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[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.
Figure 1 shows a typical MT setup for a natural-source survey.
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.
Figure 2 shows a typical MT setup for a controlled-source survey.
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]
- Magnetotellurics
- Magnetotellurics survey measurements
- 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)
References[edit]
- ↑ Vozoff, K., 1972, The magnetotelluric method in the exploration of sedimentary basins: Geophysics, vol. 37, no. 1, p. 98–141., 10., 1190/1., 1440255
- ↑ 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]
| find literature about Magnetotelluric data acquisition |
|
|
|
|