For exclusively long-period MT (frequencies below approximately 0.1 Hz), the three discrete broadband magnetic field sensors may be substituted by a single compact triaxial fluxgate magnetometer. In many situations, only the telluric sensors will be used, and magnetic data borrowed from other nearby soundings to reduce acquisition costs. A complete five-component set of MT equipment can be backpack-carried by a small field team (2 to 4 persons) or carried by a light helicopter, allowing deployment in remote and rugged areas. Most MT equipment is capable of reliable operation over a wide range of environmental conditions, with ratings of typically −25 °C to +55 °C, from dry desert to high-humidity (condensing) and temporary full immersion. Data processing and interpretation Post-acquisition processing is required to transform raw time-series data into frequency-based inversions. The resulting output of the processing program is used as the input for subsequent interpretation. Processing may include the use of remote reference data or local data only. Processed MT data is modelled using various techniques to create a subsurface resistivity map, with lower frequencies generally corresponding to greater depth below ground. Anomalies such as faults, hydrocarbons, and conductive mineralization appear as areas of higher or lower resistivity from surrounding structures. Various software packages are used for interpretation (inversion) of magnetotelluric data, where apparent resistivity is used to create a model of the subsurface. Instrument and sensor manufacturers Three companies supply most of the commercial-use world market: one in the United States (Zonge International, Inc.), one in Canada; (Phoenix Geophysics, Ltd.), and one in Germany (Metronix Messgeraete und Elektronik GmbH).) Government agencies and smaller companies producing MT instrumentation for internal use include Vega Geophysics, Ltd. in Russia and the Russian Academy of Sciences (SPbF IZMIRAN); and the National Space Research Institute of Ukraine. See also Electrical resistivity tomography, another geophysical technique of imaging Exploration geophysics, a branch of geophysics for discovering and mapping mineral resources Geophysics Geophysical Imaging Geothermal exploration Other types of imaging Reflection seismology Seismo-electromagnetics Transient electromagnetics References External links MTNet site hosted by Complete MT Solutions Inc. OpenEM Virtual Institute for Electromagnetic Geophysics. National Geoelectromagnetic Facility. Chave, A.D. and Jones, A.G. 2012. The Magnetotelluric Method:Theory and Practice. Cambridge University Press, Cambridge, U.K. Simpson, F. and Bahr, K. 2005. Practical magnetotellurics. Cambridge University Press, Cambridge. Southern African Magnetotelluric Experiment (SAMTEX). Magnetotellurics at the University of Washington. MELT Experiment at mid-ocean ridge. Canadian Exploration Geophysical Society University of Toronto