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[ai-geostats] practical range vs range

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  • Els Verfaillie
    Hi list, I want to do ordinary kriging with an anisotropic variogram with GSLIB. My variogram is an exponential model with a practical range of 1800 m in
    Message 1 of 5 , Mar 23, 2005
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      Hi list,

      I want to do ordinary kriging with an anisotropic variogram with GSLIB. My
      variogram is an exponential model with a practical range of 1800 m in
      direction 50 and 880 m in direction 320. I'm not sure whether I have to use
      the practical range (which is 3a) or the value a, which is respectively 733
      m and 293 m. Furthermore I wonder which maximum search radius I have to
      choose: the 3a or the a value?

      Any suggestions?

      Cheers,
      Els

      ___________________________________________________

      Els Verfaillie, PhD student
      Renard Centre of Marine Geology - Ghent University
      Krijgslaan 281-S8
      B-9000 Gent - Belgium
      tel: +32-9-2644573 fax: +32-9-2644967
      e-mail: Els.Verfaillie@...
      url: http://www.rcmg.ugent.be/
      ___________________________________________________

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    • Digby Millikan
      Els, Which GSLIB program are you using? When you refer to the practical range a and 3a this is normally associated with the exponential model, the GSLIB
      Message 2 of 5 , Mar 23, 2005
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        Els,

        Which GSLIB program are you using? When you refer to the practical range
        a and 3a this is normally associated with the exponential model, the GSLIB
        programs appear to be setup to use spherical model parameters, so you
        should fit spherical models to your data, in the case of fitting a spherical
        model
        to your variogram (you can fit nested models if you are not happy with a
        single
        structure spherical model) the input to the programs, e.g. in the case of
        kt3d,
        spherical models only have one range i.e. a.
        For the search radius most will tell you the range, but it really depends
        on how
        much confidence you are prepared to have for your estimates, you can extend
        your search radius further than your range, it's just that those points
        estimated
        which use values greater than the range distance will have a lower
        confidence.
        You can even have points estimated which only use data at distances greater
        than the range, in which case these estimates will have a low confidence. It
        depends on how desperate you are to get estimates into data points on how
        far you extend the search radius beyond the range. In mining we classify all
        estimates with a confidence, either by associating it with the search radius
        that
        was allowed for a data point e.g. a geologist from visual assesment of the
        continuity of the geology of an ore zone may draw a polygon extending 10m
        either side of the drillhole, and say in that case that everything in that
        polygon
        may fall into the Joint Ore Reserves Committes code as the classification as
        "Measured which means the entire polygon has the go ahead for mining
        depending on its economics, in which case the search radius would be
        extended
        beyond the range, if necessary, so that all blocks within that polygon get
        filled
        with grade. Note also that the confidence of the estimate at each point is
        provided by the kriging variance, so if you do extend the search radius
        beyond the range, you will have the kriging variance as another method of
        classifying the resource.
        i.e. You don't have to limit your search radius to the range, it's just
        that
        estimates based on samples using some data greater than the range will have
        a lower confidence, indicated by the kriging variance, which in some cases
        may be better than having no estimate.


        Regards Digby
      • Colin Badenhorst
        Hi Els, To add to Digby s comments. For a given block you can run several estimates, each with a different search ellipsoid dimension. In my case, I tend to
        Message 3 of 5 , Mar 23, 2005
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          Hi Els,

          To add to Digby's comments.

          For a given block you can run several estimates, each with a different
          search ellipsoid dimension. In my case, I tend to run 3 estimates (passes)
          for each block being estimated:

          1st pass = 0.5 x range in the XYZ dimension
          2nd pass = 1.0 x range in the XYZ dimension
          3rd pass = 2.0 x range in the XYZ dimension

          I have higher confidence in the 1st pass, moderate confidence in the 2nd
          pass, and lower confidence in the 3rd pass.

          I am sure that many people have several variants of this - there are no hard
          rules here, simply what suites your data and its setting. I would however
          caution you not to make your sample search ellipsoid dimensions too large. I
          have seen a few instances where because of this, samples located near the
          very edge of the ellipsoid are assigned negative weights, and the result is
          a very small negative block estimate.

          Regards,
          Colin

          -----Original Message-----
          From: Digby Millikan [mailto:digbym@...]
          Sent: 23 March 2005 11:46
          To: ai-geostats; Els Verfaillie
          Subject: Re: [ai-geostats] practical range vs range

          Els,

          Which GSLIB program are you using? When you refer to the practical range a
          and 3a this is normally associated with the exponential model, the GSLIB
          programs appear to be setup to use spherical model parameters, so you should
          fit spherical models to your data, in the case of fitting a spherical model
          to your variogram (you can fit nested models if you are not happy with a
          single structure spherical model) the input to the programs, e.g. in the
          case of kt3d, spherical models only have one range i.e. a.
          For the search radius most will tell you the range, but it really depends
          on how much confidence you are prepared to have for your estimates, you can
          extend your search radius further than your range, it's just that those
          points estimated which use values greater than the range distance will have
          a lower confidence.
          You can even have points estimated which only use data at distances greater
          than the range, in which case these estimates will have a low confidence. It
          depends on how desperate you are to get estimates into data points on how
          far you extend the search radius beyond the range. In mining we classify all
          estimates with a confidence, either by associating it with the search radius
          that was allowed for a data point e.g. a geologist from visual assesment of
          the continuity of the geology of an ore zone may draw a polygon extending
          10m either side of the drillhole, and say in that case that everything in
          that polygon may fall into the Joint Ore Reserves Committes code as the
          classification as "Measured which means the entire polygon has the go ahead
          for mining depending on its economics, in which case the search radius would
          be extended beyond the range, if necessary, so that all blocks within that
          polygon get filled with grade. Note also that the confidence of the estimate
          at each point is provided by the kriging variance, so if you do extend the
          search radius beyond the range, you will have the kriging variance as
          another method of classifying the resource.
          i.e. You don't have to limit your search radius to the range, it's just
          that estimates based on samples using some data greater than the range will
          have a lower confidence, indicated by the kriging variance, which in some
          cases may be better than having no estimate.


          Regards Digby
        • "Julián Ortiz C."
          Dear Els, GSLIB uses the practical range for the exponential model.The best way to check this is to calculate your experimental variograms with gamv, run the
          Message 4 of 5 , Mar 23, 2005
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            Dear Els,

            GSLIB uses the practical range for the exponential model.The best way to
            check this is to calculate your experimental variograms with gamv, run
            the model with vmodel and then plot them together using vargplt. If the
            model fits correctly your data, then you can copy the parameters from
            vmodel to whatever program you are using next (kt3d, sgsim, or any other).
            In any case, you can check the source code to see which parameter each
            model requires (the fortran 77 code is available for free at
            www.gslib.com; you can check the cova3 subroutine that calculates the
            variogram values).

            Hope this helps! Regards,

            Julián.

            --------------------------------------
            Julián Ortiz C., Ph. D.
            Assistant Professor
            Department of Mining Engineering
            University of Chile

            www.ualberta.ca/~jmo1
            www.minas.cec.uchile.cl



            Els Verfaillie wrote:

            >Hi list,
            >
            >I want to do ordinary kriging with an anisotropic variogram with GSLIB. My
            >variogram is an exponential model with a practical range of 1800 m in
            >direction 50 and 880 m in direction 320. I'm not sure whether I have to use
            >the practical range (which is 3a) or the value a, which is respectively 733
            >m and 293 m. Furthermore I wonder which maximum search radius I have to
            >choose: the 3a or the a value?
            >
            >Any suggestions?
            >
            >Cheers,
            >Els
            >
            >___________________________________________________
            >
            >Els Verfaillie, PhD student
            >Renard Centre of Marine Geology - Ghent University
            >Krijgslaan 281-S8
            >B-9000 Gent - Belgium
            >tel: +32-9-2644573 fax: +32-9-2644967
            >e-mail: Els.Verfaillie@...
            >url: http://www.rcmg.ugent.be/
            >___________________________________________________
            >
            >
            >
          • Pierre Goovaerts
            Hi Els, The key question here is the sampling density and how many data will be included in this search window. If there are many, the screening effect will
            Message 5 of 5 , Mar 23, 2005
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              Hi Els,

              The key question here is the sampling density and how many data will
              be included in this search window. If there are many, the screening effect
              will greatly attenuate the impact of the data further away, hence using a or
              3a won't make a big difference. If data are sparser, then usually I set up my
              search strategy in terms of maximum number of data, not maximum search
              radius, at least in 2D (in 3D setting the search ellipsoid right is very important).
              Although simple kriging weights become zero beyond the range, it is not
              the case for ordinary kriging, which is a reason why you shouldn't systematically
              discard the observations outside the range of autocorrelation, in particular
              if the sampling density is low..

              Regards,

              Pierre

              -----Original Message-----
              From: Els Verfaillie [mailto:els.verfaillie@...]
              Sent: Wed 3/23/2005 5:08 AM
              To: ai-geostats@...
              Cc:
              Subject: [ai-geostats] practical range vs range



              Hi list,

              I want to do ordinary kriging with an anisotropic variogram with GSLIB. My
              variogram is an exponential model with a practical range of 1800 m in
              direction 50 and 880 m in direction 320. I'm not sure whether I have to use
              the practical range (which is 3a) or the value a, which is respectively 733
              m and 293 m. Furthermore I wonder which maximum search radius I have to
              choose: the 3a or the a value?

              Any suggestions?

              Cheers,
              Els

              ___________________________________________________

              Els Verfaillie, PhD student
              Renard Centre of Marine Geology - Ghent University
              Krijgslaan 281-S8
              B-9000 Gent - Belgium
              tel: +32-9-2644573 fax: +32-9-2644967
              e-mail: Els.Verfaillie@...
              url: http://www.rcmg.ugent.be/
              ___________________________________________________

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              Checked by AVG Anti-Virus.
              Version: 7.0.308 / Virus Database: 266.8.0 - Release Date: 21/03/2005
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