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Transient electromagnetics, (also time-domain electromagnetics / TDEM), is a geophysical exploration technique in which electric and magnetic fields are induced by transient pulses of electric current and the subsequent decay response measured. TEM / TDEM methods are generally able to determine subsurface electrical properties, but are also sensitive to subsurface magnetic properties in applications like UXO detection and characterization.
TEM/TDEM surveys are a very common surface EM technique for mineral exploration, groundwater exploration, and for environmental mapping, used throughout the world in both onshore and offshore applications.
Two fundamental electromagnetic principles are required to derive the physics behind TEM surveys: Faraday's law of induction and Lenz's Law. A loop of wire is generally energized by a direct current. At some time (''t0'') the current is cut off as quickly as possible. Faraday's law dictates that a nearly identical current is induced in the subsurface to preserve the magnetic field produced by the original current (eddy currents). Due to ohmic losses, the induced surface currents dissipate—this causes a change in the magnetic field, which induces subsequent eddy currents. The net result is a downward and outward diffusion of currents in the subsurface which appear as an expanding smoke ring when the current density is contoured.
These currents produce a magnetic field by Faraday's law. At the surface, the change in magnetic field luxwith time is measured. The way the currents diffuse in the subsurface is related to the conductivity distribution in the ground.
This is a basic view of the physical principles involved. When conductive bodies are present, the diffusion of the transients is changed. In addition, transients are induced in the conductive bodies as well. This is only the most basic overview. The paper by McNeill is freely available from the Geonics website{{cite web, url=http://geonics.com/html/technicalnotes.html, title=Geonics Limited Publications, accessdate=13 June 2014 explaining the basics of the method.
Physical principles
Two fundamental electromagnetic principles are required to derive the physics behind TEM surveys: Faraday's law of induction and Lenz's Law. A loop of wire is generally energized by a direct current. At some time (''t0'') the current is cut off as quickly as possible. Faraday's law dictates that a nearly identical current is induced in the subsurface to preserve the magnetic field produced by the original current (eddy currents). Due to ohmic losses, the induced surface currents dissipate—this causes a change in the magnetic field, which induces subsequent eddy currents. The net result is a downward and outward diffusion of currents in the subsurface which appear as an expanding smoke ring when the current density is contoured.
These currents produce a magnetic field by Faraday's law. At the surface, the change in magnetic field luxwith time is measured. The way the currents diffuse in the subsurface is related to the conductivity distribution in the ground.
This is a basic view of the physical principles involved. When conductive bodies are present, the diffusion of the transients is changed. In addition, transients are induced in the conductive bodies as well. This is only the most basic overview. The paper by McNeill is freely available from the Geonics website{{cite web, url=http://geonics.com/html/technicalnotes.html, title=Geonics Limited Publications, accessdate=13 June 2014 explaining the basics of the method.
TEM/TDEM instrumentation and sensors
TEM/TDEM systems consist of atransmitter
In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna (radio), antenna with the purpose of sig ...
instrument, transmitting coil or transmitting wire, receiver coil or antenna, and receiver instrument. Depending on subsurface resistivity, current induced, receiver sensitivity and transmitter-receiver geometry, TEM/TDEM measurements allow geophysical exploration from a few metres below the surface to several hundred metres of depth.
Low-power TEM/TDEM instruments can operate using C-cell batteries, and mid-range systems (approx. 2.5 kW) can operate with automotive batteries; more powerful systems (20 kW~150 kW) require a separate generator usually mounted on a truck, plane, or helicopter to provide the necessary current for deep investigations.
Commercial applications
*Mining
Mining is the Resource extraction, extraction of valuable geological materials and minerals from the surface of the Earth. Mining is required to obtain most materials that cannot be grown through agriculture, agricultural processes, or feasib ...
(mineral location and characterization)
* Groundwater
Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
characterization
* HVDC
A high-voltage direct current (HVDC) electric power transmission system uses direct current (DC) for electric power transmission, in contrast with the more common alternating current (AC) transmission systems. Most HVDC links use voltages betwe ...
injection point mapping
* Oil and gas exploration
See also
*Geophysics
Geophysics () is a subject of natural science concerned with the physical processes and Physical property, properties of Earth and its surrounding space environment, and the use of quantitative methods for their analysis. Geophysicists conduct i ...
* Exploration geophysics
Exploration geophysics is an applied branch of geophysics and economic geology, which uses physical methods at the surface of the Earth, such as seismic, gravitational, magnetic, electrical and electromagnetic, to measure the physical properties o ...
, a branch of geophysics for discovering and mapping mineral resources and useful geological structures
* Magnetotellurics
* Seismo-electromagnetics
* Reflection seismology
* Electrical resistivity tomography, another geophysical technique of imaging
* SNMR
References