Geometrical notation used in the specification of the paraxial Fréchet kernel, eq. (31). Every single scatterer x is projected onto the nearest point on the straight-line source–receiver ray; the position of x relative to the projection point is described by the two orthogonal ray-centred coordinates q₁, q₂. The quantities L′ and L″=L−L′ are the distances from the projected point to the source s and receiver r, respectively.