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Seismic anisotropy is a term used in seismology to describe the directional dependence of the velocity of seismic waves in a medium (rock) within the Earth. ==What is seismic anisotropy?== A material is said to be anisotropic if the value of a vector measurement of a rock property varies with direction. Anisotropy differs from the rock property called heterogeneity in that anisotropy is the variation in vectorial values with direction at a point while heterogeneity is the variation in scalar or vectorial values between two or more points. Seismic Anisotropy can be defined as the dependence of velocity on direction or upon angle.〔Thomsen, L. (2002). Understanding seismic anisotropy in exploration and exploitation. SEG-EAGE Distinguished Instructor Series 5. Society of Exploration Geophysicists.〕 There are two main types of anisotropy, both of them are called transverse isotropy (it is called transverse isotropy because there is isotropy in the horizontal or vertical plane) or polar anisotropy. The difference between them is in their axis of symmetry, which is an axis of rotational invariance such that if we rotate the formation about the axis, the material is still indistinguishable from what it was before. The symmetry axis is usually associated with regional stress or gravity. * TIV- transverse isotropy with a vertical axis of symmetry, this is also called VTI (vertical transverse isotropy). This kind of anisotropy is associated with layering and shale and is found where gravity is the dominant factor. * TIH- transverse isotropy with a horizontal axis of symmetry, this is also called HTI (horizontal transverse isotropy). This kind of anisotropy is associated with cracks and fractures and is found where regional stress is the dominant factor. The transverse anisotropic matrix has the same form as the isotropic matrix, except that it has five non-zero values distributed among 12 non-zero elements. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「seismic anisotropy」の詳細全文を読む スポンサード リンク
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