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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 | + | | frompg = 16-6 |
− | | topg = 16-12 | + | | 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. | + | 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. | + | 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== |
− | 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. | + | |
| + | [[file:applying-magnetotellurics_fig16-3.png|300px|thumb|{{figure number|1}}Typical MT setup for a natural-source survey.]] |
| + | |
| + | 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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| 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. | | 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|Figure 1]] shows a typical MT setup for a natural-source survey. |
| + | |
| + | ==Controlled-source surveys== |
| | | |
− | [[file:applying-magnetotellurics_fig16-3.png|thumb|{{figure number|16-3}}]] | + | [[file:applying-magnetotellurics_fig16-4.png|thumb|300px|{{figure number|2}}Typical MT setup for a controlled-source survey.]] |
| | | |
− | ==Controlled-source surveys==
| + | 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. |
− | 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. | | 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|Figure 2]] 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?== | | ==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. | + | 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 | + | ! Locations || Reasons for Using MT |
− | ! Reasons for Using MT
| |
| |- | | |- |
− | | Carbonate terrains | + | | Carbonate terrains || Poor-quality seismic data |
− | | Poor-quality seismic data | |
| |- | | |- |
− | | Volcanic terrains | + | | Volcanic terrains || Poor-quality seismic data |
− | | Poor-quality seismic data | |
| |- | | |- |
− | | Granite overthrusts | + | | Granite overthrusts || Poor-quality seismic data |
− | | Poor-quality seismic data | |
| |- | | |- |
− | | Regional surveys | + | | Regional surveys || Less expensive than seismic; generates prospects to detail with seismic |
− | | Less expensive than seismic; generates prospects to detail with seismic | |
| |- | | |- |
− | | Remote areas | + | | Remote areas || Less expensive than seismic |
− | | Less expensive than seismic | |
| |- | | |- |
− | | Rugged terrains | + | | Rugged terrains || Less expensive than seismic |
− | | Less expensive than seismic | |
| |- | | |- |
− | | Fracture zones | + | | [[Fracture]] zones || Excellent tool for mapping |
− | | Excellent tool for mapping | |
| |} | | |} |
| | | |
| ==See also== | | ==See also== |
− | * [[What is magnetotellurics?]] | + | * [[Magnetotellurics]] |
− | * [[What does an MT survey measure?]] | + | * [[Magnetotellurics survey measurements]] |
− | * [[Case history: frontier basin analysis (Amazon Basin, Colombia)]] | + | * [[Magnetotellurics case history: frontier basin analysis (Amazon Basin, Colombia)]] |
− | * [[Case history: rugged carbonate terrain (Highlands of Papua New Guinea)]] | + | * [[Magnetotellurics case history: rugged carbonate terrain (Highlands of Papua New Guinea)]] |
− | * [[Case history: Precambrian overthrust (Northwestern Colorado)]] | + | * [[Magnetotellurics case history: Precambrian overthrust (Northwestern Colorado)]] |
− | * [[Case history: volcanic terrain (Columbia River Plateau)]] | + | * [[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]] |