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 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 (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, sub-glacial water flow mapping, and earthquake precursor research.
History
The magnetotelluric technique was introduced independently by Japanese scientists in 1948 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
At a basic level of interpretation, resistivity is correlated with different rock types. High-velocity layers are typically highly resistive, whereas
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.[12]
Mining exploration
MT is used for various
Diamond exploration, by detecting kimberlites, is also a proven application.[14]
Geothermal exploration
MT
Other
MT is also used for
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 and crust. These include investigating the composition and distribution of melts,[31][32] understanding fault mechanics and earthquake generation,[33] imaging deep lithospheric architecture and composition, which can be tied to many geodynamic processes.[34][35] Large investigations have focused on the conterminous US (e.g. the National Science Foundation EarthScope MT Program and its successor NASA and USGS MTArray[36]), the East Pacific Rise, Australia (AusLAMP MT Program[37]), Southern Africa (SAMTEX MT Project[38]), China (Part of the Sinoprobe project[39]) and the Tibetan Plateau.
Earthquake precursor prediction
Fluctuations in the MT signal may be able to predict the onset of seismic events.
Additional MT earthquake precursor monitoring stations in Japan are located in
POLARIS is a Canadian research program investigating the structure and dynamics of the Earth's lithosphere and the prediction of earthquake ground motion.[46]
Theory and practice
Energy sources
Solar energy and lightning cause natural variations in the Earth's magnetic field, inducing electric currents (known as telluric currents) under the Earth's surface.[47]
Different rocks, sediments and geological structures have a wide range of different
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.
A subsurface
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
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.[49] 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.[50]
Controlled source electromagnetics
CSEM controlled source electro-magnetic is a deep-water offshore variant of controlled source audio magnetotellurics; CSEM is the name used in the offshore oil and gas industry.[51] and for onshore exploration mostly Lotem is used in Russia, China the USA and Europe[52][53]
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,[54][page needed] uses instruments and sensors in pressure housings deployed by ship into shallow coastal areas where water is less than 300 m deep.[6][55][56][57][58] 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.[59]
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
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
Equipment
A typical full suite of MT equipment (for a "five component" sounding) consists of a receiver instrument with five
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, 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
Instrument and sensor manufacturers
Four companies supply most of the commercial-use world market: one in the United States (Zonge International, Inc.[60]), one in Canada; (Phoenix Geophysics, Ltd.[61]); one in Germany (Metronix Messgeraete und Elektronik GmbH).[62]) and One in Russia (Vega Geophysics, LLC).[63]
Government agencies and smaller companies producing MT instrumentation for internal use include the Russian Academy of Sciences (SPbF IZMIRAN); and the National Space Research Institute of Ukraine.
See also
- Electrical resistivity tomography, another geophysical technique of imaging
- Exploration geophysics, a branch of geophysics for discovering and mapping mineral resources
- Geophysics
- Geophysical Imaging
- Geothermal exploration
- Other types of imaging
- Reflection seismology
- Seismo-electromagnetics
- Transient electromagnetics
References
- ^ Rikitake, T. (1948). "Notes on electromagnetic induction within the Earth". Bull. Earthq. Res. Inst. 24 (1): 4.
- NAID 10004593077.
- .
- ^ Archived 21 July 2011 at the Wayback Machine[dead link]
- ^ Unsworth, Martyn (April 2005). "New developments in conventional hydrocarbon exploration with electromagnetic methods". CSEG Recorder. 30 (4): 34–38.
- ^ a b http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=LEEDFF000025000004000438000001&idtype=cvips&gifs=yes[dead link]
- ^ "Geothermal Exploration with Electromagnetic Methods" (PDF). 2008. Retrieved 18 October 2011.
- ^ "Oil and Gas exploration". Phoenix-geophysics.com. Retrieved 18 October 2011.
- doi:10.1071/eg991375.
- ^ "3-D MT SURVEY IN UZBEKISTAN". Phoenix-geophysics.com. Retrieved 18 October 2011.
- )
- ^ "SYSTEM 2000 FUELS EXPLORATION BOOM". Phoenix-geophysics.com. Retrieved 18 October 2011.
- S2CID 54005958.
- ^ "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.
- ^ a b "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; 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.
- ^ "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.
- ^ [dead link]http://www.bgp.com.cn/download.aspx?id=156
- ^ "Geothermal Mt Survey in Peru". Phoenix-geophysics.com. Retrieved 18 October 2011.
- ^ Sinharay, Rajib K; Bhattacharya, Bimalendu B. (2001). "An analysis of magnetotelluric (MT) data over geothermal region of Bakreshwar, West Bengal". Journal of Geophysics. 22 (1). Hyderabad: 31–39. INIST 1145977.
- S2CID 7541303.
- ^ "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.
- .
- S2CID 135191963.
- S2CID 206653863.
- S2CID 54882515.
- S2CID 247419367.
- S2CID 254777582.
- ^ "Oregon State University MTArray Status". 28 June 2023.
- ^ "AusLAMP Project Website". 15 May 2014.
- ^ "SAMTEX Website".
- ^ "Sinoprobe Website".
- ^ "Terrestrial, Atmospheric and Oceanic Sciences". Tao.cgu.org.tw. 21 September 1999. Retrieved 18 October 2011.
- .
- OSTI 20222530.
- ^ "Sawauchi Automated Stationary MT Data and Earthquake Activity (>4.0M) during May ~ August, 2008" (PDF). 2008.
- ^ "Archived copy". Archived from the original on 25 February 2010. Retrieved 25 February 2010.
{{cite web}}
: CS1 maint: archived copy as title (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
- . 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.
- ISBN 0444895418.
- ^ "Exploration with controlled-source electromagnetics under basalt cover in India". The Leading Edge. 26.
- ISBN 978-3-319-45355-2.
- .
- ^ "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.
- ^ "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". geo-metronix.de.
- ^ "Vega Geophysics Official Web Site". Retrieved 28 March 2012.
External links
- MTNet site hosted by 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.
- 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)