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ABSTRACT With the continuous development of petroleum exploration, seismic prospecting on land is mainly operated in the more and more complicated mountain, desert, Gobi, loess plateau and swamp areas. The difficulties for seismic exploration include the very complicated near surface and subsurface structures, the difficult data acquisition, too much coherent and random noise, and the seismic data with poor S/N ratio and resolution. However the exploratory development in these areas has a high requirement on the precise of seismic data. Under these challenges, the geophysicists of China have undertaken large amount of technical research on optimization of acquisition geometry design, selection of shooting parameters, static correction, noise suppression and pre-stack depth migration etc. in the recent years. All these have further improved the imaging precision of seismic data in complex areas and greatly increased our exploration capability in complex areas on land.
- Asia > China (0.36)
- North America > United States > Colorado (0.17)
- Geophysics > Seismic Surveying > Seismic Processing (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (0.72)
Summary This paper presents a case history of integrating the multifarious VSP images with the surface seismic data acquired at Tarantula Prospect in the Gulf of Mexico. Introduction The Anadarko Tarantula #1 well in the Gulf of Mexico (Figure 1) was drilled on the flank of a massive salt structure. Although the top of salt is well imaged through 3D poststack time migration, interpretation of the salt flank is ambiguous (Figure 1). These comprehensive data sets include one zero offset VSP (Z0), two offset VSP (O1 and O2), a 3D salt proximity survey (SP), and two walkaway VSP lines: north-south (W1) and west-east (W2). Figure 1 shows a 3D view of the survey geometry.
The Influence of Stacking Velocity Uncertainties On Structural Uncertainties
Bube, Kenneth P. (University of Washington, Seattle, WA) | Kane, Jonathan A. (Cambridge, MA) | Nemeth, Tamas (ChevronTexaco, San Ramon, CA) | Medwedeff, Don (ChevronTexaco, San Ramon, CA) | Mikhailov, Oleg (ChevronTexaco, San Ramon, CA)
ABSTRACT Errors in the velocity field used to migrate seismic data are a leading cause of errors in the positionining of structural events in the processing of seismic data: uncertainty in the velocity field leads to structural uncertainty. In this paper, we investigate the broader question of how errors in stacking velocity, time to an event in a stacked section, and the slope of an event in a time section lead to errors in the positioning of structural events for an isotropic medium. We perform a sensitivity analysis, obtaining simple formulas for the errors in structure that are first-order in the errors in stacking velocity, zero-offset time, and slope. These formulas are geometrically explicit: if we make a small change in stacking velocity (or time or slope), we then know the direction and magnitude of the resulting change to each point on the selected event. Being the result of sensitivity analysis, these formulas are linear. Thus if we had a probability distribution for the errors in velocity (i.e., we knew the uncertainty in velocity), we could use these formulas to obtain a probability distribution for the errors in position for points on the selected event (i.e., the uncertainty in structure). Our analysis focuses on the neighborhood of a single point on an event and assumes a homogeneous velocity field. Although the analysis is based on a very simple model, numerical experiments show that the relationships are valid approximately for moderate heterogeneities in the velocity field. In a companion paper (Bube et al., 2004), we use these results to investigate errors in structural location due to uncertainty in weak anisotropy.
- Geophysics > Seismic Surveying > Seismic Processing (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (1.00)
ABSTRACT This case history describes the time and depth processing of 2D seismic data from a complicated overthrust area in Assam, India. Conventional techniques including Prestack Depth Migration (PreSDM) are compared to a comprehensive CRS approach which comprises the whole range of time domain imaging, velocity model building, and depth imaging. In the model building, a tomographic inversion method derives a reliable depth model from CRS attributes which well recovers the characteristic velocity inversion at the main thrust fault. In depth migration, the smooth model from CRS attributes proves to be largely equivalent to the discontinuous model derived by horizon-based inversion of residual depth moveout during iterative PreSDM. In imaging, the CRS stack and its poststack depth migration (PostSDM) are compared to the conventional time and PreSDM depth sections. Both in time and depth imaging, advantages of the CRS method are observed in the sub-thrust noise zone, and at steeply dipping sediments in the low fold zone near the surface.
- Geophysics > Seismic Surveying > Seismic Processing (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (1.00)
- Asia > India > Assam > Upper Assam Basin > Digboi Field > Nahor Oil Sand (0.99)
- Asia > India > Assam > Upper Assam Basin > Digboi Field > Digboi Oil Sand Group (0.99)
- Asia > India > Assam > Upper Assam Basin > Digboi Field > Bappapung Sandstone (0.99)
A Seismic Reflection Imaging Workflow Based On the Common-Reflection-Surface (CRS) Stack: Theoretical Background And Case Study
Hertweck, Thomas (Geophysical Institute, University of Karlsruhe, Germany) | Jäger, Christoph (Geophysical Institute, University of Karlsruhe, Germany) | Mann, Jürgen (Geophysical Institute, University of Karlsruhe, Germany) | Duveneck, Eric (Geophysical Institute, University of Karlsruhe, Germany) | Heilmann, Zeno (Geophysical Institute, University of Karlsruhe, Germany)
ABSTRACT In recent years, many case studies have demonstrated that the Common-Reflection-Surface (CRS) stack produces reliable stack sections with an excellent signal-to-noise ratio. In addition, an entire set of physically interpretable stacking parameters, so-called kinematic wavefield or CRS attributes, is determined. These attributes can be applied in further processing in such a way that a complete and consistent seismic reflection imaging workflow can be established which leads from the preprocessed multicoverage data in the time domain to migrated sections in the depth domain. The basic steps of this CRS-stack-based seismic reflection imaging workflow are the CRS stack itself, the determination of a smooth macrovelocity model by means of CRS attributes, and limited-aperture pre- and poststack Kirchhoff-type depth migration where the aperture is possibly optimized by means of the determined attributes. Our workflow approach has been applied to a recently acquired seismic dataset and revealed superior results compared to standard processing based on NMO/DMO/stack with a subsequent time migration and depth conversion.
- Europe > Germany (0.18)
- North America > United States > Colorado (0.16)
ABSTRACT Imaging complex geologic structures requires accurate velocity analysis and migration that will image steeply dipping strata and faults. Combine these requirements with hundreds of metres of topographic relief, strong lateral-velocity variation at surface, and steeply dipping anisotropic strata in the overburden and you have the Canadian Foothills imaging problem. Compound these issues with lava flows at the surface, limited penetration and illumination of seismic energy, and a more dramatic tectonic history and you are imaging geologic structures in the Colombian Foothills. This 2D processing case history outlines our struggles with the noise that nearly overwhelmed the limited subsurface illumination and velocity model building under a low signal-to-noise condition in a complex geologic setting. Throughout this process, close interaction between interpreter and processor was critical for velocity-model interpretation and for discriminating between signal and noise throughout the project. Where we could define the overburden dip accurately, anisotropic Kirchhoff depth migration yielded improved imaging over the time migration. In other areas with limited signal and high noise, we found the noise generated too much Kirchhoff-operator noise. We tested Gaussian Beam migration on these datasets with promising results for this migration algorithm in noisy rough-topography settings.
- South America > Colombia (0.42)
- North America > United States > Colorado (0.17)
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (1.00)
3D Global Tomographic Velocity Model Building
Meng, Zhaobo (Upstream Technology, ConocoPhillips Company) | Valasek, Paul A. (Upstream Technology, ConocoPhillips Company) | Whitney, Steve A. (Upstream Technology, ConocoPhillips Company) | Sigler, Carl B. (Upstream Technology, ConocoPhillips Company) | Macy, Brian K. (Upstream Technology, ConocoPhillips Company) | Whitmore, N. Dan (Upstream Technology, ConocoPhillips Company)
ABSTRACT Accurate subsurface velocity estimation is crucial in seismic exploration, especially for prestack depth migration, depth conversion, geopressure prediction and AVO attribute computation. 3D reflection tomography is the most accurate method available for velocity estimation. The challenge that we face is in designing an accurate tomographic system capable of efficiently handling large volumes of seismic data acquired over increasingly complex targets. To address this need we have developed a 3D reflection tomography system using a tetrahedra-based model parameterization. Tetrahedra are capable of accurately representing complex velocity models with far fewer control points compared with other methods. The input for our tomography comes from automatically scanned migration residuals and structural dips generated from Kirchhoff prestack depth migration. An adaptive layer-based method is employed with flexible geological constraints, which enables global tomographic inversion of both compaction and competent geological environments. Application of global tomography to synthetic data demonstrates its ability to resolve strong and rapid velocity variations. Examples from 3D field data are shown to illustrate the benefits of this global tomographic system in estimating velocity models for large-scale prestack depth migration projects.
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (1.00)
ABSTRACT Stereotomography, which is based on the concept of locally coherent events, appeared to be a fast and powerful method for velocity macro-model estimation. However, in presence of low signal-to-noise ratio and coherent noise, automatic event picking on prestack data does not guarantee reliable information and may lead to wrong velocity models. In this paper, we present a new implementation of Stereotomography where the picking is performed in poststack time domain. It allows a robust and reliable picking procedure. We show results obtained using “poststack Stereotomography” on synthetic and real examples and compare these results with the conventional “prestack Stereotomography”.
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (1.00)
ABSTRACT In many deepwater areas in the Gulf of Mexico, geology is poorly resolved because most of the reservoirs are subsalt and the standard seismic method has difficulty in unraveling the complex ray paths generated by irregular salt bodies. Over the last few years, the development of 3D wave equation migration has been recognized as a major step to increase imaging accuracy subsalt. In order to mature a potentially very large but risky prospect in ultra-deepwater, a dual processing project was launched combining a conventional approach (Kirchhoff migration) and a potentially more robust approach (full wave equation migration). These parallel projects were carefully managed to be able to compare and improve the results of each method at each stage of the processing. The use of wave equation migration in conjunction with Kirchhoff depth migration resulted in an accurate velocity model that correctly defined the convoluted salt canopy and the highly variable velocity structure of the sedimentary section. Both methods gave impressive images of the subsalt geology and permitted an unambiguous interpretation of the Longhorn prospect.
- North America > United States > Louisiana (0.25)
- North America > United States > Colorado (0.17)
- North America > United States > Gulf of Mexico (0.15)
ABSTRACT This paper compares the depth migrated images produced using isotropic and anisotropic algorithms on seismic data generated by both raytracing and finite difference methods. Raytracing and finite difference produce similar seismic reflection data given the same anisotropic model. Three progressively more realistic models are used to investigate the positioning accuracy of images migrated from anisotropic seismic data by isotropic depth migration. Migrated images using Kirchhoff depth migration show that isotropic migration using either the model P-wave velocity or the interval velocity derived from the short-spread NMO velocity causes significant distortion. Only anisotropic migration recovers the image correctly. The absolute mispositioning of distorted images is reported for all studied models
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (1.00)