Figure 1.

Spherical coordinate system, where the radial distance from a fixed origin, is the magnitude of v, θ is the inclination angle measured from a fixed zenith direction (n₃), and ϕ is the azimuth angle of its orthogonal projection on a reference plane that passes through the origin and is orthogonal to the zenith, measured from a fixed reference direction (n₁) on that plane (a). Phase (v) and group (V^gr) wave velocities of quasi-waves propagating in a dispersive anisotropic medium, exhibiting different magnitude and direction Ψ (b).