Magnetotellurics (MT) is a natural-source, electromagnetic geophysical method of imaging structures below the earth's surface. Natural variations in the earth's magnetic field induce electric currents (or telluric currents) under the earth's surface. Concurrent measurements of orthogonal components of the electric and magnetic fields allow for the calculation of the impedance tensor,[1] which is complex and frequency-dependent. Using this tensor, it is possible to gain insight into the resistivity structure of the surrounding material.

Functioning Edit

Electrical conductivity is an important physical parameter of the Earth's rocks and sediments. Rocks and sediments display a wide range of electrical conductivities, thus making it an attractive parameter to distinguish different rock types. Imaging of the Earth's subsurface conductivity is an important step in identifying rock types, and understanding tectonic processes and geologic structures.

In magnetotellurics, the Earth's naturally varying electric and magnetic fields are measured over a wide range of frequencies (1/10,000 to 10,000 Hz). These fields are due to electric currents (telluric currents) that flow in the Earth and the magnetic fields that induce these currents. The magnetic fields are produced mainly by the interaction between the solar wind and the magnetosphere. In addition, worldwide thunderstorm activity causes magnetic fields at frequencies above 1 Hz. These natural phenomena create strong MT source signals over the entire frequency spectrum.

The ratio of the electric field to magnetic field can give simple information about the subsurface conductivity. Because of the skin effect phenomenon that affects electromagnetic fields, the ratio at higher frequency ranges gives information on the shallow Earth, whereas deeper information is provided by the low-frequency range. The ratio is usually represented as MT-apparent resistivity and phase as a function of frequency. The technique was introduced by the French geophysicist Louis Cagniard [2] and Russian geophysicist Tikhonov in the early 1950s. With advances in instrumentation, processing and modeling, MT is now considered one of the most important tools in deep Earth research.

Applications Edit

Originally mainly used for academic research, the MT method was used successfully for the mapping of geothermal reservoirs starting in the early 1980s and became standard for this application. In recent years magnetotellurics has also become increasingly popular in oil and mineral exploration. Another application lies in environmental geophysics, where MT is used for groundwater exploration and monitoring. The method has also been applied to investigate the distribution of silicate melts in the Earth's mantle and crust; large investigations have focused on the East Pacific Rise and the Tibetan Plateau.

Geophysicists commonly use magnetotellurics for exploration of economic commodities. The frequency range of 1 Hz to approximately 20 kHz is part of the audio-magnetotelluric (AMT) range. These are parallel to the Earth surface and move towards the Earths centre. The large frequency band allows for a range of depth penetration from several meters to several kilometers below the Earth surface. However, the readings are unreliable. Due to the nature of magnetotulluric source, the waves generally fluctuate in amplitude height. Long recording times are needed to ascertain usable reading due to the fluctuations and the low signal strength. Generally, the signal is weak between 1 to 5 kHz, which is crucial in detecting the top 100m of geology. Magnetotelluric method is also used in marine environment for hydrocarbon exploration and lithospheric studies. [3] Due to the screening effect of the electrically conductive sea water, usable upper limit of the spectrum is around 1 Hz.

Controlled Source Audio Magnetotellurics (CSAMT) is also used as a more reliable geophysical exploration method. A extensive grounded wire (2 km or more) has currents at a range of frequencies (0.1 Hz to 100 kHz) passed through it. The electric field parallel to the source and the magnetic field which is at right angles are measured. The resistivity is calculated as well as conductivity. The higher the conductivity the more likely a metallic source (graphite, nickel ore or iron ore).

References Edit

  1. Cantwell, T. (1960) Detection and Analysis of Low-Frequency Magnetotelluric Signals, Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, Massachusetts
  2. Cagniard, L. (1953) Basic theory of the magneto-telluric method of geophysical prospecting, Geophysics, 18, 605-635
  3. Welcome to the Scripps Institution of Oceanography Marine EM Laboratory website

See also Edit

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