An Introduction to Velocity Model Building

by Ian F. 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.


Summary

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 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.


Course outline

  • Why do we need a detailed velocity model?
    • Review of migration schemes: ray-theory versus wave-theory
    • 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
    • Image uncertainty: putting error-bars on images (and how meaningful are the error bars?)
  • 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
    • Parametric versus non-parametric picking of residual moveout
    • Complex water layers
    • Near-surface velocity anomalies
    • Current use of waveform inversion
    • The seismic response to strong vertical velocity contrast (especially in shallow water)
    • Imaging in and around salt bodies
    • Broadband imaging: Q-tomography & Q-migration
  • The Future: emerging R&D directions
    • Waveform inversion developments
    • Least squares migration
    • Migrating multiples
    • Full wavefield imaging
    • Scattered wavefield imaging
    • ‘Adaptive optical imaging’


Learner Outcomes

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 geolocal 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


Instructor Bio

Ian F. Jones