Illustration of incident P or S scattering at a point ξ on a seismic discontinuity to produce converted S and converted P waves, respectively. The imaging conditions of eqs (3) and (4) are designed to recover the locations and structural contrasts across hypothetical discontinuities embedded in a smooth background structure using the single scattering approximation. |$\hat{\mathbf {n}}$| and |$\hat{\mathbf {t}}$| are unit normal and tangent vectors to the discontinuity. As defined in Appendix A, |$\hat{\mathbf {n}}$| is chosen to point upward and |$\hat{\mathbf {t}}$| is chosen to point in the direction of incident (and converted) wave propagation.