The Tornado view is a deterministic sensitivity analysis tool. A formula is first calculated with all of its input parameters at their baseline values, and then one parameter at a time is varied between its minimum and its maximum while the remaining parameters stay at their baseline. The length of each bar shows how much that single parameter changes the result, so the most influential parameters can be recognized at a glance.

To create a Tornado view go to the Views bar, navigate to Graphs and click on Tornado. The new view appears in the View manager of the active workspace and stays empty until a formula has been calculated.

To define the formula and its parameters, click Formula in the view functional panel. The Tornado Formula dialog opens:

The dialog is organized in the following groups:

• Formula - select a ready-made expression on the Predefined tab (Porosity, Water saturation, Net-to-gross, Pore Radius or Permeability), or type your own expression on the Custom tab, using the same syntax as the Well log calculator. The expression that will be calculated is shown in the read-only field below the tabs.
• Wells - the well chosen in the Primary list provides the depth limits and the markers used by the range tabs. Select All wells to include every well of the project, or clear the check box and select one or several wells in the list below.
• Show - choose whether the sensitivity is expressed in Absolute output values or in Percent of the baseline result.
• Parameters - one row is created for every parameter name that is used in the formula.
• The MD range, TVD range, TVDSS range and Markers range tabs at the bottom of the dialog optionally restrict the well log interval that is used for the calculation.

The Parameters table has the following columns:

• Name - the parameter name as it is written in the formula.
• Source - the data the values were taken from: Grid, Log, Map or Constant. When the same name exists in several data types, grid properties are used first, then well logs, then maps, and a message informs you when a grid property was used although a log curve of the same name exists. A name that is not found in the project is treated as a constant, and its values have to be entered manually.
• Baseline, Min and Max - the mean, the minimum and the maximum value found in the selected wells. Any of these cells can be edited, and a value that you type is kept for the following calculations.
• ΔY - the change of the result that is produced by this parameter, given in percent of the baseline result when Percent is selected.
• Visible - clear the check box to hide the parameter in the chart. The parameter stays in the table and in the calculation.

Click Calculate to fill in the table and refresh the chart while the dialog stays open. Save applies the calculated result to the view and closes the dialog, and Cancel closes the dialog and restores the last saved result. If the formula cannot be evaluated, or a parameter has no baseline or equal Min and Max values, a message describes the problem and the result is not applied.

In the chart each row is one visible parameter, sorted from the strongest to the weakest influence. The vertical grey line marks the baseline result. The blue bar covers the range in which the result falls below the baseline and the red bar the range in which it rises above it, and the labels next to the ends of the bars give the corresponding values.