Magnetotellurics survey measurements

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

What is measured?

Two basic alternating current (AC) measurements are taken in an MT survey: a horizontal magnetic field (H-field) measurement and an electrical field (E-field) measurement. The E-field is always measured perpendicular to the H-field data.

The H-field

The H-field is the “source” signal, or the primary field. It propagates across the surface of the earth. Because it does not travel in the subsurface, the H-field data do not provide information about the subsurface geology.

Very limited information about the subsurface geology can be interpreted from the vertical H-field if this component is measured. The vertical H-field is called the tipper.

The horizontal H-field is measured with a horizontally oriented magnetic coil. The tipper is measured with a vertically oriented coil.

Be careful not to confuse an MT survey with a magnetic survey. An MT survey does not measure the magnetic properties of the subsurface rocks, as does a magnetic survey.

The E-field

Figure 1 Relationship between the E- and H-fields.

The E-field is the secondary field, generated by the H-field propagating across the surface. Each time the primary H-field (an AC signal) switches polarity, a secondary E-field (current flow) is generated in the subsurface. Thus, the horizontal E-field data provides information about the subsurface geology.

This is the same physical principle as the alternator in a car. An alternating or spinning magnetic field (H-field) sets up current flow in the wire windings in the alternator, which in turn charges the battery. In the case of an MT survey, the “wire” is the earth.

The E-field is measured with a grounded dipole typically 50–200 m long. All subsurface geology information is contained in the E-field data. However, without the H-field data, we cannot calculate resistivity.

Figure 1 shows the relationship between the E- and H-fields.

Resistivity calculation

The resistivity calculation is a simple ratio of the primary source signal (H-field) and the secondary current flow in the earth (E-field), with a modifier for the frequency at which the data were acquired:

${\mbox{Apparent Resistivity}}=\left({\frac {\mbox{E}}{\mbox{H}}}\right)^{2}\times {\frac {1}{5{\mbox{f}}}}$

where:

• E = magnitude of the E-field
• H = magnitude of the H-field
• f = frequency

See also

• Magnetotellurics
• 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

find literature about Magnetotellurics survey measurements

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