Magnetotellurics
Since first being created in the 1950s, magnetotelluric sensors, receivers and data processing techniques have followed the general trends in electronics, becoming less expensive and more capable with each generation.
Major advances in MT instrumentation and technique include the shift from analog to digital hardware, the advent of remote referencing, GPS time-based synchronization, and 3D data acquisition and processing.
While high-velocity layers are an acoustic barrier and make seismic ineffective, their electrical resistivity means the magnetic signal passes through almost unimpeded.
[9] 3-D MT survey results in Uzbekistan (32 x 32 grid of soundings) have guided further seismic mapping of a large known gas-bearing formation with complex subsurface geology.
Since then, both major and junior mining companies are increasingly using MT and audio-magnetotellurics (AMT) for both brownfields (near known deposits) and greenfields (uncharted land) exploration.
[20][21][22] Geothermal exploration with MT has also been done extensively in the United States, Iceland,[23] New Zealand, Hungary,[16] China,[24] Ethiopia, Indonesia, Peru,[25] Australia, and India.
[43] The raw geophysical time-series data from these monitoring stations is freely available to the scientific community, enabling further study of the interaction between electromagnetic events and earthquake activity.
[45] POLARIS is a Canadian research program investigating the structure and dynamics of the Earth's lithosphere and the prediction of earthquake ground motion.
Very deep, very long-period measurements (mid-crust through upper mantle depths), may require recordings of several days to weeks or more to obtain satisfactory data quality.
Long recording times are needed to ascertain usable reading due to the fluctuations and the low signal strength.
[49] Due to the screening effect of the electrically conductive sea water, a usable upper limit of the spectrum is around 1 Hz.
An offshore variant of MT, the marine magnetotelluric (MMT) method,[54][page needed] uses instruments and sensors in pressure housings deployed by ship into shallow coastal areas where water is less than 300 m deep.
This greatly improves data quality, and may allow acquisition in areas where the natural MT signal is difficult to detect because of man-made EM interference.
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.
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.
Processed MT data is modelled using various techniques to create a subsurface resistivity map, with lower frequencies generally corresponding to greater depth below ground.
Four companies supply most of the commercial-use world market: one in the United States (Zonge International, Inc.[60]), one in Canada; (Phoenix Geophysics, Ltd.[61]); one in Germany (Metronix Messgeraete und Elektronik GmbH).
[63] Government agencies and smaller companies producing MT instrumentation for internal use include the Russian Academy of Sciences (SPbF IZMIRAN); and the National Space Research Institute of Ukraine.
