A numerical model for the gravimetric recovery of sub-lithospheric mantle structures

It is a well-known fact that the long-wavelength terrestrial geoid undulations are mainly attributed to deep mantle density heterogeneities, while more detailed features in the geoid geometry are associated with the topography and the lithospheric density structure. To enhance a gravitational signature of mantle density heterogeneities below the lithosphere, the gravitational contributions of topography and lithospheric density heterogeneities should be modelled and subsequently removed from Earth's gravity field. The refined gravity field obtained after this numerical procedure is more suitable for a recovery of a mantle density structure (below the lithosphere). Following this idea, methods for a spherical harmonic analysis and synthesis of gravity field and lithospheric density structures are presented, and a theoretical relation between gravity field and mass density structure is formulated. Since a gravimetric recovery of inner density structure has a non-unique solution, we propose an alternative method based on a conversion of seismic velocities to mass densities. A forward modelling approach is then employed to find the mantle density configuration that generates the gravitational field that best approximates the corresponding refined gravitational field obtained from observed gravity field after subtracting the gravitational signal of the lithosphere.