Advanced Estimation - Investigate Anisotropy

To access this dialog:

Using the Advanced Estimation dialog, select the Investigate Anisotropy menu item.

Investigate Anisotropy interpolates a 3D Variogram in all directions, which is used to identify directions of spatial continuity with color bands of the average variance. The chosen orientation should align with the maximum grade continuity (i.e. direction where similar grades can be seen).

This 3D Variogram model should be interpreted with the sample data, to ensure the interpreted anisotropy is expected relative to the data and the geological setting. This orientation is used for setting the reference plane for variogram modelling.

This panel is only visible if Supervisor data is not being imported. You decide this using the Scenario Setup screen.

If you wish to view your samples in conjunction with the generated variogram map in the 3D Variogram window, you will need to ensure that the borehole identification attribute is called "BHID". Non-standard borehole IDs are not supported.

If no BHID field can be found, the map will still be generated, but you will not be able to view the samples file.

Getting Started

You start with the Investigate Anisotropy panel, and this topic assumes that you have already defined a scenario and determined samples and the attribute/domain to be modelled.

The first step is to select a grade field and (optionally) a zone value from the lists that were chosen in the previous stage.

The 3D variogram map is created using Create 3D Map and is displayed automatically in the 3D Variogram window.

Note: Advanced Estimation is part of the Studio RM toolset. Additional licensing modules aren't required.

Defining Lag Values

The 3D Variogram window shows a variance model colored according to standard legend bins representing bands of continuity from high correlation between samples (below sill) to no correlation between sills (above sill). These legend colors and thresholds are adjustable with the legend slider.

The separation distance of sample data pairs is referred to as “lag”, and the plotted value is the average variance at that separation distance. As the lag distance increases, so the variances changes. Typically, the variance increases up to the sill after which the variance falls above the sill. Once the variance increases above the sill, the sample separation is such that there is no longer relationship between samples. Often at large distances the where the variance drops below the sill which implies correlation, but this is more likely noise and should not be interpreted as correlation.

By default, Lag is calculated as 1/10 of the Maximum Lag distance (see Field Details, below) but can be adjusted as required.

A suitable initial value is in the order of your sample spacing. Choosing a suitable value should show an area of high continuity in the centre of the 3D variogram. The default Maximum lag value at half the maximum distance between samples, however this may be reduced so the variance map is focused around the area of interest (for example, the maximum lag might be set as 10 x the lag value).

Lag can also be adjusted once a variogram map exists, using the 3D Variogram ribbon's Lag value.

If applying a new lag setting using the 3D Variogram ribbon (i.e. when a 3D view already exists), there may be a small delay whilst all views are recreated and re-displayed.

For example, choosing a lag value that is to high hides a highly-continuous area that would otherwise be used for aligning to a major axis. Reducing the lag allows the high continuity zone to be displayed and then either manually or automatically aligned.

The semivariogram below shows the average similarity of nearby sample pairs at low values of Lag at low separation distances in the area of interest (y(h)), and is calculated for a series of lags and plotted against the Lag to create a variogram plot.

It also shows the systematic increase in y(h) as sample spacing increases and the eventual plateauing of variability beyond a given sample spacing (a). (a), therefore, represents the point where no correlation can be established between the samples. This is the sill.

Co is a random component of variability for all spacings. The average range of influence of a sample is therefore Lag = a):

In addition to the above, you can also determine the number of blocks that will exist in the output model. This will be a cube of a specified n blocks in each of the 3 axes.

As an input to the estimation process, a variogram is invaluable; in Studio you can create a 3-dimensional variogram representing three orthogonal directions. The resulting 3D result is shown in the 3D Variogram window.

The aim is to calculate variability for any sample separation distance, in any direction. These are called the Primary, Secondary and Tertiary views.

Field Details:

Variogram map file: the file name is generated automatically; this is a 3D variogram map (block model) file that will be generated for the grade and zone selected below.

Select zone: if a zone field has been defined for the current estimation scenario, you need to select a zone value using this drop-down list. If you are using 2 domain fields, you will need to choose one of the combinations provided. In this case, valid values in this list will be shown as "domain 1/domain 2" format, e.g.:

Select the combination you wish to map.

Select Zone: if zones have been specified during the Select Samples step, a zone value can be selected here. This will constrain variogram calculation to the zone (or zone combination if specified during the Define Custom Zones stage.

Select variable: select a grade from the drop-down list defined previously on the Select Samples panel.

Max distance between samples: This distance is calculated from the input samples data set (drillholes or points) and is used to determine an appropriate Maximum lag and default Lag value for this context (see below).

Maximum lag: this is the maximum permitted lag value permitted for the input data set. It is calculated as 50% of the Max distance between samples (see above). Values above Max distance between samples (see above) will be unattainable. Typically, a value of 10 x the lag is sufficient. You can edit this value if required.

Lag: the default Lag value to be used for anisotropic analysis and 3D map creation. Typically, a value of the average sample spacing between drillholes or sample points is suitable. This is an editable field. You can also edit and apply a new global lag value after the 3D variogram is created, using the 3D Variogram ribbon's Lag field.

Number of blocks: choose how big your want your output block model to be. The specified value will determine the cube size for the output model, using the value to denote the number of blocks present along each major axis.

Create Variogram Map: click to start calculation of a variogram map, to be displayed in the 3D Variogram window.

	Related Topics
	3D Variogram ribbon The 3D Variogram Window Advanced Estimation Introduction Advanced Estimation - Select Samples