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

Velocity analysis result

Reflection velocity analysis

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

Reflection velocity analysis

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

Reflection processing quiz - How should the panels be arranged?

Practical processing - Promax demo

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

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

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

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

Introduction to Seismic migration

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