Translations:The eikonal equation - book/6/en

Because u is a unit vector in the same direction as that of the gradient, it follows that $n{\rm {=}}{\rm {|}}\mathrm {grad} {\textit {t}}{\rm {|}}$. In other words, the slowness is equal to the magnitude of the gradient of traveltime. If we take the square of each side, we obtain the eikonal equation (in scalar form)