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Re: [ai-geostats] Diamond drilling

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  • Stephen Henley
    Well yes, it is common practice, but there s no real justification for it. That is a completely arbitrary value. It is very unlikely to have any relationship
    Message 1 of 2 , Feb 2 5:57 AM
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      Well yes, it is common practice, but there's no real justification for it.
      That is a completely arbitrary value. It is very unlikely to have any
      relationship to the 'real' distribution of values that are below the
      detection limit, and it can potentially do nasty things to the overall
      statistics if you have a lot of these values - by using a single substitute
      value such as 0.5*DL you risk finishing up with an artificially low
      variance.

      Fortunately for values below the detection limit, the problem usually arises
      in an area that is of no economic interest anyway. Unless of course what you
      are modelling is a contaminant where it is precisely these low-value areas
      that you are looking for !

      One (fudge) solution that you could adopt is multiple indicator kriging,
      with the lowest indicator threshold set at the detection limit. However,
      using MIK brings its own can of worms and I wouldn't necessarily advise you
      to use it.

      Another possibility, if your data appear to have a well-defined probability
      distribution, is to simulate the below-detection-limit values by sampling
      from the theoretical distribution. So the values above the DL would be the
      real reported values, those below the DL would be replaced by simulated
      values. Of course you wouldn't want to do this just once - you'd want to
      generate multiple simulated data sets.

      Hope that helps.


      ----- Original Message -----
      From: "Digby Millikan" <digbym@...>
      To: "Stephen Henley" <Stephen.Henley@...>
      Sent: Friday, January 28, 2005 10:47 PM
      Subject: Re: [ai-geostats] Diamond drilling


      > So if your detection limit was dtermined to be say 0.01%Cu for example,
      > if you received a below detection limit for a sample is usual practice to
      > use half the detection limit for modelling? e.g. 0.005%Cu.
      >
      > ----- Original Message -----
      > From: "Stephen Henley" <stephen.henley@...>
      > To: "Digby Millikan" <digbym@...>
      > Sent: Saturday, January 29, 2005 7:29 AM
      > Subject: Re: [ai-geostats] Diamond drilling
      >
      >
      >> Good question. I guess the answer is "it all depends". If the laboratory
      >> sends back both 'below-detection-limit and 'trace' in the assay results,
      >> the best thing is to ask them the precise definition they are using for
      >> each.
      >>
      >> However, generally, 'below detection limit' refers to instrumental
      >> analyses (such as XRF) which are below the practical detection limit for
      >> the element and method used. 'Trace' tends to be used with
      >> non-instrumental methods where there is too low a grade to report but the
      >> analyst can't honestly say it is zero.
      >>
      >> If you'd like to discuss this further, perhaps you could contact me
      >> off-list on stephen.henley@...
      >>
      >> best regards
      >>
      >> Steve Henley
      >>
      >>
      >>
      >> ----- Original Message -----
      >> From: "Digby Millikan" <digbym@...>
      >> To: "ai-geostats" <ai-geostats@...>
      >> Sent: Friday, January 28, 2005 8:41 PM
      >> Subject: [ai-geostats] Diamond drilling
      >>
      >>
      >>> In a diamond drilling campaign are assay results below detection limit
      >>> and trace the same thing, or do these have seperate meanings.
      >>>
      >>>
      >>>
      >>
      >>
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