A measure of the anellipticity of the P-wave phase slowness—the inverse of P-wave phase velocity—in rock exhibiting vertical transverse isotropy.

η = (ε − δ) / (1 + 2δ)

Anellipticity of P-wave phase slowness for a medium in which the elastic properties exhibit vertical transverse isotropy. Eta (η) is the anellipticity and ε and δ are the P-wave anisotropy parameters. When ε and δ are equal, η = 0 and the P-wave phase slowness is an ellipse. When ε = δ = 0, the P-wave phase slowness is isotropic.

Reference: Alkhalifah T and Tsvankin I: “Velocity Analysis for Transversely Isotropic Media,” Geophysics 60, no. 5 (September–October 1995): 1550–1566.