Subsalt Imaging Via Seaborne Electromagnetics
Authors: Hoversten G.M. , Unsworth Martyn 
Published: October 2008

ABSTRACT
In areas where seismic imaging of the base of salt structures is difficult seaborne electromagnetic techniques offer complementary as well as independent structural information. Numerical models of 2D and 3D salt structures demonstrate the capability of seaborne electromagnetic techniques to map the base of salt structures with an average depth accuracy of better than 20 percent. The seaborne electromagnetic systems base salt mapping capability are virtually unaffected by internal variation within the salt. Three dimensional structures who's horizontal aspect ratio is greater than 2 can be adequately interpreted via two dimensional inversions of seaborne EM data.

INTRODUCTION
Seismic imaging below intrusions of salt is often difficult to impossible due to the large velocity contrasts between the high velocity salt and the lower velocity sediments beneath. Seaborne electromagnetic's (SEM) offer the capability of imaging the base of salt structures with an average depth error of less than 20 percent. The proposed application of these systems is targeted on areas of the GOM where salt intrusions are laterally extensive. These intrusions diffier from the more familiar salt diapers in that they have intruded horizontally rather than vertically into the sedimentary sequence and can extend for tens and even hundreds of kilometers. The high contrast between the electrical resistivity of salt and the surrounding sediments allows SEM, through inverse modeling, to provide good estimates of the base of salt.

APPLICATION
While SEM can independently provide structural interpretations of lateral salt intrusions in the GOM in seismic no record areas, it is best used in conjunction with existing seismic data. The seismic data can define the structure down to the top of the salt, allowing these parameters to be fixed at known values in the SEM inversions, resulting in improved base salt structure from the SEM. In deep plays where velocity estimations for migration is inherently problematic the SEM resistivity structure, when used with velocity -resistivity relations derived from log information, can provide independent velocity estimates which can constrain the possible migration velocities.
Figure 1(Available in full paper) shows a depth converted seismic section for the GOM. In this example coherent reflections exist beneath the top of salt which can be interpreted, however uncertainty exists as to which reflector represents base of salt. As we will demonstrate, the accuracy of SEM interpretations would provide a definitive answer as to which seismic event corresponded to base of salt. Additionally the SEM data will provide an independent velocity model which could improve the seismic time-depth conversion.

THE SYSTEMS
Two possible SEM systems are considered in this paper. One, referred to as Magnetotelluric's (MT), relies on the natural electromagnetic radiation from the interaction of the earth's ionosphere and the solar winds. The oscillations of the ionosphere induced by the solar wind produces plane waves of EM energy which constantly radiate the earth. The earth's electromagnetic impedance, Z, can be estimated by measuring orthogonal electric (E) and magnetic (H) fields.