In the Central-Loop Transient Electromagnetic method, a steady current is transmitted in a wire loop, laid on the ground at the area to be examined (below). This current is allowed to flow for a sufficiently long time to allow turn-on transients in the ground to dissipate. This steady current is then abruptly terminated in a controlled fashion.
At the instant of transmitter turn-off, eddy currents reproduce a static magnetic field due to the transmitter loop but then decay rapidly. The decaying primary magnetic field induces eddy currents immediately below the transmitter loop in accordance with Faraday’s law. As the initial near-surface eddy current decays, its distribution in the ground in turn induces a secondary magnetic field which also decays with time. This process continues over time with an ever-weakening secondary magnetic field inducing currents at increasing depths.
The magnitude and rate of decay of the secondary magnetic field is monitored by measuring the voltage induced in a receiver coil, placed at the centre of the transmitter loop, as a function of time after the transmitter current is turned off. This is then interpreted in terms of a subsurface resistivity structure.
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