Mathias Alerini Gemini kollokvium 28 02 2008 Seismic angle migration: an adaptive limitation of the operator aperture Mathias Alerini Gemini kollokvium 28 02 2008

PostDoc work... With Pr. Bjørn Ursin Sponsored by NFR (grants 159117/I30 and 162612/S30 and the ROSE project) July 2005 to December 2007... in parallel to many other things...

Seismic Data Amplitude time

Seismic Data

Imaging process Seismic survey Seismic processing Subsurface Data

+ Velocity of seismic waves Seismic migration From seismic trace to image position (& dynamic information) x + Velocity of seismic waves

Seismic migration Kirchhoff type M s r T + Velocity For all M points along the curve, the travel times T(s,M,r) are equal Recorded energy comes from somewhere along the curve!

Seismic migration Kirchhoff type Recorded energy is somewhere along the curve! + Velocity

Seismic migration Common Image gather offset s s s s s s s s s s r r r z0 x z

Seismic migration Common Image gather angle s s s s s s s s s s r r r z0 x z

Angle Migration: Advantages Complex geological setting (Strong lateral velocity variations) 1 2 h s r Triplications: 1 offset, 2 (e.g.) angles Artifacts AVO/AVA: Reflection coefficients depends on the angle and not the offset !

Limitation of the migration operator The traveltime tells us that the recorded energy comes from somewhere along the curves but the migration result tells us the energy comes from the envelop

Limitation of the migration operator We just need this information. The rest : increases the computation time adds noise

Limitation of the migration operator The noise is not completely destroyed by the summation process

Limitation of the migration operator We want to keep only constructive interferences between the specular ray (respects Snell-Descartes) and the migrated ray s r migrated ray vs specular ray

Limitation of the migration operator at the image point s s r r   Difference in traveltime between the specular ray and the migrated ray relation to the migration operator aperture

Limitation of the migration operator at the image point, looking at one side isochrone = same traveltime  but...

Limitation of the migration operator sub-horizontal rays... isochrone = same traveltime  a small angles produces a large traveltime difference

Limitation of the migration operator sub-horizontal rays... isochrone = same traveltime  tangent to the isochrone

Limitation of the migration operator sub-horizontal rays... isochrone = same traveltime  tangent to the isochrone ... it is equivalent to the Young’s experiment Observed interferences Signals in phase

Limitation of the migration operator results: 25m between srcs and recs No LAM LAM

Limitation of the migration operator results: 100m between srcs and 100m between recs No LAM LAM

Conclusion Our Limitation of the migration aperture is easy to implement in an existing code easy to use can handle multi-orientations (faults) can include easily a local error on the geological dip gave very good results !

... finally codes have been written, rewritten in a coherent way with new tools and then ported to the cluster... ... or how to do 3 times the same work...

... finally codes have been written, rewritten in a coherent way with new tools and then ported to the cluster... ... or how to do 3 times the same work... “Through continuous efforts we succeed eventually. Thus, the more it fails, the greater chance that it succeeds”