physics

Transient electromagnetics

Transient electromagnetics is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Transient electromagnetics rather than just read about it. In short: 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 detecti…

Key takeaways

Transient electromagnetics belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Transient electromagnetics to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Transient electromagnetics from memory before moving on to harder problems.

Reference excerpt

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. In the 1960s and 1970s, the method was actively developed in the works of Soviet scientists, including A. A. Kaufman and V. A. Sidorov.

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 [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.

TEM/TDEM instrumentation and sensors TEM/TDEM systems consist of a transmitter 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 (mineral location and characterization) Groundwater characterization HVDC injection point mapping Oil and gas exploration

See also Geophysics Exploration geophysics, 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

Worked examples

Example 1 — a first encounter with Transient electromagnetics

Start with the simplest possible case. Write down what Transient electromagnetics claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Transient electromagnetics before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Transient electromagnetics ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Transient electromagnetics

In research: Transient electromagnetics appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Transient electromagnetics in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Transient electromagnetics is common in secondary-school and first-year university syllabi. It links to neighbouring topics Geophysical imaging, so understanding it makes those chapters shorter.
In everyday life: Look for Transient electromagnetics outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Transient electromagnetics in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Transient electromagnetics means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Transient electromagnetics out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Transient electromagnetics in simple terms?

Why does Transient electromagnetics matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Transient electromagnetics?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Transient electromagnetics.