Assuming a zero-offset configuration of sources/receivers, we present an approach to forwardmodelling based on a one-way wavefront construction process progressing upward from aselected target reflector. Subsequent simulation of migration amplitudes along this targetreflector yields reliable estimates of amplitudes from a zero-offset depth migration and aquick (but less accurate) indication of small-offset depth migration amplitudes. The one-waywavefront construction process is based on theory permitting quantities related to the two-waywave propagation (accumulated reflection/transmission coefficients, geometric spreading, andphase shift due to caustics) to be continued step by step in the upward direction of normal-incidencerays. The upward continuation yields in addition the isochron curvature matrix,which is essential for the migration amplitude simulation. Numerical tests demonstrate thatone-way wavefront construction is far more efficient than conventional two-point raytracing,especially when the number of sources/receivers is large. This makes it feasible to compareseveral scenarios with respect to model parameters in reasonable time.
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