Friday, July 4, 2008

SEG Polarity

We care a lot about the polarity of seismic data. It is the first thing I check and the last thing I check when sending a dataset back out again. But I can't always remember what I'm looking at. I use references all the time: whether it is from textbooks, notes I've made or bookmarked web pages, I can't remember everything I need to know. I just learn how to find things quickly!

Look at a reflection from an increase in acoustic impedance, like the water-bottom (after all we know that sediment has a faster seismic velocity than water); what does it look like?

SEG normal polarity has a negative first break for minimum phase waveform. For a zero-phase waveform the central peak will have positive amplitudes.

These are opposite for SEG reverse, otherwise known as North Sea normal due to its common usage in displaying North Sea data.

If you are in particularly shallow water and suffering with contamination of the water-bottom reflection you can also examine the direct arrival to do the same analysis.

Bear in mind that field data is likely to be minimum phase, but after designature it will most likely be zero-phase so you must change whether looking for the first-breaks or waveform centre.

These standards have been written down, but are pretty dry reading. Still, if you've got nothing else to do then look for the 1975 and 1993 polarity standards.

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1 Comments:

Blogger mike.sexton said...

Hi,
I agree with what you have written, as it gels with what I have read elsewhere.
You are the only person I have come across who states the case for minimum and zero phase wavelets. So perhaps you can resolve my problem.

If the minimum phase waveform gives rise to a negative first break (at the water bottom, say) on a field record, then this is called NORMAL.

If I then deconvolve the field record to make the data zero phase (I'm not saying I can actually do this - but let's pretend that I can), then presumably the negative first break becomes a negative peak of a symmetric wavelet. This is the REVERSE polarity situation.

So even sticking with the SEG convention, a NORMAL field record will become a REVERSE processed record after deconvolution. Is this the case? Am I missing something? Is my assumption of how the deconvolution works false? Were the standards for minimum phase and zero phase derived in isolation?

Any help would be greatly appreciated.

Mike Sexton (mike.sexton@ga.gov.au)
Geoscience Australia

October 16, 2009 12:42 AM  

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