Airborne electromagnetic methods: applications to minerals, water and hydrocarbon exploration
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Résumé
Electromagnetic methods have a transmitter that carries a current that varies in magnitude as a function of time.This current has an associated (primary) magnetic field that has a similar time dependence.According to Faraday's law of induction, this time varying field induces currents in conductive features in the ground.These currents have an associated (secondary) magnetic field that can be detected by an electromagnetic receiver.There is no need for the transmitter or the receiver to touch the ground, so electromagnetic systems can be mounted on aircraft and used to cover large areas quickly and efficiently.In time domain EM systems, the time variation of the current is a switch on, followed by a rapid switch off.In general, good conductors have secondary field responses which decay slowly after the switch off.One example from the Shea Creek area of the Athabasca Basin (northern Saskatchewan) shows that these good conductors can be detected at depths as great as 700m when the conductor is large and the intervening material is highly resistive.Poor conductors have responses that decay away rapidly.The alteration above the Millennium deposit (also in the Athabasca Basin) is an example of a response that decays away in about 300 microseconds.Historically, airborne electromagnetic methods have been most successful for massive sulphide exploration.Electromagnetic methods are being utilized in the search for fresh water.In an example from Denmark, the method has been used to successfully map the thickness of a freshwater aquifer.Using airborne methods, an area of more than 100 km2 was covered in a few days surveying, where it would have taken months to cover the same area using ground methods.In sedimentary basins, the decay of the electromagnetic response can be used to infer the conductivity as a function of depth.The shallower part of the section is inferred from the early time data and the late time data is used to estimate the deeper parts of the section.When the ground is about 10 Ohmm, airborne electromagnetic systems can only see about 300 m deep.Hence they cannot see most hydrocarbon deposits.However, electromagnetic methods have been a useful compliment to high resolution seismic in the search for shallow gas in some of the paleochannels in Alberta.They have also been used to better understand the oil sands environment.
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| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,001 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,001 | 0,000 |
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