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.

Data File:
After the drillholes are selected, the window appears to select the strata or bed to analyze. Once the seam is selected, then the Choose Value to Process appears. This allows for selecting the field to analyze, such as Thickness, Elevation, or any Quality Attribute.

Variogram Type:

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.

• Semivariogram
• Cross Semivariogram
• Correlogram
• General Relative Semivariogram
• Parwise Relative Semivariogram
• Semivariogram of Logarithms
• Semirodogram
• Seminadogram
  

Variogram Attributes:

Trimming limits will provide a range selection to process. A value of 0 trimming will delete null values. Set the number of lags and lag separation distance. Lags are the number of intervals, the separation is the distance between the intervals. Normally the Lag Tolerance is Lag Separation Distance divided by 2. OmniDirectional will look in multiple circular groups for pairs. If it is off, then it will look just directional for the pairs. The number of directions, Angle Tolerance and Bandwidth apply to the directional.

Pressing Graph will show the plot as a red line. The curve must then be fit to the red line manually with left mouse clicking to locate the nugget, sill and range.

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: Select the Head Variable, the variogram plots redraw by clicking on the lines on the large directional circle so the 4 angles would be: 0 Azimuth, 135 Azimuth, 90 azimuth and 45 azimuth. The resulting values are displayed above for geostatistical modeling.

Prompts

Command: vgramrun
Command: Select drillholes, channel samples and strata polylines.
Select objects: Specify opposite corner: 177 found
Select objects:
Reading drillhole 176
Finding splits ...
Drop-Down Menu Location: StrataCalc in Geology