From the Variogram panel or step, click Points to calculate the points of empirical variograms. Then click Curve to fit a model. Different models may be tried without repeating the Points calculation. If parameters for calculation of empirical variogram is changed, the Points must be repeated.
There are three aspects of the control how the variogram is calculated. First, if markers are used to define zones, selections are available from the Options pull down menu Marker panel. Secondly, the depth range and vertical spacing may be controlled from the Options pull down menu Analysis panel. Thirdly, a subset of wells may be selected for the calculation by selecting the wells and check the Selected check box on the upper left of the Variogram panel. If there are multiple data types within the project, only the primary data will be used for the calculation, which is selected from the Options pull down menu Data&Units panel.
There are limits that can be placed on the calculation of the variograms, from the Tools pull down menu Preferences panel. These limits include the number of wells, the minimum and maximum horizontal distance, and the maximum vertical points. The minimum horizontal distance limit is used to avoid biasing the areal anisotropy fitting from well patterns. The maximum distance is to avoid fitting data that are too far apart. To analyze the vertical and horizontal variograms separately, use a vertical limit=1 to concentrate on the horizontal and then use horizontal limit of 1 to concentrate on the vertical.
Besides the automatic curve and model fit, you can modify the variogram parameters from the GridOption panel. The variogram parameters from the GridOption panel are actually used in gridding. To graph the user input parameters from the GridOption panel instead of the automatic fit parameters, check the Input check box on the upper left of the Variogram panel.
Nugget is defaulted to zero because using too large nugget is a common problem. To fit a nugget to the variogram, from the Options pull down menu Analysis panel turn on Fit Nugget in the Variogram section. In the Variogram step, the Nugget and Sill are shown on the right hand side unless the Steps are turned off. When the nugget is larger than 0, the data reliability for gridding is modified. To change the nugget, go to the GriddingData panel from the Options pull down menu and make the Reliability for hard data less than one. It should be emphasized that the nugget effect should be as small as possible. Large nugget effect means unreliable data. More effort should be put into getting the data correct rather than using the unreliable data with large nugget effect. In general, the Reliability parameter should be 0.5 or higher.
To calculate indicator variogram, use indicator data. If the data is a continuous variable, convert the data using the Change button on the left of DataQC panel, select Funct.Transform, the select Indicator.
To calculate madogram (use absolute difference instead of squared difference) rather than variogram, from the Options pull down menu Analysis panel Variogram box turn on ABS( ).