The planes become curved when velocity varies with depth. Interfaces within the earth are just planes of constant z '. Mathematically, one equation in three unknowns defines a plane. To get some practice, we will look at the intersection of two planes. When coordinate planes are nonorthogonal, the coordinate system is said to be affine.
Successive planes are successive shot points.
With affine coordinates I find it easiest to forget about the coordinate axis, and think instead about the perpendicular plane. The shot axis can be thought of as a plane of constant geophone, say, cg .