Magnetotelluric data acquisition

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

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

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

Natural-source surveys

Figure 1 Typical MT setup for a natural-source survey.

The natural-source data-acquisition system typically measures four components: E_x, E_y, H_x, and H_y. The E_x component is oriented perpendicular to the E_y 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 H_x and H_y components are measured), the vertical H-field component—if measured—provides information on the surface geology.

Figure 1 shows a typical MT setup for a natural-source survey.

Controlled-source surveys

Figure 2 Typical MT setup for a controlled-source survey.

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 E_x (parallel to the transmitter dipole) and H_y components are measured.

Figure 2 shows a typical MT setup for a controlled-source survey.

Which method is better?

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

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

• Magnetotellurics
• Magnetotellurics survey measurements
• 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 Magnetotelluric data acquisition

• Original content in Datapages
• Find the book in the AAPG Store

middle tubing