Conventional Prestack Time Migration Research Articles

Improving prestack time migration by introducing a new velocity-related parameter: Parameter picking and 3D real data application

Prestack time migration (PSTM), a commonly used tool for seismic imaging, has been widely applied in 3D seismic data processing. However, the conventional PSTM algorithms use only one effective velocity parameter (i.e., root-mean-square [rms] velocity) for each imaging point, which may not be accurate when stronger lateral variations occur in seismic velocities. In this paper, we introduce a new parameter called the velocity variation factor that considers velocity variations in inhomogeneous media to improve PSTM. This new parameter, together with the rms velocity, describes the propagation Green’s function at an imaging point with two effective parameters rather than one effective parameter as in conventional PSTMs. This provides a more accurate traveltime calculation for the wave propagating through media with moderate lateral velocity variation. Unlike the conventional bending-ray PSTM, the additional effective parameter is fully independent of the rms velocities. We estimate the two effective parameters at each imaging point by flattening the neighboring image gathers with a global optimization algorithm. The objective function is built at each imaging point using a selective crosscorrelation-based time shift, which can quantitatively describe the slight bending of events in the local migrated gathers regardless of the quality of the gathers. We estimate the two effective parameters using the very fast simulated annealing algorithm and multiscale approach, thus avoiding the local minimum caused by the noises in the migrated gathers. We apply our two-parameter PSTM to a real 3D land data set to demonstrate its industrial applicability. A comparison of the new imaging result with the conventional prestack depth migration also is presented.

Seismic profile denoising based on common-reflection-point gathers using convolution neural networks

Abstract With the development of seismic surveys and the decline of shallow petroleum resources, high resolution and high signal-to-noise ratio have become more important in seismic processing. To improve the quality of seismic data, stationary-phase migration based on dip-angle gathers can be used to separate the reflected waves and noise. However, this method is very computationally intensive and heavily dependent on expert experience. Neural networks currently have powerful adaptive capabilities and great potential to replace artificial processing. Certain applications of convolution neural networks (CNNs) on stack profiles lead to a loss of amplitude information. Therefore, we have developed CNNs for noise reduction based on common-reflection-point (CRP) gathers. We used CRP gathers of stationary-phase migration as labels and CRP gathers of conventional prestack time migration as inputs. In addition, we analyzed the seismic amplitude properties and demonstrated the neural network optimization process and results. The results showed that our methods can achieve fast and reliable denoising and produce high-quality stack profiles that contain true amplitude information. Furthermore, the predicted high-quality CRP gathers can be used for further processing steps, such as normal moveout correction and amplitude variation with offset.

Time-lapse velocity analysis — Application to onshore continuous reservoir monitoring

In 4D seismic, the velocity model used for imaging and reservoir characterization can change as production from the reservoir progresses. This is particularly true for heavy oil reservoirs stimulated by steam injection. In the context of sparse and low-fold seismic acquisitions, conventional migration velocity analyses can be inadequate because of a poorly and irregularly sampled offset dimension. We update the velocity model in the context of daily acquisitions with buried sources and receivers. The main objective is to demonstrate that subtle time-lapse effects can be detected over the calendar time on onshore sparse acquisitions. We develop a modified version of the conventional prestack time migration to detect velocity changes obtained after crosscorrelation of the base and monitor surveys. This technique is applied on a heavy oil real data set from the Netherlands and reveals how the steam diffuses over time within the reservoir.

High-resolution imaging: An approach by incorporating stationary-phase implementation into deabsorption prestack time migration

We have improved the so-called deabsorption prestack time migration (PSTM) by introducing a dip-angle domain stationary-phase implementation. Deabsorption PSTM compensates absorption and dispersion via an actual wave propagation path using effective [Formula: see text] parameters that are obtained during migration. However, noises induced by the compensation degrade the resolution gained and deabsorption PSTM requires more computational effort than conventional PSTM. Our stationary-phase implementation improves deabsorption PSTM through the determination of an optimal migration aperture based on an estimate of the Fresnel zone. This significantly attenuates the noises and reduces the computational cost of 3D deabsorption PSTM. We have estimated the 2D Fresnel zone in terms of two dip angles through building a pair of 1D migrated dip-angle gathers using PSTM. Our stationary-phase QPSTM (deabsorption PSTM) was implemented as a two-stage process. First, we used conventional PSTM to obtain the Fresnel zones. Then, we performed deabsorption PSTM with the Fresnel-zone-based optimized migration aperture. We applied stationary-phase QPSTM to a 3D field data. Comparison with synthetic seismogram generated from well log data validates the resolution enhancements.

Compensation for absorption and dispersion in prestack migration: An effective Q approach

We have developed a migration scheme that can compensate absorption and dispersion caused by intrinsic attenuation in subsurface media. The scheme was developed by adapting prestack time migration (PSTM) in the frequency domain. Instead of applying a commonly used [Formula: see text] factor, we devised an effective [Formula: see text] parameter to compensate absorption and dispersion. The effective [Formula: see text] determines the frequency-dependent traveltime and amplitude at one imaging location by only one value. As a result, the effective [Formula: see text] can be estimated by scanning technology. We designed an index that can remove the effects of interferences of the reflections resulting from stacked thin layers in extracting the effective [Formula: see text] parameter from scanning results. The proposed scheme can thus determine an effective [Formula: see text] model using surface seismic data during migration. Stabilization is achieved by introducing a smooth, maximum-limited gain function that matches the exact amplitude compensation factor when it is less than the user-specified gain limit. The proposed scheme can be incorporated into conventional PSTM workflow. Synthetic and field data sets were used to test the proposed deabsorption PSTM. Higher-resolution imaging results are obtained.

Migration from 3D irregular surfaces: A prestack time migration approach

We have developed a modified 3D prestack time migration (PSTM) scheme that can handle rugged topography as well as high near-surface velocities in land seismic imaging. The proposed topography PSTM can be applied to seismic data recorded on a 3D irregular surface without static corrections. Two effective velocity parameters were found to describe wave propagation through inhomogeneous media above and below a chosen datum. As a result, wave propagation phenomena in the complex near surface, such as near-vertical incidences through a weathering layer and raypaths bending away from vertical in the presence of high near-surface velocities, are correctly considered. The two effective velocity parameters can be estimated by flattening events in imaging gathers. Hence, it is not necessary to have detailed knowledge of the near-surface velocity model and velocity field below the datum when applying topography PSTM. We integrated residual static corrections into topography PSTM. This eliminated the distortions along the events better than conventional residual static corrections, which are usually applied before migration. The computational cost of the topography PSTM was only slightly higher than that of conventional PSTM due to the use of a table-driven algorithm. Three-dimensional synthetic and field data sets were used to test the proposed topography PSTM. High-quality imaging results were obtained.

Prestack Time Migration in Anelastic Media Based on Designing for Short Operator

AbstractIn order to raise the computational efficiency of frequency domain prestack time migration in anelastic media, the high‐accuracy optimum short convolution operators in time domain are designed by weighted least squares method. Therefore a table‐driven time domain prestack time migration scheme in anelastic media is presented. This scheme can transform large amount of multiplication operation frequency by frequency into less convolution operation in time domain and split calculation of travel time, amplitude and short convolution operator from the inverse Q compensation migration, which greatly improved the computational efficiency of time‐domain pre‐stack time migration in anelastic media. A stabilization controlling strategy is proposed to ensure the stability of algorithm in compensation for absorption and dispersion. Numerical examples indicate that the short convolution operator designed by weighted least squares method has very high accuracy. The algorithm in time domain can achieve the same compensation result as in frequency domain while its computational cost is close to the conventional prestack time migration.