Michael, Kriging with External Drift, as implemented in the GSLIB set of programs (Deutsch & Journel) is distinct from Universal Kriging, in that the "External Drift" is intended to be defined by a secondary variable that you, the user, "believe" incorporates some information of relevance to the primary variable that you are working with.
The "External Drift" could also be a kriged array of values based on kriging some sparse secondary variable (onto a regular grid) that you also feel contains information of relevance. You can also specify a "trend" mathematically, but the original intent was to incorporate a secondary variable "relevant" to estimation of the first. An example would be: you have sparse porosity measurements in an oil field, but you have a regular array of 3-D seismic amplitudes covering that same area. Your external drift variable would be seismic amplitude and your primary variable would be porosity. Obviously the two are not precisely the same, but "hopefully" they are related. It is up to you, the user, to specify external drift terms that "make sense" physically. Obviously, this is a matter of interpretation, and you are the one responsible for justifying your choices. I suggest you read the sections on Kriging with External Drift beginning on page 70 and 96 of the Deutsch and Journel (1998) book, "GSLIB Geostatistical Software Library and User's Guide," by Oxford University Press. I'm sure there are other papers out there on this methodology, but if you are talking about the GSLIB program, then it's best to go to the documentation associated with that program. Best regards, Chris Christopher A. Rautman, Ph. D., P.G. Underground Storage Technology Department Sandia National Laboratories P. O. Box 5800; MS-0706 Albuquerque, New Mexico 87185-0706 505-844-2109; fax: 505-844-4426 -- * To post a message to the list, send it to [EMAIL PROTECTED] * As a general service to the users, please remember to post a summary of any useful responses to your questions. * To unsubscribe, send an email to [EMAIL PROTECTED] with no subject and "unsubscribe ai-geostats" followed by "end" on the next line in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list * Support to the list is provided at http://www.ai-geostats.org
