Geoscience Reference
In-Depth Information
Fig. 4.16
An example of higher uncertainty (and higher grade) near contacts
Fig. 4 .17
Example of Au grade
trend based on bench composites
timation using ordinary kriging and limited search neighbor-
hoods, trends are accounted for by the implicit re-estimation
of the mean within the search neighborhood (see Chap. 8 and
Journel and Rossi
1989
).
Some trends can be inferred from geological knowledge.
For example, the distribution of nitrate, borate, and iodine
in evaporitic-type deposits is predictable. More common-
ly, trends are detected and modeled directly from the data.
Trends can be described using plots of grade versus distance
along a relevant coordinate direction. Figure
4.17
shows the
gold grade trend in the vertical direction in a low-grade por-
phyry Au deposit. The data show that the Au grade declines
for lower elevations at an approximate rate of about 0.1 g/t
per 100 m. This trend may persist even after defining the
final estimation domains. If not taken into account, the trend
may result in overestimation of the Au resource for the lower
benches.
If trends must be accounted for explicitly, then the fol-
lowing approach is commonly applied in presence of a trend:
• Develop a deterministic trend model and remove it from
the data;
• Model the residual component; and
• Add the deterministic trend to obtain the final model.
There are some common deterministic methods for building
a trend model. They include hand or computerized contour-
ing, and fitting simple polynomial models. In practice, we
