Changes

  | part    = Predicting the occurrence of oil and gas traps

  | part    = Predicting the occurrence of oil and gas traps

  | chapter = Applying magnetotellurics

  | chapter = Applying magnetotellurics

  | frompg  = 16-1

  | frompg  = 16-6

  | topg    = 16-12

  | topg    = 16-7

  | author  = Arnie Ostrander

  | author  = Arnie Ostrander

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch16/ch16.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch16/ch16.htm

==Acquisition instrumentation==

==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.

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

==Types of surveys==

==Types of surveys==

There are two types of MT surveys: natural source.<ref name=ch16r6>Vozoff, K., 1972, The magnetotelluric method in the exploration of sedimentary basins: Geophysics, vol. 37, no. 1, p. 98–141., 10., 1190/1., 1440255</ref> and controlled source<ref name=ch16r3>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</ref> The equipment and the operational procedures for these two types differ considerably.

There are two types of magnetotelluric (MT) surveys: natural source.<ref name=ch16r6>Vozoff, K., 1972, The magnetotelluric method in the exploration of sedimentary basins: Geophysics, vol. 37, no. 1, p. 98–141., 10., 1190/1., 1440255</ref> and controlled source<ref name=ch16r3>Goldstein, M. A., and D. W. Strangway, 1975, Audio-frequency magnetotellurics with a grounded electrical dipole source: Geophysics, vol. 40, p. 669–683., 10., 1190/1., 1440558</ref> The equipment and the operational procedures for these two types differ considerably.

==Natural-source surveys==

==Natural-source surveys==

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 natural-source data-acquisition system typically measures four components: two in the horizontal field, E_x, E_y, and two in the electrical field, H_x, and H_y. (See [[Magnetotellurics survey measurements]].) 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.

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

[[:file:applying-magnetotellurics_fig16-3.png|Figure 1]] shows a typical MT setup for a natural-source survey.

[[:file:applying-magnetotellurics_fig16-3.png|Figure 1]] shows a typical MT setup for a natural-source survey.

[[file:applying-magnetotellurics_fig16-4.png|thumb|{{figure number|2}}]]

==Controlled-source surveys==

==Controlled-source surveys==

[[file:applying-magnetotellurics_fig16-4.png|thumb|300px|{{figure number|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.

 

The controlled-source system uses a high-power transmitter and motor or generator set to transmit a discrete alternating current 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.

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

==Which method is better?==

==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.

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; see [[Magnetotellurics case history: frontier basin analysis (Amazon Basin, Colombia)]]). 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 [[Wikipedia:Volcanic_rock|volcanic]], [[carbonate]], or [http://geology.about.com/od/more_igrocks/a/granite.htm granite] [[Thrust belt|overthrust]] terrain. Natural-source data have considerably deeper penetration but poorer resolution at shallower depths.

==Where to use MT==

==Where to use MT==

{| class = "wikitable"

{| class = "wikitable"

|-

|-

! Locations

! Locations || Reasons for Using MT

! Reasons for Using MT

|-

|-

| Carbonate terrains

| Carbonate terrains || Poor-quality seismic data

| Poor-quality seismic data

|-

|-

| Volcanic terrains

| Volcanic terrains || Poor-quality seismic data

| Poor-quality seismic data

|-

|-

| Granite overthrusts

| Granite overthrusts || Poor-quality seismic data

| Poor-quality seismic data

|-

|-

| Regional surveys

| Regional surveys || Less expensive than seismic; generates prospects to detail with seismic

| Less expensive than seismic; generates prospects to detail with seismic

|-

|-

| Remote areas

| Remote areas || Less expensive than seismic

| Less expensive than seismic

|-

|-

| Rugged terrains

| Rugged terrains || Less expensive than seismic

| Less expensive than seismic

|-

|-

| Fracture zones

| [[Fracture]] zones || Excellent tool for mapping

| Excellent tool for mapping

|}

|}

==See also==

==See also==

* [[What is magnetotellurics?]]

* [[Magnetotellurics]]

* [[What does an MT survey measure?]]

* [[Magnetotellurics survey measurements]]

* [[Case history: frontier basin analysis (Amazon Basin, Colombia)]]

* [[Magnetotellurics case history: frontier basin analysis (Amazon Basin, Colombia)]]

* [[Case history: rugged carbonate terrain (Highlands of Papua New Guinea)]]

* [[Magnetotellurics case history: rugged carbonate terrain (Highlands of Papua New Guinea)]]

* [[Case history: Precambrian overthrust (Northwestern Colorado)]]

* [[Magnetotellurics case history: Precambrian overthrust (Northwestern Colorado)]]

* [[Case history: volcanic terrain (Columbia River Plateau)]]

* [[Magnetotellurics case history: volcanic terrain (Columbia River Plateau)]]

==References==

==References==

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Applying magnetotellurics]]

[[Category:Applying magnetotellurics]]