When geologists create a block model, one of the most important decisions is the block size.
Usually this is defined by one or more of the following block size considerations:
- Data density (e.g. sampling, drilling pattern etc.)
- Variography
- Computational limitations
- Geological controls
- Mining considerations (e.g. bench height)
- Geostatistical parameters such as Kriging efficiency or conditional bias slope.
Geologists should also consider the block size variation in a model and whether they should have a single uniform user block size across the entire model, if they should apply sub-blocking or use an alternative methodology (e.g. partial percentages).
Each of these methods has its merits, however the most widely used method across the industry is sub-blocking. A sub-blocked model estimates the block value into the user block size, which in turn fills the sub-blocks.
Why Sub-Celling?
Sub-blocking is a valid approach when the data density (e.g. drilling pattern) is consistent and the user block size is representative across the entire model. However, nature is not consistent and geology is not usually regular.
To help manage this within a single block model where different domains are drilled with different patterns, GEOVIA has introduced the variable sub-celling feature into Surpac 6.7.2. This allows a block attribute to be estimated into different domains, using different block sizes in each, within the same block model, thus ensuring appropriate sample support for each domain.
The variable sub-celling feature allows the user to select a block size, as an alternative to the user block size, for estimating block values within a constraint. The selected block size can be either a multiplication or division of the user block size. It should also be noted this feature does not change the reported volume within a constraint and only the geometry of internal blocks within a constraint are modified (non sub-blocks).
How can I use variable sub-celling?
Prerequisites:
- GEOVIA Surpac 6.7.2 or newer should be installed.
- Dataset containing:
- Surpac block model (*.mdl) pre-populated with attributes
- Multiple constraint files for different domains (*.con).
- The associated composite string files for each domain (*.str).
Workflow: Variable Sub-celling
1. Open your block model in GEOVIA Surpac software. In this example, I am using Surpac 6.8.
2. Run an Inverse Distance, Ordinary Kriging, Nearest Neighbour or Simple Kriging estimation. In this example we will run Inverse Distance on one domain.
3. Start an Inverse Distance estimation by selecting: Block model > Estimation > Inverse Distance.
In the first form, select an input string file and the relevant d-field(s).
Then within the ‘Search parameters’ form, to apply variable sub-celling, select a block size in the drop down box that differs from the user block size (highlighted in red below)
4. This estimation should be constrained within the relevant domain constraint. You can repeat this process for each domain in the model.
By using this methodology, the geologist has the capability to select an appropriate block size for each domain. An example of the result is provided below:
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