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

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

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.

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

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

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

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

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

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

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

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

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

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

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

Examples of seismic migration Model with diffractions, anticlines, synclines

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

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

Examples of seismic migration Example of diffractions associated with faulting

Examples of seismic migration Example of diffractions associated with faulting

Examples of seismic migration A, B are multiples

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