Reflection velocity analysis

1. Reflection velocity analysis “Velocity Spectrum” • Stack power, or “semblance” is contoured as a function of (to, vrms) • Processor must pick local maxima, to find the velocity function

2. Velocity analysis result

3. Reflection velocity analysis

4. Reflection velocity analysis “Velocity Spectrum” • The spectrum is quite robust in the presence of noise

5. Reflection velocity analysis

6. Reflection velocity analysis “Velocity Spectrum” • Stack power, or “semblance” is contoured as a function of (to, vrms) • Processor must pick local maxima, to find the velocity function

7. Reflection processing quiz - How should the panels be arranged?

8. Practical processing - Promax demo

9. Multiples • Multiple reflections are a serious problems for reflection seismics • Processing assumes only a single (“primary”) bounce • Multiples will interfere with primaries • Most damaging multiples are often “free surface” multiples

10. Multiples • Simple, free surface multiples have exactly the same moveout equation as the primaries: • If they arise from the same interface, to is doubled and vrms remains the same • If they share to with a primary, vrms will likely be reduced

11. If multiples arise from the same interface, to is doubled and vrms remains the same • If they share to with a primary, vrms will likely be reduced • NMO correction of multiples will not flatten them

12. Example of multiples in real data • CMP gathers with multiples • Velocity analysis at CMP186 • CMP gathers after NMO correction • CMP stack • Note the presence of multiples even after stack

13. Introduction to Seismic migration

14. Stacked sections are zero offset sections • Stacked section contains (mainly) only those reflections that have been “flattened” • Times have all been corrected to zero offset • Multiples have been (partially) removed

15. Stacked sections are zero offset sections • We need to understand what zero-offset (“stack”) sections look like • Although stack sections look like “pictures” of the earth, they suffer from a number of distortions

16. For a given reflection time, the reflection point may lie anywhere on the arc of a circle – the reflection nevertheless appears directly below the source/receiver (CMP) location. Stacked sections are zero offset sections • A zero-offset section has co-incident sources and receivers • Energy travels down, and back up on the same ray path • Energy does not necessarily travel vertically down and up

17. Stacked sections are zero offset sections • Wherever there is structure, energy will appear at the incorrect subsurface point on the stack section • For example, a sharp, synclinal structure will result in a “bow-tie” shape of the reflection event on the stack section

18. Stacked sections are zero offset sections • For example, a sharp, synclinal structure will result in a “bow-tie” shape of the reflection event on the stack section

19. Stacked sections are zero offset sections Diffractions: • Discontinuities (faults, etc) scatter energy in all directions • The energy generates hyperbolic events at the receivers

20. Stacked sections are zero offset sections Diffractions: • The energy generates hyperbolic events at the receivers

21. Stacked sections are zero offset sections • Because of the distortions due to structure, and diffractions, stack sections only approximate the true subsurface • Distortion can be extreme in structurally complex areas • Solution is “seismic migration” (topic of the next lecture(s)) Model Events on stack section