RE: [ai-geostats] Unusual Ordinary Kriging Results

[Reid, David W]

Thank you for you replies. Especially Bruce Ramsey for putting me on the right path.

 

It turned out to be something that I was overlooking.   The estimation run had the "variable weighting" option selected and the composite length was selected as the weighting.  I had taken the weights from a report window, these were not the weights used in the estimation. The actual weights applied to estimate the grade are the (length) applied weights which are located in a "explain" text file.  The estimate is correct given the input data, but not reasonable! (80% fe in hematite is a miracle).

 

Variable 1 of 1: input='FE' output='fe'    variable weighting
   Value     Weight  variable Applied weight
  64.000    0.317    0.048    0.015
  64.000    0.048    3.000    0.143
  65.900    0.097    1.107    0.107
  51.200    0.079    0.037    0.003
  63.900    0.058    1.467    0.085
  58.200    0.084    0.048    0.004
  55.100    0.117    0.048    0.006
  56.700    0.183    0.048    0.009
  60.000    0.086    0.048    0.004
  58.200   -0.036    3.000   -0.108
  55.405   -0.004    3.000   -0.012
  56.700    0.028    3.000    0.084
  60.000   -0.021    3.000   -0.064
  62.300   -0.052    3.000   -0.155
  61.000    0.041    0.048    0.002
  61.000   -0.024    3.000   -0.073
   #samples=16, total weight=0.050778 estimate=80.769272

In the case of this block there were some very short composites (0.048m) that should have been excluded from the estimation.  Once length weighting is turned off (or short composites are excluded)  the estimation result is reasonable. 

 

It appears that the length weighting is amplifying the effect of the negative weights.

 

We try and use a sample search that minimises occurrence of negative weights, but this is often a compromise with variable sample density + orientation.  It would be nice to be able to estimate with a  search optimised  for each block.  Negative weights are usually more of a problem with less well behaved elements like P (mitigated somewhat by higher nugget variograms).

 

Thanks again for your help, I will be able to rest easily at night knowing Vulcan has been producing correct estimates.

 

Regards 

 

David Reid
-----Original Message-----
From: Reid, David W [mailto:[EMAIL PROTECTED]
Sent: Tuesday, 4 October 2005 4:45 PM
To: AI Geostats mailing list
Subject: [ai-geostats] Unusual Ordinary Kriging Results

Hello,

 

After running ordinary kriging estimations using Vulcan mine planning software it was noticed there were some unusual estimated grades. I was hoping that someone can confirm that I on the right path + not heading up the yellow brick road to Oz.

 

The estimated value reported/calculated by Vulcan for one block was 80.77.  I thought this unusual as the grade of the 16 samples selected for the estimation range from 51.2 to 65.9 (mean 59.6).   I calculated the estimated grade by summing the products of sample grade and sample weight (given by the software) and got a value of 60.13 which seems far more reasonable.

 

Maptek the software vendor's response was to suggest that negative weights were responsible for the high estimation.

 

Details of the samples are below.

 

Have I overlooked something in my calculation or is there some other explanation for the result?

 

Regards

 

David Reid

 

Number	X	Y	Z	Grade	Distance	weight	weight * grade
1	51325.6	19954.3	205.28	64	8.755	0.316811	20.27589
2	51325.6	19954.3	206.8	64	12.751	0.047555	3.043512
3	51310	19953.7	206.95	65.9	15.699	0.097072	6.397071
4	51308.2	19968.6	206.58	51.2	19.183	0.078537	4.021086
5	51309.4	19970.1	206.67	63.9	19.557	0.05828	3.724066
6	51325.3	19939	205.08	58.2	19.948	0.08432	4.907436
7	51310	19939.3	204.78	55.1	21.197	0.117351	6.466049
8	51325.3	19977.5	204.98	56.7	21.267	0.182511	10.34838
9	51310.3	19938.7	204.88	60	21.646	0.085906	5.154343
10	51325.3	19939	206.6	58.2	21.843	-0.03607	-2.09928
11	51310	19939.3	206.3	55.405	22.767	-0.00408	-0.22581
12	51325.3	19977.5	206.5	56.7	22.98	0.028131	1.59505
13	51310.3	19938.7	206.4	60	23.259	-0.02124	-1.27434
14	51309.4	19970.1	208.9	62.3	24.395	-0.0517	-3.22108
15	51340.2	19939	205.48	61	28.172	0.040817	2.489816
16	51340.2	19939	207	61	29.772	-0.0242	-1.4764
TOTAL						1	60.12579
			Ave	59.60031		block estimate	80.77

 

 

 

David Reid

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