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1841[ai-geostats] Re: variogram analysis

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  • Rajive Ganguli
    Dec 13, 2004
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      Isobel and others,

      Sorry for the delay. Since my posting, I have had an opportunity to
      discuss the geology (we found a "mini" report).

      Various types of deposits have been suggested: paleo-alluvial and
      paleofluvial channels, submerged strand lines, "false" bedrock
      combined with longshore current among others.

      Additionally, the holes were along a narrow (approx) NW-SE line.
      Omni-directional variogram should not have been done since, really,
      there was no lateral extent to layout of the holes.

      Data is lognormal. Click here
      (http://www.faculty.uaf.edu/ffrg/prelim_analysis.zip) for basic
      plots/varios. Wavy nature still present. There is big difference in
      the nature of the variogram in 135 deg direction vs. 150 deg.
      direction. Cannot explain that at this time except blame it on

      We will be in that region doing geophyics and others next summer.
      Hopefully we will know more.


      On Wed, 8 Dec 2004 12:29:17 +0000 (GMT), Isobel Clark
      <drisobelclark@...> wrote:
      > 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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