2017
61 - 80 of 144 results
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Data Space Reflectivity and the Migration based Travel Time approach to FWI
By G. ChaventFull wave inversion (FWI) has received in the last ten years a renewed interest, due to its potential ability to perform simultaneously imaging and migration velocity analysis, But it appeared quickly that its resolution by local optimization methods is hampered by the cycle skip problem unless a very good initial guess of the background velocity is available. One approach suggested to overcome this difficulty is Migration Based Travel Time (MBTT), which uses a data space change of reflectivity unknown to alleviate the cycle skip problem of FWI. The method was originally presented in the time domain and with rather obscure notations, which made it difficult to understand to a geophysical audience. The author's reading of the literature on FWI has confirmed him that the method was essentially not understood. So the objective of this talk is to present the MBTT approach and its key feature, the data space reflectivity, with unified, and hopefully more geophysical notations, and to give, as far as possible, a geophysical insight into the motivations which presided to its inception, and into the reasons which make MBTT able to determine both the long and short wavelength of the velocity starting from a poor initial velocity model.
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Extended Waveform Inversion
By B. SymesFull waveform inversion has evolved into a processing commodity with a firm foothold in the exploration workflow. However FWI still faces several major challenges. One of these is the occasional stagnation of inversion algorithms where no local model perturbation improves data fit. This "cycle-skip" phenomenon can hide kinematic information inherent in the data that would permit large updates in wave velocity fields. One approach to alleviating cycle-skip, extended inversion, transfers the data kinematics to the an extended model with more degrees of freedom than physical models. Variants fall into two categories, according to whether extra degrees of freedom are added to subsurface parameter fields or to source representations. A common theme is that the model should be so extended that the data can be fit throughout the inversion process, thus rendering cycle-skipping impossible. Model updates reduce a penalty that measures distance from the extended model to the original, physical model subspace. Some penalties are mathematically equivalent to a traveltime tomography objective. I will give a rough taxonomy of extended waveform inversion, with several examples, and describe some of the successes achieved and challenges facing this class of methods.
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Optimizing the coefficients of the leading terms of the Born Series: FWI+MVA+more
More LessThe scattering series theoretically utilizes a model perturbation framework to explain the difference between the seismic modeled data corresponding to a background model and those measured in the field corresponding to the real Earth. These perturbations include short wavelength features like those predicted by full waveform inversion (FWI) gradients, and long wavelength features often constrained by migration velocity analysis (MVA) objectives. The Born series, however, is not a convergent series. If the perturbations are large, we probably will not be able to explain the data difference. Thus, using the leading terms of the Born in an iterative process, in which they are scaled properly, allows us to avoid such limitations and update the short and long wavelength components of the velocity model. In fact, the FWI update is manifested in the first term of the Born series, and the MVA update is represented by the transmission (first Fresnel zone) part of the second term. In this case, FWI and MVA are code names for dividing the optimized update to reflectivity based portions and those adequate for the background, respectively. Examples on synthetic and real data demonstrate this logic.
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FWI for elastic media: macrovelocity reconstruction
Authors V. Cherveda, G. Chavent and K. GadykshinCurrently the common approach to perform FWI is nonlinear least square minimization of the standard data misfit functional which characterizes L2 residual between observed data and synthetized one for a current velocity model. As was aforementioned this approach has been developed and studied in a great number of publications. But up to now there are problems with reliable reconstruction of macro velocity component via FWI for realistic frequency bandwidth and offsets. The reason is the structure of the forward map, which transforms the velocity model to the data – it is almost quadratic with a well-conditioned matrix with respect to perturbations of reflectors, but has very complicated nonlinear behavior with respect to propagator perturbation . Intuitively this can be explained by the so-called “cycle- skip” effect when phase shifts of the observed and synthetic data may result in local minima. To mitigate the problem earlier was introduced a multiscale inversion strategy in time and frequency domain when the frequency of the input data is increased progressively and the inversion result for lower frequency becomes an initial guess for the higher frequency. However, such sequential inversion approach also fails due to lack of low frequencies in the data.
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Extending the reach of FWI with reflection data: Potential and challenges
Authors A. Gomes and N. ChazalnoelIn this work, we present a Reflection FWI (RFWI) workflow to update the velocity model using the low-wavenumber component of the FWI gradient of reflection data, the so-called rabbit ears. This is achieved by alternately using high-wavenumber and low-wavenumber components of the gradient to update density and velocity models, respectively. We apply this method to a deep-water survey on the Mexican side of the Gulf of Mexico. The initial model is obtained after diving wave FWI and deep ray-based tomography. However, some discontinuities remain at the deep Wilcox and Cretaceous events. After RFWI application, we observe a significant improvement in these events, both in the migrated image and gathers. Finally, we discuss our observations on the requirements and limitations of RFWI, such as the poor vertical resolution. Furthermore, we make a parallel between this method, conventional FWI and ray-based tomography, identifying potential improvements for RFWI and discussing how to combine these approaches.
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Application of tomographic FWI (TFWI) to large-scale field datasets: challenges and insights.
Authors B. Biondi, A. Almomin and R. SarkarIn the past few years we have demonstrated with blind-test synthetic examples and 3D field-data examples that a time-lag extension of the velocity model can lead to robust convergence of FWI (we called our method Tomographic FWI or TFWI) even when the starting model is far from the correct one. Computational cost and convergence rate are still major obstacles to a routine TFWI application to large-scale projects. We will discuss recent advances in these areas.
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Updating velocity fields beyong the diving waves
More LessConventional least-squares based full waveform inversion (FWI) is not suitable to construct low-wavenumber back- ground model when recorded data is dominated by reflected energy. We present a new approach to address the challenge of building kinematically correct background model with FWI for reflection-dominant seismic data. The new approach de- composes a subsurface model into a smooth background, which is updated via minimizing a new objective function, and a rough reflectivity, which is computed through a migration or least-squares migration at current background. With such a model decomposition strategy and the Born modeling, we are able to directly compute the reflection-based low-wavenumber components of a conventional FWI gradient. To guarantee that these low-wavenumber components contribute to updating background model in correct directions, we developed a new optimization strategy, which consists of two essential compo- nents: first, computing an offset-dependent matching filter to match the predicted Born wavefield and observed reflections; second, measuring the incoherency of this offset-dependent fil- ter along offset and time, and then updating the background to minimize this incoherency.Real data demonstrate the success of the proposed algorithm in constructing kinematically correct models.
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Wave-Equation Migration Velocity Analysis in the Surface Common Offset Domain: Application to Viking Graben Field Data
Authors P. Nandi and U. AlbertinThe quality of subsurface seismic images depends greatly on the accuracy of the velocity model used for imaging, but the highly nonlinear relationship between recorded data and the velocity model makes estimating accurate velocities a nontrivial task. Two methods, full-waveform inversion (FWI) and wave-equation migration velocity analysis (WEMVA), are commonly used to estimate velocities. These methods, both iterative and wave-equation based, estimate velocities by minimizing an objective function to achieve a desired level of accuracy. Full waveform inversion (FWI) can fail in complex environments when the initial model is an inaccurate representation of actual subsurface velocities. Methods based on wave-equation migration velocity analysis (WEMVA), however, use pre-stack image focusing in order to converge to a more global solution regardless of the starting model. We develop WEMVA in the surface common-offset domain by parsing data into separate bins containing a limited range of offset and processing each bin independently. We circumvent cycle-skipping, which can lead to an erroneous result, by using a sufficient number of bins to limit the amount of residual moveout between neighboring traces. We present the results of WEMVA in the common-offset domain on a 2D marine field data set recorded offshore Norway in the Viking Graben area.
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Mitigating the gradient artefacts of Migration Velocity Analysis by Gauss-Newton update
Authors R. Soubaras and B. GratacosThis paper shows how the artefacts present in the gradient of the cost function when performing a Migration Velocity Analysis can be strongly attenuated by using a second-order Gauss-Newton scheme. The artefacts on the velocity gradient are strongly attenuated when the total gradient is deconvolved by the approximate total Hessian. At each iteration, a least-squares migration is produced rather than a migration. The proposed algorithm is illustrated on the Marmousi dataset.
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Image-domain versus data-domain velocity analysis based on true-amplitude subsurface extended migration
Authors H. Chauris, Y. Li and E. CocherWe compare the traditional image-domain migration velocity analysis technique with a new approach developed in the data domain. From a reflectivity model obtained after true-amplitude migration and multiplied by an annihilator, new data are modelled under the Born approximation in the extended domain. The new objective function is simply the l2-norm of the data to be minimised. We discuss the advantages and limitations of the new approach versus the image-domain approach on two synthetic 2d models. The two approaches differ by the nature of the oscillations observed in the velocity gradients and by the relative weights contributing to the velocity update.
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Reflection Waveform Inversion method: solutions to the reflectivity-background coupling problem and consequences on the convergence
Authors R. Valensi, R. Baina and V. DupratA new formulation of the Full-Waveform Inversion (FWI) method called the Reflection Full Waveform Inversion (RFWI) method has been recently introduced in order to enable background velocity updates from reflection events. However, it has been observed (even with state of the art optimization methods) that numerous iterations in the RFWI workflow are necessary to converge towards a correct background model. This numerous iterations makes the cost of the current approach prohibitive for real scale applications. In this contribution, using an analytic model, we explain how the high number of RFWI iterations is related to the background-reflectivity models coupling. We propose two solutions for solving this issue. One solution considers a joint optimization in an extended space considering independent de-migration and migration background models. Another solution consists in a variable projection technique which enables an optimization in a reduced model parameter space where the background and the reflectivity models are always consistent. In our tests, both solutions provide convergence rates of a least one order of magnitude faster than the conventional RFWI method. Furthermore, we provide a geometrical interpretation of the relation between these different solutions and explain why the methodology based on the variable projection should be preferable.
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From RWI to JFWI: including diving waves in reflection-based velocity model building
Authors R. Brossier, W. Zhou, S. Operto, J. Virieux and P. YangIn this presentation, we will review the main limitations of classical FWI for imaging at depth, and the main concepts behind RWI and JFWI. Based on synthetic and real data applications on a 2D OBC datasets collected across a gas cloud, we will show the added value of such schemes and also their limitations mainly associated with the misfit function definition (cycle-skipping and amplitude dependency).
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Frontier Play Concepts and the Tools for Regional Screening
Authors O. Sutcliffe, A. Davies, D. Hay and M. SimmonsThe ability to infill “white space” using testable geological models is necessary for identifying frontier plays. Three principal tools can help explorationists with this task. These tools include gross depositional environment (GDE) maps to define the organisation of facies, regional depth models to aid maturity and reservoir predictions, and geodynamic models to assess the nature and timing of tectonic events. These resources are used to populate a global database of >3,150 proven and unproven predicted plays. This database identified >50 basins that have multiple unproven plays, representing a significant target for frontier exploration. Through the integration of these tools, basins will be high-graded, and the location of the next exploration frontiers will be identified.
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A method for estimating yet-to-find in hydrocarbon plays based on historical results of exploration
By D. QuirkThe background and description of method for calculating, reporting and checking yet-to-find petroleum resources.
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The Value Statement for Applying a Fully Quantitative Approach to Play Mapping
Authors P. Brown, I. Longley and R. YoungWe will show the impact upon real time decision- making of understanding the chance dependencies between prospects, particularly in undrilled or lightly drilled geologic plays. The impact of potential success upon a family of undrilled prospects is often not fully understood or appreciated. GIS technology, along with consistent play mapping using a fully quantified approach (including splitting of chance in to play and prospect components) sometimes reveals that the optimal drilling inventory is not being selected to maximize potential volumes and value.
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The Norwegian shelf - 50 years of exploration, and still attractive
By G. SoilandThe Norwegian Petroleum Directorate (NPD) is a government institution responsible for maintaining a complete inventory of petroleum resources in Norway. This is done in accordance with established resource classification routines, regular reporting from the oil companies and NPD’s geological mapping. NPD compile and publish annually figures on field reserves, contingent resources and YTF resources. Assessments of undiscovered resources is done by applying a stochastic play model method. The remaining undiscovered resources reflect the exploration potential with today’s knowledge and understanding. As new exploration activity proves up new petroleum plays, the potential is adjusted accordingly. This is an estimate of what will be technically possible to find and produce if all prospects are identified and drilled.
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A play-based approach to future oil and gas exploration on the UK Continental Shelf
Authors P. Herrington, J. Bagguley and D. QuirkThe Oil & Gas Authority (OGA) has the remit to ensure exploration activity in the UK is carried out effectively by managing future potential, optimizing value for all stakeholders and ensuring all operations are safe. As part of the objectives, a 2½ year project carrying out play analyses and yet-to-find (YTF) evaluations for petroleum-prone areas of the offshore continental shelf has started. The plans will be presented in this paper.
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Historical review of Play Fairway analysis using modern Play Based Exploration techniques in the Central North Sea, UKCS.
Authors C. Jepps, A. Shafi, F. Lottaroli and L. MecianiThe aim of this project, undertaken in 2015, was to review the accuracy of a petroleum prospectivity study in the UKCS CNS conducted by Lasmo in 1993 by comparing its predicted results to discoveries made since, and to re-predict the results of the original study using modern Play Based Exploration software and best practices. Four plays were analyzed for prospectivity during the project; the Triassic (TRI), Middle Jurassic (J2), Upper Jurassic Shallow (J3s) and Upper Jurassic Deep (J3d). The results show that between 1993 and 2015 an additional 2,082 MMBOE was discovered in the study area for the plays in question. The results validate the YTF concept as all the estimated YTF values based on the 1993 dataset (in the range 2,048-2,476 MMBOE) are “in the right ballpark”. However, if the YTF estimate for the Modern study is accurate, it is likely that all the modelled YTF values have underestimated the remaining hydrocarbon potential. In addition, the results show that the use of modern technologies and best practices produced a more accurate result.
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The 2014 North Carnarvon Basin License Round – a real-world example of the application of modern play tools and techniques in a competitive mature basin exploration arena.
Authors J. Bradshaw and I. LongelyThe 2014 Australian offshore gazettal round was announced in May 2013 with thirteen of the 31 blocks in the North Carnarvon Basin, Australia’s premier oil and gas offshore basin. 12 bids were subsequently received on 6 blocks, which were later awarded with firm and contingent total work program commitments of $33 million and $150 million, respectively.
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