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Re: AI-GEOSTATS: interpretation/testing robustness of variogram

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  • Isobel Clark
    Juliann Three obvious possibilities: (1) you have a large scale trend which only shows up when you start taking pairs at larger distances. This would be the
    Message 1 of 3 , Nov 29, 2001
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      Juliann

      Three obvious possibilities:

      (1) you have a large scale trend which only shows up
      when you start taking pairs at larger distances. This
      would be the case if your second 'rise' tends to curve
      upwards rather than flattening.

      (2) you have anisotropy. If you did an
      'omni-directional' semi-variogram, your smaller
      distances will have pairs from all directions and you
      will see an 'average' sort of semi-variogram. When you
      go to larger distances you are getting lots of pairs
      in whatever direction lies between the two clusters
      and not much in any other direction. So your larger
      scale semi-variogram may be the one in that direction
      only.

      (3) you are actually dealing with two different
      population groups within the sampled areas. If they
      have much the same internal structure but differ from
      one another in behaviour, you would get this type of
      behaviour. In geological applications, you would look
      (perhaps) for a fault line or geological boundary
      between the two.

      As Pierre says, you must ask some basic questions.
      Mine would be: why were those two areas so heavily
      sampled with comparatively sparser sampling elsewhere?

      Hope this helps
      Isobel
      http://uk.geocities.com/drisobelclark

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