ESS 135 Lecture 13

1. ESS 135 Lecture 13 Reflection Seismology Migration

2. Unmigrated: Note diffraction hyperbolas ‘bow ties’ Migrated section

3. Refractions and reflections from the sea floor (Sheriff and Geldart, Vol1) Sonobuoy refraction profile Baffin Bay

4. Velocity Stacking • Apply normal moveout using • various velocities to seismograms • and sum amplitudes to form a stacked • Seismogram. • 2. Power is sum of amplitudes squared. • 3. Plot power as a function of velocity (and dip) • The velocity that gives maximum power is the • RMS velocity.

5. Velocity stacking from Kearey and Brooks

7. Apparent Dip

8. Note: Unmigrated has diffraction Hyperbolas (draped downwards)

10. Reflection from a point gives rise to a diffraction or reflection hyperbola

11. Diffraction Migration (basis of tomography) Place hyperbola on reflection seismogram and sum along it placing the energy at the peak Move and repeat. Note at greater depth the hyperbolas have less moveout (are flatter) Migration collapses diffraction hyperbolas to a point, puts reflectors in their correct positions and reveals changes clearly such as anticlines, synclines and faults.

12. Migration drape hyperbola over diffraction hyperbolas

14. Common depth point shooting stacks all receiver seismograms for which shots And receivers a re equally spaced about A common depth point. At the middle of a survey with n geophones with a shot at each geophone the fold Is the number of traces stacked. n/2 7.14 ship moves shots are fired dx/2 where dx is separation between geophones.

15. Reflection Refraction Coefficients Normal Incidence Ai Ar AT At surface R is negative

16. Synthetic Seismograms

17. Sea Level through Time

18. Global Change Sea Level 100 m Rise and Fall • Why might it change • Ice caps • Tectonics (more ridges)

20. Sea Level Rise Melt Icecaps

21. Hallam et al. (1983) and Exxon eustatic (global) sea level reconstructions for the Phanerozoiceon.

22. Top lap on lap bottom lap Sediment is deposited where flow slackens pace at shelf edge