Main: Compute>From Logs>Cross Over Flag Curve

A Crossover Flag Curve is a binary curve that shows where one log crosses over another.  One of the most useful applications of this function is to calculate a “gas effect” curve from neutron and density porosity curves.  This curve is useful for petrophysical calculations like log footages.

Data tab

The Data tab is used to select log curves and their scales.  Petra scales the curves to these values and calculates where one curve crosses another.  In other words, cross over is calculated as if the curves were actually plotted.  In the example below, cross over is established when neutron porosity (NPHI) is to the right of density porosity (DPHI) even though NPHI is on a 0-30 scale and DPHI is on a 0-0.3 scale.

Store Output Flag Curve As - The Output Curve sets the name and specific values of a pay curve.  By default, Petra will pick the first alphabetized curve name in the database.  Click the drop down box and assign the output curve a new curve or type in a name.  In this example, the output curve will be named “Crossover.”

Use Log Curve Aliases - This option causes log aliases to be used.  When a specified curve is missing, an alias log will be used.  This is very useful for large projects.  Log curve names are aliased under the Log tab in the main module.

Options tab

The Output Curve Values section sets the specific values of a cross over curve.  

When Cross Over Condition - This entry sets the curves value when the criteria on the Input Curves tab is satisfied. By default, pay is 1.

When Not Cross Over Condition - This entry sets the curves value when the criteria on the Input Curves tab is not  satisfied.  By default, non-pay is zero

Make NULL - This entry box will assign null values to non-pay.

Output Curve Update Mode – When the cross over curve named above already exists, you have the option of updating the curve or replacing the curve.  Updating the curve will merge the new pay curve into the existing one, leaving the pay curve outside of the depth limits untouched.  Replacing the curve, on the other hand, will null the rest of the pay curve outside of the depth limits.