Magnetotellurics

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 magnetotellurics differs from electric logs

Figure 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:

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

Subsurface layers are resolved by inverse modeling of MT data acquired across a spec- trum of frequencies, as illustrated in Figure 1.

Magnetotellurics 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

• 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)
• Case history: Precambrian overthrust (Northwestern Colorado)
• Case history: volcanic terrain (Columbia River Plateau)

External links

find literature about Magnetotellurics

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