Instrumentation was devised for a Navy program, in the 1990 time frame, to search for the ELF signatures of re-entering spacecraft in the range of about 0.1 Hz to 40 Hz.

There were three orthogonal coils, each wound with about 90,000 turns on a hi permeability core.

The view here is only of one coil.  The coil and a pre-amplifier were contained within an electrostatic shield.  The output was fed to another amplifier and through a low pass/ 60 Hz notch filter, located some hundreds of feet away in a field house, to a spectrum analyzer.

The system apparently detected not only re-entering spacecraft, but project personnel began to wonder if they might also be detecting earthquake precursory signals.