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

Abstract

Standard depth-velocity modelling can be challenging for the upper part of the section in shallow water marine environments (water depth up to few hundred meters). Problems are caused by the limited number of offsets which are available for analysis and the wide-spread presence of strong multiples. Seismic acquisition that is designed for an optimal illumination of much deeper target intervals does not allow accurate velocity estimations in the shallow part of the section. However all deeper target reflections travel through the upper part of the model, so having an accurate velocity model right from the seafloor is important. 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 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 20m to 1100m.