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In a two-dimensional earth model, the ray parameter is given at any point
of the ray by the relationship

| |
(1) |

where is the inclination of the ray path with respect to the
vertical axis and *v* is the local velocity. We can see that the ray
parameter is simply the projection length of the ray path vector () on the earth surface. In a 3-D v(z)
model of the earth, the rays travel in a vertical plane. In this case,
the ray parameter, being the projection of the ray path vector,
is a two-dimensional vector that can be expressed in either cartesian
or polar coordinates, as follows:
| |
(2) |

where is the strike of the vertical plane containing the ray and
.

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Stanford Exploration Project

11/17/1997