Anisotropic Prestack Time Migration Research Articles

The key technology of 3D seismic data contiguous processing and its application: Taking the northern slope area of Zhahaquan in Qaidam Basin as an example

The surface conditions in the northern slope area of Zhahaquan in the Qaidam Basin are complex, and the underground is affected by tectonic extrusion movements, resulting in the development of faults and fractures. The existing six blocks of 3D seismic data in this area have a significant time span in acquisition and varying data quality. The existing single-block processing results indicate a low signal-to-noise ratio in the 3D seismic data connection area, along with substantial differences infrequency, phase, and energy. The fracture imaging is poor, making it challenging to accurately identify and track layer positions and fault planes in space, thereby restricting further exploration in this region. Based on a detailed analysis of the characteristics and existing problems of the original data, we conducted key technical research on continuous static correction, pre-stack noise purification, consistency processing, data regularization, and anisotropic pre-stack time migration for continuous processing of six blocks of 3D seismic data in this area. The processing results demonstrate good consistency in frequency, phase, energy, and other aspects, highlighting prominent reflection characteristics and clear imaging of complex structures in the middle and deep layers. Clear breakpoints and fault planes are also evident, solving the inconsistency of frequency, phase, energy, and incomplete coverage in the block connection section. Additionally, this processing has resolved the problem of inaccurate migration positioning caused by inconsistent migration velocity fields, providing high-quality data for subsequent structural interpretation and reservoir prediction.

Joint anisotropic amplitude variation with offset inversion of PP and PS seismic data

With the increase in exploration target complexity, more parameters are required to describe subsurface properties, particularly for finely stratified reservoirs with vertical transverse isotropic (VTI) features. We have developed an anisotropic amplitude variation with offset (AVO) inversion method using joint PP and PS seismic data for VTI media. Dealing with local minimum solutions is critical when using anisotropic AVO inversion because more parameters are expected to be derived. To enhance the inversion results, we adopt a hierarchical inversion strategy to solve the local minimum solution problem in the Gauss-Newton method. We perform the isotropic and anisotropic AVO inversions in two stages; however, we only use the inversion results from the first stage to form search windows for constraining the inversion in the second stage. To improve the efficiency of our method, we built stop conditions using Euclidean distance similarities to control iteration of the anisotropic AVO inversion in noisy situations. In addition, we evaluate a time-aligned amplitude variation with angle gather generation approach for our anisotropic AVO inversion using anisotropic prestack time migration. We test the proposed method on synthetic data in ideal and noisy situations, and find that the anisotropic AVO inversion method yields reasonable inversion results. Moreover, we apply our method to field data to show that it can be used to successfully identify complex lithologic and fluid information regarding fine layers in reservoirs.

Velocity anisotropy characteristics and pre-stack time migration imaging of the marine sedimentary strata in the South Yellow Sea Basin

The South Yellow Sea is a superimposed basin overlying Mesozoic-Cenozoic continental sediments, which in turn overlie Paleozoic-Mesozoic marine deposits that are now the target of hydrocarbon exploration. Strongly modified by multiple tectonic events, the marine sediments feature a large tectonic relief, with obvious horizontal anisotropy in seismic velocity, which significantly affects the seismic image quality. In this study, the sedimentary velocity anisotropy and its influence on image quality were analyzed using an analytical theory method, assuming transversely isotropic medium with vertical axis of symmetry (VTI), and using seismic and well-log data. Additionally, an anisotropic prestack time migration was used for the imaging of the field data. The results showed that the anisotropic pre-stack time migration processing could be used to significantly improve the accuracy of the seismic images in areas with distinct faults, offering clear images of accurately located fault planes and fault edges, thereby improving the lateral resolution of the seismic data and its signal-to-noise ratio.

Full-azimuth anisotropic prestack time migration in the local-angle domain and its applications on fracture detection

Considering that geologic structures disturb prestack amplitude relationships, anisotropic migration is thus advocated not only for extracting azimuth-preserved common image gathers (CIGs), but also for preserving fracture-induced amplitude responses. However, most conventional anisotropic migration methods are hindered by their inefficiency in either modeling azimuthal traveltime variations at large offsets or characterizing subsurface reflections. Given that prestack time migration is widely applied for most practical purposes, we began with reformulations on a quartic traveltime formula, through which a new set of anisotropic parameters was developed. Then, an anisotropic migration method was established in the local-angle domain (LAD) for more reasonable uses of subsurface wavefield information. We also used a traveltime inversion scheme to estimate those anisotropic parameters required by anisotropic migration. Using this methodology on a physical model with a fracture medium, we derived better focused CIGs by thoroughly correcting the anisotropic effects of overburden. As a result, predicted properties of the fracture medium showed fewer interventions of geologic impacts. In a field example, a comprehensive study was performed on a deep carbonate reservoir to examine influences of different anisotropic migration algorithms on ultimate fracture prediction. Comparisons of the signal-to-noise ratio and agreements with formation microimage information reconfirmed the superiority of LAD anisotropic migration in recovering true properties of subsurface fractures, relative to routine methods (i.e., azimuth-sectored migration and anisotropic migration in the surface-offset domain).

Anisotropic PP and PSv prestack depth migration of 4C seismic data; Pamberi, Trinidad

SummaryIn November 2004, EOG Resources contracted WesternGeco to acquire an ocean-bottom cable (OBC) 4C swath survey across the Pamberi-1 well in the Lower Reverse L block of the Columbus basin, eastern offshore Trinidad. The purpose of the 4C survey was to evaluate the potential of long-offset multicomponent technology for resolving lithology and stratigraphic detail in an area perturbed by shallow gas, overpressure and illumination shadows from normal regional faults and major anticlinal ridge trends acting as pressure seals. A key motivation for EOG was the realization that a conventional 3D towed-streamer survey acquired the previous year failed to adequately image the target reflectors comprising the reservoir under the main fault.Details of the P- and PSv-wave processing of this dataset through anisotropic prestack time migration were previously described (Johns et al., 2006) in which it was demonstrated that there existed a qualitative correlation between derived parameters and attributes from P and Sv anisotropic migration velocities and known regional geology. This observation was quite remarkable considering only a limited effort to constrain or validate parameters (in this case, velocities to the Pamberi-1 well checkshots) was performed. Under the “Future work” section of the previous publication, it was suggested that further data quality enhancement in preparation for more quantitative rock property classification could only be achieved after prestack depth imaging.In this paper, we present precisely that next phase in the 4C processing, advancing the P- and PSv-wave data through anisotropic prestack depth migration, using cellbased tomography with a top-down approach. The Pamberi-1 well was used to constrain the anisotropy in the shallow section, with the deeper, spatial trend away from the proximity of the well determined from the anisotropy derived previously in the time processing.Prior to proceeding with the anisotropic depth imaging, the magnitude of shear splitting from the presence of azimuthal anisotropy (HTI) was first examined to assess its potential impact on the radial rotated P-S signal. The shear wave splitting analysis revealed a principal angle of polarization that was closely aligned with the regional stress direction delineated by the normal major faults blocks acting as pressure seals.

Converted‐wave imaging in anisotropic media: theory and case studies

ABSTRACTCommon‐conversion‐point binning associated with converted‐wave (C‐wave) processing complicates the task of parameter estimation, especially in anisotropic media. To overcome this problem, we derive new expressions for converted‐wave prestack time migration (PSTM) in anisotropic media and illustrate their applications using both 2D and 3D data examples.The converted‐wave kinematic response in inhomogeneous media with vertical transverse isotropy is separated into two parts: the response in horizontally layered vertical transverse isotrophy media and the response from a point‐scatterer. The former controls the stacking process and the latter controls the process of PSTM. The C‐wave traveltime in horizontally layered vertical transverse isotrophy media is determined by four parameters: the C‐wave stacking velocity VC2, the vertical and effective velocity ratios γ0 and γeff, and the C‐wave anisotropic parameter χeff. These four parameters are referred to as the C‐wave stacking velocity model. In contrast, the C‐wave diffraction time from a point‐scatterer is determined by five parameters: γ0, VP2, VS2, ηeff and ζeff, where ηeff and ζeff are, respectively, the P‐ and S‐wave anisotropic parameters, and VP2 and VS2 are the corresponding stacking velocities. VP2, VS2, ηeff and ζeff are referred to as the C‐wave PSTM velocity model. There is a one‐to‐one analytical link between the stacking velocity model and the PSTM velocity model. There is also a simple analytical link between the C‐wave stacking velocities VC2 and the migration velocity VCmig, which is in turn linked to VP2 and VS2.Based on the above, we have developed an interactive processing scheme to build the stacking and PSTM velocity models and to perform 2D and 3D C‐wave anisotropic PSTM. Real data applications show that the PSTM scheme substantially improves the quality of C‐wave imaging compared with the dip‐moveout scheme, and these improvements have been confirmed by drilling.

Imaging Lomond Field using C-wave anisotropic PSTM

In this paper we present the results from an anisotropic converted-wave (C-wave) prestack time migration (PSTM) of a 2D multicomponent (4-C) line over Lomond Field. This is a gas-condensate field 233 km east of Aberdeen, in the Central North Sea, Block 23/21, close to the border between the UK and the Norwegian sectors, on the east flank of the Central Graben. The geologic setting is excellent for testing C-wave imaging tools; the quite complex structure is dominated by a salt-induced anticline (fractured on top) and a large gas cloud (leaked from the fractured reservoir). In addition, a thick overburden of laminated shales is likely to produce polar anisotropic (VTI) effects. The large gas cloud was the main motivation for C-wave acquisition. In fact, the imaging results obtained using P-waves (Figure 1) include a large zone of amplitude dimming, circled in black, and the push-down effects on the above events. This cloud partially obscures the area of the reservoir, which is composed of Paleocene sandstones of the Forties Formation, 200 m in thickness, and sealed by mudstones from the Rogaland Group. The presence of polar (VTI) anisotropy was confirmed by observations of angular-dependent velocity in deviated sonic logs.