Magnetotellurics
Encyclopedia
Magnetotellurics is an electromagnetic
geophysical
method of imaging
the earth
's subsurface by measuring natural variations of electrical and magnetic fields at the Earth's surface. Investigation depth ranges from 300m below ground by recording higher frequencies down to 10,000m or deeper with long-period soundings. Developed in Russia
and France during the 1950s, MT is now an international academic discipline and is used in exploration surveys around the world. Commercial uses include hydrocarbon
(oil and gas) exploration, geothermal
exploration, mining exploration, as well as hydrocarbon
and groundwater
monitoring. Research applications include experimentation to further develop the MT technique, long-period deep crustal exploration, and earthquake precursor prediction research.
n geophysicist Andrey Nikolayevich Tikhonov in the early 1950s. With advances in instrumentation, processing and modeling, MT has become one of the most important tools in deep Earth research.
Since the 1950s sensors, receivers and data processing techniques have followed the general trends in digital electronics, becoming less expensive and more capable with each generation. Major advances in MT instrumentation and technique include the advent of remote referencing, GPS time-based synchronization, and 3D data acquisition and processing.
exploration, MT is mainly used as a complement to the primary technique of reflection seismology
exploration. 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
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 – 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, complimenting 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
(CNPC) uses onshore MT more than any other oil company in the world, conducting thousands of MT soundings for hydrocarbon
exploration and map
ping throughout all of China.
) mapping.
INCO's 1991 proof-of-concept study in Sudbury, Ontario, Canada sensed a 1750 meter-deep nickel deposit. Falconbridge
followed with a feasibility study in 1996 that accurately located two Ni-Cu mineralized zones at ~800 m and ~1350 m. Since then, both major and junior mining companies are increasingly using MT and AMT for both brownfields and greenfields exploration. Significant MT mapping work has been done on areas of the Canadian Shield
.
Diamond exploration, by detecting kimberlites, is also a proven application.
measurements allow detection of resistivity
anomalies associated with productive geothermal
structures, including faults and the presence of a cap rock, 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
since the early 1980s, helping to identify several hundred megawatts of renewable power at places such as the Hatchobaru plant on Kyushu
and the Togonang plant on Leyte
. Geothermal exploration with MT has also been done in the United States, Iceland, New Zealand, Hungary, China, Ethiopia
, Indonesia, Peru
, Australia, and India
.
(HVDC) transmission systems, carbon dioxide
sequestration, and other environmental engineering applications (e.g. nuclear blast site monitoring and nuclear waste disposal site monitoring).
in the Earth's mantle and crust; large investigations have focused on the East Pacific Rise
and the Tibetan Plateau
. Other research work aims to better understand the plate-tectonic processes in the highly complex three-dimensional region formed by the collision of the African and European plates.
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 EM events and earthquake activity. The MT time series data from the GSIJ earthquake monitoring stations is available online at http://vldb.gsi.go.jp/sokuchi/geomag/menu_03/mt_data-e.html
Additional MT earthquake precursor monitoring stations in Japan are located in Kagoshima, in Sawauchi, and on Shikoku
. Similar stations are also deployed in Taiwan
on Penghu Island, 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
and the prediction of earthquake ground motion.
cause natural variations in the earth's magnetic field
, inducing electric currents (known as telluric current
s) under the Earth's surface. Simultaneous measurements of orthogonal components of the electric and magnetic fields are recorded, with the results calculated as the impedance tensor. A subsurface resistivity
model is then created using this tensor
.
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 processes and geologic
structures.
The Earth's naturally varying electric and magnetic fields
are measured over a wide range of magnetotelluric frequencies from 10,000 Hz to 0.0001 Hz (10,000s). 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
and the magnetosphere
. In addition, worldwide 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
phenomenon affects the electromagnetic field
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.
Horizontal resolution of MT mainly depends on the distance between sounding locations- closer sounding locations increase the horizontal resolution. Continuous profiling (known as dipole-to-dipole) has been used, with only metres between the edges of each telluric dipole.
Vertical resolution of MT mainly depends on the frequency being measured, as lower frequencies have longer wavelengths. Accordingly, vertical resolution decreases as depth of investigation increases.
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.
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.
is a deep-water offshore
variant of controlled source audio magnetotellurics– CSEM is the named used in the offshore oil and gas industry.
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, MMT uses instruments and sensors in pressure housings deployed by ship into shallow coastal areas where water is less than 300m deep. A derivative of MMT is offshore single-channel measurement of the vertical magnetic field
only (the Hz, or "tipper"), which eliminates the need for telluric measurements and horizontal magnetic measurements. While the theory is sound, no commercial system is yet available. Furthermore, any such system would require a solution providing for the precise orientation and stabilization of the magnetic sensor.
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).
(electric) sensors. 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 (3 to 4 persons) or carried by a light helicopter
(such as the MD Helicopters MD 500
), 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 −20C to +45C, from dry desert to high-humidity (condensing) and partial 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. Anomalies such as faults, hydrocarbons, and conductive mineralization appear as areas of higher or lower resistivity from surrounding structures.
); and the National Space Research Institute of Ukraine
.
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
geophysical
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
method of imaging
Geophysical imaging
Geophysical imaging is geophysical technique that investigates the subsurface. There are many different kinds of imaging techniques, all which are based on applied physics.Types of geophysical imaging include:...
the earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's subsurface by measuring natural variations of electrical and magnetic fields at the Earth's surface. Investigation depth ranges from 300m below ground by recording higher frequencies down to 10,000m or deeper with long-period soundings. Developed in Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
and France during the 1950s, MT is now an international academic discipline and is used in exploration surveys around the world. Commercial uses include hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
(oil and gas) exploration, geothermal
Geothermal
Geothermal is related to energy and may refer to:* The geothermal gradient and associated heat flows from within the Earth- Renewable technology :...
exploration, mining exploration, as well as hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
and groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
monitoring. Research applications include experimentation to further develop the MT technique, long-period deep crustal exploration, and earthquake precursor prediction research.
History
The magentotelluric technique was introduced by the French geophysicist Louis Cagniard and RussiaRussia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
n geophysicist Andrey Nikolayevich Tikhonov in the early 1950s. With advances in instrumentation, processing and modeling, MT has become one of the most important tools in deep Earth research.
Since the 1950s sensors, receivers and data processing techniques have followed the general trends in digital electronics, becoming less expensive and more capable with each generation. Major advances in MT instrumentation and technique include the advent of remote referencing, GPS time-based synchronization, and 3D data acquisition and processing.
Hydrocarbon exploration
For hydrocarbonHydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
exploration, MT is mainly used as a complement to the primary technique of reflection seismology
Reflection seismology
Reflection seismology is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite/Tovex, a specialized air gun or a...
exploration. 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
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
variations in subsurface structures, which can differentiate between structures bearing hydrocarbons and those that do not.
At a basic level of interpretation, resistivity
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
is correlated with different rock types. High-velocity layers are typically highly resistive, whereas sediments – 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, complimenting 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
China National Petroleum Corporation
China National Petroleum Corporation is a state-owned fuel-producing corporation and the largest integrated oil and gas company in the People's Republic of China...
(CNPC) uses onshore MT more than any other oil company in the world, conducting thousands of MT soundings for hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
exploration and map
Map
A map is a visual representation of an area—a symbolic depiction highlighting relationships between elements of that space such as objects, regions, and themes....
ping throughout all of China.
Mining exploration
MT is used for various base metals (nickel and precious metals exploration, as well as for kimberliteKimberlite
Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond in 1871 spawned a diamond rush, eventually creating the Big Hole....
) mapping.
INCO's 1991 proof-of-concept study in Sudbury, Ontario, Canada sensed a 1750 meter-deep nickel deposit. Falconbridge
Falconbridge Ltd.
Falconbridge Limited was a Toronto, Ontario-based natural resources company with operations in 18 countries, involved in the exploration, mining, processing, and marketing of metal and mineral products, including nickel, copper, cobalt, and platinum. It was listed on the TSX and NYSE , and had...
followed with a feasibility study in 1996 that accurately located two Ni-Cu mineralized zones at ~800 m and ~1350 m. Since then, both major and junior mining companies are increasingly using MT and AMT for both brownfields and greenfields exploration. Significant MT mapping work has been done on areas of the Canadian Shield
Canadian Shield
The Canadian Shield, also called the Laurentian Plateau, or Bouclier Canadien , is a vast geological shield covered by a thin layer of soil that forms the nucleus of the North American or Laurentia craton. It is an area mostly composed of igneous rock which relates to its long volcanic history...
.
Diamond exploration, by detecting kimberlites, is also a proven application.
Geothermal exploration
MT geothermal explorationGeothermal exploration
Geothermal Exploration is a multidisciplinary process used to explore the subsurface in search of viable active geothermal regions with the ultimate goal of building a geothermal power plant, where hot fluids drive turbines to create electricity...
measurements allow detection of resistivity
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
anomalies associated with productive geothermal
Geothermal
Geothermal is related to energy and may refer to:* The geothermal gradient and associated heat flows from within the Earth- Renewable technology :...
structures, including faults and the presence of a cap rock, 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
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
since the early 1980s, helping to identify several hundred megawatts of renewable power at places such as the Hatchobaru plant on Kyushu
Kyushu
is the third largest island of Japan and most southwesterly of its four main islands. Its alternate ancient names include , , and . The historical regional name is referred to Kyushu and its surrounding islands....
and the Togonang plant on Leyte
Leyte
Leyte is a province of the Philippines located in the Eastern Visayas region. Its capital is Tacloban City and occupies the northern three-quarters of the Leyte Island. Leyte is located west of Samar Island, north of Southern Leyte and south of Biliran...
. Geothermal exploration with MT has also been done in the United States, Iceland, New Zealand, Hungary, China, Ethiopia
Ethiopia
Ethiopia , officially known as the Federal Democratic Republic of Ethiopia, is a country located in the Horn of Africa. It is the second-most populous nation in Africa, with over 82 million inhabitants, and the tenth-largest by area, occupying 1,100,000 km2...
, Indonesia, Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....
, Australia, and India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
.
Other commercial applications
MT is also used for groundwater exploration and mapping, hydrocarbon reservoir monitoring, deep investigation (100 km) of the electrical properties of the bedrock for high-voltage direct currentHigh-voltage direct current
A high-voltage, direct current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses...
(HVDC) transmission systems, carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
sequestration, and other environmental engineering applications (e.g. nuclear blast site monitoring and nuclear waste disposal site monitoring).
Crustal research
MT has been used to investigate the distribution of silicate meltsMagma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
in the Earth's mantle and crust; large investigations have focused on the East Pacific Rise
East Pacific Rise
The East Pacific Rise is a mid-oceanic ridge, a divergent tectonic plate boundary located along the floor of the Pacific Ocean. It separates the Pacific Plate to the west from the North American Plate, the Rivera Plate, the Cocos Plate, the Nazca Plate, and the Antarctic Plate...
and the Tibetan Plateau
Tibetan Plateau
The Tibetan Plateau , also known as the Qinghai–Tibetan Plateau is a vast, elevated plateau in Central Asia covering most of the Tibet Autonomous Region and Qinghai, in addition to smaller portions of western Sichuan, southwestern Gansu, and northern Yunnan in Western China and Ladakh in...
. Other research work aims to better understand the plate-tectonic processes in the highly complex three-dimensional region formed by the collision of the African and European plates.
Earthquake precursor prediction research
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. These stations measure fluctuations in the Earth's electromagnetic fieldElectromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
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 EM events and earthquake activity. The MT time series data from the GSIJ earthquake monitoring stations is available online at http://vldb.gsi.go.jp/sokuchi/geomag/menu_03/mt_data-e.html
Additional MT earthquake precursor monitoring stations in Japan are located in Kagoshima, in Sawauchi, and on Shikoku
Shikoku
is the smallest and least populous of the four main islands of Japan, located south of Honshū and east of the island of Kyūshū. Its ancient names include Iyo-no-futana-shima , Iyo-shima , and Futana-shima...
. Similar stations are also deployed in Taiwan
Taiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...
on Penghu Island, 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
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
and the prediction of earthquake ground motion.
Energy sources
Solar energy and lightningLightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
cause natural variations in the earth's magnetic field
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
, inducing electric currents (known as telluric current
Telluric current
A telluric current , or Earth current, is an electric current which moves underground or through the sea. Telluric currents result from both natural causes and human activity, and the discrete currents interact in a complex pattern...
s) under the Earth's surface. Simultaneous measurements of orthogonal components of the electric and magnetic fields are recorded, with the results calculated as the impedance tensor. A subsurface resistivity
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
model is then created using this tensor
Tensor
Tensors are geometric objects that describe linear relations between vectors, scalars, and other tensors. Elementary examples include the dot product, the cross product, and linear maps. Vectors and scalars themselves are also tensors. A tensor can be represented as a multi-dimensional array of...
.
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 processes and geologic
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
structures.
The Earth's naturally varying electric and magnetic fields
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
are measured over a wide range of magnetotelluric frequencies from 10,000 Hz to 0.0001 Hz (10,000s). 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
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
and the magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
. In addition, worldwide thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...
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
Skin effect
Skin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
phenomenon affects the electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
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.
Depth and resolution
MT measurements can investigate depths from about 300m down to tens of kilometres, though investigations in the range of 500m to 10,000m are typical. Greater depth requires measuring lower frequencies, which in turn requires longer recording times. Very deep, very long-period measurements (15km or more), may require recordings of several days 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 dipole-to-dipole) has been used, with only metres between the edges of each telluric dipole.
Vertical resolution of MT mainly depends on the frequency being measured, as lower frequencies have longer wavelengths. 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 to 5 kHz, which is a crucial range in detecting the top 100m of geology. The magnetotelluric method is also used in marine environments for hydrocarbonHydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
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.
Audio-magnetotellurics (AMT)
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-magneticControlled source electro-magnetic
The Controlled Source Electromagnetic method is an offshore geophysical technique , employing electromagnetic remote-sensing technology to indicate the presence and extent of hydrocarbon accumulations below the seabed....
is a deep-water offshore
Offshore (hydrocarbons)
"Offshore", when used relative to hydrocarbons, refers to an oil, natural gas or condensate field that is under the sea, or to activities or operations carried out in relation to such a field...
variant of controlled source audio magnetotellurics– CSEM is the named used in the offshore oil and gas industry.
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, MMT uses instruments and sensors in pressure housings deployed by ship into shallow coastal areas where water is less than 300m deep. A derivative of MMT is offshore single-channel measurement of the vertical magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
only (the Hz, or "tipper"), which eliminates the need for telluric measurements and horizontal magnetic measurements. While the theory is sound, no commercial system is yet available. Furthermore, any such system would require a solution providing for the precise orientation and stabilization of the magnetic sensor.
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 dependant 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 200m spacings, or even adjacent soundings (dipole-to-dipole). MT surveys are carried out year-round in the Americas, Europe, Asia, Africa and Oceania.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.Survey equipment
A typical full suite of MT equipment (for a "five component" sounding) consists of a receiver instrument with five sensors: three magnetic sensors, and two telluricTelluric current
A telluric current , or Earth current, is an electric current which moves underground or through the sea. Telluric currents result from both natural causes and human activity, and the discrete currents interact in a complex pattern...
(electric) sensors. 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 (3 to 4 persons) or carried by a light helicopter
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
(such as the MD Helicopters MD 500
MD Helicopters MD 500
The MD Helicopters MD 500 series is an American family of light utility civilian and military helicopters. The MD 500 was developed from the Hughes 500, a civilian version of the US Army's OH-6A Cayuse/Loach...
), 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 −20C to +45C, from dry desert to high-humidity (condensing) and partial 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.
Instrument and sensor manufacturers
MT instrumentation design and construction is a specialized international activity, with only a small number of companies and scientific organizations having the necessary expertise and technology. Three companies supply most of the commercial-use world market: one in the United States (Zonge Engineering and Research Organization, 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 IZMIRANIZMIRAN
Institute of Earth magnetism, ionosphere and radiowaves propagation named after Nikolay Pushkov of the Russian Academy of Sciences or IZMIRAN for short - is a scientific institution of RAS founded in 1939 on the basis of the General Magnetic Survey Bureau and Pavlovskaya Magnetic Observatory of...
); and the National Space Research Institute of Ukraine
Lviv Centre of Institute for Space Research
The Lviv Centre of Institute of Space Research of the National Academy of Sciences of Ukraine and National Space Agency of Ukraine was formed in 1996 at the basis of Special Design Bureau of Karpenko Institute for Physics and Mechanics of NASU as a scientific experimental division of Institute...
.
See also
- GeophysicsGeophysicsGeophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
- Exploration geophysicsExploration geophysicsExploration geophysics is the applied branch of geophysics which uses surface methods to measure the physical properties of the subsurface Earth, in order to detect or infer the presence and position of ore minerals, hydrocarbons, geothermal reservoirs, groundwater reservoirs, and other geological...
, a branch of geophysics for discovering and mapping mineral resources - Seismo-electromagneticsSeismo-electromagneticsSeismo-electromagnetics is the study of electromagnetic phenomena associated with seismic activity such as earthquakes and volcanos, and also the use of electromagnetic methods in seismology such as magnetotellurics. Links between the electromagnetic fields in the lithosphere and those in the...
- Reflection seismologyReflection seismologyReflection seismology is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite/Tovex, a specialized air gun or a...
- Transient/Time-Domain ElectroMagnetics, TDEM
- Other types of imagingGeophysical imagingGeophysical imaging is geophysical technique that investigates the subsurface. There are many different kinds of imaging techniques, all which are based on applied physics.Types of geophysical imaging include:...
- Electrical resistivity tomographyElectrical resistivity tomographyElectrical resistivity tomography or electrical resistivity imaging is a geophysical technique for imaging sub-surface structures from electrical measurements made at the surface, or by electrodes in one or more boreholes. It is closely related to the medical imaging technique electrical...
, another geophysical technique of imaging - Geothermal ExplorationGeothermal explorationGeothermal Exploration is a multidisciplinary process used to explore the subsurface in search of viable active geothermal regions with the ultimate goal of building a geothermal power plant, where hot fluids drive turbines to create electricity...
External links
- MTNet site hosted by the Dublin Institute for Advanced StudiesDublin Institute for Advanced StudiesThe Dublin Institute for Advanced Studies Dublin, Ireland was established in 1940 by the Taoiseach of the time, Éamon de Valera under the . The Institute consists of 3 schools: The , the and the . The directors of these schools are currently Professor Werner Nahm, Professor Luke Drury and...
. - 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 EM Geophysics
- USGS Magnetotelluric Surveys / Reports (Open Files)
- Practical Magnetotellurics textbook