Difference between revisions of "Magnetotellurics"
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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- | + | | frompg = 16-4 |
| − | | topg = 16- | + | | topg = 16-4 |
| 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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| isbn = 0-89181-602-X | | isbn = 0-89181-602-X | ||
}} | }} | ||
| − | Magnetotellurics (MT) is an electrical geophysical technique that measures the resistivity of the subsurface. Although MT cannot provide the resolution of [[Seismic data|seismic surveys]], it is less expensive and, more importantly, can be used in places where seismic data collection is impractical or gives poor results. | + | Magnetotellurics (MT) is an electrical geophysical technique that measures the [[Electrical_methods#Electrical_properties_of_materials|resistivity]] of the subsurface. Although MT cannot provide the resolution of [[Seismic data|seismic surveys]], it is less expensive and, more importantly, can be used in places where seismic data collection is impractical or gives poor results. This is the same physical parameter that is measured in a borehole [[Basic_open_hole_tools#Resistivity|resistivity log]]. |
| − | ==How | + | ==How magnetotellurics differs from electric logs== |
| − | [[File:Applying-magnetotellurics fig16-1.png|thumbnail|{{figure number|1}}Simplified relationship between a lithologic log, an electric log, an MT sounding, and an inversion run using the MT sounding data.]] | + | [[File:Applying-magnetotellurics fig16-1.png|thumbnail|300px|{{figure number|1}}Simplified relationship between a lithologic log, an electric log, an MT sounding, and an inversion run using the MT sounding data.]] |
MT differs from an inductive electric log in three major ways: | MT differs from an inductive electric log in three major ways: | ||
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| − | [[:Applying-magnetotellurics fig16-1.png|Figure 1]] shows the simplified relationship between a lithologic log, an electric log, an MT sounding, and an inversion run using the MT sounding data. We can also take electric log data and run a forward MT model to produce an MT sounding curve. | + | [[:file:Applying-magnetotellurics fig16-1.png|Figure 1]] shows the simplified relationship between a lithologic log, an electric log, an MT sounding, and an inversion run using the MT sounding data. We can also take electric log data and run a forward MT model to produce an MT sounding curve. |
==Subsurface layers resolved== | ==Subsurface layers resolved== | ||
| − | Subsurface layers are resolved by inverse modeling of MT data acquired across a | + | Subsurface layers are resolved by inverse modeling of MT data acquired across a spectrum of frequencies, as illustrated in [[:file:Applying-magnetotellurics fig16-1.png|Figure 1]]. |
| − | == | + | ==Magnetotellurics resolution== |
| − | The rule | + | The rule of thumb for MT resolution of depth of burial vs. layer thickness is 10:1. For example, to “see” a layer at a depth of 1,500 m (5,000 ft), the thickness of the layer needs to be approximately 150 m (500 ft) or more. Low-resistivity layers are more easily delineated than high-resistivity layers. It is difficult for MT to resolve more than three or four subsurface layers. |
==See also== | ==See also== | ||
| − | * [[ | + | * [[Magnetotellurics survey measurements]] |
| − | + | * [[Magnetotelluric data acquisition]] | |
| − | * [[ | + | * [[Magnetotellurics case history: frontier basin analysis (Amazon Basin, Colombia)]] |
| − | * [[ | + | * [[Magnetotellurics case history: rugged carbonate terrain (Highlands of Papua New Guinea)]] |
| − | * [[ | + | * [[Magnetotellurics case history: Precambrian overthrust (Northwestern Colorado)]] |
| − | * [[ | + | * [[Magnetotellurics case history: volcanic terrain (Columbia River Plateau)]] |
| − | * [[ | ||
==External links== | ==External links== | ||
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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]] | ||
Latest revision as of 17:53, 25 January 2022
| Exploring for Oil and Gas Traps | |
| |
| Series | Treatise in Petroleum Geology |
|---|---|
| Part | Predicting the occurrence of oil and gas traps |
| Chapter | Applying magnetotellurics |
| Author | Arnie Ostrander |
| Link | Web page |
| Store | AAPG Store |
Magnetotellurics (MT) is an electrical geophysical technique that measures the resistivity of the subsurface. Although MT cannot provide the resolution of seismic surveys, it is less expensive and, more importantly, can be used in places where seismic data collection is impractical or gives poor results. This is the same physical parameter that is measured in a borehole resistivity log.
How magnetotellurics differs from electric logs[edit]
MT differs from an inductive electric log in three major ways:
| Magnetotellurics Measurements | Electric Log Measurements |
|---|---|
| Made from the surface | Made subsurface from inside a borehole |
| Depth of investigation is a function of both frequency at which the measurement is taken and the average resistivity of the subsurface | Depth of investigation is the depth of the borehole measuring device below the surface |
| Respond only to changes in average bulk resistivity | Respond to individual rock layers along the wall of the borehole |
Figure 1 shows the simplified relationship between a lithologic log, an electric log, an MT sounding, and an inversion run using the MT sounding data. We can also take electric log data and run a forward MT model to produce an MT sounding curve.
Subsurface layers resolved[edit]
Subsurface layers are resolved by inverse modeling of MT data acquired across a spectrum of frequencies, as illustrated in Figure 1.
Magnetotellurics resolution[edit]
The rule of thumb for MT resolution of depth of burial vs. layer thickness is 10:1. For example, to “see” a layer at a depth of 1,500 m (5,000 ft), the thickness of the layer needs to be approximately 150 m (500 ft) or more. Low-resistivity layers are more easily delineated than high-resistivity layers. It is difficult for MT to resolve more than three or four subsurface layers.
See also[edit]
- Magnetotellurics survey measurements
- Magnetotelluric data acquisition
- Magnetotellurics case history: frontier basin analysis (Amazon Basin, Colombia)
- Magnetotellurics case history: rugged carbonate terrain (Highlands of Papua New Guinea)
- Magnetotellurics case history: Precambrian overthrust (Northwestern Colorado)
- Magnetotellurics case history: volcanic terrain (Columbia River Plateau)
External links[edit]
| find literature about Magnetotellurics |
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