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What is anisotropy?

definition

Seismic anisotropy is the speed of propagation of seismic waves in a medium with particles polarization direction related.

Physical mechanism

Most mineral crystals in nature are anisotropic, and this anisotropy is microscopic anisotropy. When a large number of anisotropic crystals are randomly arranged, they exhibit macroscopically isotropic characteristics.

There are two mechanisms for macroscopic anisotropy:

1. LPO (lattice-preferred orientation): The mineral crystal itself is anisotropic, forming macroscopic anisotropy due to the orientation of crystal particles.
2. SPO (shape-preferred orientation): Orientation of isotropic media with different wave velocities causes macroscopic anisotropy

The anisotropy and inhomogeneity of the medium are completely different concepts. Anisotropy describes the change of media properties at a certain point with direction. The non-uniformity describes the difference in properties of media between two points.

classification

Anisotropic classification:

1. An isotropic medium, the elastic parameter requires two parameter representations
2. VTI, Vertical Transverse Isotropy, the axis of anisotropic symmetry is vertical, and the different directions in the horizontal plane are isotropic. The elastic parameters need to be represented by 5 parameters. The common VTI is caused by media layering.
3. HTI, Horizontal Transverse Isotropy, the axis of anisotropic symmetry is horizontal, and the different directions in the vertical plane are isotropic. The elastic parameters need to be represented by 5 parameters. The common HTI is caused by vertical cracks.
4. TTI, Tilted Transverse Isotropy, tilt anisotropy, anisotropic symmetry axis is oblique
5. Fully anisotropic media, elastic parameters require 21 parameters

Anisotropy research method

Pn phase

S-wave splitting

Receive function method

Surface wave dispersion curve method

S wave splitting

Tangential energy minimum method

Silver & Chan, 1991, JGR

Splitting Intensity

Chevrot, 2000, JGR

Cross correlation method

references