Magnetotellurics

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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 (MTmiddle tubing) is an electrical geophysical technique that measures the resistivity of the subsurface. Although MTmiddle tubing 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

Figure 1 Simplified relationship between a lithologic log, an electric log, an MTmiddle tubing sounding, and an inversion run using the MTmiddle tubing sounding data.

MTmiddle tubing 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 MTmiddle tubing sounding, and an inversion run using the MTmiddle tubing sounding data. We can also take electric log data and run a forward MTmiddle tubing model to produce an MTmiddle tubing sounding curve.

Subsurface layers resolved

Subsurface layers are resolved by inverse modeling of MTmiddle tubing data acquired across a spectrum of frequencies, as illustrated in Figure 1.

Magnetotellurics resolution

The rule of thumb for MTmiddle tubing 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 MTmiddle tubing to resolve more than three or four subsurface layers.

See also

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