Difference between revisions of "Magnetotellurics"
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==How MT differs from electric logs== | ==How MT differs from electric logs== | ||
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| + | [[Applying-magnetotellurics fig16-1.png|thumb|{{figure number|1}} Simplified relationship between a lithologic log, an electric log, an MT sounding, and an inversion run using the MT sounding data.]] | ||
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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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| Respond only to changes in average bulk resistivity || Respond to individual rock layers along the wall of the borehole | | Respond only to changes in average bulk resistivity || Respond to individual rock layers along the wall of the borehole | ||
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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. | ||
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| + | ==Subsurface layers resolved== | ||
| + | Subsurface layers are resolved by inverse modeling of MT data acquired across a spec- trum of frequencies, as illustrated in [[:Applying-magnetotellurics fig16-1.png|Figure 1]]. | ||
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| + | ==MT resolution== | ||
| + | The rule-of-thumb for MT resolution for 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== | ||
Revision as of 18:59, 17 April 2014
| 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 resis- tivity log.
How MT 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 spec- trum of frequencies, as illustrated in Figure 1.
MT resolution[edit]
The rule-of-thumb for MT resolution for 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]
- What is magnetotellurics (MT)?
- What does an MT survey measure?
- How are MT data acquired?
- 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)
External links[edit]
| find literature about Magnetotellurics |
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