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Environmental Monitoring (Environmental Monitoring)
fiori, paoletti, renato.romero, tringali, direnzo - 19:52 Saturday 27 April 2024 (64116) Print this report
low frequency magnetic disturbance site-wide

A new noise is seen in the external magnetometer starting April 22 19:56 UTC. Figures 1 and 2. The noise has been detected and studied by Renato and Federico. Here is a report of the investigation so far.

The noise is impulsive, as seen in Figures 3,4,5. Observed in the time domain it consists of a carrier at about 8Hz switching on-off with a cycle of approximately 4.4s (0.23 Hz): on for 2.2s and off for 2.2s.

The noise is seen by all magnetometers site-wide. Internal magnetometers sees it a factor up to about 10 larger than the external "N" magnetometer, in Figure 6 and Figure 7 (the EXT_N magnetometer is the gray spectrum in all 3 plots). Magnetometers that see it a bit more intense are those close to NI, WI and WE towers.

A close look to the narrow spectral structure of the noise shows that it does not follow the mains, in Figure 8.

These hints suggest a DC-powered PWM source (Federico).

Bruco runs using ENV_NI_MAG_V as target, and EXCLUDING all magnetometers (ENV_*MAG*) channels, show that this disturbance is not coherent with any other ENV channel (https://scientists.virgo-gw.eu/DataAnalysis/DetCharDev/bruco/users/fiori/ENV_NI_MAG_V_1397853318_ENV/) with the exception of a small coherence with the ENV_CEB_ELECTRIC. In particular no coherence, nor evidence of the noise is seen in all IPS and UPS probes. No coherence is also measured with any other Virgo signal (https://scientists.virgo-gw.eu/DataAnalysis/DetCharDev/bruco/users/fiori/ENV_NI_MAG_V_1397853318/). While s is a good news for the interferometer, it does not help much locating the source.

Measurements will be done also to check for possible external sources, like it was for galvanic currents in the natural gas pipelines (https://logbook.virgo-gw.eu/virgo/?r=55542).

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fiori, graziaplena, paoletti, tringali, renato.romero - 17:23 Thursday 09 May 2024 (64217) Print this report

We performed some measurements with a portable magnetometer: 3-axial magnetic probe FL3 100 Stefan Mayer. 

Figure 1 compares spectra recorded in two locations:

  • "EXTMAG chs X,Y,Z" the probe was placed on the soil nearby the buried external magnetometer
  • "GUARD3 chs X,Y,Z" the probe was on the soil next to the vent tube of the methan gas pipeline, this is located inside the EGO fence next the EGO guardiania, see the attached pictures.

The GUARD3 spectra clearly show the noise: 0.23 Hz spaced lines peaking at 8Hz. The peak amplitude (spectra are obtained with 25s window) is approx 2 nT/sqrtHz. Which is a factor approx 100 larger than measured in the location EXTMAG, but similar to that measured by the magnetometers on the NI and WI tower base. Might be due to telluric currents in the chambers?  

As a check of the probe calibration, in Figures 2 and 3 we compare the EXTMAG measurement with that of the buried magnetometer, ENV_EXT_MAG_N. The fluxgate probe is more noisy (intrinsic noise) but around 8Hz they are measuring the same.

The indication is that the magnetic disturbance is radiated from the methan gas pipelines. A noise clearly associated to the power supply for galvanic currents in these pipelines was observed in the year 2022 (55542 and 57973). At that time the noise was shaped as approx. 5Hz comb. 

Federico has contacted the methan gas company (SNAM) to investigate further the source.

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