Well, as the author of the "green bible" I guess I should help out a little bit here... The key idea is that there exists an analytical expression that allows you to compute a priori, for any threshold of a multigaussian random function, the indicator semivariogram models. You only need to know the threshold and the normal score semivariogram of the variable. Then, you just compare the "expected" or "theoretically-derived" indicator semivariogram models to the "empirical" or "derived from the data" ones. Note that you don't even need to go through the burden of computing the "theoretically-derived" indicator semivariogram models to know that the underlying assumptions of the multigaussian model are not fulfilled. In many situations, you will notice that your experimental indicator semivariograms are not symmetric with respect to the median; for example the 0.1 decile semivariogram might have a longer range than the 0.9 decile semivariogram. This happens frequently since the low background values tend to be better connected in space than the high values... The next question is "what do we do with that?"... or in other words "How do we know that the differences between expected and empirical indicator semivariograms are significant". You could test it, but I don't think it's worth it in practice... Well, cross-validation has taught me that even if the indicator semivariograms don't look like expected under the multigaussian model, multigaussian kriging might still give you better results than indicator kriging.. so it's hard to come up with "cast-in-stone" rules regarding the relative merits of parametric and non-parametric approaches.. but I am sure that everyone who has some experience with geostatistics has already realized that.. As I often say during my short-course, geostatistics provides you with a toolbox, and cross-validation and experience will teach you wich tools to use in any particular situation... Cheers, Pierre
-----Original Message----- From: Perry Collier [mailto:[EMAIL PROTECTED] Sent: Tue 3/22/2005 7:55 PM To: ai-geostats@unil.ch Cc: Subject: [ai-geostats] bi-Gaussian assumption for non-mathematicians Hi all from Oz (Australia) First post on this list. I am a mine geo currently doing some post-grad geostats study (Edith Cowan Uni in WA, hi Dr Ute, Prof. Lyn!). Expanding on some very useful feedback from my Uni course director, I would be interested in your learned "from the horse's mouth" comments (what, why, how, when) regarding the bi-Gaussian assumption for Gaussian simulation and the various means of checking it. I am slightly "mathematically challenged", so if anyone could explain the whole thing without too much scary maths, it would be much appreciated. I have Goovaerts' green geostats bible, which is good stuff, but I'm trying to convert some of the maths to English. Any comments from mining practitioners would be interesting... Cheers Perry Collier Senior Mine Geologist Ernest Henry Mine Xstrata Copper Australia Ph:(07) 4769 4527 Fax: (07) 4769 4555 E-mail: [EMAIL PROTECTED] Web: http://www.xstrata.com PO Box 527 Cloncurry QLD 4824 Australia "I like rich people. I like the way they live. I like the way I live when I'm with them..." From Roger & Hammerstein's Sound of Music ********************************************************************** The information contained in this e-mail is confidential and is intended only for the use of the addressee(s). If you receive this e-mail in error, any use, distribution or copying of this e-mail is not permitted. You are requested to forward unwanted e-mail and address any problems to the Xstrata Queensland Support Centre. Support Centre e-mail: [EMAIL PROTECTED] Support Centre phone: Australia 1800 500 646 International +61 2 9034 3710 **********************************************************************
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