(an, ī so’ tr∂p ē or a nī’ sō tr∂p ē) Anisotropy is the variation of a physical property depending on the direction in which it is measured; it is closely related to heterogeneity. Anisotropy is the directional variation at a single point in space, whereas heterogeneity is the variation from point to point. Both anisotropy and heterogeneity are matters of scale, and so their usage relates to the wavelengths involved. All anisotropy is the expression of small-scale heterogeneity with preferred orientation(s), observed at much larger scale. The synonym aeolotropy is also used to describe the same idea.
Crystals exhibit so-called intrinsic anisotropy although it derives from the atomic-scale heterogeneity, as organized into unit cells. Similarly, unfractured shales may be called intrinsically anisotropic, although this derives from preferred orientation of grains, e.g. clays. And fractured formations may also be called intrinsically anisotropic, although this derives from preferred orientation(s) of small-scale fractures. By contrast, sequences of thin layers may be called extrinsically anisotropic, when sampled with wavelengths much larger than the bed thicknesses.
The small-scale heterogeneity may lead to various symmetry properties of the medium, depending on its preferred orientation(s). These symmetry properties may be classified into a few symmetry systems. In geophysics, the most important of these are:
- Isotropy (e.g. an unfractured sandstone)
- Polar anisotropy (aka VTI) (e.g. an unfractured shale, or a thin-bed sequence with stationary statistics)
- Orthorhombic (aka Orthotropic) (e.g. a shale with a set of vertical parallel fractures, or with two orthogonal sets of vertical parallel fractures)
- Monoclinic (e.g. a shale with two or more non-orthogonal sets of vertical parallel fractures)
- Thomsen, L., 2014. Seismic Anisotropy in Exploration and Exploitation, the SEG/EAGE Distinguished Instructor Short Course #5 Lecture Notes, 2nd Edition, Soc. Expl. Geoph., Tulsa