Translations:Dictionary:Anisotropy (seismic)/22/en

• Polar anisotropy (transverse isotropy) is the simplest geophysical case of anisotropy. It has elastic properties that are independent of the azimuth about a polar axis (hence the name) of symmetry, which is usually vertical. It is associated with, for example, unfractured shales or thinly-bedded sequences (see Physical Causes of Anisotropy, below). An example is given in Figure A-14. Polar anisotropy has five independent constants among 12 nonzero elements of the stiffness (or compliance) matrix. However, the anisotropic seismic behaviour is governed directly by certain combinations of these five stiffnesses; for body waves they may be taken as the Thomsen anisotropic parameters.
• Azimuthal anisotropy is a general term describing all lower symmetries, which all have azimuthal variation of elastic properties. In exploration geophysics, it is usually caused by aligned fractures (see Physical Causes of Anisotropy).
• The simplest plausible case of azimuthal anisotropy, in exploration geophysics, is that of Orthorhombic anisotropy (more properly: "orthotropic"). It has the symmetry of a brick, with nine independent constants among 12 nonzero elements of the stiffness (or compliance) matrix. In exploration geophysics, it is usually caused by a single set of aligned fractures in an otherwise polar anisotropic medium, or perhaps two such sets, orthogonal to each other (see Physical Causes of Anisotropy). Despite the complexity, it is commonly feasible to analyze modern wide-azimuth (WAZ) datasets in terms of orthorhombic anisotropy .