Fig. 4From: Cross-validation of independent ultra-low-frequency magnetic recording systems for active fault studiesa Amplitude spectra and b phase spectra of the response function of EMI BF4, EMI BF7, Zonge ANT4, and QFido3 coils. Phase shifts are plotted between − 180° and 180°. The QuakeFinder system low-pass filters manifest as “phase wrapping” near 10 Hz in bBack to article page