If the Spatial Variability Analysis option has been selected in Project Settings, this will allow you to define Correlation Length for materials in the Material Statistics dialog.
Select Statistics > Materials.
In the Material Statistics dialog, select a material and select the Spatially Variable Material checkbox.
Enter a Correlation Length in the X and Y directions.
If you select the Pick button beside the Correlation length input, a dialog will appear with measured correlation length data for some material types.
The Correlation Length represents the distance within a material over which values of a spatially random variable (e.g. cohesion) are expected to be highly correlated (i.e. similar in magnitude). In practice, two samples taken from adjacent locations in a soil profile tend to have very close values and as the sampling distance increases this tendency decreases. Small values of spatial correlation length mean that soil properties are highly fluctuated; on the other hand, large values of spatial correlation length mean that soil properties vary smoothly within the soil profile.
If the Correlation lengths are equal in the X and Y directions, this will generate isotropic random fields (i.e. no preferred direction, same spatial variability in the horizontal and vertical directions). In practice, it is often observed that horizontal correlation lengths are much larger than vertical correlation lengths (e.g. 10 times larger). This means that properties in the vertical direction have a greater variability over shorter distances; in the horizontal direction properties tend to be more uniform (less variability).
Correlation Length Sensitivity Analysis
The user can automatically create scenarios with different correlation lengths in order to see the effect correlation length has on the results. To do this, first make sure the Spatial Variability Analysis option has been selected in Project Settings. Also ensure you are using a Multi-Scenario file from the Scenarios tab in Project Settings. Finally ensure at least one material has been defined as spatially variable in the Material Statistics dialog.
Now select Statistics > Spatial Sensitivity Scenarios. You will see a dialog allowing you to change the correlation length in either direction, or isotropically for a given material. After clicking Create, the specified scenarios will be created.
The results can be graphed along the scenarios using the Graph Scenarios option in Interpret.