Seismic Depth Imaging and Anisotropic Velocity Model Building
|Mr Etienne Robein (ERT, Pau, France)|
|Geophysics – Surface Imaging|
|The EET 4 book is available in the EAGE Bookshop|
|10 CPD points|
ANISOTROPY DEPTH MIGRATION INVERSION KIRCHHOFF MIGRATION OFFSET REFLECTION TOMOGRAPHY WAVE PROPAGATION
A short version of this course has been recorded as an E-Lecture. Watching this video will give you a clear introduction of what the course is about and it will help you to prepare yourself if you are going to attend it!
As the search for new resources forces to maximize the production of discovered reservoirs and explore new ones in domains that are increasingly complex, seismic imaging is becoming more and more important.
Seismic imaging is inherent part of the data processing sequence that aims to produce clear and accurate images of the Earth's subsurface suitable for interpretation by geoscientists.
This course will provide the audience with a unified overview of today's most popular seismic depth imaging techniques used in the oil and gas industry. These requires an estimate of how fast the seismic waves travel at any given point in the Earth and at any direction (anisotropy): the velocity model. Recent advances in seismic acquisition, imaging technology and high-performance computing allow us to correctly assess a much greater complexity of subsurface models and consequently, improve the accuracy of seismic images and detect structures that were previously invisible.
The course will present in simple terms (cartoons rather than equations!) the principle of different techniques in each class of methods (Kirchhoff, Beam Migrations, WEM, RTM), while pointing out their respective merits and limitations. Similarities and differences between time- and depth-imaging will be briefly reviewed. Special emphasis will be on methods used to build the necessary anisotropic velocity models. Both ray-based techniques (linear and non-linear tomography) and wavefield extrapolation-based ones, including full-waveform inversion, will be addressed.
The relationship between recent developments in acquisition and imaging on the one hand and interpretation and imaging on the other will also be stressed.
Upon completion of the course, participants will be able to:
- Evaluate the potential value of the principal techniques used in seismic imaging;
- Understand differences between time- and depth-processing and select the best option for a given problem;
- Be aware of key steps and issues in building anisotropic depth velocity models with borehole control;
- Understand the complementarity between ray-based and wavefield extrapolation-based velocity model building;
- Be aware of current results and issues in full-waveform inversion;
- Evaluate the impact of recent breakthroughs in data acquisition on seismic imaging.
- An image of the changes in elastic properties in the subsurface;
- Various techniques to simulate wave propagation: Ray tracing, Gaussian beams, Wavefield Extrapolation;
- An introduction to seismic anisotropy; Thomsen’s parameters; VTI, TTI,TOR models;
- Illumination; Signal/noise; Resolution; Accuracy; Amplitude fidelity;
- Wavefield separation; deghosting; broadband seismic;
- Data sorting for imaging: shots; binning; common offset; offset vectors; CIGs.
Ray-based Kirchhoff Depth Migrations (KPreSDM)
- Principles of Kirchhoff depth migration in the Common-Offset domain;
- Common Image Gathers;
- What if the velocities or anisotropic parameters are wrong;
- Kirchhoff summation in the Scattering Angle domain.
Time processing: Kirchhoff migration in ‘vertical time’ (PreSTM)
- Definition and use of ‘vertical time’;
- The ‘multi-1D’ assumptions made in PreSTM: benefit and impact on the seismic image;
- Definition and measurement of the Vnmo and Eta fields;
- CRS and multifocusing (velocity independent) methods. Zero and non-zero offset.
The various Beam Migrations
- The concept of ray-based migration of ‘locally coherent events’ in the shot and common offset domains;
- The use of tau-p transform;
- Conceptual differences between Kirchhoff and Beam migrations
- A range of methods to build the image and increase efficiency: fast beam, Gaussian Beam, controlled beam migrations;
- Comparisons on real cases; discussion of respective merits and limitations.
Velocity Model Building (VMB) by Ray-based tomography
- Definition, flowchart and principles of linear and non linear (‘slope’) tomography;
- Non-flatness or Residual Move Out analysis of Common Image Gathers;
- A variety of ‘strategies’ and VMB flowcharts: examples and discussion;
- Accounting for anisotropy: estimation of the delta and epsilon fields with borehole control.
Wavefield extrapolation-based methods: WEM and RTM
- Claerbout’s imaging principle;
- Principles of one-way Shot point migration ‘WEM’. Synthetic and real case comparisons: merits and limitations;
- The RTM sequence illustrated on a synthetic example; two-way propagation; numerical issues; frequency limitation in RTM;
- Benefits of the ‘two-way propagator’ (turning waves; prism waves): synthetic and real case examples;
- Illumination studies to improve RTM results;
- RTM and the migration of multiples.
Wave-equation based Velocity Model Building and FWI
- Scattering Angle CIGs after RTM: Poynting vectors; Extended Imaging condition;
- VMB after RTM. The case of subsalt imaging. Recent examples and advances;
- Concept, flowchart and techniques of Full Waveform Inversion (FWI);
- Main issues in FWI: numerical, initial model, cost function. Synthetic and real case studies: comparison and discussion;
- Impact of recent advances in acquisition (low frequencies).
The course is aimed at geoscientists involved in exploration and production projects where seismic play a role and who wish to:
- Learn more about seismic imaging concepts and the terminology used by seismic processors;
- Be aware of the variety of techniques and tools at our disposal in seismic imaging;
- Have a well-argued selection of the imaging method to apply to the seismic data acquired for their projects;
- improve their critical view on the benefits and limitations of the seismic data sets they are using in their projects;
- Have a better appreciation of what they can expect from reprocessing vintage data sets with modern tools.
The course will also benefit students who want to have a first acquaintance to reflection seismic in general and seismic imaging in particular.
The course can be understood by geoscientists with a moderate mathematical background. Physical concepts are presented without equations but with a maximum of simple schemes and animated graphic illustrations. However, some basic knowledge of wave propagation theory may help. A comprehensive list of references is given in the book and updated in the presentation for those who are interested in more rigorous and mathematical approaches.
About the instructor
Etienne Robein graduated from Ecole Nationale Supérieure d'Aéronautique et Espace and Ecole Nationale Supérieure Pétrole et Moteurs / IFP in Paris in 1973. He started his career with Shell in the Hague, before joining Elf, now Total, where he has worked on operational, research and managerial assignments in France, Italy, the UK and Azerbaijan. His professional experience covers seismic acquisition, processing and interpretation. His last position with Total was R&D programme manager in Geology and Geophysics. Etienne now works as a freelance tutor in Geophysics, having set-up his own Company “ERT”.
Etienne is the author or co-author of several presentations in International Conferences, including the SEG, EAGE, WPC, AAPG, and Petroleum Geology Conference and he contributed to the EAGE’s “Distinguished Lecture Programme” and “Education days”. In 2003, he published a text book on “Velocities, Time-imaging and Depth-imaging in Reflection Seismics,” which became a best-seller EAGE Edition. Etienne was President of EAGE in 2000. He was also Chairman of EAGE’s Research Committee, member of the EAGE Awards Committee and Europe’s representative at the SEG Council.
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