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Successful application of joint reflection/refraction tomographic inversion in a shallow water marine environment

We show how refraction tomography (also called first arrival travel time tomography) helps to produce more accurate and detailed depth velocity models below a shallow seafloor. We do not use refractions by themselves to build a complete shallow velocity model. In our proposed workflow, refraction tomography complements standard reflection tomography and the priority remains with the reflections to guarantee stability of the solution and to avoid uncertainties associated with refracted or diving waves in complex media. We use wave equation modelling to calculate synthetic gathers and estimate the travel time mismatch between real and synthetic first arrivals. It leads to a robust workflow which can be easily introduced into production depth-velocity processing. We show how this joint reflection/refraction velocity inversion works using a real 1000sq.km 3D marine seismic dataset acquired in an area where the water depth varies from 20 m to 1100 m.