Prestack Imaging Research Articles

Pre-Stack Imaging for the Messinian Sub Salt Sequences in the Levantine Basin of the East Mediterranean: A Case Study, Offshore Lebanon

The Mediterranean Sea has a progressive geological significance; it is considered a vast extensive promising basin for gas exploration. Herodotus and Levantine basins are the main basins in the eastern Mediterranean basin, which are similar in their evolution stages and accommodate similar sedimentary sequences. The recent geophysical and petrophysical studies proved that Herodotus and Levantine basins are promising for gas with possible great reserves exceeding 2,000 bcm. Recent discoveries and increased exploration activities including seismic acquisition and offshore drilling made the Eastern Mediterranean a hot area for gas exploration, especially after the discovery of Zohr gas Field in 2015, offshore Egypt. However, the regional extension of the complex Messinian salt beds and domes make the exploration of high risk. The subsalt seismic imaging in a time domain and the resolution of the underlying carbonate/clastic reservoirs' primary reflections are a big challenge in this area. Therefore, the main objective of the present study is to improve sub-salt imaging with much more accurate interval velocity estimation. In the study area, offshore Lebanon, the delineated structures in the sub-salt imaging after the Pre-stack Depth Imaging were more significant compared to the Pre-Stack Time Imaging which provided optimum sub-salt imaging and enabled the accurate interpretation of the main geologic features which may increase the certainty level of the hydrocarbon exploration and discoveries.

Key technologies for complex surface seismic acquisition in the Sichuan Basin and their application effect

The earth surface in the Sichuan Basin and its periphery is geologically complex, which causes low signal-to-noise ratio of seismic data, poor imaging quality of seismic profiles and great difficulty in data acquisition. In order to ensure high-quality operation of the seismic survey project in the complex surface of the Sichuan Basin, researchers have been working continuously to solve the bottleneck problems in the seismic acquisition technology for complex surface in recent years. In addition, the upgrading of acquisition technology and operation capacity is promoted effectively by means of technical innovation, and a series of key technologies for seismic acquisition are developed. And the following application results of this series of technologies are obtained. First, the optimization design technology of the geometry based on wave equation forward modeling and actual data, combined with prestack migration imaging can show the influence of different acquisition schemes on the prestack imaging effect of complex targets more directly, which makes the seismic acquisition scheme more economic and effective. Second, by extracting landform risks and surface obstacle information intelligently, combined with field fine reconnaissance, landform risk identification and hierarchical evaluation and indoors intelligent optimization of well-shot physical point deployment are conducted, and thus the GIS+ intelligent deployment technology of shot point in complex surface is formed, which effectively reduces the risks of operation. Third, the cavity excitation technology which reduces the initial pressure of explosive pulse and extends the explosive action time of explosives can increase the conversion rate of rock excitation energy into effective elastic wave energy and the reflected energy and improve the quality of seismic single shot data. Fourth, combination of the automatic evaluation technology for field seismic data acquired in mountainous regions and the KL-GMLiveQC1.0 software can improve the evaluation efficiency, reduce the evaluation cost and ensure the high-quality acquisition of seismic data in complex surface. In conclusion, this series of key seismic acquisition technologies greatly improve the signal-to-noise ratio and resolution of seismic data, lay a foundation for fine reservoir prediction, and ensure the continuous important breakthrough of three-dimensional natural gas exploration and development in the gas fields of marine carbonate rock, shale gas, volcanic rock and shallow tight sandstone gas in the Sichuan Basin.

Seismic field data displaying azimuthal anisotropy, 1986–2020

The azimuthal (az’l) processing of 3D full-azimuth full-offset P-P reflection seismic data can enable better imaging, thus yielding improved estimates of structure, lithology, porosity, pore fluids, in situ stress, and aligned porosity that flows fluids (macrofracture porosity). In the past 34 years, the oil and gas industry has significantly advanced in the use of seismic azimuthal anisotropy, in particular, to gain information concerning unequal horizontal stresses and/or vertically aligned fractures, and possibly more importantly, to improve the prestack imaging especially in complex structure. The important development stages during the past 40 years were enabled by industry advancements in acquisition, processing, theory, and interpretation. The typical important techniques became evident in PP amplitude variation with angle and azimuth (AVAaz) and orthorhombic imaging. These techniques addressed the complications due to wave propagation in birefringent media. PP AVAaz, now industry standard for vertically aligned fracture characterization, is accompanied by a near-angle azimuthal amplitude variation when aligned connected porosity that flows fluids is present. Birefringence is present with unequal horizontal stresses and/or vertically aligned fractures that flow fluids. I have focused on the field-data documentation of the relationships among azimuthal P-P reflection data, S-wave birefringence, and hydrocarbon production. With increases and improvements in acquisition and processing, plus today’s powerful versatile interpretation platforms, continual advances beyond orthorhombic (ORT) into monoclinic and triclinic symmetries are to be expected. The use of 3D azimuthal seismic for time-lapse changes of the in situ stress field, fracture populations, and pore fluids, as rocks undergo production processes (oil and gas reservoir production processes, wastewater disposal, etc.) and at plate boundaries where stresses change, offers great potential to benefit not just the oil and gas industry but all of humanity.

Improved prospect evaluation and drilling results in the Dutch North Sea with diffraction imaging

Abstract We present an integrated interpretation case study of pre-stack depth migration and diffraction imaging in two Dutch North Sea gas fields. Diffraction imaging, as the detector of discontinuities at very high resolution and unlimited by illumination constraints, is the ideal technique for mapping gas accumulations at fault cuts (DI as DHI). The diffraction image, being a migrated physical wavefield, has amplitude, frequency, phase, polarity and tuning properties which can all be used to enhance its interpretation value. We use the diffraction imaging response of elementary models to build up an edge diffraction calculus and thus model and predict responses from more complex structures. This confirms the details seen in the diffraction images and well data from the studied gas fields. Diffraction imaging allows for a significant refinement of the interpretation resulting in more accurate definition of drilling targets and lower uncertainty in resource volumes.

A new scheme for velocity analysis and imaging of diffractions

Seismic diffractions are the responses of small-scale inhomogeneities or discontinuous geological features, which play a vital role in the exploitation and development of oil and gas reservoirs. However, diffractions are generally ignored and considered as interference noise in conventional data processing. In this paper, a new scheme for velocity analysis and imaging of seismic diffractions is proposed. Two steps compose of this scheme in our application. First, the plane-wave destruction method is used to separate diffractions from specular reflections in the prestack domain. Second, in order to accurately estimate migration velocity of the diffractions, the time-domain dip-angle gathers are derived from a Kirchhoff-based angle prestack time migration using separated diffractions. Diffraction events appear flat in the dip-angle gathers when imaged above the diffraction point with selected accurate migration velocity for diffractions. The selected migration velocity helps to produce the desired prestack imaging of diffractions. Synthetic and field examples are applied to test the validity of the new scheme. The diffraction imaging results indicate that the proposed scheme for velocity analysis and imaging of diffractions can provide more detailed information about small-scale geologic features for seismic interpretation.

Study and application of PS-wave pre-stack migration in HTI media and an anisotropic correction method

Anisotropy correction is necessary during the processing of converted PSwave seismic data to achieve accurate structural imaging, reservoir prediction, and fracture detection. To effectively eliminate the adverse effects of S-wave splitting and to improve PSwave imaging quality, we tested methods for pre-stack migration imaging and anisotropic correction of PS-wave data. We based this on the propagation rules of seismic waves in a horizontal transverse isotropy medium, which is a fractured medium model that reflects likely subsurface conditions in the field. We used the radial (R) and transverse (T) components of PS-wave data to separate the fast and slow S-wave components, after which their propagation moveout was effectively extracted. Meanwhile, corrections for the energies and propagation moveouts of the R and T components were implemented using mathematical rotation. The PS-wave imaging quality was distinctly improved, and we demonstrated the reliability of our methods through numerical simulations. Applying our methods to three-dimensional and three-component seismic field data from the Xinchang-Hexingchang region of the Western Sichuan Depression in China, we obtained high-quality seismic imaging with continuous reflection wave groups, distinct structural features, and specific stratigraphic contact relationships. This study provides an effective and reliable approach for data processing that will improve the exploration of complex, hidden lithologic gas reservoirs.

The design of L1-norm visco-acoustic wavefield extrapolators

Explicit depth frequency–space (f − x) prestack imaging is an attractive mechanism for seismic imaging. To date, the main focus of this method was data migration assuming an acoustic medium, but until now very little work assumed visco-acoustic media. Real seismic data usually suffer from attenuation and dispersion effects. To compensate for attenuation in a visco-acoustic medium, new operators are required. We propose using the L1-norm minimization technique to design visco-acoustic f − x extrapolators. To show the accuracy and compensation of the operators, prestack depth migration is performed on the challenging Marmousi model for both acoustic and visco-acoustic datasets. The final migrated images show that the proposed L1-norm extrapolation results in practically stable and improved resolution of the images.

Interpretation value of diffractions and sub-specular reflections – applications on the Zhao Dong field

We provide an overview of integrated pre-stack depth migration and diffraction imaging for the Zhao Dong field, Bohai Bay, China. This field is highly compartmentalized by complex faulting and further characterized by channel systems, fractures and volcanic features. The objective of the diffraction imaging is to better define these small-scale features. Tools to facilitate interpretation include displays with pre-stack depth migration and diffraction images overlain in different colour scales, as well as a weighted blending of them into a single volume. An important concept is that of the sub-specular reflection, which is obtained alongside the pure diffraction image by applying ultra-weak specularity tapers. Tuning properties of elementary diffractor images together with sub-specular reflectors provide a decisive uplift of diffraction imaging for the interpreter.

Harmonising diverse 3D geometries in a hard rock environment for pre-stack imaging

Four 3D seismic surveys were acquired by HiSeis Pty Ltd over two years across an existing mining camp. The main objective of these seismic surveys was definition of structures which could assist characterization of mineralised zone that consists mainly of massive sulphides. Initial processing of seismic data started shortly after acquisition in 2011. Preliminary products have shown great promise in resolving complex structural environments and showed potential for direct targeting form seismic data. Our motive for reprocessing the dataset was in integration of all 2011 and 2012 seismic data in an amplitude consistent routine which could bring new value for amplitude based analysis of massive sulphide bodies. Conventional 3D deep-move out corrections (DMO), followed by a post-stack migration algorithm has not been successful in merging diverse datasets. Considerable improvement was achieved through the application of pre-stack time migration (PSTM) algorithm. This allowed us to use unique bin size for all merged seismic surveys. Successful imaging of merged datasets has been challenged. The main reason was that 3D seismic datasets had vastly diverse offset and azimuth distribution. Highly irregular migration fold coverage is an obvious problem which had to be overcome by exclusion of unsuccessful imaged events prior to stacking.

Least‐Squares Reverse Time Migration in Visco‐Acoustic Media

AbstractAs the subsurface medium is widely viscous, it is necessary to compensate absorption and to correct dispersion of seismic waves in true‐amplitude pre‐stack imaging. The instability problem encountered in conventional inverse‐Q migration hinders the application of the method. In this work, imaging was treated as a linear inverse problem named least‐squares reverse time migration (LSRTM). Firstly, we linearized the wave equation and defined the cost function. Then based on the derived equations of the adjoint wave propagation operator, iterative solution was derived using the adjoint‐state method. At last dynamical phase encoding technology was used to reduce computation cost. This method breaks a new way for imaging in visco‐acoustic media while avoiding the instability problem. The true‐amplitude imaging results can be obtained while compensating absorption and correcting dispersion automatically. It is also a good way to suppress the imaging noise and correct amplitude unbalance caused by geometrical spreading or weak illumination. Compared with conventional reverse time migration (RTM), this method can yield results with higher resolution and lower imaging noise. The validity of the method was demonstrated by the numerical test on synthetic seismic data.

Prestack wavefield approximations

The double-square-root (DSR) relation offers a platform to perform prestack imaging using an extended single wavefield that honors the geometrical configuration between sources, receivers, and the image point, or in other words, prestack wavefields. Extrapolating such wavefields, nevertheless, suffers from limitations. Chief among them is the singularity associated with horizontally propagating waves. I have devised highly accurate approximations free of such singularities which are highly accurate. Specifically, I use Padé expansions with denominators given by a power series that is an order lower than that of the numerator, and thus, introduce a free variable to balance the series order and normalize the singularity. For the higher-order Padé approximation, the errors are negligible. Additional simplifications, like recasting the DSR formula as a function of scattering angle, allow for a singularity free form that is useful for constant-angle-gather imaging. A dynamic form of this DSR formula can be supported by kinematic evaluations of the scattering angle to provide efficient prestack wavefield construction. Applying a similar approximation to the dip angle yields an efficient 1D wave equation with the scattering and dip angles extracted from, for example, DSR ray tracing. Application to the complex Marmousi data set demonstrates that these approximations, although they may provide less than optimal results, allow for efficient and flexible implementations.

An effective denoising strategy for wave equation migration based on propagation angles

We present an effective denoising strategy for two-way wave equation migration. Three dominant artifact types are analyzed and eliminated by an optimized imaging condition. We discuss a previously unsolved beam-like artifact, which is probably caused by the cross-correlation of downward transmitting and upward scattering waves from both the source and receiver side of a single seismic shot. This artifact has relatively strong crosscorrelation but carries no useful information from reflectors. The beam-like artifact widely exists in pre-stack imaging and has approximately the same amplitude as useful seismic signals. In most cases, coherent artifacts in the image are caused by directionally propagating energy. Based on propagation angles obtained by wavefield gradients, we identify the artifact energy and subtract its contribution in the imaging condition. By this process most artifacts can be accurately eliminated, including direct wave artifacts, scattering artifacts, and beamlike artifacts. This method is independent of the wavefield propagator and is easy to adapt to almost all current wave equation migration methods if needed. As this method deals with the physical artifact origins, little damage is caused to the seismic signal. Extra k-domain filtering can additionally enhance the stacking result image quality. This method succeeds in the super-wide-angle one-way migration and we can expect its success in other two-way wave equation migrations and especially in reverse time migration.

Impact of modelling shallow channels on 3D prestack depth migration, Elgin-Franklin fields, UKCS

A case study from the Elgin-Franklin fields is used to illustrate the influence of seabed low velocity anomalies on imaging deep targets below 5000 m depth. In the uppermost 400 m of sediment, glacial channels are present and generate locally low velocity anomalies resulting in pull-up/push-down effects observed on seismic reflection data. The original 3D prestack depth migration did not take into account these shallow velocity anomalies, but used a smooth interval velocity in the uppermost layer of sediment. In the southern area, the results were locally better than with prestack time migration, but in the northern area they were worse, or at best comparable. A new velocity model with a channel layer was built using well velocities for the initial model. Tomographic and migration iterations were used to refine all velocities, including the channel velocities, and for introducing anisotropy. Imaging of the pre-Cretaceous section was clearly an improvement over prestack time migration. For correct imaging, local shallow velocity variations must be identified and taken into account in the velocity model. Even though the overburden appears to be rather flat, prestack depth migration imaging performs better than prestack time migration overall.

Plane wave shot PSDM and controlled illumination techniques

The Pre-Stack Depth Migration (PSDM) method based on wavefield continuation is the most reliable method for imaging complex structure in the subsurface, although there are large computational costs and poorly adaptive geometry. Plane wave shot migration is another method to perform exact wave equation prestack imaging with high computational efficiency and without the migration aperture problem. Moreover, wavefield energy can be compensated at the target zone by controlled illumination. In this paper, plane wave shot PSDM was implemented by the control of the plane down-going wavefield and selection of number and range of the raypaths in order to optimize the imaging effect. In addition, controlled illumination techniques are applied to enhance the imaging precision of interesting areas at different depths. Numerical calculation indicates that plane wave shot imaging is a rapid and efficient method with less computational cost and easy parallel computation compared to the single-square-root operator imaging for common shot gathers and doublesquare-root operator imaging for common midpoint gathers.

Data stacking beyond CMP

Stacking has been used in seismic data processing for a long time. In fact, stacked sections (or volumes in 3D) are standard deliverables in the industry, and the concepts of common-midpoint (CMP) gathers and normal moveout (NMO) correction are mentioned in almost every textbook on seismic processing. Although the general trend is toward prestack imaging (either in time or in depth), the construction of stacked sections remains an important step within the seismic processing flow, since they are almost always the first available interpretable images of the subsurface.

Time-shift imaging condition in seismic migration

Seismic imaging based on single-scattering approximation is in the analysis of the match between the source and receiver wavefields at every image location. Wavefields at depth are functions of space and time and are reconstructed from surface data either by integral methods (Kirchhoff migration) or by differential methods (reverse-time or wavefield extrapolation migration). Different methods can be used to analyze wavefield matching, of which crosscorrelation is a popular option. Implementation of a simple imaging condition requires time crosscorrelation of source and receiver wavefields, followed by extraction of the zero time lag. A generalized imaging condition operates by crosscorrelation in both space and time, followed by image extraction at zero time lag. Images at different spatial crosscorrelation lags are indicators of imaging accuracy and are also used for image-angle decomposition. In this paper, we introduce an alternative prestack imaging condition in which we preserve multiple lags of the time crosscorrelation. Prestack images are described as functions of time shifts as opposed to space shifts between source and receiver wavefields. This imaging condition is applicable to migration by Kirchhoff, wavefield extrapolation, or reverse-time techniques. The transformation allows construction of common-image gathers presented as functions of either time shift or reflection angle at every location in space. Inaccurate migration velocity is revealed by angle-domain common-image gathers with nonflat events. Computational experiments using a synthetic data set from a complex salt model demonstrate the main features of the method.

An integrated method for resolving the seismic complex near‐surface problem

ABSTRACTWe describe an integrated method for solving the complex near‐surface problem in land seismic imaging. This solution is based on an imaging approach and is obtained without deriving a complex near‐surface velocity model. We start by obtaining from the data the kinematics of the one‐way focusing operators (i.e. time‐reversed Green's functions) that describe propagation between the acquisition surface and a chosen datum reflector using the common‐focus‐point technology. The conventional statics solutions obtained from prior information about the near surface are integrated in the initial estimates of the focusing operators. The focusing operators are updated iteratively until the imaging principle of equal traveltime is fulfilled for each subsurface gridpoint of the datum reflector. Therefore, the seismic data is left intact without any application of time shifts, which makes this method an uncommitted statics solution. The focusing operators can be used directly for wave‐equation redatuming to the respective reflector or for prestack imaging if determined for multiple reflecting boundaries. The underlying velocity model is determined by tomographic inversion of the focusing operators while also integrating any hard prior information (e.g. well information). This velocity model can be used to perform prestack depth imaging or to calculate the depth of the new datum level. We demonstrate this approach on 2D seismic data acquired in Saudi Arabia in an area characterized by rugged topography and complex near‐surface geology.

Least-squares joint imaging of multiples and primaries

Multiple reflections contain subsurface reflectivity information which often complements that found in primary reflections. Previous attempts to combine the additional information by summing prestack images of the primaries and multiples have generally failed because of crosstalk leakage between the images. We present a general linear least-squares joint imaging of multiples-and primaries (LSJIMP) inversion method to simultaneously suppress crosstalk noise and combine pegleg multiples and primaries in a prestack sense. In general, LSJIMP is compatible with a wide variety of prestack imaging methods and can be extended to jointly image primaries and other embedded wave modes such as shear-wave conversions. We present a particular LSJIMP implementation that utilizes an efficient linear operator to model and image pegleg multiples in a true relative-amplitude sense. Applied to a given type of pegleg multiple in the data, our imaging operator produces an image directly comparable to primaries after NMO. Our operator's kinematic component is an extension of the NMO equation that independently images split peglegs in a moderately heterogeneous earth. Its amplitude component corrects multiples for their differences in angle-dependent reflection strength and illumination, relative to a primary. We test our LSJIMP implementation on 2D and 3D prestack field data examples and show that the method cleanly separates primaries and multiples and also uses joint information in the events to interpolate the signal in acquisition gaps.

Multiple diffractions and coherent noise in marine seismic data

Summary Multiple reflections from irregularities on the sea floor, (multiple diffractions), can share a number of characteristics with the primary scattering that is also generated by such irregularities. The kinematics of the primary scattering were investigated in a classic 1983 Geophysics paper by Ken Larner and his colleagues, as part of a study of coherent noise in marine seismic data. Where the sea floor is generally smooth except for isolated irregularities, many of the conclusions of that paper can also be applied to the multiple scattering. In particular, strategies for removal of scattered noise from sea-floor irregularities can also be applied to attenuate sea floor and other multiple diffractions. Like the primary scattering, the multiple diffractions are organised in mid-point gathers in such a way that mid-point stacking may enhance portions of the multiple. Also like the primary scattering, the multiples can be distinguished from primary reflection events in the data by their dip in pre-stack shot and receiver gathers. Pre-stack dip-filtering in those domains can be used to remove the portions of the multiple diffractions that would otherwise be enhanced by the mid-point stack, in the same way that dip-filtering can be used to remove the primary scattering. Shot and receiver dip-filtering can, however, damage diffractions and lateral amplitude changes from geologyrelated primaries in the data. A partial strategy, in less complex structure, is to apply pre-stack imaging in conjunction with multiple removal, with the aim of simplifying the geology-related primaries and hence increasing the separation between the primaries and the multiples. The pre-stack imaging does however increase the complexity of the multiple diffractions, and can also introduce aliasing and dispersion-related artefacts. The multiples are mis-migrated, but the degree of mis-migration varies smoothly with offset. Multiple removal can still be applied, but with a reduced potential for primary damage.

The common focal point and common reflection surface methodologies: subsets or complements?

ABSTRACTThe common focal point (CFP) method and the common reflection surface (CRS) stack method are compared. The CRS method is a fast, highly automated procedure that provides high S/N ratio simulation of zero‐offset (ZO) images by combining, per image point, the reflection energy of an arc segment that is tangential to the reflector. It uses smooth parametrized two‐way stacking operators, based on a data‐driven triplet of attributes in 2D (eight parameters in 3D). As a spin‐off, the attributes can be used for several applications, such as the determination of the geometrical spreading factor, multiple prediction, and tomographic inversion into a smooth background velocity model. The CFP method aims at decomposing two‐way seismic reflection data into two full‐aperture one‐way propagation operators. By applying an iterative updating procedure in a half‐migrated domain, it provides non‐smooth focusing operators for prestack imaging using only the energy from one focal point at the reflector. The data‐driven operators inhibit all propagation effects of the overburden. The CFP method provides several spin‐offs, amongst which is the CFP matrix related to one focal point, which displays the reflection amplitudes as measured at the surface for each source–receiver pair. The CFP matrix can be used to determine the specular reflection source–receiver pairs and the Fresnel zone at the surface for reflection in one single focal point. Other spin‐offs are the prediction of internal multiples, the determination of reflectivity effects, velocity‐independent redatuming and tomographic inversion to obtain a velocity–depth model. The CFP method is less fast and less automated than the CRS method. From a pointwise comparison of features it is concluded that one method is not a subset of the other, but that both methods can be regarded as being to some extent complementary.