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The frequency-dependent eikonal equation is,
| |
(10) |

where A and are the amplitude and phase functions, respectively.
Defining parameter and using , where
is angular frequency and *s* is slowness, one may use the
definition of *K* in equation (10) to write the following
as the ray path equation (see equation (4) )
| |
(11) |

To examine how equation (11) varies as a function of
frequency, a frequency derivative is applied to yield,
| |
(12) |