The magnetotelluric (MT) geophysical method utilizes the natural variations of the earth’s electromagnetic field. No artificial EM source is needed.
The solar wind interacts with the earth’s magnetosphere and results in electric current systems in the Ionosphere. These current systems act as the source for an electromagnetic signal propagating down into the earth. Electric currents are then induced within the earth, depending on the resistivity distribution in the subsurface.
The MT signal measured by the same seafloor receivers as used for CSEM can be used to reconstruct a subsurface resistivity model through a numerical inversion process. Such data can be used to map salt and volcanic structures and reconstruct the subsurface resistivity distribution in several tens of kilometers depth.