Subsalt Full-tensor Electromagnetic Inversion in the Red Sea

D. Colombo¹, G. McNeice¹, and D. Rovetta¹

¹Saudi Aramco

Abstract

The Red Sea basin is characterized by thick salt bodies above Tertiary formations deposited over deep basement structures. The Red Sea “salt”, typically referred to as the layered evaporite sequence, starts at shallow depths below the water bottom and its velocity can be fairly complex and laterally-varying. Full tensor gravity gradiometry, magnetotelluric and controlled-source electromagnetic were acquired in three different blocks in the Red Sea with coincident wide azimuth seismic surveys. The goal is the quantitative integration of the different geophysical measurements into a common-structure multi-parameter model that validates current interpretations and enhances the seismic image of the subsurface. The combined broadband magnetotelluric and controlled-source electromagnetic acquisition with long offsets, in both inline and broadside directions, was specifically designed to be sensitive to the subsalt sedimentary section. The overlap in frequency content and the commonality of propagation mode at far offsets provide redundant sensitivity to the pre-salt section. The 3D inversion of the electromagnetic datasets with minimal a-priori information recovers the general geologic framework of the Red Sea. The results show high sensitivity to the salt overburden structures, including the base of salt, and to the conductive subsalt sediments. Moreover, magnetotelluric inversion also recovers the basement structures, characterized by tilted fault blocks typical of a rift basin. Good consistency is found between the depth domain seismic image and the estimated resistivity structure, which is also verified by recent drilling results. The deep basement structures imaged by magnetotelluric inversion are also confirmed by independent gravity inversion results. The completeness and redundancy of the acquired datasets play an important role in exploration de-risking and imaging of the pre-salt section in the Red Sea by validating interpretative hypothesis and by providing independent information for velocity model building.