Band-limited scattered wavefield reconstruction beneath complex overburdens using the Marchenko method
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<p>Structural imaging beneath complex overburdens, such as sub-salt or sub-basalt, typically characterized by high-impedance contrasts represents a major challenge for state-of-the-art seismic methods. Reconstructing complex geological structures in the vicinity of and below salt bodies is challenging not only due to uneven, single-sided illumination of the target area but also because of the imperfect removal of surface and internal multiples from the recorded data, as required by traditional migration algorithms. In such tectonic setups, most of the downgoing seismic wavefield is reflected toward the surface when interacting with the overburden's top layer. Similarly, the sub-salt upcoming energy is backscattered at the salt's base. Consequently, the actual energy illuminating the sub-salt reflectors, recorded at the surface, is around the noise level. In diapiric trap systems, conventional seismic extrapolation techniques do not guarantee sufficient quality to reduce exploration and production risks; likewise, seismic-based reservoir characterization and monitoring are also compromised. In this regard, accurate wavefield extrapolation techniques based on the Marchenko method may open up new ways to exploit seismic data.</p><p>The Marchenko redatuming technique retrieves reliable full-wavefield information in the presence of geologic intrusions, which can be subsequently used to produce artefact-free images by naturally including all orders of multiples present in seismic reflection data. To achieve such a goal, the method relies on the estimation of focusing operators allowing the synthesis of virtual surveys at a given depth level. Still, current Marchenko implementations do not fully incorporate available subsurface models with sharp contrasts, due to the requirements regarding the initialization of the focusing functions. Most importantly, in complex media, even a fairly accurate estimation of a direct wave as a proxy for the required initial focusing functions may not be enough to guarantee sufficiently accurate wavefield reconstruction.</p><p>In this talk, we will discuss a scattering-based Marchenko redatuming framework which improves the redatuming of seismic surface data in highly complex media when compared to other Marchenko-based schemes. This extended version is designed to accommodate for band-limited, multi-component, and possibly unevenly sampled seismic data, which contain both free-surface and internal multiples, whilst requiring minimum pre-processing steps. The performance of our scattering Marchenko method will be evaluated using a comprehensive set of numerical tests on a complex 2D subsalt model.</p>Keywords:
Overburden
Multiple
Passive seismic
Seismic migration
Reflection
Geophysical Imaging
Compared to primary arrivals, multiples have longer propagation paths and smaller reflection angles, leading to a wider illumination area in the horizontal direction and higher resolution in the vertical direction. Hence, it is better to make full use of the multiples rather than suppressing them. However, seismic attenuation exists widely in the subsurface medium, especially directly below the deep sea bottom. Therefore, to compensate for the attenuation effect during multiple imaging, we have developed a viscoacoustic reverse time migration (RTM) method of different-order multiples. Following the multiple propagation paths, we compensate for the attenuation during source wavefield forward propagation and receiver backward propagation, and we introduce a regularization operator to automatically eliminate the exponential high-frequency noise during the attenuation compensation process. Taking advantage of the full wavefield information, we jointly use the different-order multiples and primaries when implementing viscoacoustic RTM. In numerical examples, we validate the viscoacoustic RTM of different-order multiples in a three-layer attenuation model and an attenuating Sigsbee2B model. Our results suggest that our method can image the models using different-order multiples separately, which suppresses crosstalk artifacts, balances energy, raises resolution, and improves subsalt images dramatically.
Multiple
Seismic migration
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Marine seismic acquisitions record both primary and multiple wavefields. In a typical processing sequence, multiple energy is removed from the data before migration. However, valuable information might be contained in the multiple wavefield. A modification is proposed to the standard reverse time migration (RTM) algorithm to enable correct imaging between the primary wavefield and the first-order multiple wavefield. The advantages of this modification, reverse time migration of multiples (RTMM), are evaluated through three real data-processing projects and identified three key advantages. First, RTMM can recover small-angle reflections critical for shallow-water imaging that are missing in the primary wavefield. Second, RTMM has a wider illumination coverage, which significantly extends the image for an ocean-bottom node (OBN) project. Third, RTMM produces an image complementary to the primary image in a complex geologic setting, possibly assisting with interpretation. In addition, a synthetic study is presented of two types of cross-talk noise that hinder the full potential of RTMM, and corresponding practical strategies are proposed to handle them.
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Seismic migration
Ocean bottom
Geophysical Imaging
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Both vertical well seismic imaging and deviated well seismic imaging are needed in oilfield development.At the same time,it is not possible for us to obtain detailed reservoirs and interlayers with high resolution when only surface seismic data are used.In this research,we investigated cross well staggered-grid finite-difference reverse-time migration algorithm and absorbing boundary conditions.In the end,we obtained reverse-time migration image from the theory model and field seismic data.The result shows that the migration algorithm is correct,and the signal-to-noise ratio and resolution ratio of reverse-time migration imaging profile are higher than that of surface seismic imaging profile.What is more,for the reverse-time imaging profile,the details of layers are clearer,and it coincides well with the actual strata,which makes it much more credible.This means we can use the above method to get detailed reservoirs and interlayers with high resolution.
Seismic migration
Geophysical Imaging
Seismic exploration
SIGNAL (programming language)
Passive seismic
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Abstract Multiples have longer propagation paths and smaller reflection angles than primaries for the same source–receiver combination, so they cover a larger illumination area. Therefore, multiples can be used to image shadow zones of primaries. Least‐squares reverse‐time migration of multiples can produce high‐quality images with fewer artefacts, high resolution and balanced amplitudes. However, viscoelasticity exists widely in the earth, especially in the deep‐sea environment, and the influence of Q attenuation on multiples is much more serious than primaries due to multiples have longer paths. To compensate for Q attenuation of multiples, Q ‐compensated least‐squares reverse‐time migration of different‐order multiples is proposed by deriving viscoacoustic Born modelling operators, adjoint operators and demigration operators for different‐order multiples. Based on inversion theory, this method compensates for Q attenuation along all the propagation paths of multiples. Examples of a simple four‐layer model, a modified attenuating Sigsbee2B model and a field data set suggest that the proposed method can produce better imaging results than Q ‐compensated least‐squares reverse‐time migration of primaries and regular least‐squares reverse‐time migration of multiples.
Multiple
Seismic migration
Least-squares function approximation
Reflection
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Some hydrocarbon reservoirs are trapped beneath salt bodies, where seismic imaging is greatly challenged due to poor illumination. Multiple reflections have different propagation wave paths from primary reflections and thus can be used to complement the illuminations where primary reflections from beneath the salt are not acquired. Consequently, migration of multiples can sometimes provide better subsalt images compared to conventional migration which uses primary reflections only. In this paper, we propose to modify conventional reverse time migration so that multiples can be used as constructive reflection energy for subsalt imaging. This new approach replaces the impulsive source wavelet with the recorded data containing both primaries and multiples and uses predicted multiples as the input data instead of primary reflections. In the reverse time migration process, multiples recorded on the surface are extrapolated backward in time to each depth level, and the observed data with both primaries and multiples are extrapolated forward in time to the same depth levels, followed by a crosscorrelation imaging condition. A numerical test on the Sigsbee2B data set shows that a wider coverage and a more balanced illumination of the subsurface can be achieved by migration of multiples compared with conventional migration of primary reflections. This example demonstrates that reverse time migration of multiples might be a promising method for complex subsalt imaging.
Multiple
Seismic migration
Reflection
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In the implementation of migration of multiples, reverse time migration (RTM) is superior to other migration algorithms because it can handle steeply dipping structures and offer high-resolution images of the complex subsurface. However, the RTM results using two-way wave equation contain high-amplitude, low-frequency noise and false images generated by improper wave paths in migration velocity model with sharp velocity interfaces or strong velocity gradients. To improve the imaging quality in RTM of multiples, we separate the upgoing and downgoing waves in the propagation of source and receiver wavefields. A complex function involved with the Hilbert transform is used in wavefield decomposition. Our approach is cost effective and avoids the large storage of wavefield snapshots required by the conventional wavefield separation technique. We applied migration of multiples with wavefield decomposition on a simple two-layer model and the Sigsbee 2B synthetic data set. Our results demonstrate that the proposed approach can improve the image generated by migration of multiples significantly.
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Seismic migration
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In the process of conventional reverse-time migration,the numerical impulsive source is replaced with the recorded data including primaries and multiples on the surface and are extrapolated forward in time to each depth level,while the recorded primary reflections are replaced with multiples and are extrapolated backward in time to the same depth levels,then we select the appropriate imaging condition to realize the multiples imaging based on RTM The above process is Multiple RTM.In this paper,we analyzes the conventional RTM's imaging capability of multiples and then discuss the theory of Multiple RTM.Numerical examples show that the multiples can be imaged by RTM Only the pairs of the wave field energy between the extrapolated forward source wavefield and the extrapolated backward predicted multiples can be crosscorrelated to produce the real imaging values.Multiple RTM can provide more vast and precise underground lighting and imaging for the deeper and more complex seismic exploration.
Multiple
Seismic migration
Geophysical Imaging
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One of the challenges with reverse-time migration based on finite-difference method is the problems of computational costs, in terms of free disk space and/or computational time. This report discusses the principle of a new imaging condition, referred as ‘first arrival imaging condition’, and shows the advantage of less computational costs of this method, compared to the widely used source-normalized crosscorrelation imaging condition for reverse-time migration. Principally, with crosscorrelation imaging conditions, all the multiples inside both forward modelling and reverse-time migration wavefields are involved; on the other hand, with the first arrival imaging condition, the multiples in the wavefields are not included.
Seismic migration
Multiple
Geophysical Imaging
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