Offset class determination for imaging

Oil and natural gas exploration -- geology and geophysics
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jefry123
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Offset class determination for imaging

Post by jefry123 »

Before applying pre-stack migration, we creates offset classes to have regular folds to reduce migration artifacts. For 2D they are common offset gathers and for 3D they are common offset vectors, Right?

And, How do we calculate the offset range/interval for offset class creation?

GuyM
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Re: Offset class determination for imaging

Post by GuyM »

The short answer is you don't.

Or rather most pre-stack migrations function a lot better when the input dataset has been regularized and bin centred first, whether that's into common offset planes or offset-vector tiles. Holes and irregularities create artifacts that you have to handle in migration.

So in a crude sense you could just make sure that the offset classes are full populated by looking at foldmaps and playing around a little bit; making sure there's not any large gaps in the data and so on. That's essentially what you are doing if you just rely on "offset class"

In practice some kind of N-dimensional spatial resample that will infill large gaps from the surrounding data and also bin-centre (while adjusting the source and receiver location in a smart way) gives better results. There's different families of algorithm, with the earliest generations using fourier based methods, and later ones using smarter analysis of the surrounding structure.

So - basically this is resampling the wavefield to make regular offset classes first, then using those classifications in the migration.

jefry123
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Re: Offset class determination for imaging

Post by jefry123 »

Thank you,
In some documents there are some rule of thumbs, as two time the shot intervals or somethings like this. How do these things work?

GuyM
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Re: Offset class determination for imaging

Post by GuyM »

Can't think of a reason why that would be the case.

Usually the issue is related to the offset distribution across CDPs, which is turn is a function of the SP spacing AND the CDP interval.

So for example on a 2D marine line, if there's 2 CDP per shot, then one CDP will have channels 1,3,5,7,9... and the next CDP will have channel 2,4,6,8,10...

If there were 120 channels in total (and so 60 fold data) you might set things up so the first offset plane corresponded to halfway between the offsets for channel 1 and channel 2, and the next one between channels 3 and 4, and so on.

So you'd output 60 offset planes, and each offset plane would be two receiver spacings apart.

jefry123
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Re: Offset class determination for imaging

Post by jefry123 »

Thank you, that was the thing I was trying to understand. How about in 3D surveys? Is it different in land and marine?!

GuyM
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Re: Offset class determination for imaging

Post by GuyM »

Yes - the binning is more complex than the simple 2D case; that's why you need to examine the offsets (or offset vector tiles) to determine the best distribution (ie with the least holes) and/or use some kind of interpolation to fill in the gaps.

Mostly people use interpolation schemes to regularise the data, which avoids the issue.

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