Imaging first order multiples in VSP data: a detailed field study
Summary A detailed field study focused on constructively using the water surface multiples recorded in marine seismic VSP data for improved subsurface depth imaging is presented. Traditionally, up-going and down-going wavefields are separated during VSP signal processing and only the up-going wavefield is used to image the subsurface below the receivers. However with using the down-going wavefield, structures above the receivers are also imaged and wider horizontal coverage is achieved. Our field study consists of two perpendicular 2D walk away surveys: SW-NE and NW-SE. Images constructed using first order water surface multiples are presented and compared with each other as well as with the 3D surface seismic cube. VSP images have higher resolution than the 3D surface seismic image. Both VSP surveys image the shallower part of the subsurface very well. However, in the deeper parts of the sections, qualities of the VSP images are substantially different from each other. The SWNE line images the deeper section of the subsurface rather well whereas the NW-SE line fails to image the deeper section of the subsurface. 3D ray tracing modeling shows that there are considerable amounts of unfocused and out-plane energies reflected from the deeper reflectors in NW-SE survey. This can explain why line NW-SE fails to image the deeper reflectors.