The global conductivity distribution

We still do not have a global conductivity distribution that can be shown to be sufficiently close to a world average that it can be used as a standard. Continental magneto-telluric results show a wide divergence of conductivity distributions with depth, according to the local geology. The recent marine magneto-telluric investigations have also increased the variability of sub-oceanic conductivity. They tend to show that the overall average conductivity of the oceanic mantle differs little from its continental counterpart. Recent work based on diurnal and storm-time variations have generally confirmed earlier estimates. The conductivity at a depth of 1000 km appears to be close to 1 S/m. The secular variation jerk in 1969 has afforded data for a reassessment of lower mantle conductivity. The consensus of opinion puts the conductivity at the base of the mantle close to 100 S/m.