Electromagnetic sensing at Earth
Earth's salty oceans are electrically conductive fluid passing through Earth's variable main magnetic field-- so they induce electric currents and magnetic fields of their own. These oceanic magnetic fields can then be used to probe the inner structure of our planet. Oceans cover 70% of Earth's surface and both ground-based data and satellite data can be used to perform electromagnetic (EM) sensing of Earth's interior using marine EM fields. EM sensing depends on the electrical conductivity of materials; it provides information on the chemical composition and temperature of Earth's interior.
The largest challenging in using marine EM fields for EM sensing is properly constraining the marine EM fields. Magnetic fields from Earth's ionosphere, among other sources, must be removed or their error must be constrained.
Figure from Grayver+2016 showing a 1-D estimate of Earth's interior electrical conductivity, as determined from satellite-measured tidal magnetic signals. The 1-D profile is compared to bounds for a dry and water saturated mantle.
Publications
N. R. Schnepf, M. Nair, A. Maute, N. M. Pedatella, A. Kuvshinov, & A. D. Richmond (2018). An analysis of ionospheric versus oceanic tidal magnetic signals. Geophysical Research Letters, 45, 15, 7257-7267, doi.org/10.1029/2018GL078487.
A. Grayver, N. R. Schnepf, A. Kuvshinov, T. Sabaka & N. Olsen (2016). Results of 1-D inversions using satellite-detected ocean tidal magnetic signals. Science Advances, 2, e1600798, doi.org/10.1126/sciadv.1600798.
N. R. Schnepf, A. Kuvshinov, & T. Sabaka (2015). Can we probe the conductivity of the lithosphere and upper mantle using satellite tidal magnetic signals? Geophysical Research Letters, 42, 3233-3239, doi.org/10.1002/2015GL063540.
N. R. Schnepf, M. C. Nair, A. Kuvshinov, H. Toh, & S. Maus (2014). Tidal signals in ocean bottom magnetic measurements of the Northwestern Pacific: Observations versus predictions, Geophysical Journal International, 198, 1096-1110, doi.org/10.1093/gji/ggu190.