The heterogeneous electrical conductivity structure of the lower mantle

We developed a new three-dimensional electromagnetic time domain technique to invert 32 years of magnetic monthly mean values (1958-1990) from 120 geomagnetic observatories to image the mid-lower mantle. A preliminary global analysis of the data provided a source field model and an initial one-dimensional (1-D) conductivity profile for the inversion. The 1-D analysis showed that monthly mean magnetic data could resolve the lower mantle conductivity structure in the depth range ∼900-1400 km. We run a series of synthetic data inversion to test the approach. The inversion of the true data with different initial conditions and different fractions of the data set was found fairly stable. Regions of low and high conductivity values were identified. The most resistive structures are beneath the Australian region, near Japan, North and Central America while highly conductive structures are identified beneath Eastern Africa and Eurasia. Some of these significant variations in the lower mantle conductivity are well correlated with structures in seismic tomography model. The uncorrelated features suggest that temperature and compositional effects known as predominant on both conductivity and velocity could be associated with other processes affecting mainly the conductivity, probably in relation with minor phases.