Transient electromagnetics
Encyclopedia
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 area very common surface EM technique for mineral exploration, groundwater exploration, and for environmental mapping, used throughout the world in both onshore
and offshore
applications.
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.
These currents produce a magnetic field by Faraday's law. At the surface, the change in magnetic field [flux] with 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 explaining the basics of the method: Geonics technical notes.
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. 2.5kW) can operate with automotive batteries; more powerful systems (20kW~150kW) require truck-mounted generators to provide the necessary current for deep investigations.
Transient (oscillation)
A transient event is a short-lived burst of energy in a system caused by a sudden change of state.The source of the transient energy may be an internal event or a nearby event...
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 area very common surface EM technique for mineral exploration, groundwater exploration, and for environmental mapping, used throughout the world in both onshore
Onshore
Onshore is a term used in finance to denote the jurisdiction in which a company is domiciled and in which it pays a significant rate of tax. It is the opposite of offshore, which denotes a tax haven...
and offshore
Offshore drilling
Offshore drilling refers to a mechanical process where a wellbore is drilled through the seabed. It is typically carried out in order to explore for and subsequently produce hydrocarbons which lie in rock formations beneath the seabed...
applications.
Physical principles
Two fundamental electromagnetic principles are required to derive the physics behind TEM surveys: Faraday's law of inductionFaraday's law of induction
Faraday's law of induction dates from the 1830s, and is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators...
and Lenz's Law
Lenz's law
Lenz's law is a common way of understanding how electromagnetic circuits must always obey Newton's third law and The Law of Conservation of Energy...
. 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.
These currents produce a magnetic field by Faraday's law. At the surface, the change in magnetic field [flux] with 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 explaining the basics of the method: Geonics technical notes.
TEM/TDEM instrumentation and sensors
TEM/TDEM systems consist of a transmitterTransmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
instrument, transmitting coil or transmitting wire, receiver
Receiver
-Technology:* The listening device part of a telephone** The handset that sometimes contains the above device** The headset that can also contain the above device.* Audio-Visual receiver* Digital media 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. 2.5kW) can operate with automotive batteries; more powerful systems (20kW~150kW) require truck-mounted generators to provide the necessary current for deep investigations.
Commercial applications
- MiningMiningMining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...
(mineral location and characterization) - GroundwaterGroundwaterGroundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
characterization - HVDCHigh-voltage direct currentA high-voltage, direct current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses...
injection point mapping
See also
- GeophysicsGeophysicsGeophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
- Exploration geophysicsExploration geophysicsExploration geophysics is the applied branch of geophysics which uses surface methods to measure the physical properties of the subsurface Earth, in order to detect or infer the presence and position of ore minerals, hydrocarbons, geothermal reservoirs, groundwater reservoirs, and other geological...
, a branch of geophysics for discovering and mapping mineral resources and useful geological structures - MagnetotelluricsMagnetotelluricsMagnetotellurics is an electromagnetic geophysical method of imaging the earth's subsurface by measuring natural variations of electrical and magnetic fields at the Earth's surface. Investigation depth ranges from 300m below ground by recording higher frequencies down to 10,000m or deeper with...
- Seismo-electromagneticsSeismo-electromagneticsSeismo-electromagnetics is the study of electromagnetic phenomena associated with seismic activity such as earthquakes and volcanos, and also the use of electromagnetic methods in seismology such as magnetotellurics. Links between the electromagnetic fields in the lithosphere and those in the...
- Reflection seismologyReflection seismologyReflection seismology is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite/Tovex, a specialized air gun or a...
- Electrical resistivity tomographyElectrical resistivity tomographyElectrical resistivity tomography or electrical resistivity imaging is a geophysical technique for imaging sub-surface structures from electrical measurements made at the surface, or by electrodes in one or more boreholes. It is closely related to the medical imaging technique electrical...
, another geophysical technique of imaging