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 ranges from 300 m below ground by recording higher frequencies down to 10,000 m 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 (oil and gas) exploration, geothermal exploration, carbon sequestration, mining exploration, as well as hydrocarbon and groundwater monitoring. Research applications include experimentation to further develop the MT technique, long-period deep crustal exploration, deep mantle probing, and earthquake precursor prediction research.
- 1 History
- 2 Commercial applications
- 3 Research applications
- 4 Theory and practice
- 5 Variants
- 6 Exploration surveys
- 7 Instrument and sensor manufacturers
- 8 See also
- 9 References
- 10 External links
The magnetotelluric technique was introduced independently by Japanese scientists in the 1940s (Hirayama, Rikitake), and Russian geophysicist Andrey Nikolayevich Tikhonov in 1950 and the French geophysicist Louis Cagniard With advances in instrumentation, processing and modelling, MT 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.
For hydrocarbon 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, 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 (CNPC) and Nord-West Ltd use onshore MT more than any other oil company in the world, conducting thousands of MT soundings for hydrocarbon exploration and mapping throughout the globe.
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 about 800 m and 1350 m depth. Since then, both major and junior mining companies are increasingly using MT and audio-magnetotellurics (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.
MT geothermal exploration 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.
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 current (HVDC) transmission systems, carbon dioxide sequestration, and other environmental engineering applications (e.g. nuclear blast site monitoring and nuclear waste disposal site monitoring).
MT has been used to investigate the distribution of silicate melts in the Earth's mantle and crust; large investigations have focused on the continental US (National Science Foundation EarthScope MT Program), 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.
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 (GSIJ). These stations measure fluctuations in the Earth's electromagnetic field 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. The MT time series data from the GSIJ earthquake monitoring stations is available online at https://web.archive.org/web/20100225080738/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.
Theory and practice
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 (10,000 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 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 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 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 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 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 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) 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
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 only (the Hz, or "tipper"), which eliminates the need for telluric measurements and horizontal magnetic measurements. While the theory is sound and many case histories exist, no commercial systems are available by KMS Technologies. Furthermore, any such system would require a solution providing for the precise orientation and stabilization of the magnetic sensor.
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.
A typical full suite of MT equipment (for a "five component" sounding) consists of a receiver instrument with five sensors: three magnetic sensors (typically induction coil sensors), and two telluric (electric) sensors. For long-period MT (frequencies below approximately 1–10 Hz), the three discrete magnetic field sensors can typically be replaced with 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 (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 −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, and conductive mineralization appear as areas of higher or lower resistivity from surrounding structures. For interpretation(inversion) of magnetotelluric data a number of software are used (WinGlink, Geotools MT, ZondMT2D).
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 International, Inc.), one in Canada; Phoenix Geophysics, Ltd.), and one in Germany (Metronix Messgeraete und Elektronik GmbH). High power CSEM array systems above 100 KVA for hydrocarbon and geothermal expploration are manufactured by KMS Technologies, USA. Government agencies and smaller companies producing MT instrumentation for internal use include Vega Geophysics, Ltd. in Russia and the Russian Academy of Sciences (SPbF IZMIRAN); and the National Space Research Institute of Ukraine. Geometrics, Inc. of San Jose, California USA produces a high-frequency, hybrid-source AMT system combining both natural fields and a high-frequency (1 to 70 kHz) transmitter, as well as a multi-channel distributed network CSAMT instrument.
- Electrical resistivity tomography, another geophysical technique of imaging
- Exploration geophysics, a branch of geophysics for discovering and mapping mineral resources
- Geothermal exploration
- Other types of imaging
- Reflection seismology
- Transient electromagnetics
- Tikhonov, A.N., 1950. in 1953 On determining electrical characteristics of the deep layers of the Earth's crust, Doklady, 73, 295-297.
- Cagniard, L (1953). "Basic theory of the magneto-telluric method of geophysical prospecting". Geophysics. 18: 605–635. Bibcode:1953Geop...18..605C. doi:10.1190/1.1437915.
- (PDF) http://www.hagi.or.id/download/JGeofisika/2004_2/2004_2_3.pdf. Retrieved 26 November 2009. Missing or empty
- "Archived copy" (PDF). Archived from the original (PDF) on 17 May 2006. Retrieved 2009-11-26.CS1 maint: Archived copy as title (link)
- "Geothermal Exploration with Electromagnetic Methods" (PDF). 2008. Retrieved 18 October 2011.
- "Oil and Gas exploration". Phoenix-geophysics.com. Retrieved 18 October 2011.
- "CSIRO PUBLISHING – Exploration Geophysics". Publish.csiro.au. Retrieved 18 October 2011.
- "3-D MT SURVEY IN UZBEKISTAN". Phoenix-geophysics.com. Retrieved 18 October 2011.
- "CSIRO PUBLISHING – ASEG Extended Abstracts". Publish.csiro.au. Retrieved 18 October 2011.
- "SYSTEM 2000 FUELS EXPLORATION BOOM". Phoenix-geophysics.com. Retrieved 18 October 2011.
- "Imaging the geometry and structure of kimberlite pipes using audio-MT". Homepages.dias.ie. Retrieved 18 October 2011.
- "Geothermal Exploration with Electromagnetic Methods" (PDF). 2008. Retrieved 18 October 2011.
- "Mapping geothermal reservoirs using broadband 2-D MT and gravity data" (PDF).
- "Characterizing a geothermal reservoir using broadband 2D MT survey in Theistareykir, Iceland" (PDF). Retrieved 18 October 2011.
- "Magnetotelluric Soundings in the Takigami Geothermal Area, Japan" (PDF). International Geothermal Association. 24 April 2005. Retrieved 24 January 2018.
- "Science Links Japan | Geothermal Reservoirs Modeling in the Western Side of Mt. Aso, SW Japan by Magnetotelluric Method". Sciencelinks.jp. 18 March 2009. Archived from the original on 29 February 2012. Retrieved 18 October 2011.
- Josephine B. Rosell and Maribel C. Zaide-Delfin (24 April 2005). "Resource Potential of the Southern Leyte Geothermal Prospect, Philippines: A Geologic Evaluation" (PDF). International Geothermal Association. Retrieved 24 January 2018.CS1 maint: Uses authors parameter (link)
- "Philippine National Oil Company". Pnoc.com.ph. Archived from the original on 2 October 2011. Retrieved 18 October 2011.
- "Geothermal | The Energy Development Corporation Website". Energy.com.ph. Archived from the original on 4 November 2015. Retrieved 18 October 2011.
- "Characterizing a geothermal reservoir using broadband 2-D MT survey in Theistareikir, Iceland:". SEG Expanded Abstracts. 2008.
- "Geothermal Mt Survey In Peru". Phoenix-geophysics.com. Retrieved 18 October 2011.
- "An analysis of magnetotelluric (MT) data over geothermal region of Bakreshwar, West Bengal". Cat.inist.fr. Retrieved 18 October 2011.
- H. Thunehed; et al. "Geophysical and geological pre-investigations for HVDC-electrodes" (PDF). PowerAfrica 2007. IEEE. doi:10.1109/PESAFR.2007.4498123.
- "Energy Sector: Science and Technology: Cleaner Fossil Fuels". Natural Resources Canada. 4 May 2010. Archived from the original on 11 August 2011. Retrieved 18 October 2011.
- "MT SURVEY IN TAIWAN EVALUATES THE POSSIBILITY OF CO2 SEQUESTRATION". Phoenix-geophysics.com. Retrieved 18 October 2011.
- "Hydrogeologic assessment of the Amchitka Island nuclear test site (Alaska) with magnetotellurics". Cat.inist.fr. Retrieved 18 October 2011.
- "PICASSO- Phase I: MT Investigation of the Betic-Rif mountain system. Comparison of actual robust processing algorithms" (PDF).
- "Terrestrial, Atmospheric and Oceanic Sciences". Tao.cgu.org.tw. 21 September 1999. Retrieved 18 October 2011.
- "Short-term earthquake prediction: Current status of seismo-electromagnetics". Tectonophysics. 470: 205–213. Bibcode:2009Tectp.470..205U. doi:10.1016/j.tecto.2008.07.019.
- "A critical review of Electric Earthquake Precursors" (PDF).
- "Sawauchi Automated Stationary MT Data and Earthquake Activity (>4.0M) during May ~ August, 2008" (PDF). 2008.
- http://tao.cgu.org.tw/pdf/v133p367.pdf[permanent dead link]
- "Polaris Consortium". Polarisnet.ca. Retrieved 18 October 2011.
- Cantwell, T. (1960) Detection and Analysis of Low-Frequency Magnetotelluric Signals, PhD Thesis, Massachusetts Institute of Technology, Cambridge, Massachusetts
- "Basic Theory Of The Magneto-Telluric Method Of Geophysical Prospecting". Scitation.aip.org. Retrieved 18 October 2011.
- "Marine EM laboratory". Scripps Institution of Oceanography. 23 April 2010. Retrieved 18 October 2011.
- "EMpulse Geophysics – Saskatoon". Empulse.ca. Archived from the original on 27 August 2011. Retrieved 18 October 2011.
- "Research | Concepts |CSEM and MT Exploration for Petroleum". Scripps Institution of Oceanography. 6 May 2009. Retrieved 18 October 2011.
- Stéphane Sainson, Electromagnetic seabed logging, A new tool for geoscientists. Ed. Springer, 2016
- Constable, Stephen; et al. "Marine magnetotellurics for petroleum exploration Part I: A sea-floor equipment system" (PDF). Geophysics. Bibcode:1998Geop...63..816C. doi:10.1190/1.1444393.
- "Gemini Prospect Marine MT and CSEM Surveys". Marineemlab.ucsd.edu. 6 May 2009. Retrieved 18 October 2011.
- "Marine Mt In China With Phoenix Equipment". Phoenix-geophysics.com. Retrieved 18 October 2011.
- "Integrated Electromagnetic Services, WesternGeco". Westerngeco.com. Archived from the original on 30 October 2009. Retrieved 18 October 2011.
- "CA2006000042 DETECTION OF RESISTIVITY OF OFFSHORE SEISMIC STRUCTURES MAINLY USING VERTICAL MAGNETIC COMPONENT OF EARTH'S NATURALLY VARYING ELECTROMAGNETIC FIELD". Wipo.int. Retrieved 18 October 2011.
- Strack, Kurt (1992). Exploration with Deep Transient Elctromagnetics. Elsevier. ISBN 0444895418 – via copyright released versions at: http://www.kmstechnologies.com/Files/Technology_Library/Publications/Exploration_with_Deep_Transient_Electromagnetics/Introduction_w_cover.pdf.
- "Exploration with controlled-source electromagnetics under basalt cover in India". The Leading Edge. 26.
- www.kmstechnologies.com http://www.kmstechnologies.com/. Retrieved 2016-09-26. Missing or empty
- Leo Fox. "PRECISION MEASUREMENT OF Hz IN MARINE MT" (PDF).
- "Surveys | AMT and MT". Zonge. Archived from the original on 3 October 2011. Retrieved 18 October 2011.
- "Phoenix products : The MTU Receiver". Phoenix-geophysics.com. Retrieved 18 October 2011.
- "Metronix Geophysics : Technology for Geophysics and Magnetotellurics". Metronix.
- "Vega Geophysics Official Web Site". Retrieved 28 March 2012.
- MTNet site hosted by Complete MT Solutions Inc.
- 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.
- Sainson S. 2016. Electromagnetic seabed logging, A new tool for geoscientists. Ed. Springer.
- 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)