Re: GEOSTATS: Effective range
- In response to Abedini's question on effective range:
1) As a rough measure, some people take the intercept between the variance
of the data and the variogram as the "effective range." Doesn't always
work; the variance might not intercept the variogram. Also, some
human judgment need to be applied; not any intercept makes sense (e.g.,
power law behavior, presence of a drift, etc).
2) You can eyeball the range; there is nothing wrong with this. This is
probably better than (1).
3) The minute you have "nested" structures, you potentially might have
different heirachies of variability in your data. Maybe you can explain
the physics behind this; maybe you wouldn't be able to. But it always
pays to know why you have such behavior. E.g., when you have different
ranges, are they because: of a laminae system, which makes up a bedset,
which further makes up a sequence?
4) Keep the model variograms simple. Using just one model usually suffices.
No need to model every kink in the sample variograms. Get the major
features right (early behavior, sill, and range) by using one model.
Keep in mind the parameters that would most affect your kriging results
and work with these parameters. Maybe you're using a small search
neighborhood and modeling the smaller lags correctly is more important
in your study. Maybe you wouldn't even need a "correct" range.
5) The same applies to anisotropy. Is it really going to affect your
predictions? How about using the same variogram and trying out an
anisotropic search neighborhood? Use more samples in your predictions
along the major axis of anisotropy.
6) Why would something be anisotropic? Know the "why" is usually more
important than modeling the anisotropy blindly. Knowing the "why"
guarantees that your predictions would make sense. Not knowing the
"why" doesn't guarantee anything. In fact your predictions could be
garbage.It's always good to make sure that the "anisotropy" is not
an artifact of the data itself.
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