The simplest physically realizable anisotropic system is hexagonal
symmetry anisotropy, known as *transverse isotropy*
(TI). Hexagonal symmetry has an axis of cylindrical symmetry, which
implies that the properties of the medium in a given direction
depend only on the angle between that direction and the symmetry
axis direction. The symmetry direction is usually a result of some
dominant global directed influence, such as gravity, regional stresses
or parallel cracks. If gravity causes it, then the symmetry axis lies
vertically, and the medium has transverse isotropy with a vertical axis of
symmetry (VTI). If the direction of symmetry results from horizontal
regional stresses or vertically parallel cracks, the symmetry axis
lies horizontally, and the medium has transverse isotropy with a horizontal
axis of symmetry (HTI).

11/12/1997