Salt detection and interactive interpretation by seismic-gravity simultaneous joint inversion

We have applied simultaneous joint inversion to two 2D seismic lines and gravity data in the South Gabon sub-basin, West Africa. There are poor velocity contrasts between a shallow carbonate layer and juxtaposed salt structures, and the lateral change in density impacts acoustic propagation to deeper layers. Consequently, conventional seismic imaging yields poor continuity below the salt. Accurate interpretation of the salt bodies is a key task in obtaining the velocity model for pre-stack depth migration, but is complicated due to the adverse reflectivity setting and the almost uniform velocity conditions. In the study area, the geological setting poses particular challenges for identifying the salt body, whether by seismic reflections or velocity. Gravity offers an additional source of information for the position of salt domes, and reference velocity and density inside a salt body are available through well logs. With this additional information, simultaneous joint inversion can be coupled interactively with interpretation until the two techniques converge to a common solution. Our simultaneous joint inversion workflow, operating entirely in depth, allows the recognition of salt shapes through their anomalous petrophysical properties, and the complete removal of the image distortion in pre-salt horizons.