Re: GEOSTATS: Variogram Modelling for Kriging with External
- 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?In Isatis use the Collocated Co-Kriging with the Markov-Bayes
> (I tried collocated co-kriging, but the st. dev. map has very small
> values which I thought are not reflective of the uncertainties.)
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
> 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.
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