Question 6 of 10: The Electromagnetic Method — Governing Physics, Transmitter/Receiver Operation and a Case History

Question 6: The Electromagnetic Method — Governing Physics, Transmitter/Receiver Operation and a Case History (16.66% of paper)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Governing physics. The EM method rests on three linked electromagnetic laws. Faraday's law of induction states that a time-varying magnetic field induces an electromotive force (and hence, in a conductor, a current) in any closed loop threading that field — this is what allows a transmitter's changing field to induce currents in the ground at all. Ampere's law (with Maxwell's displacement-current correction) states that an electric current, in turn, produces its own magnetic field — this is what allows the eddy currents induced in the ground to generate a secondary magnetic field of their own. Lenz's law states that the induced current, and hence the secondary field it produces, always opposes the change in the primary field that created it — which fixes the secondary field's phase relationship (phase-lagged, and of opposing sense) to the primary, the basis for separating primary and secondary signal at the receiver.

Transmitter excitation and receiver response. The transmitter is a coil (or long grounded wire) carrying a controlled, time-varying current, which by Ampere's law radiates a primary magnetic field into the ground. Where this changing primary field threads a conductive body or layer, Faraday's law induces eddy currents in it; those currents, by Ampere's law, generate their own secondary magnetic field, phase-lagged per Lenz's law relative to the primary. A receiver coil some distance away then measures the combination of the (known) primary field plus this secondary field. In a frequency-domain (continuous-wave) system, the receiver resolves the response into in-phase and quadrature components relative to the transmitter waveform; in a time-domain system, the transmitter current is switched off abruptly, and the receiver measures only the decaying secondary field during the "off-time" window, once the primary field itself has vanished — so there is no large primary signal to subtract, which lets the system record the slow late-time decay of good conductors and gives the depth penetration airborne TDEM is known for.

Case history. Airborne time-domain EM (e.g. VTEM- or SkyTEM-type systems) is a workhorse tool in massive-sulphide exploration: a large conductive sulphide lens produces a strong, slowly-decaying secondary-field response distinguishable from the faster-decaying response of the surrounding, more resistive host rock, allowing rapid reconnaissance mapping of conductor targets over large, often heavily vegetated or lake-covered terrain that would be slow and expensive to cover on the ground. The same physics is applied geotechnically at the ground scale — e.g. a Slingram/ground-conductivity survey to trace a buried, highly conductive contaminant plume in groundwater, or to locate buried metallic drums/utilities — wherever the target of interest presents a strong conductivity contrast with its surroundings.