The Magnetotelluric Facility is composed of seven permanent and 20 portable sensors that record electromagnetic fields. It is the electromagnetic equivalent of the seismic arrays. The portable sensors are moved in a rolling grid similar to the Transportable Array grid, but are only in place about a month before they are moved to the next location. A magnetotelluric station consists of a magnetometer, four electrodes, and a data recording unit that are buried in shallow holes. The electrodes are oriented north-south and east-west and are saturated in a salt solution to improve conductivity with the ground.