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Oilfield Glossary
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gamma (γ)

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1. n. [Geophysics]

An S-wave parameter for a medium in which the elastic properties exhibit vertical transverse isotropy. Gamma (γ) is the S-wave anisotropy parameter and is equal to half the ratio of the difference between the horizontally and vertically traveling SH-wave velocities squared divided by the vertically traveling SH-wave velocity squared; an SH-wave is a shear wave that is horizontally polarized.

γ ≡ ½ [(C66C44) / C44] = ½ [(VSH2VSH2) / VSH2]

S-wave parameter (γ) for a medium in which the elastic properties exhibit vertical transverse isotropy, where C66 is the modulus for a horizontally polarized and horizontally traveling S-wave (perpendicular to the symmetry axis), C44 is the modulus for a horizontally polarized and vertically traveling S-wave (parallel to the symmetry axis), VSH is the velocity for a horizontally polarized and horizontally traveling S-wave and VSH is the velocity for a horizontally polarized and vertically traveling S-wave.

Reference: Thomsen L: “Weak Elastic Anisotropy,” Geophysics 51, no. 10 (October 1986): 1954–1966.