Magnetotelluric data acquisition

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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

Acquisition instrumentation[edit]

The data are collected using a microprocessor-controlled voltmeter. The voltmeter is in fact a system of complex hardware/software devices that includes amplification, filtering. A/D conversion, stacking and averaging, and various data-enhancement algorithms.

Types of surveys[edit]

There are two types of MT surveys: natural source.[1] and controlled source[2] The equipment and the operational procedures for these two types differ considerably.

Natural-source surveys[edit]

The natural-source data-acquisition system typically measures four components: Ex, Ey, Hx, and Hy. The Ex component is oriented perpendicular to the Ey component. This is also true for the H-field components.

The predominant low-frequency ( 1.0 Hz) source is equatorial thunderstorm activity.

Although H-field data do not provide information on the subsurface geology (when only Hx and Hy components are measured), the vertical H-field component—if measured—provides information on the surface geology.

The figure below shows a typical MT setup for a natural-source survey.

Figure 16-3 

Controlled-source surveys[edit]

The controlled-source system uses a high-power transmitter and motor/generator set to transmit a discrete AC waveform. This signal is transmitted into a grounded dipole typically 600–1,200 m (2,000–4,000 ft) long. The transmitter is normally located 3–6 km (2–4 mi) from the survey line.

Normally, only the Ex (parallel to the transmitter dipole) and Hy components are measured.

The figure below shows a typical MT setup for a controlled-source survey.

Figure 16-4 

Which method is better?[edit]

The choice of MT method depends on the survey objectives. Natural-source data are best suited for regional surveys where the stations are widely spaced (e.g., frontier basin analysis). Controlled-source data are best suited for mapping structural detail where the stations lie along a continuous profile at 100–200-m (300–600-ft) spacings. The maximum depth of exploration for the controlled-source method is 3,000–4,500 m (10,000–15,000 ft) in a typical volcanic, carbonate, or granite overthrust terrain. Natural-source data have considerably deeper penetration but poorer resolution at shallower depths.

Where to use MT[edit]

MT can be valuable in areas that yield poor-quality seismic data and where acquiring seismic data is very expensive. The following table indicates where to use MT and the reasons for using it.

Locations Reasons for Using MT
Carbonate terrains Poor-quality seismic data
Volcanic terrains Poor-quality seismic data
Granite overthrusts Poor-quality seismic data
Regional surveys Less expensive than seismic; generates prospects to detail with seismic
Remote areas Less expensive than seismic
Rugged terrains Less expensive than seismic
Fracture zones Excellent tool for mapping

See also[edit]

References[edit]

  1. Vozoff, K., 1972, The magnetotelluric method in the exploration of sedimentary basins: Geophysics, vol. 37, no. 1, p. 98–141., 10., 1190/1., 1440255
  2. Goldstein, M., A., Strangway, D., W., 1975, Audio-frequency magnetotellurics with a grounded electrical dipole source: Geophysics, vol. 40, p. 669–683., 10., 1190/1., 1440558

External links[edit]

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Magnetotelluric data acquisition
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