Rajive I haven't read the other responses yet, so this may be redundant.
Two possibilities: (1) anisotropy: if this is shallow marine data there should be a difference between longshore drift and off-shore deepening of sea-bed. You have an omni-directional semi-variogram. It is possible that the sampling grid is irregular enough to be highlighting directional differences?? (2) mega-ripples: I have seen similar behaviour in off-shore marine diamonds which tend to hug the bottom of trenches or ripples. Major ocean beds have mega-ripples on the kilometre scale, which is what you are seeing here. More worrying, I would say, is the fact that your graph is dropping with distance. This suggests that you also have some underlying trend (non-stationarity) which is causing closely spaced samples to be 'more different' than those further apart. I notice you are using a log transform. What does your probability plot look like? Isobel
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