Variation of an electrical property depending on the direction in which it is measured. The resistivity anisotropy coefficient is the square root of the ratio of the resistivity measured perpendicular to the bedding to that parallel to the bedding; also called pseudo-anisotropy. It usually has a value between 1 and 2. For a sequence of isotropic layers with thicknesses zi and resistivities the unit resistance RT is
and the pseudo-anisotropy is
See dar Zarrouk. The anisotropy of induced polarization in rocks is less than the anisotropy of resistivity. In layered rocks the resistivity parallel to the layering is less than that perpendicular to the layering. Anisotropy as measured in a borehole is caused by cyclic thin sequences of alternating sand and shale, sorting of sand grains, and fractures (healed or fluid-filled).