Variogram model

Use the Variogram Model form (model > Facies > Variogram Model) to assign a variogram model to your Volume of Interest (VOI) if you assigned a variogram-based modeling method (SIS or TGS) to your facies model. A VOI with a variogram-based modeling method but with no variogram assigned, will be blocked from running in the last step of the modeling workflow. Input to the variogram is the upscaled grid property. When you are using SIS, one variogram model per indicator (discrete property class) needs to be created; for TGS you only need one variogram model.

Upon opening the Variogram Model form, the Facies Variogram View is opened automatically. If you have saved an experimental variogram in the previous step, the variogram model is visualized as a black curve. If you have not done so, the view will open with nothing on display.

To assign a variogram - overall workflow

Variogram modeling requires first to calculate an experimental variogram (data points) and then finding the parameters describing the variogram model (curve). The intended workflow on the form is as follows:

  1. After opening the Variogram Model form, first select the Facies Model from the Model drop-down list at the top of the form. If you arrive on this form by proceeding through the workflow, the model and the VOI will already be selected for you.
    2. Using the context menu options Copy and Paste, you can copy-paste variogram model settings between the VOIs which have variogram-based modeling methods (i.e. SIS, TGS). Once you have copied settings, you can select Paste Settings Options. This will open a separate form, which allows you to paste the settings to multiple VOIs at once. Settings are copied only between the facies classes that match. Note that checkbox selection can be done in bulk by holding down SHIFT while selecting a range of VOIs (they turn blue), then checking the box of one of them. All other VOIs in the selected range will get checked simultaneously.
  2. Select a Facies in the Facies table (only when the selected method for your VOI is SIS).
    3. You can use the context menu to copy and paste the parameters from one facies to another.
  3. At the right side of the form you have two options: you can either fill in all values yourself, or you can use the Autofit to Data or Autofill buttons at the base of the form. Filling in the required fields creates a curve-fitting to determine a variogram model (which you can alter later).
    • Autofit to Data - Select this option to have the application fill in the 'Parameters' section for you. You must first specify an Azimuth and Dip for the major direction and a curve type yourself to activate this option.
      • Autofitting the variogram parameters for a TGS model is not recommended because the experimental variogram and the variogram model represent different properties. The experimental variogram is calculated from upscaled facies indicators, but the variogram model is used to simulate a continuous Gaussian variable to be truncated. Consequently the sills and variogram shapes are not expected to be the same. Nevertheless, the apparent range of the experimental variogram judged by visual inspection can be used to estimate the range for the variogram model.
    • Autofill - Select this option to copy all the settings of an existing facies variogram model to the currently selected facies. Clicking the button will open a new form where you select the facies model, VOI and facies to copy the settings from.
  4. Customize the variogram model parameters. In case you are modeling facies variograms, you need to do this for each facies. You can use the context menu to copy and paste the parameters from one facies to another.
  5. Assign the variogram model(s) to the selected facies in the VOI by clicking Apply at the base of the form. Click OK to go the next step in the workflow.
Only when you click Apply, the variogram model is assigned to the VOI.

Variogram Calculation Settings

Variogram Model Directions

  • Azimuth - Specify the azimuth (angle with Northing direction) of the direction. By default, the azimuth of the Major direction is 0°, and the azimuth of the Minor direction is always perpendicular to it. The major direction is defined as the direction of the greatest spatial continuity.
    • Uncertainty   The Azimuth of the Major direction can be propagated as a parametric uncertainty in a probabilistic volumetric calculation with the study strip. Click the uncertainty symbol next to the Azimuth entry field to open the Uncertainty Parameter dialog. For how to use the controls on the dialog, see The Uncertainty Parameter dialog. For information about parametric uncertainties and how to use them in JewelSuite Subsurface Modeling, see Incorporating uncertainty in static or dynamic modeling.
  • Dip - Specify the dip angle. By default, the dip of the Vertical direction is -90°.

Curve Type

Choose the type of variogram model (Spherical, Exponential or Gaussian) that you want to auto-fit as a curve to your experimental variogram. This curve is displayed in black.

Note on the Gaussian variogram - The Gaussian variogram is not allowed for SIS because it uses the model directly to simulate the facies indicators, and it is not possible for a facies indicator to have a Gaussian variogram. For TGS, the Gaussian variogram is allowed because it uses the model indirectly to simulate a Gaussian variable that is truncated to make facies indicators. The Gaussian variogram is useful for TGS if you wish to make facies objects with smooth edges.

Parameters

The following controls allow you to create an alternative variogram model, independent of the experimental variogram. After changing a value in any of these boxes, press Tab on your keyboard; the alternative variogram model that you just created will be displayed as a black curve in the Facies Variogram View.

  • Nugget - The semi-variance when distance between data points is below the sampling resolution. The value indicates the minimum semi-variance of the model.
  • Sill - The semi-variance where the variogram curve levels off and becomes a flat curve. The value indicates the maximum semi-variance of the data set.
  • Major range - The lag distance at which property values become independent (the variogram reaches its plateau) in the major direction. The major direction is the direction with the largest spatial continuity.
    • Uncertainty   This parameter can be propagated as a parametric uncertainty in a probabilistic volumetric calculation with the study strip. Note that the variogram range impacts the Search Ellipsoid, as such the number of data points included in the grid point calculation, see 'Search Factor' in Controlling the facies modeling method. Click the uncertainty symbol next to the 'Major range' entry field to open the Uncertainty Parameter dialog. For how to use the controls on the dialog, see The Uncertainty Parameter dialog. For information about parametric uncertainties and how to use them in JewelSuite Subsurface Modeling, see Incorporating uncertainty in static or dynamic modeling.
  • Minor range - The lag distance at which property values become independent (the variogram reaches its plateau) in the minor direction. The minor direction is the direction with the least spatial continuity (by definition perpendicular to the major direction).
    • Uncertainty   This parameter can be propagated as a parametric uncertainty in a probabilistic volumetric calculation with the study strip. Note that the variogram range impacts the Search Ellipsoid, as such the number of data points included in the grid point calculation, see 'Search Factor' in Controlling the facies modeling method. Click the uncertainty symbol next to the 'Minor range' entry field to open the Uncertainty Parameter dialog. For how to use the controls on the dialog, see The Uncertainty Parameter dialog. For information about parametric uncertainties and how to use them in JewelSuite Subsurface Modeling, see Incorporating uncertainty in static or dynamic modeling.
    For uncertainty studies and other sensitivity scenarios, you can specify a Minor range value greater than the Major range value if you have assigned TGS and SIS method for the selected VOI's. A warning message is displayed on the form in such cases.
  • Vertical range - The lag distance at which property values become independent (curve reaches its plateau) in vertical direction.
    • Uncertainty   This parameter can be propagated as a parametric uncertainty in a probabilistic volumetric calculation with the study strip. Note that the variogram range impacts the Search Ellipsoid, as such the number of data points included in the grid point calculation, see 'Search Factor' in Controlling the facies modeling method. Click the uncertainty symbol next to the 'Vertical range' entry field to open the Uncertainty Parameter dialog. For how to use the controls on the dialog, see The Uncertainty Parameter dialog. For information about parametric uncertainties and how to use them in JewelSuite Subsurface Modeling, see Incorporating uncertainty in static or dynamic modeling.
  • Same major and minor range - Select this option to use the same value for the major range and the minor range. This option will grey out the 'Minor range' text box (it is updated when you change the value for the major range).

After making changes to any of the settings of the variogram model, click Apply or OK to save it to your selected VOI. The variogram in the Facies Variogram View is updated immediately.