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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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==Acquisition instrumentation==
 
==Acquisition instrumentation==
   −
The data are collected using a microprocessor-controlled voltmeter. The voltmeter is in fact a system of complex hardware/software devices that includes amplification, filtering. A/D conversion, stacking and averaging, and various data-enhancement algorithms.
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The data are collected using a microprocessor-controlled voltmeter. The voltmeter is in fact a system of complex hardware and software devices that includes amplification, filtering, analog to digital conversion, stacking and averaging, and various data-enhancement algorithms.
    
==Types of surveys==
 
==Types of surveys==
There are two types of 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_fig16-3.png|thumb|{{figure number|1}}Typical MT setup for a natural-source survey.]]
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[[file:applying-magnetotellurics_fig16-3.png|300px|thumb|{{figure number|1}}Typical MT setup for a natural-source survey.]]
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The natural-source data-acquisition system typically measures four components: E<sub>x</sub>, E<sub>y</sub>, H<sub>x</sub>, and H<sub>y</sub>. The E<sub>x</sub> component is oriented perpendicular to the E<sub>y</sub> component. This is also true for the H-field components.
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The natural-source data-acquisition system typically measures four components: two in the horizontal field, E<sub>x</sub>, E<sub>y</sub>, and two in the electrical field, H<sub>x</sub>, and H<sub>y</sub>. (See [[Magnetotellurics survey measurements]].) The E<sub>x</sub> component is oriented perpendicular to the E<sub>y</sub> component. This is also true for the H-field components.
    
The predominant low-frequency ( 1.0 Hz) source is equatorial thunderstorm activity.
 
The predominant low-frequency ( 1.0 Hz) source is equatorial thunderstorm activity.
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==Controlled-source surveys==
 
==Controlled-source surveys==
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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[[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 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 volcanic, carbonate, or granite 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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{| class = "wikitable"
 
{| class = "wikitable"
 
|-
 
|-
! Locations
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! Locations || Reasons for Using MT
! Reasons for Using MT
   
|-
 
|-
| Carbonate terrains
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| Carbonate terrains || Poor-quality seismic data
| Poor-quality seismic data
   
|-
 
|-
| Volcanic terrains
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| Volcanic terrains || Poor-quality seismic data
| Poor-quality seismic data
   
|-
 
|-
| Granite overthrusts
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| Granite overthrusts || Poor-quality seismic data
| Poor-quality seismic data
   
|-
 
|-
| Regional surveys
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| Regional surveys || Less expensive than seismic; generates prospects to detail with seismic
| Less expensive than seismic; generates prospects to detail with seismic
   
|-
 
|-
| Remote areas
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| Remote areas || Less expensive than seismic
| Less expensive than seismic
   
|-
 
|-
| Rugged terrains
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| Rugged terrains || Less expensive than seismic
| Less expensive than seismic
   
|-
 
|-
| Fracture zones
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| [[Fracture]] zones || Excellent tool for mapping
| Excellent tool for mapping
   
|}
 
|}
    
==See also==
 
==See also==
* [[What is magnetotellurics?]]
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* [[Magnetotellurics]]
* [[What does an MT survey measure?]]
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* [[Magnetotellurics survey measurements]]
* [[Case history: frontier basin analysis (Amazon Basin, Colombia)]]
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* [[Magnetotellurics case history: frontier basin analysis (Amazon Basin, Colombia)]]
* [[Case history: rugged carbonate terrain (Highlands of Papua New Guinea)]]
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* [[Magnetotellurics case history: rugged carbonate terrain (Highlands of Papua New Guinea)]]
* [[Case history: Precambrian overthrust (Northwestern Colorado)]]
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* [[Magnetotellurics case history: Precambrian overthrust (Northwestern Colorado)]]
* [[Case history: volcanic terrain (Columbia River Plateau)]]
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* [[Magnetotellurics case history: volcanic terrain (Columbia River Plateau)]]
    
==References==
 
==References==
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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]]
 +
[[Category:Treatise Handbook 3]]

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