We took measurements in two locations chosen to be far enough from evident infrastructural sources of magnetic noise: a first location is along the "scolmatore" bank, about 500m E from the EGO gate,
a second location is in the coutryside, at least 1km far (judging by eye)
from electroducts (Figure 2 provides a map). We laid the magnetometer on ground along NS and then EW directions.
Figure 3 is a plot of the magnetic field modulus (square root of the sum of ew and ns amplitude spectra squared). Our measurements (in red and blue) are compared with the noise measured inside the Central Building, both in standard conditions and when in a previous experiment all CB electric loads were powered on the IPS line (UPS switch off). These measurements are also compared to the spectrum of "natural magnetic field variations on a quiet day" as reported in the magnetometer manual.
Our measurements seem consistent with the low natural noise model.
The drop below 4 Hz is due to the sensor response function, while the sensor noise is (as from specs) at least one order of magnitude below the measured noise. The 50Hz noise is at 0.2nT (still the most intense component). But has peculiar sidebands at +-8Hz which seem present in both locations, more pronounced in the "countryside" location.
