Common Reflection Surface Method Research Articles

Using common-reflection-surface stack for enhanced near-surface seismic reflection imaging: Examples from consolidated and unconsolidated environments

The small geophone spacing and spread lengths commonly used to investigate ultrashallow layers in conjunction with the limited bandwidth of surface impact sources do not allow for clear separation of different seismic arrivals. The interference of events and the high noise levels due to near-source offsets make shallow seismic data processing a challenging task. The wavefront attributes of seismic waves commonly used in conventional seismic processing are suggested to improve near-surface seismic reflection imaging. These wavefront attributes are often used as the stacking parameters in multidimensional time imaging methods such as common-reflection surface (CRS). It is shown in this paper that the CRS method can improve near-surface seismic data processing by enhancing: (1) unstacked gathers via a CRS-based local stacking scheme, (2) semblance picking for velocity model building, and (3) zero-offset stacked data via the CRS global stacking. The CRS-based local stacking can mitigate the loss of data associated with optimum-windowing-based muting of coherent noise. The CRS local stacking infills the muted zones via its robust data interpolation and regularization features and enhances the image quality. Furthermore, the CRS-based enhanced data allow for improved near-surface velocity model building by producing higher coherence and more focused semblance peaks. The CRS global stacking is shown to further smooth and provide more continuity of events in the zero-offset data. Applications to a synthetic and two field data sets collected from high- and low-velocity environments indicate the efficiency and feasibility of the proposed approach.

Application of common reflection surface (CRS) to velocity variation with azimuth (VVAz) inversion of the relatively narrow azimuth 3D seismic land data

Abstract The fracture direction and its intensity are critical properties related to hydrocarbon characterization and identification. Both these properties have an essential role in identifying the direction of hydrocarbon migration, determining the sweet spot area, and optimizing the drilling design. The velocity variation with azimuth (VVAz) is a well-known method to estimate the fracture direction and its intensity. This method is of widespread interest because it predicts the properties based on seismic data without any practical constraints. Despite this interest, the technique requires rich azimuth 3D seismic data in our case, which is rare. This study aims to apply regularization and interpolation by including the wave front attributes based on the Common Reflection Surface (CRS) method before the VVAz inversion. The motivation of using the CRS method is to enrich the current azimuth of the 3D seismic data and improve the S/N ratio. The synthetic and the real 3D seismic data are evaluated to examine the interpolation scheme of the proposed CRS method’s performance. Based on the evaluation of the 3D seismic data after regularization, the amplitude versus offset (AVO) phenomena, and the VVAz inversion results are relatively consistent (or matched) with the model. A similar result is found for the case of real 3D seismic data. A significant positive correlation between the fracture intensity of FMI and the real seismic data of about 0.9 is obtained. Therefore, CRS can be used as a regularization and interpolation method before the VVAz inversion of the relatively narrow azimuth 3D seismic data.

Common reflection surface methods in low fold coverage seismic data of complex marine geological structures

The primary objective of seismic data processing is to generate seismic cross-section with an optimum signal to noise ratio to model geological structures imaging as accurately as possible with the actual condition. However, we may look onto complex geological structures with less than adequate number of fold coverage seismic data. In this study, Common Reflection Surface (CRS) method is applied to generate 2D marine seismic cross-sections with better reflector continuity of 3 (three) real data seismic lines. It is based on obtaining the suitable aperture value along the attributes namely RN, RNIP, and a. Its results are compared to conventional Common Mid-Point (CMP) ones. The final products are presented in multiple- attenuated and time-migrated cross-section with Surface Related Multiple Elimination (SRME) and Kirchhoff Migration techniques, respectively. Comparison of final time-migrated both CRS and CMP stacks show the CRS method is more effective and reliable to model accurate complex marine geological structures with high signal to noise ratio based on reflector continuity and noise reduction.

Преимущества специальной обработки сейсмических данных методом CRS на лицензионном участке АО «Самаранефтегаз»

Преимущества специальной обработки сейсмических данных методом CRS на лицензионном участке АО «Самаранефтегаз»

Velocity-model estimation with NIP-wave tomography inversion – A real data example

The common-reflection-surface (CRS) stack in addition to high signal-to-noise ratio stacked sections creates the curvature and local dip of a reflector. With this information, so-called kinematic wavefield attributes, it is possible to obtain a subsurface velocity model for laterally inhomogeneous media using normal-incidence-point (NIP) wave inversion. The result of CRS provides some location points on the stacked section as input for the inversion. In each iteration, dynamic ray tracing is applied along the central ray according to an input data point. With this technique, the parameter needed for forward modeling will be obtained. By updating the velocity model and minimizing the misfit between input data and the forward model parameter, it is possible to achieve a final velocity model consistent with input data. In this paper, we applied the CRS method on complex structure in the northeast of Iran, and then we performed the NIP-wave tomography inversion on this data set by means of CRS attributes. The result clearly shows the ability of NIP-wave tomography in velocity-model inversion.

Seismic imaging of complex structures with the CO-CDS stack method

Various seismic imaging methods are introduced to resolve some of the possible ambiguities of seismic interpretation in complex structures. Reducing dependency of imaging techniques on velocity or using diffraction energy for imaging more structural details are the main topics of the imaging research. In this study, we try to improve the seismic image quality in semi-complex structures by combining the common reflection surface (CRS) method with a diffraction based scheme in the common-offset domain. Previously introduced partial CRS and common offset CRS methods exhibited reliable performance in imaging complex media. Here, we were looking for stable and efficient solutions, preserving advantages of the previous methods. Herewith, the proposed operator fits better to diffractions than to reflections. Therefore, we call it the commonoffset common diffraction surface stack (CO CDS). In a previous study, improvement of the quality of seismic image by the CRS method was achieved by combination of the CDS method with the partial CRS. This resulted in the introduction of the partial CDS. Initially, in this study, the common-offset CRS traveltime equation was modified to the common-offset CDS. The hypothetical shot reflector experiment in the CRS method was changed to shot diffraction point experiment. In the introduced operator, two wavefront curvatures, observed at receivers positions, are set equal in order to satisfy the diffraction condition. In the proposed method, we search for accurate attribute sets for each considered offset individually, and then form a new operator by four coherent attributes. Application of the common- offset CDS method on synthetic and field data shows more details of the geological structures with higher quality, while preserving continuity of reflection events. The proposed method is, however, more expensive than the partial and common offset CRS for large dataset.

Imaging seismic data in complex structures by introducing the partial diffraction surface stack method

The common reflection surface (CRS) stack method is known as a generalized stacking velocity analysis tool and was originally introduced as a data-driven method to simulate zero-offset sections. However, this method has some difficulties in imaging complex structures and low-quality data. The problem of conflicting dips is one of the drawbacks of the CRS method addressed in many studies. The common diffraction surface (CDS) method was explicitly introduced to overcome this problem. In one study, the problem was resolved by combination of the CDS method and the common offset CRS method. The method was called the common offset CDS method showed successful application on improving image quality in semi-complex media. In this study, we combined the partial CRS with the CDS to derive the partial CDS for more efficient resolve of the conflicting dips problem. In the partial CDS, thresholds in the angle spectrum were removed for full contribution of all possible dips to have volume of operators for a sample point. The aperture definition in the partial CDS is the same as in the partial CRS, where an offset and time variant aperture is used. The new method was applied on a simple synthetic data set with much diffraction points imbedded in the model. Then it was applied to a semicomplex data set to enhance the body of mud volcanoes and faults. For better comparison, it was applied to two more real data sets from a complex overthrust zone to improve the seismic quality and remove the geological ambiguities in the interpretation. In the synthetic data example, more conflicting dips were resolved than in the other methods. In all real data examples, the enhanced partial CDS data were depth-migrated to compare them with the pre-stack depth migration of partial CRS gathers. More details of the geological structures can be observed in the new results.

Migration-velocity building in time and depth from 3D (2D) Common-Reflection-Surface (CRS) stacking - theoretical framework

We demonstrate how the velocity concepts attached to conventional stacking assuming local smooth 1D type of Earth models, are modified within the setting of the Common Reflection Surface (CRS) method to handle lateral velocity variations. The corresponding matrix (scalar) normal moveout (NMO)- and Dix-velocities in 3D (2D) are now linked to a smooth velocity medium in depth sampled along the mapping or normal rays taking into account lateral velocities. This is rather different from the conventional 1D case where the link between the data-driven velocities and the smooth 1D velocity medium are represented by vertical mapping rays. Further and by analogy with the conventional 1D approach, where time-migration velocities are computed from NMO- or Dix-velocities after proper smoothing, its CRS counterpart is established. It is demonstrated that matrix (scalar) time-migration velocities to be used in ray-based 3D (2D) time-migration (TM) can be obtained by proper mapping of the corresponding CRS velocities. Relevant mapping equations valid for the 2D case have earlier been treated in the literature. However, to our knowledge, this is the first time such mapping equations have been derived for the 3D case. This mapping takes into account smooth lateral velocity variations, normally not properly accounted for in the initial model employed in conventional time-migration building based on NMO-velocities. Also, unlike the conventional approach, the use of the CRS method makes it feasible to build a smooth velocity field in depth beyond that of 1D. In this connection, we propose to use Normal Incidence Point (NIP) tomography (alternatively Image Incidence Point (IIP) tomography) to build a macro-velocity model upon which the time-to-depth mapping and the correction factors can be calculated. That step is then followed by construction of an updated smooth replacement medium, better able to capture local changes in velocity. The proposed approach can be used iteratively. Such depth velocities can be employed as an initial model for iterative depth migration. The main idea being that velocity-model building by using time-domain observables ensures more robustness.

Seismic imaging of mud volcano boundary in the east of Caspian Sea by common diffraction surface stack method

The problem of seismic imaging in complex structures such as folded faulting areas, salt domes, or mud volcano-bearing areas cannot be properly solved by the conventional seismic processing methods. The reverse time migration, Gaussian beam migration, the extended search strategy in the optimized common reflection surface (CRS) stack, and the partial CRS stack methods are among the new methods introduced for seismic imaging in complex structures. Among them, the CRS stack method is frequently used for seismic imaging. However, besides its great advantage in enhancing the quality of seismic section, it faces with some problem in imaging of complex structures, especially in the existence of conflicting dips. The CRS method transforms pre-processed multi-coverage data into a zero-offset section by summing along stacking surfaces instead of only along a common reflection point trajectory. The extended parameter search strategy in the CRS method and the partial CRS stack method were introduced to solve the problem of conflicting dips in the CRS method. However, some examples showed that these methods could not completely solve the problem of conflicting dips. In this study, a new introduced method is used to completely solve the problem of conflicting dips in complex structures. The new stacking surface is the approximation of diffraction response of a diffraction point. The surface is made by well-known three CRS attributes, where the radius of normal and normal incidence point waves is equals. The third parameter is the emergence angle of the central ray that the stacking surface is built for each angle, thus making an operator volume. Therefore, the operator gathers more energy that might get lost by other stacking surfaces. The new method was applied on a seismic data from a complex geological structure from northeast of Iran. This area contains mud volcano, an indicator for gas reservoir. Mud also absorbs the seismic energy and deteriorates the quality of the seismic section. Therefore, imaging the boundary of the mud volcano is questionable. By applying post-stack time migration on the stacked section obtained by the new method, the advantage of the stacking operator has been evaluated for imaging in complex structures. These advantages were more highlighted in depth migration applied on the new introduced operator volume common diffraction surface (CDS)-stacked section. The results showed that not only the boundary of the mud but also the faults that were completely unclear in previous results were imaged well.

Recovering diffractions in CRS stacked sections

The common reflection surface (CRS) method has been proposed as an alternative to the classical common midpoint method to further improve the signal-to-noise ratio, as well as to obtain additional kinematic parameters that are useful for a number of imaging purposes. In the CRS method, reflected events are enhanced by stacking along a generalized hyperbolic moveout, referred to as the CRS moveout. However, during the process of CRS stacking, diffractions are likely to be attenuated or even suppressed. Diffracted events are important since they carry high-resolution information about the subsurface structure. By the use of a modified version of the CRS technique, diffractions can be enhanced in the same way as reflections. This paper proposes a combined approach where the CRS stack is superimposed on the CRS diffraction-enhanced stack. In this way we can recover the diffractions in CRS stacked sections. The potential of the method has been demonstrated using marine seismic data acquired offshore Brazil.

Part I — CRS stack: Global optimization of the 2D CRS-attributes

Abstract The common-reflection-surface (CRS) stacking method simulates zero-offset (ZO) seismic traces by means of summing the amplitudes of seismic reflection events of multi-coverage data by using a stacking operator defined for the midpoint and half-offset coordinates. For two-dimensional media, this operator depends on three kinematic attributes, also referred to as CRS attributes. The main problem in using the CRS stacking method is the determination of the three CRS attributes from multi-coverage seismic data for each sample point in the simulated ZO section. Usually, the CRS attributes are determined in several steps, through searching individual attributes on the prestack seismic gathers and on post-stack sections. This multi-step solution may accumulate errors or inaccuracies in the CRS attributes, which can compromise the quality of the ZO stacked section and affects their successful application to other processes. In this study, the optimization problem of the CRS method is solved by means of multidimensional global optimization, which uses an objective function based on a coherence measure (semblance) of the traces in the multi-coverage data. The simultaneous determination of the three CRS attributes is performed by applying the Simulated Annealing (SA), Modified Simulated Annealing (MSA) and Very Fast Simulated Annealing (VFSA) global optimization algorithms. These algorithms were compared in terms of their effectiveness, efficiency and robustness. The results show that the VFSA and MSA algorithms are more suitable for determining the CRS attributes and generate high signal-to-noise ratio ZO stacked sections. Both algorithms are highly effective, but VFSA is more efficient than MSA. The classical SA algorithm has a low efficiency and effectiveness for determining the CRS attributes. In our proposed single-step global optimization strategy to search CRS attributes, we use the stacking velocity model derived from conventional velocity analysis to determine the lower and upper limits of the optimization search space for RNIP‐ We use real data with low fold and high noise level to illustrate the proposed optimization strategy and show that its result is of higher quality than that obtained with CRS stack based on the well-known extended-pragmatic search strategy.

Common-reflection-surface imaging of shallow and ultrashallow reflectors

We analyzed the feasibility of the common-reflection-surface (CRS) stack for near-surface surveys as an alternative to the conventional common midpoint (CMP) stacking procedure. The data-driven, less user-interactive CRS method could be more cost efficient for shallow surveys, where the high sensitivity to velocity analysis makes data processing a critical step. We compared the results for two field data sets collected to image shallow and ultrashallow reflectors: an example of shallow P-wave reflection for targets in the first few hundred meters, and an example of SH-wave reflection for targets in the first 10 m. By processing the shallow P-wave records using the CMP method, we imaged several nearly horizontal reflectors with onsets from 60 to about 250 ms. The CRS stack produced a stacked section more suited for a subsurface interpretation, without any preliminary formal and time-consuming velocity analysis, because the imaged reflectors possessed greater coherency and lateral continuity. With CMP processing of the SH-wave records, we imaged a dipping bedrock interface below four horizontal reflectors in unconsolidated, very low velocity sediments. The vertical and lateral resolution was very high, despite the very shallow depth: the image showed the pinchout of two layers at less than 10 m depth. The numerous traces used by the CRS stack improved the continuity of the shallowest reflector, but the deepest overburden reflectors appear unresolved, with not well-imaged pinchouts. Using the kinematic wavefield attributes determined for each stacking operation, we retrieved velocity fields fitting the stacking velocities we had estimated in the CMP processing. The use of CRS stack could be a significant step ahead to increase the acceptance of the seismic reflection method as a routine investigation method in shallow and ultrashallow seismics.

Fast estimation of common-reflection-surface parameters using local slopes

Present-day techniques to estimate the traveltime parameters of the common-reflection-surface (CRS) stack are tedious, time-consuming, and expensive processes based on local coherence analyses along a large number of trial surfaces. With the 2D CRS method, faster and cheaper determination is possible. The complete set of CRS parameters can be extracted from seismic data by an application of modern local-slope-extraction techniques. The necessary information about the CRS parameters is contained in the slopes of the common-midpoint section at the central point and one or several common-offset sections in its vicinity. We studied two procedures for the CRS parameter extraction technique. Their difference lies in the way the common-offset parameters are determined. One technique requires slope-derivative information (a possible source of instability); the other uses slope information at two different locations and less data redundancy. Testing on a synthetic data example proved that the procedures are sufficiently robust to allow for high-quality extraction of all CRS parameters from the extracted slope fields. In this way, the CRS parameter extraction can be sped up by several orders of magnitude as compared to the conventional procedure based on coherence analysis along trial surfaces.

Detecção de camadas delgadas usando sísmica de reflexão

Improvement of vertical resolution in seismic data is one of the major challenges for the seismic reflection method. The main problem is related to the larger loss of high frequencies relative to the lower ones during seismic wave propagation. As a consequence, the contribution of lower frequencies in the final section is significantly greater than the corresponding one due to higher frequencies. In principle, this situation should not constitute an obstacle to the full recovery of the spectrum, because of the powerful tools available in seismic processing, designed to equalize all frequencies that are present in the data volume. The main reason why these tools in general do not succeed is the presence of noise, for example random noise. Such situation gives rise to a critical frequency, such that all frequencies above it are impossible to recover and, as a consequence, the seismic pulse cannot be made short enough to allow for optimal resolution. Methods to solve the problem by means of random noise attenuation have been extensively investigated and tested without relevant results. The alternative to such techniques is the enhancement of the multiplicity that can be used for stacking. Since the amplitude decays exponentially with frequency, a large multiplicity is necessary in order to attain a significant effective frequency gain. The capability to attain extremely large multiplicity is exactly the main virtue of the Common Reflection Surface (CRS) method. In this paper, we propose the strategy of combining the CRS method with the spectral whitening, a known frequency-recovery technique, so as to optimize vertical resolution in seismic data and, as a consequence, improve the discrimination of thin reservoirs. First tests on synthetic and real data shown in this work are very encouraging and permit to conclude that the proposed approach has a good potential for practical application.

Velocity estimation by the common-reflection-surface (CRS) method: Using ground-penetrating radar data

In this paper, we describe the use of the common-reflection-surface (CRS) method to estimate velocities from ground-penetrating radar (GPR) data. Applied to multicoverage data, the CRS method provides, as one of its outputs, the time-domain rms velocity map, which is then converted to depth by the familiar Dix algorithm. Combination of the obtained depth-converted velocity map with electrical resistivity in-situ measurements enables us to estimate both water content and water conductivity. These quantities are essential to delineate infiltration of contaminants from the surface after industrial or agricultural activities. The method was applied to GPR data and compared with the classical NMO approach. The results show that the CRS method provides a physically more meaningful velocity field, thus improving the potential of GPR as an investigation tool for environmental studies.

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.

Imaging of complex basin structures with the common reflection surface (CRS) stack method

SUMMARY Common reflection surface (CRS) stack technology is applied to seismic data from certain areas of the Donbas Foldbelt, Ukraine, after conventional seismic methods gave unsatisfactory results. On the conventionally processed post-stack migrated section the areas of interest already showed clear features of the basin structure, but reflector continuity and image quality were poor. It was our objective to improve the image quality in these areas to better support the geological interpretation and the model building. In contrast to the standard common mid-point (CMP) stack, in which a stacking trajectory is used, the CRS method transforms pre-processed multicoverage data into a zero-offset section by summing along stacking surfaces. The stacking operator is an approximation of the reflection response of a curved interface in an inhomogeneous medium. The primary advantage of the data-driven CRS stack method is its model independence and the enhancement of the signal-tonoise ratio of the stacked sections through a stacking reflection response along traces from more than one CMP gather. The presented results show that the multifold strength of the CRS stack is of particular advantage in the case of complex inverted features of Devonian‐Carboniferous sediments in the Donbas Foldbelt data. We observe that in these areas where the confidence level for picking and interpretation of the stacking velocity model is low, imaging without a macrovelocity model gives improved results, because errors due to wrong or poor stacking velocity models are avoided.

Refinement step for parameter estimation in the CRS method

The Common Reflection Surface (CRS) method is a powerful extension of the well established Common Midpoint (CMP) method in the sense that it is able to accept, at each trace location on the zero-offset (ZO) section to be constructed, reflection data from source and receiver pairs that are arbitrarily located around that point. The CRS method uses the general hyperbolic moveout, that depends, in the 2D situation considered in this work, on three parameters. One of these parameters is the classical NMO velocity. As in the single-parameter CMP method, the CRS parameters or attributes are estimated by a direct application of suitable coherence analysis to the input multicoverage data. The estimation of the three CRS parameters is generally performed in two steps. The first step has a global character and aims in obtaining an initial estimate of the parameters. The second step has a local character, trying to refine the previous initial values to more accurate values. Here we focus on the refinement step assuming that initial estimates have been already provided. We review and compare three of these methods and compare their performances on illustrative synthetic and real data examples. Comparisons with the application of the conventional CMP method are also provided.

Multiple attenuation using Common-Reflection-Surface attributes

Applied to multicoverage data, the Common-Reflection-Surface (CRS) method obtains, besides a clear stacked section, also a number of traveltime parameters or attributes defined at each point of that section. The CRS parameters provide useful information for a variety of seismic processing purposes. We consider the use of CRS attributes in multiple identification and attenuation. In the 2D situation, the CRS method produces three parameters associated with the resulting simulated (stacked) zero-offset (ZO) section. We propose and discuss simple algorithms designed to identify and, as a next stage, attenuate or eliminate multiples. First experiments show that these algorithms have the potential of favorably replace well-established multiple suppression methods.

Multifocusing imaging with controlled reflection‐point dispersal

Multifocusing imaging proposed by Gelchinsky (Gelchinsky et al., 1999a, b; Landa et al., 1999) belongs to a group of techniques that can be characterized as macro-model independent imaging methods. These methods, which also include the common-reflection surface method (Mann et al., 1999; Jager et al., 2001), and optical stack (de Bazelaire, 1988), are described in a collection of papers published as a special issue of the Journal of Applied Geophysics (Hubral, 1999). Macro-model independent imaging methods represent a new alternative to the classical processing sequence of NMO, dip moveout (DMO), and stacking. They are based upon a new transformation of 2-D multicoverage prestack data into a simulated zero-offset stack section. This transformation involves stacking large supergathers of seismic traces, each of which can span many common-midpoint (CMP) gathers. Stacking of large supergathers is made possible by the use of a generalized moveout correction. For a given source-receiver pair, this correction depends on three parameters: the wavefront curvatures of the normal wave, the normal incidence point wave (Hubral, 1983), and the emergence angle of the central ray. For each supergather and each zero-offset time, T , these parameters are obtained through a coherence analysis of the moveout corrected supergather.