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  | part    = Predicting the occurrence of oil and gas traps
 
  | part    = Predicting the occurrence of oil and gas traps
 
  | chapter = Applying magnetotellurics
 
  | chapter = Applying magnetotellurics
  | frompg  = 16-1
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  | frompg  = 16-6
  | topg    = 16-12
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  | topg    = 16-7
 
  | author  = Arnie Ostrander
 
  | author  = Arnie Ostrander
 
  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch16/ch16.htm
 
  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch16/ch16.htm
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==Types of surveys==
 
==Types of surveys==
There are two types of magnetotelluric (MT) surveys: natural source.<ref name=ch16r6>Vozoff, K., 1972, The magnetotelluric method in the exploration of sedimentary basins: Geophysics, vol. 37, no. 1, p. 98–141., 10., 1190/1., 1440255</ref> and controlled source<ref name=ch16r3>Goldstein, M., A., Strangway, D., W., 1975, Audio-frequency magnetotellurics with a grounded electrical dipole source: Geophysics, vol. 40, p. 669–683., 10., 1190/1., 1440558</ref> The equipment and the operational procedures for these two types differ considerably.
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There are two types of magnetotelluric (MT) surveys: natural source.<ref name=ch16r6>Vozoff, K., 1972, The magnetotelluric method in the exploration of sedimentary basins: Geophysics, vol. 37, no. 1, p. 98–141., 10., 1190/1., 1440255</ref> and controlled source<ref name=ch16r3>Goldstein, M. A., and D. W. Strangway, 1975, Audio-frequency magnetotellurics with a grounded electrical dipole source: Geophysics, vol. 40, p. 669–683., 10., 1190/1., 1440558</ref> The equipment and the operational procedures for these two types differ considerably.
    
==Natural-source surveys==
 
==Natural-source surveys==
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[[file:applying-magnetotellurics_fig16-4.png|thumb|300px|{{figure number|2}}Typical MT setup for a controlled-source survey.]]
 
[[file:applying-magnetotellurics_fig16-4.png|thumb|300px|{{figure number|2}}Typical MT setup for a controlled-source survey.]]
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The controlled-source system uses a high-power transmitter and motor/generator set to transmit a discrete AC waveform. This signal is transmitted into a grounded dipole typically 600–1,200 m (2,000–4,000 ft) long. The transmitter is normally located 3–6 km (2–4 mi) from the survey line.
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The controlled-source system uses a high-power transmitter and motor or generator set to transmit a discrete alternating current waveform. This signal is transmitted into a grounded dipole typically 600–1,200 m (2,000–4,000 ft) long. The transmitter is normally located 3–6 km (2–4 mi) from the survey line.
    
Normally, only the E<sub>x</sub> (parallel to the transmitter dipole) and H<sub>y</sub> components are measured.
 
Normally, only the E<sub>x</sub> (parallel to the transmitter dipole) and H<sub>y</sub> components are measured.
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==Which method is better?==
 
==Which method is better?==
The choice of MT method depends on the survey objectives. Natural-source data are best suited for regional surveys where the stations are widely spaced (e.g., frontier basin analysis). Controlled-source data are best suited for mapping structural detail where the stations lie along a continuous profile at 100–200-m (300–600-ft) spacings. The maximum depth of exploration for the controlled-source method is 3,000–4,500 m (10,000–15,000 ft) in a typical [[Wikipedia:Volcanic_rock|volcanic]], [[carbonate]], or [http://geology.about.com/od/more_igrocks/a/granite.htm granite] [[Thrust belt|overthrust]] terrain. Natural-source data have considerably deeper penetration but poorer resolution at shallower depths.
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The choice of MT method depends on the survey objectives. Natural-source data are best suited for regional surveys where the stations are widely spaced (e.g., frontier basin analysis; see [[Magnetotellurics case history: frontier basin analysis (Amazon Basin, Colombia)]]). Controlled-source data are best suited for mapping structural detail where the stations lie along a continuous profile at 100–200-m (300–600-ft) spacings. The maximum depth of exploration for the controlled-source method is 3,000–4,500 m (10,000–15,000 ft) in a typical [[Wikipedia:Volcanic_rock|volcanic]], [[carbonate]], or [http://geology.about.com/od/more_igrocks/a/granite.htm granite] [[Thrust belt|overthrust]] terrain. Natural-source data have considerably deeper penetration but poorer resolution at shallower depths.
    
==Where to use MT==
 
==Where to use MT==
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[[Category:Predicting the occurrence of oil and gas traps]]  
 
[[Category:Predicting the occurrence of oil and gas traps]]  
 
[[Category:Applying magnetotellurics]]
 
[[Category:Applying magnetotellurics]]
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[[Category:Treatise Handbook 3]]

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