In conventional processing of ocean-bottom cable (OBC) seismic, after separation of upgoing and downgoing wavefields, the upgoing data are processed while downgoing data are discarded. However, the illumination of the shallow section is much better for downgoing data. During depth imaging of the Clair 3D OBC survey there was evidence of a shallow high-velocity layer that was invisible on conventionally processed upgoing data. Mirror imaging of downgoing data was applied, producing an improved image of the shallow sediments. The mirror-imaged downgoing data were input along with conventionally-imaged upgoing data to ray-traced common-image-point (CIP) tomography. Imaging of offset-vector tiles (OVTs) was used, to incorporate offset and azimuth information as accurately as possible. The inclusion of mirror-imaged downgoing data with upgoing data in CIP tomography has greatly improved the reliability and resolution of the velocity model. Depth-imaged results are also improved, for deep as well as shallow events, showing simpler structure and better event continuity.


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