Overthrust imaging with tomo‐datuming: A case study

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper IPTC 18293, “Near-Surface Velocity Model of Dukhan Field From Multiphysics Survey To Enhance PSDM Seismic Imaging,” by K. Setiyono, Qatar Petroleum; S. Gallo, C. Boulanger, F. Bruere, F. Moreau, and B. Rondeleux, CGG; and J. Snow, Qatar Petroleum, prepared for the 2015 International Petroleum Technology Conference, Doha, Qatar, 7-9 December. The paper has not been peer reviewed. Dukhan field affords an opportunity to assess alternative near-surface velocity-modeling work flows that specifically integrate microgravity (MGR) and vertical electrical sounding (VES). A pilot study of the Dukhan seismic data demonstrates that integration of MGR/VES into prestack depth migration (PSDM) improves the fidelity of seismic images and results in an improved depth match. The resultant MGR/VES PSDM pilot model improved the velocity model between the surface and top Simsima formation on the basis of visible impact on the deeper structure obtained from imaging and better fit between the seismic depth horizons and well measured depths. Introduction The Dukhan field (Fig. 1) along the southwest coast of Qatar is an elongated anticline that has significant hydrocarbon reservoirs in the Jurassic Arab and deeper formations. In 2007, Qatar Petroleum invested in a new full-field, land and shallow-water 3D-seismic survey of the area. Despite considerable and continuous progress in seismic technology, obtaining an accurate, quantitative, well-focused seismic depth image at reservoir level remains challenging. In land seismic, a poor characterization of the near-surface geology is detrimental to the depth imaging of underlying structures. The proper knowledge of the near surface to correct for its perturbations onto the seismic wavefield remains one of the main challenges to be addressed. The near surface influences the kinematics as well as the dynamics of the source-generated seismic waves. Knowledge of the near surface to correct for its induced distortions is critical for a reliable use of seismic images, structures, and amplitude at reservoir level. The near-surface lateral and vertical velocity variations often are not accessible with seismic methods, even to the seismic refraction survey, because their depth of investigation is limited to the top of the shallowest hard layer. In addition, rapid lateral changes and large velocity contrasts are unfavorable to seismic refraction.

Azimuthal variation in traveltimes is increasingly being used as a tool for reservoir characterization and fracture detection. One issue in interpreting the results of azimuthal normal moveout (NMO) analyses is to determine whether the apparent observed anisotropy is due to intrinsic azimuthal anisotropy or, in reality, is primarily caused by lateral velocity variations. A field data example is presented where lateral heterogeneity is suspected as the main cause of an observed azimuthal NMO anomaly. Modelling studies of isotropic media containing lateral velocity variations were performed to investigate the possibility that the observed anomaly in the field data could be caused solely by isotropic lateral velocity variation. In both the modelled and field data, the patterns observed in the apparent velocity anisotropy are very distinctive with obvious correlations between ellipse parameters (fast and slow velocities and azimuth of the fast velocity) and the structure of the lateral velocity variation. The inverted RMS and interval NMO ellipses also show a very high degree of correlation both spatially and temporally, extending below the heterogeneous layer.

Current schemes for removing near-surface effects in seismic data processing use either static corrections or wave-equation datuming (WED). In the presence of rough topography and strong lateral velocity variations in the near surface, the WED scheme is the only option available. However, the traditional procedure of WED downward continues the sources and receivers in different domains. A new waveequation global-datuming method is based on the doublesquare-root operator, implementing the wavefield continuation in a single domain following the survey sinking concept. This method has fewer approximations and therefore is more robust and convenient than the traditional WED methods. This method is compared with the traditional methods using a synthetic data example.

Anisotropic depth migration (ADM) has become more commonplace over the past four years. The data-processing examples detailed here illustrate the robustness of the method in a variety of structured settings in the Alberta Foothills. The public-domain structural line, the Husky/Talisman dataset, illustrates subtle improvements in imaging with a dramatic improvement in accuracy of horizon depths when ADM is applied to these data. A 3D survey in a difficult imaging area, Nordegg/Chungo, shows significant healing of broken basement reflectors when we correct for anisotropy in the complex-dipping clastic overburden. Finally, in the Blackstone area where we observe intense folding in the near surface, comparisons between poststack time migration, prestack time migration, prestack depth migration, and prestack anisotropic depth migration show the similar step-change improvements as we imrove the technology of our algorithm. Introduction Traditional depth migration corrects only for lateral velocity heterogeneity. Current anisotropic depth migration algorithms also correct for velocity changing with direction in the dipping clastic overburden. Seismic anisotropy is a well-quantified physical characteristic of clastic rocks. Studies of anisotropic behaviour in clastic rocks include Thomsen (1986), Vernik and Liu (1997), Leslie et al (1997). Several papers over the past seven years illustrate the improvements in imaging when we stop ignoring anisotropy and begin to correct for it (Ball, 1995; Vestrum and Muenzer, 1997; Vestrum, et al., 1998; Ferguson and Margrave, 1998; Schmid et al., 1998; Dai et al., 1998; Fei et al., 1998; Vestrum et al., 1999, Kirtland Grech et al, 2001; etc.). The seismic data processing case histories presented here outline the improvements in seismic imaging and structural positioning when we correct for seismic anisotropy in depth migration. Benjamin Creek The Husky/Talisman dataset is the focus of our first processing case history. In this example, I illustrate the improvement in correlation between well depths and seismic depths. Figure 1 shows this improvement. Isotropic depth migration for this line (Figure 1a) shows a typical result (Shultz and Canales, 1997; Etris et al., 2001) of isotropic depth migration where the seismic depths are deeper than the well depths. When we ignore anisotropy, we need to use a velocity faster than the vertical velocity if we are to flatten the image gathers and obtain a quality image. The anisotroic migration (Figure 1b), models the velocity behavour changing with direction, so it produces a clear image and correlates to the well depths. One can see in Figure 1b that there is still some uncertainty in the correlation, but we have removed a systematic error from the depth correlation. We could now go back and adjust our imaging velocities so that our final depth migration matched perfectly with well depths, without needing a separate depth-conversion step after the depth migration. We can now consider well depths as a constraint on our imaging velocity model. There are also several imaging improvements on the anisotropic depth migration over the isotropic depth migration. Figure 1: (a) isotropic depth migration and (b) anisotropic depth migration and overlain well depths to top and bottom of the carbonate sheet. Nordegg/Chungo The Nordegg/Chungo 3D is a Time Seismic data volume acquired in the Alberta Foothills. A more difficult imaging area than the Benjamin Creek area, this volume shows imaging problems that arise when the anisotropic overburden has dramatically changing dip. Figure 2 shows the comparison between isotropic and anisotropic depth migrations for a line out of this volume. Note the improvement in basement continuity beneath the steeply dipping near-surface when we use anisotropic depth migration (Figure 2b). Several other potential hydrocarbon-target events show similar improvement. Note that the target event, as indicated by the red arrow, shows more consistent imaging over the range of dips from the back limb over the crest of the structure and into the forelimb. The anisotropic velocity model more accurately represents the velocity structure of the overburden. With more accurate raytracing over a larger range of reflector angles, the migration is better able to image the range of reflector dips on this structure. Blackstone This Talisman dataset is a very dramatic anisotropic imaging example. The extensive folding of the clastic overburden creates difficulties imaging structures below. Figure 3 shows a comparison between poststack time migration (Figure 3a), prestack time migration (Figure 3a), prestack depth migration (Figure 3c), and prestack anisotropic depth migration (Figure 3d). Each step forward in complexity of the migration

An integrated procedure for migration velocity analysis in complex structures of thrust belts

Traditional pre-stack depth migration can only provide subsurface structural information. However, simple structure information is insufficient for petroleum exploration which also needs amplitude information proportional to reflection coefficients. In recent years, pre-stack depth migration algorithms which preserve amplitudes and based on the one-way wave equation have been developed. Using the method in the shot domain requires a deconvolution imaging condition which produces some instability in areas with complicated structure and dramatic lateral variation in velocity. Depth migration with preserved amplitude based on the angle domain can overcome the instability of the one-way wave migration imaging condition with preserved amplitude. It can also offer provide velocity analysis in the angle domain of common imaging point gathers. In this paper, based on the foundation of the one-way wave continuation operator with preserved amplitude, we realized the preserved amplitude prestack depth migration in the angle domain. Models and real data validate the accuracy of the method.

Seismic hazards in extensional regions are mostly posed by high‐angle normal faults and deep hanging‐wall basins that can promote significant ground‐motion amplifications. Characterized by strong lateral velocity variations, these structures represent challenging targets for seismic exploration. In this paper, we show that prestack depth migration of high‐resolution seismic‐reflection data, collected with nonconventional dense wide‐aperture arrays, can effectively assist deterministic seismic‐hazard assessment of fault‐bounded basins. Our high‐resolution 3.2 km long seismic profile was acquired along Vallo di Diano, the largest fault‐bounded intramontane basin in the southern Apennines seismic belt (Italy). The imaging strategy combines prestack depth migration with an iterative velocity‐building technique that utilizes results of multiscale refraction tomography. In particular, migration of steeply dipping reflections is essential in imaging the complex morphology of the basement rocks beneath the basin. By combining seismic‐reflection and V P tomography results we were able to evaluate (1) the structure of the Vallo di Diano basin along a representative transect from near‐surface to basement depths; (2) the shallow‐depth geometry of the Vallo di Diano fault system, which bounds the basin to the east; (3) the structure of the sediment–basement interface; and (4) the velocity structure of basement and sediments within the basin. These findings are valuable both for understanding the tectonic evolution of the Vallo di Diano and for developing earthquake‐shaking scenario studies.

In case of lateral velocity variations and of dipping events, depth migration is the adequate processing to obtain seismic images directly interpretable in terms of geological structures . However this technique involves estimation of a detailed depth model in which lateral velocity variations are taking info account. Due to difficulties in determining a correct structural model, depth migration is often used as an iterative and interpretive process. Therefor a sequential strategy of seismic processing and interpretation may be redefined as an incremental proces of structural model building. This structural model is defined in depth and is viewed as the sum of two types of model: a geometrical model which describes reflector geometry and a velocity model which gives an estimate of the intra-bed velocities used for time-depth conversion. Relations between geometrical and velocity models are guided by geological reasoning.

Getting a geologically realistic seismic image still remains a great challenge in foothills exploration. When geological structures are complex, seismic sections show distorted images and become difficult to be interpreted. Seismic processing tools, like time and depth migrations, have been developed to deal with this. Furthermore, when lateral velocity variations are strong, depth migration is definitely required. However, for a correct application of this processing, the construction of a detailed depth model describing lateral and vertical velocity variations is essential.

Accurate processing of 3-D seismic data sets is essential to meet the objectives of today's explorationist. Migration is a process that is particularly important in 3-D seismic processing. Originally, 3-D migration was performed in two separate 2-D passes but this suffered from the inaccuracies of a constant velocity assumption. The splitting algorithm is now a popular one-pass approach to 3-D migration. One-pass migration algorithms avoid the assumption of constant velocities and more correctly position the seismic energy. A more flexible approach to one-pass 3-D migration is known as k partitioning. This algorithm allows for a choice of currently available 2-D migrations to be used to perform an accurate one-pass 3-D migration. The Ky partitioning approach is also more efficient. When lateral variations in velocity become significant, a depth migration algorithm will more accurately image the data than the standard time migration algorithm. The splitting algorithm is converted into a depth migration by including the thin lens term. For the Ky partitioning algorithm this is achieved by the use of dilation. Dilation consists of small dynamic time shifts applied to the 3-D data before migration to compensate for the existence of lateral velocity variations. Application of both splitting and Ky-dilation one-pass 3-D migration algorithms to a synthetic data set show that the proposed dilation factors can produce sharp migrated images in the presence of lateral velocity variations. In this case the Ky-dilation results exhibit superior clarity compared with the image obtained from the splitting approach. The extra flexibility of the Ky-dilation approach allowed the use of a high-dip time migration algorithm which avoided the frequency dispersion effects that degrade the image of the finite-difference splitting migration. An improved image has been obtained for less computational effort.

Time migration, as opposed to depth migration, suffers from two well-known shortcomings: (1) approximate equations are used for computing Green’s functions inside the imaging operator and (2) in case of lateral velocity variations, the transformation between the image-ray coordinates and the Cartesian coordinates is undefined in places where the image rays cross. We have found that the first limitation can be removed entirely by formulating time migration through wave propagation in image-ray coordinates. Our approach constructs a time-migrated image without relying on any kind of traveltime approximation by formulating an appropriate geometrically accurate acoustic wave equation in the time-migration domain. The advantage of this approach is that the propagation velocity in image-ray coordinates does not require expensive model building and can be approximated by quantities that are estimated in conventional time-domain processing. Synthetic and field data examples demonstrate the effectiveness of the proposed approach and show that the proposed imaging workflow leads to a significant uplift in terms of image quality and can bridge the gap between time and depth migrations. The image obtained by the proposed algorithm is correctly focused and mapped to depth coordinates; it is comparable to the image obtained by depth migration.

The Guando field is in an area of rough topography and complex geology. In order to better site wells in such fields it is of value to have a sound understanding of this geological complexity. Obtaining a satisfactory seismic image to enable such interpretation requires pre-stack imaging. The geological complexity is such that post stack imaging fails due to the failure of the common mid point assumption. Is it sufficient to perform pre-stack time migration or is it necessary to perform pre-stack depth migration? In this instance both have been performed on this field and results for each of them will be shown. The basic theory of both pre-stack time and pre-stack depth migration will be described without using mathematics. Model data will then be used to illustrate the shortcomings of pre-stack time migration using a model of similar geological complexity to the Guando field. Both pre-stack migrations require a velocity field. These differ and the manner of their derivation also differs. Velocity model building for both techniques will be described. Particular emphasis will be placed on the use of tomographic and scanning techniques for pre-stack depth migration. Examples will be shown of the velocity models from both migrations. It will be shown that the pre-stack depth migration velocity model is also a valuable interpretation asset. Data from both migrations will be shown. These will demonstrate the superiority of the pre-stack depth migrated data and the reasons why it is superior.

Various migration methods have been proposed to image high-angle geological structures and media with strong lateral velocity variations; however, the problems of low precision and high computational cost remain unresolved. To describe the seismic wave propagation in media with lateral velocity variations and to image high-angle structures, we propose the generalized screen propagator based on particle swarm optimization (PSO-GSP), for the precise fitting of the single-square-root operator. We use the 2D SEG/EAGE salt model to test the proposed PSO-GSP migration method to image the faults beneath the salt dome and compare the results to those of the conventional high-order generalized screen propagator (GSP) migration and split-step Fourier (SSF) migration. Moreover, we use 2D marine data from the South China Sea to show that the PSO-GSP migration can better image strong reflectors than conventional imaging methods.

The seismic image produced by pre-stack depth migration is more accurate and has clearer geological significance than the time image. However, the waveform of the depth-domain seismic image is affected not only by depth-dependent velocity variation but also by media attenuation, resulting in strong spectral variation of depth-domain seismic data. Therefore, depth-domain seismic inversion is still challenging. We propose a wavelet extraction method of attenuation media based on the generalized seismic wavelet, to address this issue. Then, the estimated depth-domain wavelets were applied to the direct acoustic impedance inversion. First, we investigated the effect of attenuation media on depth-domain source wavelets and derived an analytical formula for the depth-domain wavelets of attenuation media. Next, the time-domain generalized seismic wavelet was extended to the depth domain, which was utilized to study the feasibility of using the generalized seismic wavelet to characterize the seismic wavelet of the depth-domain attenuation media. Based on the orthogonal matching pursuit, we propose a method to extract the depth-domain generalized seismic wavelet directly from depth-domain seismic data. Finally, we applied this method to the depth-domain direct acoustic impedance inversion of a 3D field data example. Tests on the synthetic and 3D field datasets show that the proposed method can correctly extract the depth-domain seismic wavelet of attenuation media and attain direct inversion of the depth-domain acoustic impedance with high accuracy. Therefore, our method is effective and has robust potential in reservoir characterization, fluid prediction, and attribute extraction in the depth domain.

In the mountainous terrain of west China, seismic data quality may be very poor due to severe statics problem and heavy noise especially in presence of complicated surface topography and complex subsurface structures. Moreover the complexity of subsurface structures maks the wave field much more complicated and the velocity distribution laterally varies significantly. Under this unfavorable context, it is very challenging to build a reasonable velocity model for the pre-stack depth migration to delineate the explicit structure contact relationship under complex geological settings. Here one enhanced velocity model building solution is presented. We innovatively make use of diving wave refraction tomography to build shallow velocity model and high-resolution reflection tomography to update the whole velocity model especially deep data. By tightly constraining the seismically derived velocity with available well control, one reliable velocity model can eventually obtained. After that we performed the pre-stack Kirchhoff depth migration and achieved geologically reasonable depth imaging results to help the geologists better understand the complex subsurface structures. This project is the first application of diving wave tomography on land seismic data in China. Its success has provided one robust and effective velocity model building solution and has significant meaning to other complex depth imaging land projects in China.