We finally found the source of the "Magnetic Monster" (as we friendly named it) that was plaguing Virgo external magnetometers, making them blind in the Schumann resonance band.
It is a 1V 2A DC power supply placed ... 4km from the CEB !
This is a system used to maintain an underground Natual Gas pipeline at a given voltage level relative to the ground, in order to avoid the so-called Galvanic corrosion. The pipeline starts from the SNAM Mortaiolo center station, towards Cenaia - Palaia.
This DC power supply, for unknown reason, modulate (chopper) its current a few times in one second (3 or 5 Hz). But it is not defective. Another good power supply put in its place have the same behavior. It is as if the resistance of the tube changes 5 times in one second (and this is impossible). Or, more likely, there is some other local current being injected into the tube and each power supply connected to it tries to counteract with another opposite-sign current.
Whatever the reason, we have been forced to leave things as they are for the moment. The SNAM company has become aware of the problem, they will call a meeting with the technicians to understand the reason for this behavior (not found in all the other pipellines starting from the Mortaiolo center station).
This hunt began in November 2021 when we discovered the M.M. and it took many people for many days to find the source. We measured something like a hundred places outside Virgo, with a portable battery powered system. The most active Noise Hunters were Jean-Loup Raymond and Lorenzo Pierini (thank you guys).
Attached is the map with a (presumed) drawing of the underground pipe in orange and the position of the DC power supply (the red square on the left), and the spectrogram showing the signal that disappears when we turn off the power supply.
