I've had two emails recently about quality checking seismic volumes. And last month, this question popped up on LinkedIn:
We have written before about making a data quality volume for your seismic — a handy way to incorporate uncertainty into risk maps — but these recent questions seem more concerned with checking a new volume for problems.
First things first
Ideally, you'd get to check the volume before delivery (at the processing shop, say), otherwise you might have to actually get it loaded before you can perform your QC. I am assuming you've already been through the processing, so you've seen shot gathers, common-offset gathers, etc. This is all about the stack. Nonetheless, the processor needs to prepare some things:
- The stack volume, of course, with and without any 'cosmetic' filters (eg fxy, fk).
- A semblance (coherency, similarity, whatever) volume.
- A fold volume.
- Make sure the processor has some software that can rapidly scan the data, plot amplitude histograms, compute a spectrum, pick a horizon, and compute phase. If not, install OpendTect (everyone should have it anyway), or you'll have to load the volume yourself.
There are also some things you can do ahead of time.
- Be part of the processing from the start. You don't want big surprises at this stage. If a few lines got garbled during file creation, no problem. If there's a problem with ground-roll attentuation, you're not going to be very popular.
- Make sure you know how the survey was designed — where the corners are, where you would expect live traces to be, and which way the shot and receiver lines went (if it was an orthogonal design). Get maps, take them with you.
- Double-check the survey parameters. The initial design was probably changed. The PowerPoint presentation was never updated. The processor probably has the wrong information. General rule with subsurface data: all metadata is probably wrong. Ideally, talk to someone who was involved in the planning of the survey.
- You didn't skip (2) did you? I'm serious, double check everything.
Crack open the data
OK, now you are ready for a visit with the processor. Don't fall into the trap of looking at the geology though — it will seduce you (it's always pretty, especially if it's the first time you've seen it). There is work to do first.
- Check the cornerpoints of the survey. I like the (0, 0) trace at the SW corner. The inline and crossline numbering should be intuitive and simple. Make sure the survey is the correct way around with respect to north.
- Scan through timeslices. All of them. Is the sample interval what you were expecting? Do you reach the maximum time you expected, based on the design? Make sure the traces you expect to be live are live, and the ones you expect to be dead are dead. Check for acquisition footprint. Start with greyscale, then try another colourmap.
- Repeat (5) but in a similarity volume (or semblance, coherency, whatever). Look for edges, and geometric shapes. Check again for footprint.
- Look through the inlines and crosslines. These usually look OK, because it's what processors tend to focus on.
- Repeat (7) but in a similarity volume.
Dive into the details
- Check some spectrums. Select some subsets of the data — at least 100 traces and 1000 ms from shallow, deep, north, south, east, west — and check the average spectrums. There should be no conspicuous notches or spikes, which could be signs of all sorts of things from poorly applied filters to reverberation.
- Check the amplitude histograms from those same subsets. It should be 32-bit data — accept no less. Check the scaling — the numbers don't mean anything, so you can make them range over whatever you like. Something like ±100 or ±1000 tends to make for convenient scaling of amplitude maps and so on; ±1.0 or less can be fiddly in some software. Check for any departures from an approximately Laplacian (double exponential) distribution: clipping, regular or irregular spikes, or a skewed or off-centre distribution:
- Interpret a horizon and check its phase. See Purves (Leading Edge, October 2014) or SubSurfWiki for some advice.
- By this time, the fold volume should yield no surprises. If any of the rest of this checklist throws up problems, the fold volume might help troubleshoot.
- Check any other products you asked for. If you asked for gathers or angle stacks (you should), check them too.
Last of all, before actual delivery, talk to whoever will be loading the data about what kind of media they prefer, and what kind of file organization. They may also have some preferences for the contents of the SEG-Y file and trace headers. Pass all of this on to the processor. And don't forget to ask for All The Seismic.
What about you?
Have I forgotten anything? Are there things you always do to check a new seismic volume? Or if you're really brave, maybe you have some pitfalls or even horror stories to share...