Variogram Map

Variogram map is a map (grid) of preferential continuity direction by calculating variogram region by region. The variogram map here is different from the rose diagram of variogram. The rose diagram of variogram is a single variogram. Please note that the rose diagram of variogram is also called variogram map.

To make a variogram map, there should be at least 30 wells (each well may be a single data point). The following is the procedure:

• Select the variogram model from the GridOption panel (try spherical first)
• Design the grid (x,y division) from the GridSize panel
• Modify Map Cell Ratio (ratio of window size to cell size) from the Options pull down menu Analysis panel (the ratio should be large enough to include enough data)
• If nugget effect is to be studied, turn on the Fit Nugget option from the Options pull down menu Analysis panel
• Modify top, bottom and vertical spacing from the Options pull down menu Analysis panel
• Click Map button on the left of the GridSize panel then select Variogram Map to generate the variogram map grid
• Go to GridOutput panel
• Select Variogram Map as active grid on the lower right pull down list
• GGraph then VargmMap (the ellipses) from the left of the GridOutput panel
• GGraph then Top View to graph the individual parameters: Nwell, CRa, Azimuth, CLv, CRv, Sill, Err. To change to another layer of parameters, make the graph the only graph, then move the scroll bar at the bottom of the graph. The 7^th parameter, Err, is the nugget effect as a percentage of the sill, or it is the curve fit error if nugget effect is turned off.

When the variogram map is being calculated, a box will be posted on the basemap indicating the area for collecting data for the current cell. Variograms are calculated for each cell and then automatically fit to the selected (spherical) model. The model fit of seven parameters is saved to the grid *.C1.

In general, variogram is different from location to location. By using a variogram map, different variogram can be used to fine tune the gridding. To change the variogram between layers, see Marker Reference.

To use a variogram map to guide gridding, there should a variogram map grid *.C1 available and the XY dimensions must be the same as that of the grid being built.

To introduce a variogram map grid from outside GridSTAT, besides the 7 parameter format (Nwell,CRa,Azimuth,CLv,CRv,Sill,Err), there are three other combination of parameters:

• NZ=1: only CRv (Correlation Ratio of horizontal to vertical)
• NZ=2: top CRa (Correlation Ratio Areally), second Azimuth (degrees)
• NZ=3: top CRv, second CRa, and third Azimuth.