An Introduction to Velocity Model Building
by Ian Jones
Duration: Two days
Intended Audience: Entry level
Prerequisites (Knowledge/Experience/Education Required): 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.
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 several case-studies, and we will briefly discuss the less well known and emerging techniques. 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 bias in this course is towards those techniques that have seen widespread industrial use over the past 30 years. Unfortunately, some topics will not be covered, in part due to the time constraints. The omissions will include VSP and multi-component data.
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?
- How accurate does an image need to be?
How detailed can we get?
- Sources of uncertainty
- Non-uniqueness and ambiguity
- Limits on resolution
Model building through the ages
- The iterative multidisciplinary approach
- Tomographic update
- Anisotropy versus heterogeneity (and other higher order moveout effects)
Current industrial practice
- What does tomography need to accomplish?
- Iterative model update
- Layered, gridded and hybrid tomography
- Complex water layers
- Near-surface velocity anomalies
The Future: Emerging R&D directions
- Wavepath tomography
- Waveform inversion
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.
Ian F. Jones