S U M M A R YConstructing an image of the Earth subsurface from acoustic wave reflections has previously been described as a recursive downward redatuming of sources and receivers. Most of the methods that have been presented involve reflectivity and propagators associated with oneway wavefield components. In this paper, we consider the reflectivity relation between twoway wavefield components, each a solution of a Helmholtz equation. To construct forward and inverse propagators, and a reflection operator, the invariant-embedding technique is followed, using Dirichlet-to-Neumann maps. Employing bilinear and sesquilinear forms, the forwardand inverse-scattering problems, respectively, are treated analogously. Through these mathematical constructs, the relationship between a causality radiation condition and symmetry, with respect to a bilinear form, is associated with the requirement of an anticausality radiation condition with respect to a sesquilinear form. Using reciprocity, sources and receivers are redatumed recursively to the reflector, employing left-and right-operating adjoint propagators. The exposition of the proposed method is formal, that is numerical applications are not derived.The key to applications lies in the explicit representation, characterization and approximation of the relevant operators (symbols) and fundamental solutions (path integrals). Existing constructive work which could be applied to the proposed method are referred to in the text.In Berkhout (1985) acoustic wave propagation and reflection in discretized space is represented by matrix operations. According to this model the reflected wavefield from an acoustic contrast is obtained by multiplying a multisource wavefields matrix by a product of three matrices. Evaluating this product from the right to the left, modelling a surface seismic experiment, the first matrix propagates the source wavefields downward to the reflector, the second matrix reflects these and the third matrix propagates the reflected wavefields upward to the measurement surface. Because the wavefields are considered in the temporal frequency domain, forward wavefields propagation is recursive with respect to depth. This means that propagation can be handled incrementlly through the medium by ordered matrix multiplications. Inversion for the reflection matrix is also represented by a three-matrix product. The measured wavefield matrix, in which each column represents a different common source gather, is then multiplied from the right and the left with an approximate inverse-propagation matrix, which is the Hermitian of the forward-propagation matrix.In Wapenaar (1996a) Berkhout's model is generalized to R 3 for one-way wavefields employing operators for matrices. Wavefield decomposition into one-way wavefields (Weston 1988;de Hoop 1996;Wapenaar & Grimbergen 1996) follows from a factorization of the two-way Helmholtz wave equation. Propagation in the marching direction, determined by the square-root Helmholtz operator, is then separated from scattering from medium v...