1. n. [Formation Evaluation]

A difference in vertical and horizontal resistivity within a formation and at the scale of the resistivity measurement. Although there are several possible types of anisotropy, the term usually is used when the electrical properties are the same in all horizontal directions but different in the vertical direction. For horizontal beds, this type of anisotropy is more fully known as transverse isotropy with a vertical axis of symmetry (TIV). The term may also refer to a difference in resistivities measured parallel and perpendicular to bedding.

See: bed,  horizontal resistivity,  parallel resistivity,  perpendicular resistivity,  vertical resistivity