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Re: GEOSTATS: Variogram Modelling for Kriging with External

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  • David Garner
    ... -As you suggested above using the abundant soft data is a good way to determine structure. With high density seismic data, the variogram is reasonably
    Message 1 of 1 , Jun 18, 1997
      In reply to Kian:
      > Questions: 1. What are the valid ways to model the structure in the above case?
      -As you suggested above using the abundant soft data is a good way to
      determine structure. With high density seismic data, the variogram is
      reasonably insensitive to small changes in your model. Maybe compute
      onmi-directional for reference. Use your <6km gaussian first. The
      hole effects at long distance will be handled by the seismic
      coverage. Later compare anisotropic model. The difference may be
      incremental. Short lags are the critical information to fit for
      integrating wells and seismic.

      > 2. Are four wells too few for kriging with external drift?
      No. I used 6 wells last month. The issue is one of non-stationarity. At
      the distances you have mentioned >18K and depending on
      geological/velocity structures, Ext. Drift. is likely to give a good
      result. You did say the wells and seismic velocity had a very good correlation.

      > 3. Should a collocated co-kriging be used instead?
      > (I tried collocated co-kriging, but the st. dev. map has very small
      > values which I thought are not reflective of the uncertainties.)
      In Isatis use the Collocated Co-Kriging with the Markov-Bayes
      assumption. Then you only need the variogram on the drift(seismic
      velocity) as you have computed. Don't get hung up on st. dev. maps.
      Collocated co-kriging always has a lower st. dev. than Ext. Drift. This is a model
      based error and should only be used as a qualitative guide to how
      error varies. In practice, you may find poor drilling results close
      to well control because of the scaling problems between seismic
      and wells. Comparing the above Kriging techniques is a helpful way to
      assess errors.

      > 4. Is it better to use conditional simulations?
      Better depends on your project goals. For volumetrics calculations,
      yes. Using a co-simulation with ext. drift or collocated co-sim. is
      a useful way to quantify the uncertainty of your mapping, or the
      probability of the mapped attribute occuring.

      Regards,
      David Garner
      TerraMod Consulting
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