The application of 3-D depth migration to the development of an Alaskan offshore oil field

Geophysics ◽  
1994 ◽  
Vol 59 (10) ◽  
pp. 1551-1560 ◽  
Author(s):  
David N. Whitcombe ◽  
Eugene H. Murray ◽  
Laurie A. St. Aubin ◽  
Randall J. Carroll
Keyword(s):  
Velocity Model ◽  
Oil Field ◽  
Northwestern Part ◽  
Positioning Error ◽  
Depth Migration ◽  
Velocity Models ◽  
Positioning Errors ◽  
Ray Trace ◽  
Offshore Oil ◽  
Lateral Positioning

Inconsistencies in fault positioning between overlapping 3-D seismic surveys over the northwestern part of the Endicott Field highlighted lateral positioning errors of the order of 1000 ft (330 m) in the seismic images. This large uncertainty in fault positioning placed a high and often unacceptable risk on the placement of wells. To quantify and correct for the seismic positioning error, 3-D velocity models were developed for ray‐trace modeling. The lateral positioning error maps produced revealed significant variation in the mispositioning within the Endicott Field that were mainly caused by lateral variations in permafrost thickness. These maps have been used to correct the positions of mapped features and have enabled several wells to be successfully placed close to major faults. Prior to this analysis, these wells were considered too risky to place optimally. The seismic data were 3-D poststack depth migrated with the final velocity model, producing a repositioned image that was consistent with the ray‐trace predictions. Additionally, a general enhancement of data imaging improved the interpretability and enabled the remapping and subsequent successful development of the peripheral Sag Delta North accumulation.

Kinematic interpretation in the prestack depth‐migrated domain

Geophysics ◽  
1997 ◽  
Vol 62 (4) ◽  
pp. 1226-1237 ◽  
Author(s):  
Irina Apostoiu‐Marin ◽  
Andreas Ehinger
Keyword(s):  
Signal To Noise Ratio ◽  
Velocity Model ◽  
Point Of View ◽  
Prestack Depth Migration ◽  
Depth Migration ◽  
Velocity Models ◽  
Time Domain Data ◽  
And Migration ◽  
Point To Point ◽  
Homogeneous Media

Prestack depth migration can be used in the velocity model estimation process if one succeeds in interpreting depth events obtained with erroneous velocity models. The interpretational difficulty arises from the fact that migration with erroneous velocity does not yield the geologically correct reflector geometries and that individual migrated images suffer from poor signal‐to‐noise ratio. Moreover, migrated events may be of considerable complexity and thus hard to identify. In this paper, we examine the influence of wrong velocity models on the output of prestack depth migration in the case of straight reflector and point diffractor data in homogeneous media. To avoid obscuring migration results by artifacts (“smiles”), we use a geometrical technique for modeling and migration yielding a point‐to‐point map from time‐domain data to depth‐domain data. We discover that strong deformation of migrated events may occur even in situations of simple structures and small velocity errors. From a kinematical point of view, we compare the results of common‐shot and common‐offset migration. and we find that common‐offset migration with erroneous velocity models yields less severe image distortion than common‐shot migration. However, for any kind of migration, it is important to use the entire cube of migrated data to consistently interpret in the prestack depth‐migrated domain.

Incorporating geologic information into reflection tomography

Geophysics ◽  
2004 ◽  
Vol 69 (2) ◽  
pp. 533-546 ◽  
Author(s):  
Robert G. Clapp ◽  
Biondo L. Biondi ◽  
Jon F. Claerbout
Keyword(s):  
Velocity Structure ◽  
Null Space ◽  
Velocity Model ◽  
Prestack Depth Migration ◽  
Depth Migration ◽  
Interval Velocity ◽  
Velocity Models ◽  
Regularized Inversion ◽  
Symmetric Operators ◽  
Reflection Tomography

In areas of complex geology, prestack depth migration is often necessary if we are to produce an accurate image of the subsurface. Prestack depth migration requires an accurate interval velocity model. With few exceptions, the subsurface velocities are not known beforehand and should be estimated. When the velocity structure is complex, with significant lateral variations, reflection‐tomography methods are often an effective tool for improving the velocity estimate. Unfortunately, reflection tomography often converges slowly, to a model that is geologically unreasonable, or it does not converge at all. The large null space of reflection‐tomography problems often forces us to add a sparse parameterization of the model and/or regularization criteria to the estimation. Standard tomography schemes tend to create isotropic features in velocity models that are inconsistent with geology. These isotropic features result, in large part, from using symmetric regularization operators or from choosing a poor model parameterization. If we replace the symmetric operators with nonstationary operators that tend to spread information along structural dips, the tomography will produce velocity models that are geologically more reasonable. In addition, by forming the operators in helical 1D space and performing polynomial division, we apply the inverse of these space‐varying anisotropic operators. The inverse operators can be used as a preconditioner to a standard tomography problem, thereby significantly improving the speed of convergence compared with the typical regularized inversion problem. Results from 2D synthetic and 2D field data are shown. In each case, the velocity obtained improves the focusing of the migrated image.

Practical aspects and applications of 2D stereotomography

Geophysics ◽  
2003 ◽  
Vol 68 (3) ◽  
pp. 1008-1021 ◽  
Author(s):  
Frederic Billette ◽  
Soazig Le Bégat ◽  
Pascal Podvin ◽  
Gilles Lambaré
Keyword(s):  
Estimation Method ◽  
Synthetic Data ◽  
Velocity Model ◽  
Velocity Estimation ◽  
Common Source ◽  
Depth Migration ◽  
Reflection Seismic ◽  
Velocity Models ◽  
Automatic Picking ◽  
Iterative Inversion

Stereotomography is a new velocity estimation method. This tomographic approach aims at retrieving subsurface velocities from prestack seismic data. In addition to traveltimes, the slope of locally coherent events are picked simultaneously in common offset, common source, common receiver, and common midpoint gathers. As the picking is realized on locally coherent events, they do not need to be interpreted in terms of reflection on given interfaces, but may represent diffractions or reflections from anywhere in the image. In the high‐frequency approximation, each one of these events corresponds to a ray trajectory in the subsurface. Stereotomography consists of picking and analyzing these events to update both the associated ray paths and velocity model. In this paper, we describe the implementation of two critical features needed to put stereotomography into practice: an automatic picking tool and a robust multiscale iterative inversion technique. Applications to 2D reflection seismic are presented on synthetic data and on a 2D line extracted from a 3D towed streamer survey shot in West Africa for TotalFinaElf. The examples demonstrate that the method requires only minor human intervention and rapidly converges to a geologically plausible velocity model in these two very different and complex velocity regimes. The quality of the velocity models is verified by prestack depth migration results.

Fast and accurate dynamic raytracing in heterogeneous media

1990 ◽  
Vol 80 (5) ◽  
pp. 1284-1296
Author(s):  
Claude F. Lafond ◽  
Alan R. Levander
Keyword(s):  
Shooting Method ◽  
Heterogeneous Media ◽  
Velocity Model ◽  
Travel Times ◽  
Complex Velocity ◽  
Geometrical Spreading ◽  
Imaging Condition ◽  
Depth Migration ◽  
Velocity Models ◽  
Complex Velocity Model

Abstract We have developed a fast and accurate dynamic raytracing method for 2.5-D heterogeneous media based on the kinematic algorithm proposed by Langan et al. (1985). This algorithm divides the model into cells of constant slowness gradient, and the positions, directions, and travel times of the rays are expressed as polynomials of the travel path length, accurate to the second other in the gradient. This method is efficient because of the use of simple polynomials at each raytracing step. We derived similar polynomial expressions for the dynamic raytracing quantities by integrating the raytracing system and expanding the solutions to the second order in the gradient. This new algorithm efficiently computes the geometrical spreading, amplitude, and wavefront curvature on individual rays. The two-point raytracing problem is solved by the shooting method using the geometrical spreading. Paraxial corrections based on the wavefront curvature improve the accuracy of the travel time and amplitude at a given receiver. The computational results for two simple velocity models are compared with those obtained with the SEIS83 seismic modeling package (Cerveny and Psencik, 1984); this new method is accurate for both travel times and amplitudes while being significantly faster. We present a complex velocity model that shows that the algorithm allows for realistic models and easily computes rays in structures that pose difficulties for conventional methods. The method can be extended to raytracing in 3-D heterogeneous media and can be used as a support for a Gaussian beam algorithm. It is also suitable for computing the Green's function and imaging condition needed for prestack depth migration.

Seismic velocity model building in an area of complex geology, southern Alberta, Canada

Geophysics ◽  
2008 ◽  
Vol 73 (5) ◽  
pp. VE255-VE260 ◽  
Author(s):  
J. Helen Isaac ◽  
Don C. Lawton
Keyword(s):  
Cross Sections ◽  
Model Building ◽  
Seismic Velocity ◽  
Velocity Model ◽  
Prestack Depth Migration ◽  
Depth Migration ◽  
Velocity Models ◽  
Effective Velocity ◽  
Southern Alberta ◽  
Complex Geology

We developed velocity models to prestack depth migrate two seismic lines acquired in an area of complex mountainous geology in southern Alberta, Canada. Initial processing in the time domain was designed to attenuate noise and enhance the signal in the data. The prestack and poststack time-migrated sections were poorly focused, implying the velocity models would be inadequate for prestack depth migration. The velocity models for prestack depth migration, developed by flattening reflections on common image gathers, ineffectively imaged the complex geology. We developed our most effective velocity models by integrating the mapped surface geology and dips, well formation tops, geological cross sections, and seismic-velocity information into the interpretation of polygonal areas of constant velocity on several iterations of prestack depth-migrated seismic sections. The resulting depth-processed sections show a more geologically realistic geometry for the reflectors at depth and achieve better focusing than either the time-migrated sections or the depth sections migrated with velocity models derived by flattening reflections on offset gathers.

Matrix-free anisotropic slope tomography: Theory and application

Geophysics ◽  
2019 ◽  
Vol 84 (1) ◽  
pp. R21-R43 ◽  
Author(s):  
Borhan Tavakoli F. ◽  
Stéphane Operto ◽  
Alessandra Ribodetti ◽  
Jean Virieux
Keyword(s):  
Inverse Problem ◽  
Waveform Inversion ◽  
Model Space ◽  
Velocity Model ◽  
Sensitivity Matrix ◽  
Model Quality ◽  
Depth Migration ◽  
Velocity Models ◽  
New Formulation ◽  
Matrix Free

Slope tomography uses traveltimes, source, and receiver slopes of locally coherent events to build subsurface velocity models. Locally coherent events by opposition to continuous reflections are suitable for semiautomatic and dense picking, which is conducive to better resolved tomographic models. These models can be further used as background/initial models for depth migration or full-waveform inversion. Slope tomography conventionally relies on ray tracing for traveltimes and slopes computation, where rays are traced from scatterers in depth to sources and receivers. The inverse problem relies on the explicit building of the sensitivity matrix to update the velocity model by local optimization. Alternatively, slope tomography can be implemented with eikonal solvers, which compute efficiently finely sampled traveltime maps from the sources and receivers, whereas slopes are estimated by finite differences of the traveltime maps. Moreover, a matrix-free inverse problem can be implemented with the adjoint-state method for the estimation of the data-misfit gradient. This new formulation of slope tomography is extended to tilted transverse isotropic (TTI) acoustic media, in which the model space is parameterized by four anisotropic parameters (e.g., vertical wavespeed, Thomson’s parameter [Formula: see text], [Formula: see text], and tilt angle) and the coordinates of the scatterers. A toy synthetic example allows for a first assessment of the crosstalk between anisotropic parameters and scatterer coordinates. A more realistic synthetic example indicates the feasibility of the joint update of the vertical wavespeed and [Formula: see text]. The slope tomography is finally applied to real broadband towed-streamer data to build the vertical velocity and the scatterers, while anisotropic parameters [Formula: see text] and [Formula: see text] are used as background parameters. The velocity model quality is assessed through common-image gathers computed by TTI Kirchhoff prestack-depth migration.

Geologically constrained seismic imaging — Workflow

Geophysics ◽  
2008 ◽  
Vol 73 (5) ◽  
pp. VE313-VE319 ◽  
Author(s):  
Stig-Kyrre Foss ◽  
Mark Rhodes ◽  
Bjørn Dalstrøm ◽  
Christian Gram ◽  
Alastair Welbon
Keyword(s):  
Image Quality ◽  
Model Building ◽  
Velocity Model ◽  
Seismic Interpretation ◽  
Gravity Modeling ◽  
Final Model ◽  
Depth Migration ◽  
Velocity Models ◽  
Velocity Model Building

We present the geologically constrained workflow for velocity-model building as a case study from offshore Brazil. The workflow involves basin reconstruction, gravity modeling, and seismic interpretation in addition to standard prestack depth migration (PSDM) model-building tools. Building a salt model based on seismic evidence can be highly nonunique. In a geologically constrained seismic-processing workflow, the main aim is to use geologic understanding with geophysical models and datasets to improve an input velocity realization for the PSDM loop, thereby improving image quality. All of these methods are inherently uncertain, and a final model is based on a range of subjective choices. Thus a final result that agrees with all sciences still can be completely wrong. However, an understanding of these choices enables a unique way of testing and constraining the number of antimodels: velocity models that fit the observations but are different from the final result. This can reduce time spent and uncertainty in geologic evaluation.

Removing fault shadow distortions from seismic images using depth-velocity modelling and pre-stack depth migration

2012 ◽  
Vol 52 (2) ◽  
pp. 700
Author(s):  
Sergey Birdus ◽  
Alexey Artyomov
Keyword(s):  
Real Data ◽  
Velocity Model ◽  
Velocity Inversion ◽  
Depth Migration ◽  
Velocity Models ◽  
Depth Processing ◽  
Different Types ◽  
Successful Removal ◽  
Velocity Anomalies ◽  
Additional Image

In many areas, fault shadows manifest a serious challenge to seismic imaging. The major part of this problem is caused by different types of velocity variations caused by faults. Pre-stack depth migration with sufficiently accurate velocity model successfully resolves this problem and the high resolution tomographic depth-velocity modelling is the most important component of the solution. During depth processing on a number of real 3D seismic datasets with fault shadows from Australia and other regions, the following were noticed: The appearance of the image distortions below the faults and the convergence speed of the tomographic velocity inversion depend on the acquisition direction. Sometimes, tomographic modelling produces depth-velocity models that closely follow geology, but the models contain non-geological looking anomalies in other areas. In both cases, the depth migration delivers distortion-free images. If anisotropy is present in faulted areas, it creates additional image distortions and can require extra input data and processing efforts. To examine these effects and optimise depth-processing workflow, several 3D synthetic seismic datasets were created for different types of velocity anomalies associated with the faults in isotropic and anisotropic media and different acquisition directions. On synthetic and real data from Australia, different types of fault shadows are illustrated; how they can be solved depending on the acquisition direction are also shown. Some types of the fault shadows are shown to require multi-azimuth illumination to guarantee their successful removal.

Planned seismic imaging using explicit one-way operators

Geophysics ◽  
2005 ◽  
Vol 70 (5) ◽  
pp. S101-S109 ◽  
Author(s):  
Robert J. Ferguson ◽  
Gary F. Margrave
Keyword(s):  
Fourier Transform ◽  
Seismic Imaging ◽  
Computational Cost ◽  
Velocity Model ◽  
Fixed Cost ◽  
Positioning Error ◽  
Model Accuracy ◽  
The Cost ◽  
Improved Accuracy ◽  
Lateral Positioning

A method is presented to compare the accuracy and computational cost of explicit one-way extrapolation operators as used in seismic imaging. For a given model, accuracy is measured in terms of lateral positioning error, and cost is calculated relative to the cost of the spatial fast Fourier transform. The result is a planned imaging scheme that achieves the greatest accuracy with respect to the velocity model for a fixed cost. To demonstrate, we use a 2D section of the EAGE/SEG salt model and assemble a suite of the most common operators. The data are imaged using each operator individually, and the results are compared to the result from the plan-based algorithm. The planned image is shown to return improved accuracy for no additional cost.

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