Magnetotellurics

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 , which is complex and frequency-dependent. Using this tensor, it is possible to gain insight into the resistivity structure of the surrounding material.

Functioning

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 ionosphere. 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 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

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 applications lie 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 1Hz to approximately 20 kHz are 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 5kHz which is crucial in detecting the top 100m of geology.

Magnetotelluric method is also being applied to marine environment for hydrocarbon exploration and lithospheric studies . Due the the screening effect of the electrically conductive sea water, the upper limit of the spectrum is around 1 Hz. The sensor assembly is added a buoyancy chamber and a concrete anchor. This gets dropped overboard off a ship, sinks to the sea floor, and remains in the sediment for a couple of days. A remote signal is then used to detach the anchor from the frame and the flotation chamber brings it to the surface.

Controlled Source Audio Magnetotellurics (CSAMT) is also used as a more reliable geophysical exploration method. A extensive grounded wire (2Km or more) has currents at a range of frequencies (0.1Hz to 100kHz) 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, nickle ore or iron ore).

See also

• Other types of imaging
• Electrical resistivity tomography, another geophysical technique of imaging
• Exploration geophysics, a branch of geophysics

External links

• MTNet site hosted by the Dublin Institute for Advanced Studies.
• Simpson, F. and Bahr, K. 2005. Practical magnetotellurics [1]. Cambridge University Press, Cambridge.
• Magnetotellurics at the university of Washington.
• MELT Experiment at mid-cean ridge.