(ī kōn’ ∂l) (from Greek (ikon) meaning image. An equation derived from the wave equation through the substitution of a harmonic wave trial solution, in which the local velocity is compared to a reference velocity (analogous to comparing a velocity to the speed of light in vacuum):
where is an index of refraction and is the wave function. the quantity is identified as wave propagation travel time. Valid only where the variation of properties is small within a wavelength, sometimes called the ‘‘high-frequency condition.’’
More commonly in geophysical literature, the eikonal equation (for scalar waves) is written in terms of medium velocity only where , as
Solutions to the eikonal equation yield a high-frequency or large-wavenumber asymptotic representation of the wave field as a family of rays, represented by ray position and ray direction---the so-called kinematic aspect of wave propagation.
Another form of the eikonal equation is written in terms of the ray direction vector where for
thus are the generalized coordinates and are the generalized momenta from Hamiltonian mechanics, and the eikonal equation corresponds to the Hamiltonian function or the Hamilton-Jacobi equation of analytical mechanics.