At an undisclosed test site, a helicopter takes off, towing a kite-like array of massive coils. The system, called Airborne Transient Electromagnetic (Atem) detection, works by firing a powerful pulse of electricity through a giant transmitter coil. This creates a brief, strong electromagnetic field that penetrates the ground or water. When the pulse is turned off, the magnetic field induces tiny, decaying “eddy currents” in any conductive material it hits. These currents, in turn, create their...

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