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[ai-geostats] Re: Kriging Small Blocks

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  • Isobel Clark
    Nicolau I was talking about kriging before cutoff is applied. If the cutoff is applied to the block estimates my comments stand. If you aply the cutoff to your
    Message 1 of 5 , Jul 19, 2004
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      Nicolau

      I was talking about kriging before cutoff is applied.
      If the cutoff is applied to the block estimates my
      comments stand. If you aply the cutoff to your data
      first and then krige, you get the opposite problem,
      because you will over-estimate every value and
      under-estimate the tonnage.

      My point (1) is that, if you wish to avoid conditional
      bias in your kriging, you could consider using a
      non-linear kriging method such as those mentioned. I
      have no experience with either, since I follow a
      different route in the correction of conditional bias
      in mineral resource estimation.

      Isobel
      http://geoecosse.bizland.com/whatsnew.htm



      --- nicolau.barros@... wrote: > Isobel,
      >
      > So for mining purposes can't we just krige before
      > applying the cut-off
      > criteria? I mean, for most mining applications one
      > will prefer to have a
      > more realistic geologic block model and will always
      > have the chance to
      > evaluate his/her panels under the appropriate
      > cut-off criteria, but applying
      > that criteria after estimating small blocks, right?
      >
      > Could you please explain your point in solution (1)
      > below? Thanks for
      > indicating the literature.
      >
      > Thanks
      >
      > Nicolau Barros
      > Engineer
      > Mine Planning and Production Control Department
      > Mineração Rio do Norte S.A.
      > nicolau.barros@...
      > +55 (93) 549 8215
      >
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      > -----Mensagem original-----
      > De: Isobel Clark [mailto:drisobelclark@...]
      > Enviada em: segunda-feira, 19 de julho de 2004 05:23
      > Para: Julhendra_Solin@...
      > Cc: ai-geostats@...
      > Assunto: [ai-geostats] Re: Kriging Small Blocks
      >
      > Jul
      >
      > The warning about kriging small blocks is about
      > "small" relative to the sampling density. For
      > example,
      > less than about one-third of the grid spacing.
      >
      > The warning is the same as the one about 'point'
      > kriging (mapping) The map is too smooth - or, at
      > least, a lot smoother than the real surface would
      > be.
      > High value areas will be under-estimated and low
      > value
      > areas will be over-estimated.
      >
      > If your objective in kriging is to obtain general
      > maps
      > of an area with an idea of where the highs and lows
      > are, then ordinary kriging is sufficient. The over-
      > and under- estimations cancel out on average.
      >
      > In mining applications, where block kriging
      > originated, most applications require a 'cutoff',
      > where values below a certain value are not included
      > in
      > the 'plan'. In this case, mapping or estimating
      > small
      > blocks will result in an over-estimation of
      > 'payable'
      > ground and an under-estimation in average value.
      >
      > In pollution or environmental applications, the
      > areas
      > at risk will be under-estimated as will the true
      > toxicity or risk factors.
      >
      > There are two major ways round this problem:
      >
      > (1) use a non-linear kriging approach such as
      > disjunctive kriging or the multivariate gaussian. Ed
      > Isaacs and Mohan Srivastava's book is th ebest
      > reference for the latter. Rivoirard's book for DK.
      >
      > (2) simulation. There are lots of simulation methods
      > around, which allow you to 'put back the roughness'
      > and get an idea how bad the problem might be. GSLib
      > is
      > pretty good on this.
      >
      > Isobel
      > http://geoecosse.bizland.com/course_brochure.htm
      >
      > If, as in mining, you wish to apply some sort
      >
      >
      >
      >
      >
      >
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