Common Shot Domain Research Articles

Source deghosting of coarsely sampled common-receiver data using a convolutional neural network

It is well known that source deghosting can best be applied to common-receiver gathers, whereas receiver deghosting can best be applied to common-shot records. The source-ghost wavefield observed in the common-shot domain contains the imprint of the subsurface, which complicates source deghosting in the common-shot domain, in particular when the subsurface is complex. Unfortunately, the alternative, that is, the common-receiver domain, is often coarsely sampled, which complicates source deghosting in this domain as well. To solve the latter issue, we have trained a convolutional neural network to apply source deghosting in this domain. We subsample all shot records with and without the receiver-ghost wavefield to obtain the training data. Due to reciprocity, these training data are a representative data set for source deghosting in the coarse common-receiver domain. We validate the machine-learning approach on simulated data and on field data. The machine-learning approach gives a significant uplift to the simulated data compared to conventional source deghosting. The field-data results confirm that the proposed machine-learning approach can remove the source-ghost wavefield from the coarsely sampled common-receiver gathers.

Surface-offset gathers from elastic reverse time migration and velocity analysis

Angle-domain common-image gathers (ADCIGs) from elastic reverse time migration (ERTM) are valuable tools for seismic elastic velocity estimation. Traditional ADCIGs are based on the concept of common-offset domains, but common-shot domain implementations are often favored for computational cost considerations. Surface-offset gathers (SOGs) built from common-offset migration may serve as an alternative to the common-shot ADCIGs. We have developed a theoretical kinematic framework between these two domains, and we determined that the common SOG gives an alternative measurement of kinematic correctness in the presence of incorrect velocity. Specifically, we exploit analytical expressions for the image misposition between these two domains, with respect to the traveltime perturbation caused by velocity errors. Four formulations of the PP and PS residual moveout functions are derived and provide insightful information of the velocity error, angle, and PS velocity ratio contained in ERTM gathers. The analytical solutions are validated with homogeneous examples with a series of varied parameters. We found that the SOGs may perform in the way of simplicity and linearity as an alternative to the common-shot migration. To make a full comparison with ADCIGs, we have developed a cost-effective workflow of ERTM SOGs. A fast vector P- and S-wave decomposition can be obtained via spatial gradients at selected time steps. A selected ERTM imaging condition is then modified in which the migration is done by offset groups between each source and receiver pair for each P- and S-wave decomposition. Two synthetic (marine and land) examples are used to demonstrate the feasibility of our methods.

Pre-stack Depth Migration based on VTI Media in Common Shot Domain

With the increasing complexity of seismic exploration targets, the basic imaging theory based on the basic assumption of simple isotropic media cannot meet the actual needs of oil and gas exploration at this stage. Therefore, the study of imaging methods in anisotropic media is of great significance to the exploration of various underground structures and mineral exploitation. In this paper, the SSF continuation operator for VTI anisotropic medium is derived and the one-way prestack depth migration with VTI media in common shot domain is realized. The VTI-HESS model is verified by calculation. Compared with prestack depth migration of various isotropic media, it can be seen that prestack depth migration of VTI media has better imaging effect because of considering anisotropic parameters, which is closer to the actual imaging effect of exploration data.

3D edge-diffraction coefficients in the azimuth and emergence domain

The conventional equation for 3D edge diffractions in the Cartesian coordinate system lacks angle information for studying the energy patterns in 3D seismic prestack data. Here, a new calculation method is presented for determining 3D edge-diffraction coefficients in a spherical coordinate system that can formulate the coefficients according to the azimuth and emergence angles. To avoid the singularity phenomenon, a Haar wavelet operator matrix is used for this new method. Analysis of the edge-diffraction coefficients variations with azimuth in the common-shot domain, common-receiver domain, and common-midpoint domain reveals that the variation curves of the coefficients can be used to identify the trend of a fault. The phenomenon of polarity reversal of the edge-diffraction coefficients is observed in the different domains, and the coefficients in the common-shot domain are more sensitive than in other domains.

Modeling and assessing the effects of the sea surface, from being flat to being rough and dynamic

The sea surface acts as a very strong reflector because of the large impedance contrast between water and air. The reflection coefficient is [Formula: see text] in a very good approximation. Apart from the surface multiples, the sea surface is also responsible for generating the source and receiver ghost wavefields. These cause the well-known ghost notches in the spectrum: areas where the signal-to-noise ratio is very low. To model the ghost wavefields, ghost operators can be computed and applied to ghost-free data. Modeling experiments indicate that in the case of a flat sea surface, the character of the notches in various gather types, e.g., receiver gather, common-offset gather, shot record, is largely determined by the complexity of the earth. In a simple earth, e.g., horizontally layered, the notches are always well-defined and deep, but in a complex earth, they become blurry in some of the gather types. Therefore, in the case of a complex subsurface, source deghosting is best carried out in the common-receiver domain and receiver deghosting is best carried out in the common-shot domain. In the case of a simple subsurface, deghosting can be carried out in all domains. An additional factor is that the sea surface may be rough and dynamic. This causes blurry ghost notches in all gather types, even in the case of a simple earth. To model the source ghost for this situation, an effective static rough sea surface suffices. This keeps the computations simple. The condition is that the source has an impulsive character. However, to model the receiver ghost (and the source ghost for a nonimpulsive source), the dynamics of the sea surface must be included. This can be done by composing the final result from the results computed for several “frozen” snapshots of the dynamic sea surface.

Two field trials for deblending of simultaneous source surveys: Why we failed and why we succeeded?

Currently, deblending is the main strategy for dealing with the intense interference problem of simultaneous source data. Most deblending methods are based on the property that useful signal is coherent while the interference is incoherent in some domains other than common shot domain. In this paper, two simultaneous source field trials were studied in detail. In the first trial, the simultaneous source survey was not optimal, as the dithering code had strong coherency and the minimum distance between the two vessels was also small. The chosen marine shot scheduling and vessel deployment made it difficult to deblend the simultaneous source data, and result was an unexpected failure. Next, we tested different parameters (the dithering code and the minimum distance between vessels) of the simultaneous source survey using the simulated blended data and got some useful insights. Then, we carried out the second field trial with a carefully designed survey that was much different from the first trial. The deblended results in common receiver gather, common shot gather or the final stacked profile were encouraging. We obtained a complete success in the second field trial, which gave us confidence in the further test (such as a full three dimensional acquisition test or a high-resolution acquisition test with denser spatial sampling). Remembering that failures with simultaneous sourcing seldom reported, in this paper, our contribution is the discussion in detail about both our failed and successful field experiments and the lessons we have learned from them with the hope that the experience gained from this study can be very useful to other researchers in the same field.

Wavefield separation and polarity reversal correction in elastic reverse time migration

In elastic reverse time migration (RTM), one of the problems that are often encountered is the cross-talk between P- and S-wavefields. A useful processing technique to reduce the cross-talk is separating the P- and S-wavefields by using divergence and curl operators before applying an elastic imaging condition. However, the separated wavefields lose their physical meaning because their phase and amplitude are changed. In this paper, we modify the divergence and curl operators to give the separated wavefields a clear physical meaning: the separated wavefield is the first derivative of the input wavefield with respect to time. Another problem often encountered is polarity reversals in PS and SP images, which can cause destructive interference in the final stacked image and thus destroy the migrated events. In this paper we also develop a procedure for polarity reversal correction based on the polarization vectors of the P- and S-wavefields in the common-shot domain. The correction factor is first calculated at every imaging point during the wavefield reconstruction and is then multiplied by the PS and SP images at each time step when an elastic imaging condition is applied. Numerical examples with synthetic data have shown that the modified wavefield separation method is correct, and the procedure of polarity reversal correction is effective for a complex model.

Polarity reversal correction for elastic reverse time migration

Polarity reversals in PS and SP images of elastic reverse time migration will cause destructive interference in the final stacked results, which may lead to an erroneous interpretation. We derive a polarity reversal correction based on an analysis of the polarity distribution of PS and SP images. The key aspect for a polarity reversal correction is the polarity distribution of the S-wave component. Polarity reversal correction methods can be performed according to the actual patterns of the polarity distribution in gathers. For the 2D isotropic case, a sign factor is introduced to represent the polarity distribution of the S-wave component; this sign factor can be computed using the energy flux density vector. We present a procedure to correct the polarity reversal for elastic reverse time migration in the common-shot domain. First, the sign factor is computed during the wavefield reconstruction for every imaging point. Then, the polarity reversal is corrected by multiplying the PS and SP images with the sign factor at every time step when an elastic imaging condition is applied. We also design a filter algorithm for the sign factor to improve its consistency along an event and thereby to diminish the impact of the inaccuracy of the energy flux density vector and to improve the imaging results. Numerical examples have shown that the polarity reversal correction procedure works and that the sign filter algorithm successfully eliminates the consequences of the inaccuracy of the energy flux density vector. The quality of the migration events is effectively improved.

Acquisition using simultaneous sources

We present a technique in which two or more shots are acquired during the time it normally takes to acquire one shot. The two (or more) shots are fired in a near simultaneous manner with small random time delays between the component sources. A variety of processing techniques are applied to produce the same seismic images which would have resulted from firing the simultaneous shots separately. These processing techniques rely on coherency of the wavefield in the common-shot domain and unpredictability in the common-receiver, common-offset, and common-midpoint domains. We present results of its application on synthetic 2D, real 2D, and real 3D data from the Gulf of Mexico. These results demonstrate that, in deep water with modest water-bottom reflectivity, no special processing is required, whereas in shallower water with stronger water-bottom reflectivity, the use of shot-separation techniques is necessary. We conclude that this technique can be used robustly to improve source sampling and, for example, ...

Migração PSPI baseada nas equações da onda unidirecionais com amplitude verdadeira

As equações da onda unidirecionais com amplitude verdadeira, propostas nos trabalhos mais recentes, levam em conta de forma correta a parte cinemática e dinâmica da equação completa da onda equivalentemente às obtidas a partir da aproximação de altas freqüências. Isto é, as equações iconal e de transporte obtidas para estes operadores se correspondem com as obtidas com a equação completa da onda. Num meio com variação lateral de velocidade, a implementação das equações unidirecionais com amplitude verdadeira envolve o cálculo numérico de operadores integro-diferenciais que podem ser aproximados por somatórios e resolvidos por técnicas de diferenças finitas. Para um meio que varia somente com a profundidade, as novas equações unidirecionais com amplitude verdadeira podem ser solucionadas através do tradicional método phase-shift , e as novas condições de fronteira e de imagem se reduzem a multiplicações no domínio de Fourier. Neste trabalho estamos propondo estender a migração phase-shift com amplitude verdadeira para meios com variação lateral v(x,z), através da metodologia Phase shift plus interpolation (PSPI). Experimentos numéricos no domínio de tiro comum com o novo método PSPI com amplitude verdadeira e sua extensão split-step (PSPI-SS), produzem imagens com amplitudes mais corretas em relação aos operadores unidirecionais convencionais, enquanto as características cinemáticas da migração PSPI e PSPI-SS são preservadas.