This crossplot is used to graphically determine a cementaion exponent (m) value for a set of core data representing a particular rock type.
Formation factor (F) is measured in the lab and is equal to the resistivity ratio of the 100% brine saturated rock (Ro) to the resistivity of the brine with which it is saturated (Rw). Formation factor (F=Ro/Rw) can also be determined with the equation F=a/PHI^m, where the slope equals cementation exponent (m) and the intercept at PHI=1.00 is equal to the tortuosity factor (a).
Start by inputting porosity-F data series, (green, blue and orange cells). Each data series allows an unlimited number of rows, and cut and paste functionality is available. Next, adjust the cementation exponent value until the line fits the desired data series. The tortuosity factor input is often pinned at 1.00 for core work.
Reference
Archie, G. E., “The Electrical Resistivity Log As An Aid In Determining Some Reservoir Characteristics.” Transactions of the AIME Volume 146, Issue 1, December 1942: 54-62.