Advances in Subsalt Seismic-electromagnetic Imaging: Applications to the Red Sea Exploration

Seismic exploration in the Red Sea is complicated by the presence of salt that strongly affects the propagation of seismic energy, resulting in poor imaging of the subsalt section unless accurate depth-domain velocity estimates are used to obtain reliable seismic images. This is typically difficult to obtain from seismic velocity analysis alone because of the low intra-salt reflectivity and the overall low signal-to-noise ratio. In these conditions, the velocity model building challenge can be addressed by the combination of complementary geophysical technologies to obtain enhanced velocity-depth models. Interpretation-driven or advanced multi-parameter joint inversion schemes can make use of all the available geophysical information such as seismic, electromagnetics (EM) and gravity to achieve analytical integration for common earth models. Recent applications of gravity gradiometry, magnetotellurics and controlled-source EM in the Red Sea have shown the potential for imaging the complex geologic section to aid velocity model building. The internal variations in the evaporitic overburden, the subsalt structures and basement are independently imaged by combined EM methods and confirmed by new drilling results. Controlled-source EM, in particular, provides unprecedented details of the salt overburden internal structures while magnetotellurics provide excellent reconstruction of the base of salt and the basement. Geologic/geophysical integration by means of interpretative software platform was used to provide geologically-consistent models and to enhance seismic depth imaging. The obtained multi-parameter, common-structure models provide an improved image of the complex subsurface in the Red Sea. The reciprocal consistency of the obtained results show promises for extending the work to a more analytical integration with seismic that would involve simultaneous joint inversion of the different datasets for enhanced velocity discrimination.