Geoscience Reference
In-Depth Information
13.2
Medium- and Short-term Modeling
1. Develop a Sequential Gaussian conditional simulation
model and comprising the volume corresponding to
the previous year of production, FY01, (July 1, 2000-
June 30, 2001) was prepared. The simulation grid was
1 × 1 × 15 m, and was used as a reference to compare the
alternative models and methodology developed. The sim-
ulation model not only honored the histogram and vario-
gram models of the conditioning data, but reflected actual
production figures. The simulated variables were Total
Copper (TCu), Sulfuric acid-soluble Copper (SCu), Arse-
nic (As) and Total Iron (Fe). The conditional simulation
model is not described here in detail, as it was only used
as a reference.
2. The volume to be mined in the following quarter was
defined, and a reserve block model is created within it.
The blocks can be the same size as the long-term resource
model blocks, or smaller if the additional infill and/or
blast hole data justifies it. In the case of the initial study,
for each month of the FY01 period, a quarterly model was
defined based on actual mined out volumes.
3. The geologic model is updated monthly using informa-
tion from bench and face mapping, as well as blast hole
cuttings. For example, when completing the quarterly
model for the month of January, the planned mining vol-
umes corresponding to the months of February through
April are considered, and the geologic information avail-
able up to December 31 is used.
4. The grade models (TCu, SCu, As, and Fe) were updated
using infill drill holes and blast holes through the previ-
ous month. The same methodology as used for the long-
term resource model is applied, except that smaller blocks
sizes were used as warranted by the additional drill holes
available. The long-term block model is 25 × 25 × 15 m,
while the quarterly model is based on 12.5 × 12.5 × 15 m
blocks; therefore, within each block of the long-term
model there are 4 blocks of the quarterly model. It is al-
ways convenient to define the quarterly block model in
a manner consistent with the geometry of the long-term
model, such that comparisons can be easily made.
5. The quarterly models are compared with the long-term
resource model and with the reference simulation model
to quantify the improvements obtained. In the case of the
routine, operational procedure, the comparison is done
against the monthly reconciliation figures for the prior
months, such that a closer control of the long- and the
medium-term models is maintained.
The long-term resource model historically underestimated
mine production, particularly in-situ TCu grade. The resource
estimation methodology was partly to blame, but even after
improving the estimation methodology, the resource model
still had a small TCu deficiency. This deficiency was traced
to a lower-than-expected TCu grade in the exploration drill
holes, mostly those drilled using conventional rotary tech-
Updating the long-term resource model using in-fill data
implies repeating many of the steps described in previous
Chapters. This is regardless of whether the task involves esti-
mation of values, estimation of distributions, or simulations.
However, some special considerations are required, particu-
larly if production information is used.
One of the most difficult aspects of updating short-term
models is updating the geologic model and estimation do-
mains using production data. In practice, face, bench, or
stope mapping from underground workings and a descrip-
tion of blast hole cuttings or production drill holes may be
available, but seldom used. This is partly due to data quality,
and also to the tight timeframe involved.
The grade model can be updated using both infill drill
hole data and production data. The use of blast holes can
be controversial for several reasons, including perceived
sampling quality, and discrepancies of its grade distribution
compared to the exploration drill holes grade distribution.
Despite the difference in quality of the individual samples
(drill holes vs. blast holes), often the much larger number of
blast holes available compensate for the poorer precision of
the individual sample. The key to using blast hole samples is
that there should be no significant bias.
A different issue is the estimation strategy. The imple-
mentation of any estimation method should consider the
possibility of blast holes overwhelming the infill drill holes
in certain areas; thus, an adequate estimation strategy should
carefully consider how blast holes are used.
In all cases, the medium- or short-term block model
should be updated only for the relevant portion of the de-
posit, for example, corresponding to the next three months
of production. An example is given here of a medium-term
model prepared for the Escondida copper mine in Northern
Chile and courtesy of BHP Billiton. It illustrates a practical
application of the process.
13.2.1
Example: Quarterly Reserve Model,
Escondida Mine
At Minera Escondida in early 2002, medium-term
planning was required on 13-week intervals, since this
was the forecast period used, and updated on a monthly
basis. Therefore, the quarterly planning cycle was in
fact a monthly moving-window that represented the
planned mined volumes three months at a time. In order
to develop a practical methodology and demonstrate the
usefulness of updating the long-term resource model,
an initial study was developed that consisted of the
following:
