Friday, June 27, 2008

Calculating Seismic Fold

One of the selfish reasons for having a blog like this is to record all the things that I have to look up into one place, so I know where to go. For some reason, maybe because it is so basic, this can be a really difficult equation to find. It can be found on page 57 (of the second edition at least), but most other textbooks seem to talk around the issue instead e.g., p244-245.

I have this note in a course book from when I studied for my degree. It reads:

Fold = Number of geophones in the array / 2 * no. spacings array is moved along

Now that is completely correct, except I have to interpret it every time I read the sentence.

The same calculation seems so much easier when written as:

Fold = no. geophones / 2 * pull-up

There is nothing different here, except I've used less words to say the same thing. A lesson for the blog too I expect... So this is usually the method I use to calculate fold on a daily basis. But when I was working in a university, one of my friends always used to use a different method:

Fold = geophone spacing * no. geophones / 2 * shot spacing

Yeah, it's the same thing! But just more complicated. Pick the way of doing it you are most comfortable with, just don't get confused if you see someone else doing it differently and remember you can always convert to your method.

3d fold is just a little more complicated as you have to consider the extra dimension. You can do this by defining a bin area and calculating how many shot locations fall into this bin. Once I figure this one out maybe I'll post these equations too!

Web links:
general acquisition
3d fold

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Blogger Andy said...

I'm a little confused by this post. Imagine you had an 8km cable(640 Hydrophones @ 12.5m spacing) and 25m shot-point interval.
The fold is defined by the number of traces that fit into each bin. The easiest way to calculate this is (1/2 the streamer length) / the SP interval
A more reliable and accurate way might include the number of hydrophones and the hydrophone spacing.
(No.Hydrophones*Hydrophone spacing)/ SP interval
I diagram is worth a thousand words but basically picturing a source firing and being recorded by the near trace will hit a certain point on the ground. we will hit that same bin every time we shoot every 25 meters until we reach the last trace. Trying to picture the ray path between the source and Last trace will be a triangle where the ray hits the reflector exactly half way between the source and the hydrophone. this is exactly half the streamer length after the first shot. And as we had a shot point interval of 25m it hopefully makes sense to divide half the streamer length by that interval... giving you the number of times a bin is targeted during a line.

Haha, well at the beginning of this comment I said I was a little confused, no doubt anyone who reads this comment will be even more confused now. Good work with the Geophysics posting, keep up the good work!

Reading back I think the issue is that you need to divide by the sp interval/pull up/shot spacing whatever you want to call it not multiply. that said I have just written this comment without the aid of any resources so if i am wrong feel free to correct me.

January 10, 2010 2:12 AM  
Blogger Alan said...

The following always works for me regardless of whether it is 2D land or marine:
(GI/SI)*(No Channels/2)
GI= Group Interval
SI= Shot Interval

February 18, 2010 5:32 PM  

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