EarthScope is installing a small number of permanent and portable magnetotelluric (MT) sensors across the United States in conjunction with USArray seismic instrumentation. These MT sensors record naturally occurring electric and magnetic fields, which scientists will combine with other geophysical data to identify Earth's thermal structure and to study the significance of fluids within Earth's lithosphere (upper ~70 km).
An MT station is passive with no associated noise or motion, and consists of a magnetometer, four electrodes, and a data-recording unit that are buried in shallow holes. The four electrodes are placed along two lines in a cross or "L" shape, oriented along north-south and east-west directions, and connected by a thin cable to the magnetometer and data recording unit which are usually at the intersection of the two lines. The electrodes are saturated with a common salt solution to improve conductivity with the ground.