Calculate Variogram

This command takes the selected drillholes and calculates the variogram data, outputting the Nugget, Sill and Range. These parameters are entered into the Carlson Kriging dialogs for gridding and block modeling. Variograms may be 2D or 3D. When the command is first activated, you will be prompted to select the variogram type, as shown below.

After selecting the variogram type, you will be prompted to select the drillholes, channel samples, and strata polylines to use for variogram calculation. You will then be prompted to select a strata to process, as shown below.

The type of variogram to calculate is shown in the upper left corner. First, select the Head Variable in the pull-down. Then select the type, such as Semivariogram. This should then display the head variables span from low to high. The current types of variograms to choose from is shown below. Details of each method can be found online at kriging websites such as Isobel Clark's www.kriging.com.

Head Variable: Lists the variable that is analyzed.
Ignore Zeros: Ignores any zero values in the drilling.
Validate Drilling: Lists the number of holes used.
Min/Max/Average: Lists the minimum, maximum and average values of the sample points.
Low and High Trimming Limit: Trimming limits will provide a range selection to process. A value of 0 trimming will delete null values.
Drilling Paired Dist.: This shows the number of found pairs in the drillholes.
Min/Max/Avg: This is the distance between all sampled pairs of drillholes.
Number of Lags: Set the number of lags, or pairs to model.
Lag Separation Distance: Distance between the pairs. The distance between the lags should be set to the minimum drilling paired distance.
Lag Tolerance: Lag Tolerance is Lag Separation Distance divided by 2.
Omni-Directional: Omni-Directional will look in multiple circular groups for pairs. If it is off, then it will look just directional for the pairs.
Number of Directions: Directions it will look for pairs
Angle Tolerance: angle to stay under for looking for pairs.
Band Width: How wide to look for the pairs.
Load/Save: Saves the variogram data as a VIO file that can be loaded later.

Pressing Graph will show the plot as a red line and the points as green X. They can be turned on or off to display in the window. The curve must then be fit to the red line manually with left mouse by double clicking to locate the values.

Fitting:
Omnidirectional: Click Exponential option beside big circle on the right to begin. Left click left mouse and drag on the plot to set Range, Sill and Nugget or enter them in the boxes to update the curve. To move the Nugget, left click on the far left side, at the beginning of the curve.

Directional Trend: The variogram plots redraw by clicking on the lines on the large directional window at the top, with the green line, so the 4 angles would be: 0 Azimuth, 135 Azimuth, 90 azimuth and 45 azimuth. The resulting values are displayed above for geostatistical modeling.

3D Kriging

Low Trimming Limit: This value sets the minimum attribute value to consider for the variogram.
Maximum Trimming Limit: This value sets the maximum attribute value to consider for the variogram.
Spatial Analysis: This button will scan the sample data and generate recommended values for the variogram calculation.

Prompts

Command: Select drillholes, channel samples and strata polylines.
Select objects: Specify opposite corner: 177 found
Select objects:
Reading drillhole 176
Finding splits ...

Calculate Variogram

2D Variogram

3D Kriging

Prompts