First-order effects, the lens term

Now let us be specific about what is meant by the lens term in the present context of before-stack migration in the presence of lateral velocity variation. Specializing the DSR equation to 15$^\circ$ angles gives  

$${ \partial U \over \partial z } \eq - \ \Biggl\lbrace \ \lef... ...ial^2 \ \over \partial g^2} \ \right] \ \Biggr\rbrace \ U \quad$$ (17)

Rearranging the terms to group by behavior gives

$$\begin{eqnarray} { \partial U \over \partial z } \ \ &=&\ \ - \ \left[ { i \omeg... ...uad \quad \quad + \quad \quad \quad \quad \hbox{diffraction term}\end{eqnarray}$$ (18) (19)

So you see the familiar type of lens term, but it has two parts, one for shifting at the shot, and one for shifting at the geophone.