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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.

Tuesday
Jan042011

## Will this change anything?

Stubborn as it is, I often neglect to check the weather forecast before I go out in the morning. I live within walking distance to most things, and I can bear extreme cold for a few minutes (and even run if I have to). So for me, searching for a weather forecast the night before or the first thing won't actually change my morning routine. And that is to say nothing of the reliability of the forecasts!

Every one of us can pick and choose how much information to use in our daily lives. On one end of the spectrum is no information, where uncertainty and ambiguity reigns. On the other end is total information, which can be unwieldy and noisy. One way to hone in the appropriate balance is to ask the question, "will this change anything?"

When deciding whether to run a fancy diagnostic borehole tool, say, or to redo a structure map to include new well data, the wrong thing to ask is "what will this information do for me?", or even, "will this technology or method work?" Instead, we should be asking, "will this change anything?"

Will adding (or excluding) this ingredient change the taste or outcome of my meal?

If the drilling engineer on your team is on the ball, cost conscious, and able to drill at 40 metres per hour, then LWD (logging-while-drilling) information may not actually allow you to steer the well on the fly. It's nice data to have after the fact, but it won't change how you drill the well. If your team's strategy is to drill relative time structural highs, then re-doing a velocity model for more accurate depth maps may be a waste of time.

When we talk about how information might change our plans, or change our understanding, we are talking about it's value.  Asking, "will this change anything?" is really trying to pin down, "how much do I value this information?" The weather channel might be more valuable to you than it is to me, but how valuable is it? Will it change anything if you have to get on without it?