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. 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
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.

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