Physics
The natural source fields travel as plane waves and the physics behind these are explained in high detail on the page for fundamentals of natural sources. At the surface of the Earth, the fields are transmitted vertically into the subsurface, creating vertically propagating plane waves that generate sheet currents, as shown below. This fundamentals page provides more details and reflection and transmission for MT.
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Todo
step-by-step from plane source to apparent resistivity (radio button widghet): (1) plane waves, (2) skin depth), (3) phase difference between E and H (tie back to 1D stuff Thibaut already did)
The fields propogate at different frequencies and each frequency provides different information about the subsurface. Low frequencies penetrate deeper while high frequencies provide information near the surface. The plane wave attenuates with depth and a skin depth is the distance at which the amplitude has decreased by \(1/e\). This is explained in detail on the fundamentals page. The movie below shows the magnetic field at different frequencies and how it attenuates in the subsurface.
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