Velocities, Imaging, and Waveform Inversion - The Evolution of Characterizing the Earth’s Subsurface
Course Description
In using sound waves to characterise the Earth’s subsurface, we can employ ray-theory and/or wave-theory, and both migration algorithms and velocity estimation schemes employ one or other of these theoretical descriptions. In this course, we’ll review the evolution of the industry’s approaches to building earth models via velocity estimation and imaging, outlining the evolution from ray tomography to full waveform inversion, and look towards the emerging possibilities for replacing imaging techniques with direct subsurface parameter inversion methods. The approach will be non-mathematical, concentrating on an intuitive understanding of the principles, demonstrating them via case histories. The course will commence with a brief 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 several case-studies, and we will then discuss the newer techniques such as waveform inversion and least-squares migration. The approach will not be mathematical, but rather will try to concentrate on an intuitive understanding of the principles, and demonstrate them via case histories. The course schedule will be divided into the following sections: • Why do we need a detailed velocity model? Including a brief review of migration schemes. • How accurate can we make an image and how detailed can we get? We’ll discus the sources of uncertainty, non-uniqueness and ambiguity, as well as the intrinsic limits on resolution • Current industrial practice: Iterative model update with both ray- based tomography and waveform methods in order to achieve ‘true amplitude’ subsurface information. • What might come next? Discussing direct inversion of subsurface parameters as an alternative to simply migrating data. Following this course, participants should ideally understand how migration works, in terms of the approximations involved, and how this relates to the geology to be imaged. They should appreciate the limitations of current and future imaging and velocity estimation technology, so as to be able to decide what model building technique should be employed to image a given geological objective. And finally, to appreciate the potential for a radical change in current industrial practise, moving from a sequential imaging route (via migrated images) to a closed-loop inversion route (directly delivering subsurface attributes).
Course Objectives
Upon completion of the course, participants will be able to: • understand how migration works, in terms of the approximations involved, and how this relates to the geology to be imaged. • appreciate the limitations of current and future imaging and velocity estimation technology, so as to be able to decide what model building technique should be employed to image a given geological objective. • appreciate the potential for a radical change in current industrial practise, moving from and imaging route (via migrated images) to an inversions route (delivering subsurface attributes)
Course Outline
The course will cover the building detailed velocity models for imaging, and briefly discuss the inherent limitations on our ability to build a detailed model. Current-day practice will be covered, exemplified via case-studies, and we will then discuss the newer techniques such as waveform inversion and least-squares migration. The approach will not be mathematical, but rather will concentrate on an intuitive understanding of the principles, demonstrated via case histories. The schedule includes: • Why do we need a detailed velocity model? (Review of migration schemes) • How accurate does an image need to be and how detailed can we get? (Sources of uncertainty, non-uniqueness and ambiguity, limits on resolution) • Current industrial practice: Iterative model update (ray-based tomography and waveform inversion methods) • What might come next? (direct inversion of subsurface parameters)
Participants’ Profile
The course is designed for: practising geoscientists who desire to better understand the principles and limitations of both current and emerging technologies involved in subsurface parameter estimation and imaging, and geoscience students.
Prerequisites
A general knowledge of geophysics.
Recommended Reading
• Jones, I.F., 2015, Estimating subsurface parameter fields for seismic migration: velocity model building, in: Encyclopedia of Exploration Geophysics. SEG, pp. U1-1-U1-24. Editors: Vladimir Grechka and Kees Wapenaar. http://library.seg.org/doi/ pdf/10.1190/1.9781560803027.entry3 • Jones, I.F, 2014, Tutorial: migration imaging conditions. First Break, 32, no.12, 45-55. • Brittan, J., J. Bai, H. Delome, C. Wang, and D. Yingst, 2013, Full waveform inversion — The state of the art: First Break, 31, 75–81. • Jones, I.F, 2012, Tutorial: Incorporating near-surface velocity anomalies in pre-stack depth migration models. First Break, 30, no.3, 47-58. • Jones, I.F, 2010, Tutorial: ray-based tomography. First Break, 28, no.2, 45-52 • Jones, I.F., 2010, An introduction to velocity model building, EAGE, ISBN 978-90-73781-84-9, 296 pages.
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.