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Thursday
Sep062012

## Fold for sale

A few weeks ago I wrote a bit about seismic fold, and why it's important for seeing through noise. But how do you figure out the fold of a seismic survey?

The first thing you need to read is Norm Cooper's terrific two-part land seismic tutorial. One of his main points is that it's not really fold we should worry about, it's trace density. Essentially, this normalizes the fold by the area of the natural bins (the areal patches into which we will gather traces for the stack). Computing trace density, given effective maximum offset Xmax (or depth, in a pinch), source and receiver line spacings S and R, and source and receiver station intervals s and r:

$\mathrm{traces/km^2} = \frac{\pi \times X^2_\mathrm{max} \times 10^6}{S \times R \times s \times r}$

Cooper helpfully gave ballpark ranges for increasingly hard imaging problems. I've augmented it, based on my own experience. Your mileage may vary! (Edit this table)

### Traces cost money

So we want more traces. The trouble is, traces cost money. The chart below reflects my experiences in the bitumen sands of northern Alberta (as related in Hall 2007). The model I'm using is a square land 3D with an orthogonal geometry and no overlaps (that is, a single swath), and 2007 prices. A trace density of 50 traces/km2 is equivalent to a fold of 5 at 500 m depth. As you see, the cost of seismic increases as we buy more traces for the stack. Fun fact: at a density of about 160 000 traces/km2, the cost is exactly \$1 per trace. The good news is that it increases with the square root (more or less), so the incremental cost of adding more traces gets progressively cheaper:

Given that you have limited resources, your best strategy for hitting the 'sweet spot'—if there is one—is lots and lots of testing. Keep careful track of what things cost, so you can compute the probable cost benefit of, say, halving the trace density. With good processing, you'll be amazed what you can get away with, but of course you risk coping badly with unexpected problems in the near surface.

What do you think? How do you make decisions about seismic geometry and trace density?

References

Cooper, N (2004). A world of reality—Designing land 3D programs for signal, noise, and prestack migration, Parts 1 and 2. The Leading Edge. October and December, 2004.

Hall, M (2007). Cost-effective, fit-for-purpose, lease-wide 3D seismic at Surmont. SEG Development and Production Forum, Edmonton, Canada, July 2007.

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I'll have to read the paper, But I worry that this may be a bit of an over simplification. Every Seismic shoot is going to be different, It's hard to compare costs of shooting in a swamp or a forest where you have to use dynamite versus an open plain. Also your target objectives will likely dictate what type of shooting parameters you need.

What's unfortunate is how short sighted certain operators can be. With a shallow objective depth, it's easy to want to choose a short half length. But if you later find a deeper zone is productive you may have shot yourself in the foot. The shallow survey might be usable for a structural since, but the lack of far offsets would make any avo interpretation impossible.

I'm sure this has happened quite a few times once people realized about certain subsalt productivity.

September 6, 2012 | Toastar

@Toastar: You're dead right. I didn't mean to give the impression my cost curve applies to any 3D. The numbers in that curve were the product of a giant spreadsheet that accounted for the cost of everything from helicopters to camp berths — in northern Alberta, in 2007. The parameters of that curve are sensitive to the locality, the market, the conditions, etc. I bet the shape would always be more or less the same, though. There are lots of other complications too — near surface heterogeneity, inconsistent illumination, bandwidth, etc. Anyway, sorry for the misdirection.

On the subject of planing ahead, and shooting multi-purpose seismic, Evan and I often talk about how the technology in the Lytro camera might apply to seismic... one day!

September 6, 2012 | Matt Hall

Yeah I've been thinking about getting one now that they support windows.

But isn't that concept sort of what reprocessing is all about? You can already do 2D Depth reprocessing on a laptop.

The thing about the lytro is it just doesn't seem ready for prime time. I'd love to see a version aimed at professionals rather then kids. Why doesn't it support camera flash? Why doesn't it support a screw in tripod adapter. What would it take to get to a reasonable resolution(~3MP)

I feel their whole business model is about trying to get bought by canon or nikon.

September 6, 2012 | Toastar

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