An Introduction to Velocity Model Building
The course will commence with an overview of different migration schemes, and cover the motivations for building detailed velocity models, and briefly discuss the inherent limitations on our ability to build a detailed model. Current-day practice will be covered, exemplified via many case-studies, and we will briefly discuss the less well known and emerging techniques. The approach will mostly be non-mathematical, and will rather try to concentrate on an intuitive understanding of the principles, and demonstrate them via case histories. The bias in this course is towards those techniques that have seen widespread industrial use over the past 40 years. Unfortunately, some topics will not be covered, in-part due to the time constraints: these omissions will include consideration of VSP and multi-component data, and Marchenko imaging.
The course objective is to provide the participants a firm understanding of the processes and assumptions involved in building velocity-depth models and of the limitations of various migration algorithms
For the topics listed below, real data examples will be used to demonstrate the application and limitation of each technique. Why do we need a detailed velocity model? • Review of migration schemes • The limitations of time migration and benefits of depth migration • Snell’s law and how to ignore it • How does depth migration differ from time migration? • Is depth migration always necessary? • Migration using ray methods (Kirchhoff, beam, CRAM, etc.) • Migrating using wavefield extrapolation methods (WEM, RTM, etc.) • One-way versus two-way propagation • Creating gathers in wavefield extrapolation methods • Pre-processing considerations for RTM How detailed can we get? • Sources of uncertainty • Non-uniqueness and ambiguity • Limits on resolution Resolving short-scale-length velocity anomalies: • Anisotropy versus heterogeneity (and other higher order moveout effects) • The mechanics of tomographic inversion with ray theory • Parametric versus non-parametric picking of residual moveout • Structural constraints, MAZ, OVT, and Q tomography • The mechanics of tomographic inversion with wavefield extrapolation theory (FWI) Examples of current industrial practice for various geological settings (time permitting): • Resolving near-surface velocity anomalies • Seismic response to strong vertical velocity change (e.g. chalk, basalt, salt) • Seismic response to strong lateral velocity change (e.g. salt walls, lateral terminations) • Comparisons of ray tomographic and FWI models for specific case studies The Future: emerging R&D directions (time permitting) • Waveform inversion developments • Least squares migration • Migrating multiples • Full wavefield imaging • Scattered wavefield imaging • ‘Adaptive optical imaging’ • Bayesian uncertainty estimation (putting error bars on images)
Geophysicists with an interest in migration and velocity model building and geologists (with a basic knowledge of data processing) who wish to understand a bit more about how the images they look at are created.
The course is designed to be followed by anyone with a broad geoscience background: no specific detailed foreknowledge is required, although a familiarity with geophysical terminology will be useful.
• Jones, I.F, 2014, Tutorial: migration imaging conditions. First Break, accepted. • Jones, I.F, and Davison, I., 2014, Seismic imaging in and around salt bodies. SEG Interpretation, 2, no.4, SL1-SL20. • Jones, I.F, 2013, Tutorial: The seismic response to strong vertical velocity change. First Break, 31, no.6., 43-54. • Jones, I.F, 2013, Tutorial: Transforms, orthogonality, eigenvectors, and eigenvalues. First Break, 31, no.1., 51-61. • Jones, I.F, 2012, Tutorial: Incorporating near-surface velocity anomalies in pre-stack depth migration models. First Break, 30, no.3, • Jones, I.F, 2010, Tutorial: ray-based tomography. First Break, 28, no.2, 45-52 • Jones, I. F., 2008, A modeling study of pre-processing considerations for reverse-time migration: Geophysics,. 73, NO. 6; T99— T106. • Fruehn, J.K., I. F. Jones, V. Valler, P. Sangvai, A. Biswal, & M. Mathur, 2008, Resolving Near-Seabed Velocity Anomalies: Deep Water Offshore Eastern India: Geophysics, 73, No.5, VE235-VE241.. • Jones, I. F., 2008, Effects of pre-processing on reverse time migration — a North Sea study: First Break, 26, no.6, 73-80. • Jones, I.F., Sugrue, M.J., Hardy, P.B., 2007, Hybrid Gridded Tomography. First Break, 25, no.4, 15-21. • Farmer, P., Jones, I.F., Zhou, H., Bloor, R., Goodwin, M.C., 2006, Application of Reverse Time Migration to Complex Imaging Problems. First Break, 24, no.9, 65-73. • Jones, I.F., 2003, A review of 3D preSDM velocity model building techniques First Break, 21, no.3, 45-58. • Jones, I.F., Fruehn, J., 2003, Factors affecting frequency content in 3D preSDM imaging, : The Leading Edge, 22, no.2,.128-134. Learner Outcome Upon completion of this course, the participants should be able to: 1. Describe how migration works, in terms of the underlying physics and the associated approximations involved 2. Classify model building and migration schemes in terms of the theory on which they are based (waves versus rays) 3. Decide which migration and model building scheme are appropriate for imaging a given geological environment 4. Characterize the limitations of model building and migration schemes, in terms of imaging artifacts 5. Differentiate between the current state-of-the art and future imaging and parameter estimation technologies
About the Instructor
Ian Jones received a joint honours BSc in Physics with Geology from the University of Manchester, UK, in 1977, an MSc in Seismology from the University of Western Ontario, Canada, and a PhD in Geophysical Signal Processing from the University of British Columbia, Canada. After working for ‘Inverse Theory & Applications Inc.’ in Canada for two years, he joined CGG, where for 15 years he was involved in R&D in the London and Paris offices, latterly as manager of the depth imaging research group. In 2000 he joined ION GX Technology, as a Senior Geophysical Advisor in their London office. In 2021 he joined BrightSkies Geoscience as Senior Geophysical Advisor. His interests include velocity model building and migration, and his most recent activity includes writing the text books: ‘Velocities, Imaging, and Waveform Inversion: the evolution of characterising the Earth’s subsurface’ published by the EAGE in 2018; ‘An Introduction to Velocity Model Building’ published by the EAGE in 2010; and co-editing the SEG Geophysics Reprints series volumes ‘Classics of Elastic Wave Theory’ and also ‘Pre-Stack Depth Migration and Velocity Model Building’, as well as contributing the chapter on model building to the new SEG online encyclopaedia. He has served as an associate editor for the journals ‘Geophysics’ and ‘Geophysical Prospecting’, and teaches the SEG/EAGE/PESGB continuing education course on ‘Velocity Model Building’ and was an external lecturer at the University of Leeds and Imperial College London. Ian was awarded the EAGE’s Anstey Medal in 2003 for “contributions to the depth imaging literature”, made the SEG European Honorary Lecturer in 2012 for “contributions to advancing the science and technology of geophysics”, conducted the 2018-2019 EAGE International Education Tour, and was made an Honorary Life Member by the EAGE in 2018, and received the best paper award for his 2019 First Break tutorial on FWI.