Magnetotelluric
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Magnetotellurics (MT) is an
electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...
geophysical Geophysics () is a subject of natural science concerned with the physical processes and properties of Earth and its surrounding space environment, and the use of quantitative methods for their analysis. Geophysicists conduct investigations acros ...
method for inferring the
earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
's subsurface electrical conductivity from measurements of natural geomagnetic and geoelectric field variation at the Earth's surface. Investigation depth ranges from 100 m below ground by recording higher frequencies down to 200 km or deeper with long-period soundings. Proposed in Japan in the 1940s, and France and the USSR during the early 1950s, MT is now an international academic discipline and is used in exploration surveys around the world. Commercial uses include
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...
(oil and gas) exploration,
geothermal Geothermal is related to energy and may refer to: * Geothermal energy, useful energy generated and stored in the Earth * Geothermal activity, the range of natural phenomena at or near the surface, associated with release of the Earth's internal he ...
exploration,
carbon sequestration Carbon sequestration is the process of storing carbon in a carbon pool. It plays a crucial role in Climate change mitigation, limiting climate change by reducing the amount of Carbon dioxide in Earth's atmosphere, carbon dioxide in the atmosphe ...
, mining exploration, as well as
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...
and
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
monitoring. Research applications include experimentation to further develop the MT technique, long-period deep crustal exploration, deep mantle probing, sub-glacial water flow mapping, and earthquake precursor research.


History

The magnetotelluric technique was introduced independently by Japanese scientists in 1948 (Hirayama, Rikitake),
Soviet The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
geophysicist
Andrey Nikolayevich Tikhonov Andrey Nikolayevich Tikhonov (; 17 October 1906 – 7 October 1993) was a leading Soviet Russian mathematician and geophysicist known for important contributions to topology, functional analysis, mathematical physics, and ill-posed problems ...
in 1950 and the French geophysicist Louis Cagniard in 1953. With advances in instrumentation, processing and modelling, magnetotellurics has become one of the most important tools in deep Earth research. 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.


Commercial applications


Hydrocarbon exploration

For
hydrocarbon exploration Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for hydrocarbon deposits, particularly petroleum and natural gas, in the Earth's crust using petroleum geology. Exploration methods ...
, MT is mainly used as a complement to the primary technique of
reflection seismology Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflection (physics), reflected seismic waves. The method requir ...
exploration.http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=LEEDFF000025000004000438000001&idtype=cvips&gifs=yes While seismic imaging is able to image subsurface structure, it cannot detect the changes in resistivity associated with hydrocarbons and hydrocarbon-bearing formations. MT does detect
resistivity Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity i ...
variations in subsurface structures, which can differentiate between structures bearing hydrocarbons and those that do not. At a basic level of interpretation, resistivity is correlated with different rock types. High-velocity layers are typically highly resistive, whereas
sediments Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...
– porous and permeable – are typically much less resistive. While high-velocity layers are an acoustic barrier and make seismic ineffective, their electrical resistivity means the magnetic signal passes through almost unimpeded. This allows MT to see deep beneath these acoustic barrier layers, complementing the seismic data and assisting interpretation. 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.
China National Petroleum Corporation The China National Petroleum Corporation (CNPC) () is a major national oil and gas corporation of China and one of the largest integrated energy groups in the world. Its headquarters are in Dongcheng District, Beijing, Dongcheng District, Beij ...
(CNPC) an
Nord-West Ltd
use onshore MT more than any other oil company in the world, conducting thousands of MT soundings for
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...
exploration and
map A map is a symbolic depiction of interrelationships, commonly spatial, between things within a space. A map may be annotated with text and graphics. Like any graphic, a map may be fixed to paper or other durable media, or may be displayed on ...
ping throughout the globe.


Mining exploration

MT is used for various
base metals A base metal is a common and inexpensive metal, as opposed to a precious metal such as gold or silver. In numismatics, coins often derived their value from the precious metal content; however, base metals have also been used in coins in the past ...
(e.g. nickel) and
precious metals Precious metals are rare, naturally occurring metallic chemical elements of high economic value. Precious metals, particularly the noble metals, are more corrosion resistant and less chemically reactive than most elements. They are usual ...
exploration, as well as for
kimberlite Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known as the main host matrix for diamonds. It is named after the town of Kimberley, Northern Cape, Kimberley in South Africa, where the discovery of an 83.5-Car ...
mapping.
INCO Vale Canada Limited (formerly Vale Inco, CVRD Inco and Inco Limited; for corporate branding purposes simply known as "Vale" and pronounced in English) is a wholly owned subsidiary of the Brazilian mining company Vale. Vale's nickel mining and ...
's 1991 proof-of-concept study in
Sudbury Sudbury may refer to: Places Australia * Sudbury Reef, Queensland Canada * Greater Sudbury, Ontario ** Sudbury (federal electoral district) ** Sudbury (provincial electoral district) ** Sudbury Airport ** Sudbury Basin, a meteorite impact cra ...
, Ontario, Canada sensed a 1750-meter-deep nickel deposit.
Falconbridge Falconbridge may refer to: *Falconbridge Ltd., a Canadian mining company *Falconbridge, Middlesex County, Ontario *Falconbridge, Greater Sudbury, Ontario People with the surname *Lord Falconbridge, an alternative title for barons, viscounts, and e ...
followed with a feasibility study in 1996 that accurately located two Ni-Cu mineralized zones at about 800 m and 1350 m depth. Since then, both major and junior mining companies are increasingly using MT and
audio-magnetotellurics Magnetotellurics (MT) is an electromagnetic geophysical method for inferring the earth's subsurface electrical conductivity from measurements of natural geomagnetic and geoelectric field variation at the Earth's surface. Investigation depth ran ...
(AMT) for both brownfields (near known deposits) and greenfields (uncharted land) exploration. Significant MT mapping work has been done on areas of the
Canadian Shield The Canadian Shield ( ), also called the Laurentian Shield or the Laurentian Plateau, is a geologic shield, a large area of exposed Precambrian igneous and high-grade metamorphic rocks. It forms the North American Craton (or Laurentia), th ...
. Diamond exploration, by detecting kimberlites, is also a proven application.


Geothermal exploration

MT
geothermal exploration Geothermal exploration is the exploration of the subsurface in search of viable active geothermal regions with the goal of building a geothermal power plant, where hot fluids drive turbines to create electricity. Exploration methods include a bro ...
measurements allow detection of resistivity anomalies associated with productive
geothermal Geothermal is related to energy and may refer to: * Geothermal energy, useful energy generated and stored in the Earth * Geothermal activity, the range of natural phenomena at or near the surface, associated with release of the Earth's internal he ...
structures, including faults and the presence of a
cap rock Caprock or cap rock is a hard, resistant, and impermeable layer of rock that overlies and protects a reservoir of softer organic material, similar to the crust on a pie where the crust (caprock) prevents leakage of the soft filling (softer material ...
, and allow for estimation of geothermal reservoir temperatures at various depths. Dozens of MT geothermal exploration surveys have been completed in Japan and the
Philippines The Philippines, officially the Republic of the Philippines, is an Archipelagic state, archipelagic country in Southeast Asia. Located in the western Pacific Ocean, it consists of List of islands of the Philippines, 7,641 islands, with a tot ...
since the early 1980s, helping to identify several hundred
megawatts The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named in honor o ...
of renewable power at places such as the Hatchobaru plant on
Kyushu is the third-largest island of Japan's Japanese archipelago, four main islands and the most southerly of the four largest islands (i.e. excluding Okinawa Island, Okinawa and the other Ryukyu Islands, Ryukyu (''Nansei'') Ryukyu Islands, Islands ...
and the Togonang plant on
Leyte Leyte ( ) is an island in the Visayas group of islands in the Philippines. It is eighth-largest and sixth-most populous island in the Philippines, with a total population of 2,626,970 as of 2020 census. Since the accessibility of land has been ...
. Geothermal exploration with MT has also been done extensively in
the United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
, Iceland, New Zealand, Hungary, China,
Ethiopia Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a landlocked country located in the Horn of Africa region of East Africa. It shares borders with Eritrea to the north, Djibouti to the northeast, Somalia to the east, Ken ...
, Indonesia,
Peru Peru, officially the Republic of Peru, is a country in western South America. It is bordered in the north by Ecuador and Colombia, in the east by Brazil, in the southeast by Bolivia, in the south by Chile, and in the south and west by the Pac ...
, Australia, and
India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...
.


Other

MT is also used for
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
exploration and mapping research, hydrocarbon reservoir monitoring, deep investigation (100 km) of the electrical properties of the bedrock for
high-voltage direct current A high-voltage direct current (HVDC) electric power transmission system uses direct current (DC) for electric power transmission, in contrast with the more common alternating current (AC) transmission systems. Most HVDC links use voltages betwe ...
(HVDC) transmission systems,
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
sequestration, and other environmental engineering applications (e.g. nuclear blast site monitoring and
nuclear waste disposal Radioactive waste is a type of hazardous waste that contains radioactive material. It is a result of many activities, including nuclear medicine, nuclear research, nuclear power generation, nuclear decommissioning, rare-earth mining, and nucle ...
site monitoring).


Research applications


Crust and mantle

Since the MT is highly sensitive to the composition and temperature of the Earth, it has been widely used to understand numerous geological phenomena in the
Earth's mantle Earth's mantle is a layer of silicate mineral, silicate rock between the Earth's crust, crust and the Earth's outer core, outer core. It has a mass of and makes up 67% of the mass of Earth. It has a thickness of making up about 46% of Earth's ...
and crust. These include investigating the composition and distribution of melts, understanding
fault mechanics Fault mechanics is a field of study that investigates the behavior of geologic faults. Behind every good earthquake is some weak rock. Whether the rock remains weak becomes an important point in determining the potential for bigger earthquakes. ...
and
earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
generation, imaging deep lithospheric architecture and composition, which can be tied to many geodynamic processes. Large investigations have focused on the conterminous US (e.g. the National Science Foundation EarthScope MT Program and its successor NASA and USGS MTArray), the
East Pacific Rise The East Pacific Rise (EPR) is a mid-ocean rise (usually termed an oceanic rise and not a mid-ocean ridge due to its higher rate of spreading that results in less elevation increase and more regular terrain), at a divergent tectonic plate bound ...
, Australia (AusLAMP MT Program), Southern Africa (SAMTEX MT Project), China (Part of the Sinoprobe project) and the
Tibetan Plateau The Tibetan Plateau, also known as the Qinghai–Tibet Plateau or Qingzang Plateau, is a vast elevated plateau located at the intersection of Central Asia, Central, South Asia, South, and East Asia. Geographically, it is located to the north of H ...
.


Earthquake precursor prediction

Fluctuations in the MT signal may be able to predict the onset of seismic events. Stationary MT monitoring systems have been installed in Japan since April 1996, providing a continuous recording of MT signals at the Wakuya Station (previously at the Mizusawa Geodetic Observatory) and the Esashi Station of the
Geographical Survey Institute of Japan The , or GSI, is the national institution responsible for surveying and mapping the national land of Japan. The former name of the organization from 1949 until March 2010 was Geographical Survey Institute; despite the rename, it retains the same ...
(GSIJ). These stations measure fluctuations in the Earth's
electromagnetic field An electromagnetic field (also EM field) is a physical field, varying in space and time, that represents the electric and magnetic influences generated by and acting upon electric charges. The field at any point in space and time can be regarde ...
that correspond with seismic activity. 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. Additional MT earthquake precursor monitoring stations in Japan are located in
Kagoshima , is the capital Cities of Japan, city of Kagoshima Prefecture, Japan. , the city had an estimated population of 583,966 in 285,992 households, and a population density of 1100 persons per km2. The total area of the city is . Etymology While the ...
, in Sawauchi, and on
Shikoku is the smallest of the List of islands of Japan#Main islands, four main islands of Japan. It is long and between at its widest. It has a population of 3.8 million, the least populated of Japan's four main islands. It is south of Honshu ...
. Similar stations are also deployed in
Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia. The main geography of Taiwan, island of Taiwan, also known as ''Formosa'', lies between the East China Sea, East and South China Seas in the northwestern Pacific Ocea ...
on
Penghu Island The Penghu ( , Hokkien POJ: ''Phîⁿ-ô͘''  or ''Phêⁿ-ô͘'' ) or Pescadores Islands are an archipelago of 90 islands and islets in the Taiwan Strait, about west of the main island of Taiwan across the Penghu Channel, coveri ...
, as well as in the Fushan Reserve on the island of Taiwan proper. POLARIS is a Canadian research program investigating the structure and dynamics of the Earth's
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...
and the prediction of earthquake ground motion.


Theory and practice


Energy sources

Solar energy Solar energy is the radiant energy from the Sun's sunlight, light and heat, which can be harnessed using a range of technologies such as solar electricity, solar thermal energy (including solar water heating) and solar architecture. It is a ...
and
lightning Lightning is a natural phenomenon consisting of electrostatic discharges occurring through the atmosphere between two electrically charged regions. One or both regions are within the atmosphere, with the second region sometimes occurring on ...
cause natural variations in the
Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...
, inducing electric currents (known as
telluric current Telluric (from the Latin ''tellus'', "earth") may refer to several things related to the Earth: * Telluric current, a natural electric current in the Earth's crust * Telluric contamination, contamination of astronomical spectra by the Earth's atmo ...
s) under the Earth's surface. Different rocks, sediments and geological structures have a wide range of different electrical conductivities. Measuring electrical resistivity allows different materials and structures to be distinguished from one another and can improve knowledge of
tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...
processes and
geologic Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth s ...
structures. The Earth's naturally varying electric and
magnetic fields A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
are measured over a wide range of magnetotelluric frequencies from 10,000 Hz to 0.0001 Hz (10,000 s). These fields are due to electric currents flowing in the Earth and the magnetic fields that induce these currents. The magnetic fields are produced mainly by the interaction between the
solar wind The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
and the
magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...
. In addition, worldwide
thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustics, acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorm ...
activity causes magnetic fields at frequencies above 1 Hz. Combined, these natural phenomena create strong MT source signals over the entire frequency spectrum. The ratio of the electric field to magnetic field provides simple information about subsurface conductivity. Because the
skin effect In electromagnetism, skin effect is the tendency of an alternating current, alternating electric current (AC) to become distributed within a Conductor (material), conductor such that the current density is largest near the surface of the conduc ...
phenomenon affects the
electromagnetic field An electromagnetic field (also EM field) is a physical field, varying in space and time, that represents the electric and magnetic influences generated by and acting upon electric charges. The field at any point in space and time can be regarde ...
s, 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 both apparent resistivity as a function of frequency and phase as a function of frequency. A subsurface
resistivity Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity i ...
model is then created using this
tensor In mathematics, a tensor is an algebraic object that describes a multilinear relationship between sets of algebraic objects associated with a vector space. Tensors may map between different objects such as vectors, scalars, and even other ...
.


Depth and resolution

MT measurements can investigate depths from about 300 m down to hundreds of kilometers, though investigations in the range of 500 m to 10,000 m are typical. Greater depth requires measuring lower frequencies, which in turn requires longer recording times. Very deep, very long-period measurements (mid-crust through
upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (geology), crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle (Earth), lower man ...
depths), may require recordings of several days to weeks or more to obtain satisfactory data quality. Horizontal resolution of MT mainly depends on the distance between sounding locations- closer sounding locations increase the horizontal resolution. Continuous profiling (known as Emap) has been used, with only meters between the edges of each telluric dipole. Vertical resolution of MT mainly depends on the frequency being measured, as lower frequencies have greater depths of penetration. Accordingly, vertical resolution decreases as depth of investigation increases.


Signal strength and recording times

Magnetic fields in the frequency range of 1 Hz to approximately 20 kHz are part of the audio-magnetotelluric (AMT) range. These are parallel to the Earth surface and move towards the Earth's centre. This large frequency band allows for a range of depth penetration from several metres to several kilometres below the Earth's surface. Due to the nature of magnetotelluric 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 and 5 kHz, which is a crucial range in detecting the top 100 m of geology. The magnetotelluric method is also used in marine environments for
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...
exploration and lithospheric studies. Due to the screening effect of the electrically conductive sea water, a usable upper limit of the spectrum is around 1 Hz.


2D and 3D magnetotellurics

Two-dimensional surveys consist of a longitudinal profile of MT soundings over the area of interest, providing two-dimensional "slices" of subsurface resistivity. Three-dimensional surveys consist of a loose grid pattern of MT soundings over the area of interest, providing a more sophisticated three-dimensional model of subsurface resistivity.


Variants


Audio-magnetotellurics

Audio-magnetotellurics (AMT) is a higher-frequency magnetotelluric technique for shallower investigations. While AMT has less depth penetration than MT, AMT measurements often take only about one hour to perform (but deep AMT measurements during low-signal strength periods may take up to 24 hours) and use smaller and lighter magnetic sensors. Transient AMT is an AMT variant that records only temporarily during periods of more intense natural signal (transient impulses), improving signal-to-noise-ratio at the expense of strong linear polarization.


Controlled source electromagnetics

CSEM
controlled source electro-magnetic The controlled source electromagnetic (CSEM) method, also called sea bed logging, is a mostly offshore geophysical technique, employing electromagnetic remote-sensing technology to map the electric resistivity distribution of the subsurface. The el ...
is a deep-water offshore variant of controlled source audio magnetotellurics; CSEM is the name used in the offshore oil and gas industry. and for onshore exploration mostly Lotem is used in Russia, China the USA and Europe Onshore CSEM / CSAMT may be effective where electromagnetic cultural noise (e.g. power lines, electric fences) present interference problems for natural-source geophysical methods. An 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 then calculated, and the lower the resistivity, the more likely there is a conductive target (graphite, nickel ore or iron ore). CSAMT is also known in the oil and gas industry as onshore controlled source electromagnetics (Onshore CSEM). An offshore variant of MT, the marine magnetotelluric (MMT) method, uses instruments and sensors in pressure housings deployed by ship into shallow coastal areas where water is less than 300 m deep. A derivative of MMT is offshore single-channel measurement of the vertical
magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
only (the Hz, or "tipper"), which eliminates the need for telluric measurements and horizontal magnetic measurements.


Exploration surveys

MT exploration surveys are done to acquire resistivity data which can be interpreted to create a model of the subsurface. Data is acquired at each sounding location for a period of time (overnight soundings are common), with physical spacing between soundings dependent on the target size and geometry, local terrain constraints and financial cost. Reconnaissance surveys can have spacings of several kilometres, while more detailed work can have 200 m spacings, or even adjacent soundings (dipole-to-dipole). The HSE impact of MT exploration is relatively low because of light-weight equipment, natural signal sources, and reduced hazards compared to other types of exploration (e.g. no drills, no explosives, and no high currents).


Remote reference soundings

Remote Reference is an MT technique used to account for cultural electrical noise by acquiring simultaneous data at more than one MT station. 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.


Equipment

A typical full suite of MT equipment (for a "five component" sounding) consists of a receiver instrument with five
sensors A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...
: three
magnetic Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, m ...
sensors (typically induction coil sensors), and two telluric (electric) sensors. 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 A helicopter is a type of rotorcraft in which Lift (force), lift and thrust are supplied by horizontally spinning Helicopter rotor, rotors. This allows the helicopter to VTOL, take off and land vertically, to hover (helicopter), hover, and ...
, 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 In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may b ...
, 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

Four companies supply most of the commercial-use world market: one in the United States (Zonge International, Inc.), one in Canada; (Phoenix Geophysics, Ltd.); one in Germany (Metronix Messgeraete und Elektronik GmbH).) and One in Russia (Vega Geophysics, LLC). Government agencies and smaller companies producing MT instrumentation for internal use include the Russian Academy of Sciences (SPbF
IZMIRAN The Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation of the Russian Academy of Sciences (IZMIRAN, ) is a scientific institution of the Russian Academy of Sciences. this institute was founded in 1939 by Nikolay Pushk ...
); and the National Space Research Institute of Ukraine.


See also

*
Electrical resistivity tomography Electrical resistivity tomography (ERT) or electrical resistivity imaging (ERI) is a geophysical technique for imaging sub-surface structures from electrical resistivity measurements made at the surface, or by electrodes in one or more borehol ...
, another geophysical technique of imaging *
Exploration geophysics Exploration geophysics is an applied branch of geophysics and economic geology, which uses physical methods at the surface of the Earth, such as seismic, gravitational, magnetic, electrical and electromagnetic, to measure the physical properties o ...
, a branch of geophysics for discovering and mapping mineral resources *
Geophysics Geophysics () is a subject of natural science concerned with the physical processes and Physical property, properties of Earth and its surrounding space environment, and the use of quantitative methods for their analysis. Geophysicists conduct i ...
*
Geophysical Imaging Geophysical imaging (also known as geophysical tomography) is a minimally destructive geophysical technique that investigates the subsurface of a terrestrial planet. Geophysical imaging is a noninvasive imaging technique with a high parametrical ...
*
Geothermal exploration Geothermal exploration is the exploration of the subsurface in search of viable active geothermal regions with the goal of building a geothermal power plant, where hot fluids drive turbines to create electricity. Exploration methods include a bro ...
* Other types of
imaging Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of an image). Imaging technology is the application of materials and methods to create, preserve, or duplicate images. ...
*
Reflection seismology Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflection (physics), reflected seismic waves. The method requir ...
*
Seismo-electromagnetics Seismo-electromagnetics are various electro-magnetic phenomena believed to be generated by tectonic forces acting on the Earth's crust, and possibly associated with seismic activity such as earthquakes and volcanoes. Study of these has been pro ...
*
Transient electromagnetics Transient electromagnetics, (also time-domain electromagnetics / TDEM), is a geophysical exploration technique in which electric and magnetic fields are induced by transient pulses of electric current and the subsequent decay response measured. T ...


References


External links


MTNet site
hosted b
ManoTick GeoSolutions Ltd.

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.


Magnetotellurics at the University of Washington

MELT Experiment at mid-ocean ridge

Canadian Exploration Geophysical Society

University of Toronto EM Geophysics


{{Authority control Geomagnetism Earth sciences Russian inventions Geophysical imaging