Translations:Ray equation/3/en

In this section, the position vector r always represents a point on a specific raypath and not any arbitrary point in space. As time increases, r traces out the particular raypath in question. The seismic ray at any given point follows the direction of the gradient of the traveltime field $t\mathbf {(} r)$. As before, let u be the unit vector along the ray. The ray in general will follow a curved path, and the raypath vector $n\mathbf {\ u}$ will be tangent to this curved raypath.