1. n. [Formation Evaluation]
The value a in the relation of formation factor (F) to porosity (phi): F = a / phim. The value a is derived empirically from best fits of measured values of F and phi on a group of rock samples. It has no clear physical significance, although it has been related to grain shape and tortuosity. In the saturation equation, it always occurs associated with the water resistivity as (a * Rw). It is sometimes claimed that a must be 1 since at phi = 1, F must be 1. However, a material with phi = 1 is not a rock: a is essentially an empirical factor for rocks and as such can take any value. A wide range of values has been found, from 0.5 to 5.