Joint-inversion of magnetotelluric, gravity and seismic data to image sub-basalt sediments offshore the Faroe-Islands

We report on a 2D and 3D analysis of multiple types of geophysical data in order to map sub-basalt sediments in License 6 area offshore the Faroe Islands. Integrating multiple types of data is crucial for improving sub-basalt imaging, since seismic data alone can only poorly resolve the low velocity sediment layer under the fast velocity basalt layer due to scattering at inhomogeneities within the basalt. Our analysis is based on low structural resolution types of data acquired in the same target area: magnetotelluric, full tensor gravity and first arrival tomographic seismic data. As they carry complementary information content about the geological structure, this can be efficiently harvested using a joint-inversion algorithm, which inverts the data to a common Earth model by assuming a borehole derived relationship between the physical rock parameters of electrical resistivity, seismic velocity and density. This relatively strong coupling type results in an increase of resolution for the joint Earth Model in comparison with Earth models derived from single methods data. We present two final joint Earth models derived by two different joint inversion codes with different coupling schemes: A 2D model across the Brugdan borehole and a 3D larger regional type of model. The derived Earth models show varying thickness of the basalt layer and the presence of sub-basalt sediments in most part of the regions. The Earth models compare well with the actual physical parameters measured as a function of depth in the Brugdan borehole and seismic reflection data for the 2D model and regional studies for the 3D model and 3D seismic reflection data for the 2D model and information from regional studies for the 3D model.