Magnetotelluric (MT) methods are a passive geophysical method that infer the Earth’s subsurface resistivity though measuring naturally occurring geomagnetic and geoelectrical fields at the Earth’s surface. These naturally occurring fields occur over a wide band of frequencies. Low frequencies correspond to locations deep in the crust, while high frequencies correspond to shallow depths. MT surveys focus on depth ranges from 100-10,000m and therefore have a much greater depth of penetration than other electrical or electromagnetic methods. MT measurements allow geologists to determine resistivity as a function of depth. The electrical resistivity of rocks and soils is a function of mineralogy, porosity, permeability, temperature and fluid composition.