Fig. 7From: Cross-validation of independent ultra-low-frequency magnetic recording systems for active fault studiesSpectrograms for the time interval April 5, 2014 to May 12, 2014. a QFido3 north coil; b Zonge ANT4 north and east coils; c EMI BF4 north, east, and EMI BF7 vertical coils. The black triangles along the upper plot border show times of high geomagnetic activity (Ap > 29)Back to article page