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Magnetotelluric data acquisition (view source)
Revision as of 23:02, 17 January 2014
, 23:02, 17 January 2014Initial import
{{publication
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Predicting the occurrence of oil and gas traps
| chapter = Applying magnetotellurics
| frompg = 16-1
| topg = 16-12
| author = Arnie Ostrander
| link = http://archives.datapages.com/data/specpubs/beaumont/ch16/ch16.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
==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.
==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.
==Natural-source surveys==
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.
The predominant low-frequency ( 1.0 Hz) source is equatorial thunderstorm activity.
Although H-field data do not provide information on the subsurface geology (when only H<sub>x</sub> and H<sub>y</sub> components are measured), the vertical H-field component—if measured—provides information on the surface geology.
The figure below shows a typical MT setup for a natural-source survey.
[[file:applying-magnetotellurics_fig16-3.png|thumb|{{figure number|16-3}}]]
==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.
Normally, only the E<sub>x</sub> (parallel to the transmitter dipole) and H<sub>y</sub> components are measured.
The figure below shows a typical MT setup for a controlled-source survey.
[[file:applying-magnetotellurics_fig16-4.png|thumb|{{figure number|16-4}}]]
==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.
==Where to use MT==
MT can be valuable in areas that yield poor-quality seismic data and where acquiring seismic data is very expensive. The following table indicates where to use MT and the reasons for using it.
{| class = "wikitable"
|-
! Locations
! Reasons for Using MT
|-
| Carbonate terrains
| Poor-quality seismic data
|-
| Volcanic terrains
| Poor-quality seismic data
|-
| Granite overthrusts
| Poor-quality seismic data
|-
| Regional surveys
| Less expensive than seismic; generates prospects to detail with seismic
|-
| Remote areas
| Less expensive than seismic
|-
| Rugged terrains
| Less expensive than seismic
|-
| Fracture zones
| Excellent tool for mapping
|}
==See also==
* [[Overview]]
* [[What is Magnetotellurics (MT)?]]
* [[What does an MT survey measure?]]
* [[Case history: frontier basin analysis (Amazon Basin, Colombia)]]
* [[Case history: rugged carbonate terrain (Highlands of Papua New Guinea)]]
* [[Case history: Precambrian overthrust (Northwestern Colorado)]]
* [[Case history: volcanic terrain (Columbia River Plateau)]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch16/ch16.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Predicting the occurrence of oil and gas traps]]
[[Category:Applying magnetotellurics]]
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Predicting the occurrence of oil and gas traps
| chapter = Applying magnetotellurics
| frompg = 16-1
| topg = 16-12
| author = Arnie Ostrander
| link = http://archives.datapages.com/data/specpubs/beaumont/ch16/ch16.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
==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.
==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.
==Natural-source surveys==
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.
The predominant low-frequency ( 1.0 Hz) source is equatorial thunderstorm activity.
Although H-field data do not provide information on the subsurface geology (when only H<sub>x</sub> and H<sub>y</sub> components are measured), the vertical H-field component—if measured—provides information on the surface geology.
The figure below shows a typical MT setup for a natural-source survey.
[[file:applying-magnetotellurics_fig16-3.png|thumb|{{figure number|16-3}}]]
==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.
Normally, only the E<sub>x</sub> (parallel to the transmitter dipole) and H<sub>y</sub> components are measured.
The figure below shows a typical MT setup for a controlled-source survey.
[[file:applying-magnetotellurics_fig16-4.png|thumb|{{figure number|16-4}}]]
==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.
==Where to use MT==
MT can be valuable in areas that yield poor-quality seismic data and where acquiring seismic data is very expensive. The following table indicates where to use MT and the reasons for using it.
{| class = "wikitable"
|-
! Locations
! Reasons for Using MT
|-
| Carbonate terrains
| Poor-quality seismic data
|-
| Volcanic terrains
| Poor-quality seismic data
|-
| Granite overthrusts
| Poor-quality seismic data
|-
| Regional surveys
| Less expensive than seismic; generates prospects to detail with seismic
|-
| Remote areas
| Less expensive than seismic
|-
| Rugged terrains
| Less expensive than seismic
|-
| Fracture zones
| Excellent tool for mapping
|}
==See also==
* [[Overview]]
* [[What is Magnetotellurics (MT)?]]
* [[What does an MT survey measure?]]
* [[Case history: frontier basin analysis (Amazon Basin, Colombia)]]
* [[Case history: rugged carbonate terrain (Highlands of Papua New Guinea)]]
* [[Case history: Precambrian overthrust (Northwestern Colorado)]]
* [[Case history: volcanic terrain (Columbia River Plateau)]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch16/ch16.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Predicting the occurrence of oil and gas traps]]
[[Category:Applying magnetotellurics]]