Electromagnetic exploration methods are commonly used for mineral exploration. They can also be used for research. Low frequency magnetic field measurements can, in parallel with electric field measurements, be used to construct a resistivity structure of the Earth’s crust and upper mantle. For this application, low-noise and very low-noise three-axis sensors such as the Mag-13 are commonly used to infer structural changes through the Earth’s crust, and in some cases are used to study earthquakes.