1. 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

2. 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

3. 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.

4. 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

5. 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

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

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

8. 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

9. Introduction to seismic migration
Even for a simple, dipping reflector the reflection appears in the wrong place on the stack section
Examination of the figure shows that
Events on the stack section appear vertically below midpoints; true reflection points are further updip
The reflection has a gentler dip on the stack section than in reality
The reflection points are further apart on the stack section than they are in reality

10. Introduction to seismic migration
Which of the two sections below is the original stack, and which is the migrated section?

11. Introduction to seismic migration
The Figure points to a method for manual migration:
Use a compass to draw constant traveltime circles from several points on the reflection surface
Use a ruler to find a tangent plane to all such circles

12. Introduction to seismic migration
Manual migration:
Use a compass to draw constant traveltime circles from several points on the reflection surface
Use a ruler to find a tangent plane to all such circles
The strategy works equally well for migrating diffractions
Constant traveltime circles all intersect at a point, corresponding to the true location of the discontinuity

13. Introduction to seismic migration
Any successful migration algorithm will therefore:
Move events updip
Steepen dips
Shorten events
Collapse diffractions onto points

14. Introduction to seismic migration
Any successful migration algorithm will
Move events updip
Steepen dips
Shorten events
Collapse diffractions onto points

15. Examples of seismic migration
Model with diffractions, anticlines, synclines

16. Examples of seismic migration
Example of “bowtie” associated with syncline

17. Examples of seismic migration
Example of “bowtie” associated with syncline

18. Examples of seismic migration
Example of diffractions associated with faulting

19. Examples of seismic migration
Example of diffractions associated with faulting

20. Examples of seismic migration
A, B are multiples

21. Examples of seismic migration
Example of multiples: incorrect positioning of multipes after migration