[R-sig-Geo] Best distance for a semi-variogram calculation

[Arnaud Mosnier]

Dear geoUseRs,

New in the world of variograms, I ask for some advices !

My general question is :
Is there a methodology to find the best distance to be used as a limit
for the calculation of a semi-variogram ?

Here is an example that is problematic for me :
I have a 300 km by 300 km grid regularly spaced points containing
water current speed.
I first tested for spatial trend making a linear model between current
speed value and coordinates.
As there were a high effect, I removed this trend making my variogram
on the residuals of the model.
I calculated the semi-variogram on my variable from 0 to 300 km.
The variance first increase, attain a local maximum then decrease, and
re-increase acquiring a wave form, never really converging to a sill
value ...

1) do you have ever met such a pattern ? And if yes what was the reason of it ?

2) I agree that value of points located at 300 km from each other are
certainly not linked but how to know if there is no sill in the
semi-variogram ? Do I have to consider the distance where the first
decrease occurred in the semi-variance plot as the limit of the range
?

3) With that kind of shape what kind of variogram model would you
recommend to fit on it ?