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1814Re: [ai-geostats] variogram analysis

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  • dwmccarn@aol.com
    Dec 7, 2004
      Dear Rajive:
      I cannot conclude with only 328 pairs that the feature is "wavy" because I do not know how those pairs are distributed for each point in the variogram.  Try different lag spacings, or create an "equal-n" lag variogram where each lag has the same number of pairs.  If that shows the same feature, then perhaps there is a repeating feature (faults, fractures, ore controls, etc.) occurring at regular intervals throughout the sampling domain.  I take it that you have 26 or so sample locations.
      Using "equal-distance" lags usually gives a large number of pairs to the first couple of lags, and then the n drops off rapidly, and the variogram is harder to interpret than with an "equal-n" type variogram.
      I wrote my variography codes to work both ways...
      Dan ii
      Dan W. McCarn, AIPG CPG #10245, Wyoming PG #3031, EurGeol #462
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      In a message dated 12/7/2004 3:27:31 PM Mountain Standard Time, NTGLOVER@... writes:
      Usually when I've seen a "wavy" semivariogram, it's because of a local
      feature superimposed over an existing field function - for instance, a
      release of mercury in a field  of soil with very low "natural" mercury
      content. The period of the waviness is related to the distance across
      the feature (the width of the spill, in this case).  Of course, this is
      nothing particularly earth-shattering, but useful none the less.

      I've used semivariograms like this in the past to "guestimate" the
      approximate size of a plume based on sparse data.  Not all geostatistics
      ends up in gridding and estimating at every point!  Sometimes just
      looking at the semivariogram is very useful.  

      Tim Glover
      Senior Environmental Scientist - Geochemistry
      Geoenvironmental Department
      MACTEC Engineering and Consulting, Inc.
      Kennesaw, Georgia, USA
      Office 770-421-3310
      Fax 770-421-3486
      Email ntglover@...
      Web www.mactec.com

      -----Original Message-----
      From: Rajive Ganguli [mailto:rajive.ganguli@...]
      Sent: Tuesday, December 07, 2004 4:50 PM
      To: ai-geostats@...
      Subject: [ai-geostats] variogram analysis

      My question is general.  What do you conclude if your variogram is
      wavy? Cyclic patterns?  I have what appears to be high nugget,
      followed by a wavy pattern.

      If you wish, here is more info: an offshore placer platinum deposit,
      not too many boreholes - just 29 from decades ago spanning several
      square kilometers.  The variogram (from GEOEAS) of the grade (ln) is
      given in:


      The variogram is cyclic. Goes up and down.  I tried various

      I will try to dig up the geological information and see what it says.
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