We present a two-step inversion procedure to extract subsurface velocity estimates from large-offset seismic data. The first step is the automatic iterative large-offset inverse method (AILOIM) which we test in the presence of strong water-bottom multiples. Since this method is based on finding the locus of critically reflected points, low-velocity zones cannot be resolved and the reconstruction is only an average estimate. In the presence of low-velocity zones, the second step of the inversion process is required. This second step is a generalized least-squares inverse scheme applied to the precritical reflections. We computed the inverse solution using a perturbation technique and determined the reference model from an estimate of the average velocity given by the first step.Two major features of this inversion method are: amplitudes of reflection arrivals are incorporated into analysis, and accurate results can be achieved without human interaction (picking the traveltime curves). Numerical examples using synthetic and field data demonstrate the accuracy of our inversion procedure.

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