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MIGRATION 
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Joined: Tue Jan 17, 2012 8:49 pm
Posts: 1
Post MIGRATION
I really need a diagrammatic explanation on migration in seismic processing


Sun Jan 22, 2012 8:31 pm
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Joined: Sun May 13, 2012 9:37 pm
Posts: 11
Post Re: MIGRATION
Well I can't draw a diagram. But let me try to explain ddepth migration with words. What we do is we use velocity analysis to come up with a velocity model. Now, by using the ddivergence theorem we can come up with Green's functions that allow us to express the seismic field recorded on a measuring surface as the field at some point within the Earth volume. I realice that is a confusing sentence, but all it means is that we have an equation which allow us to take our seismic data recorded at the surface (i.e. At z=0) and transform it to the data that we would have recorded if we put our geophones at some depth in the Earth (z=z1 or something). Now we must assume that our data is a recording of the upgoing field only to do this. Then we can use an estimate of the source field(i.e the wavelet) and use basically the same equation to calculate what the source field is at the same depth. So we almost have our recipe. We take our data at z=0 and back propagate it to depth z1, we forward propagate our source field to depth z1 as well. Now if there is a reflector just below z1, then the zero lage of the cross corellation of our newly propagated wave fields will be non zero. If there is not a reflector just below z1 then the zero lag of the cross correlation will be zero. Finally, all we have to do is step our data down through the Earth, using the velocity model and Greens functions, to successive depths z1, z2, z3 etc. At each step we take cross correlation of the two fields and we output zero lag value to the migrated output. Thats it. Sorry if its not too clear. Tough without equations.


Mon May 14, 2012 3:03 am
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Joined: Tue Oct 09, 2012 2:49 pm
Posts: 2
Post Re: MIGRATION
Hi,
If you have Ylmaz book 'Seismic Data Processing', you can find very simple definition with diagrams.

Best Regards,
Lukasz
www.newseismic.com


Tue Oct 09, 2012 2:55 pm
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Joined: Sat Feb 15, 2014 7:33 am
Posts: 11
Post Re: MIGRATION
Only just found this forum but these pages while a little dated are pretty useful for this type of thing:

http://www.xsgeo.com/course/mig.htm

The way I tend to think of it is that the stacked section is a "zero offset normal ray section" - that is to say we have corrected the data to simulate what things would look like is the source and receiver were co-located (via NMO). Its a normal ray because the ray-path down to any given reflector make a right-angle (normal) to that reflector.

For flat events this is fine - the ray is vertical down to the reflector, and back up again.

For dipping events it means that the data is mispositioned, as the ray make a right-angle with the reflecting interface, not the surface.

Migration tries to correct this - but its based on a mathematical approximation (and a model of the earth) so its not perfect.

Time migration takes you to the "image ray" solution, which makes a normal with the *surface* that the shot and receiver are sitting on.

This doesn't take account of any refraction that takes place at interface boundaries or within a layer, so where you have rapid lateral or vertical velocity variations the ray is curved or deflected - so the data is still mispositioned.

Depth migration takes you to a vertical ray solution, where the reflection point in the sub-surface is directly under the co-located source-receiver.

This can be extended to pre-stack data, where the migration not only corrects the normal ray to an image ray (time migration) or vertical ray (depth migration), it also corrects the data to zero offset.


Sat Feb 15, 2014 7:59 am
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