Induction coil antennas (ELF magnetometers)

The horizontal magnetic components of the SR are recorded with induction coil antennas (ELF magnetometers). The coil has many thousands of turns wound on a ferromagnetic core, which concentrates the magnetic flux and increases sensitivity. The induced voltage is proportional to the time rate of change of the magnetic induction, that is, u \propto \mathrm{d}B/\mathrm{d}t Votis 2018.

An example of a portable solution is the receiver described in Votis 2018, where two coils placed back to back with a total length of approximately 60 cm are used. The analog chain achieves a gain of around 112 dB at 10 Hz and an equivalent input noise of roughly 2.88 nV/√Hz. Thanks to this, clean peaks of the first six resonance modes are visible already with an acquisition time of around 10 minutes.

Because the induced voltage increases with frequency (the time derivative), the coil's response is frequency-dependent and must be accounted for in further processing by calibration (see #39). Two mutually perpendicular coils oriented north-south and east-west provide both horizontal magnetic components needed for directional analysis.