Spring enabled the EE shop to work on setting up power for the LEMIs, and I had a look at the new signals. The top plot in the figure shows that we can see Schumann Resonances quite well, up to quite close to 60 Hz. The bottom two plots show some transient signals that might interfere with a feed-forward system.
It looks like the signals are degraded by wind. I am not surprised because we see wind noise in buried seismometers. I think we would have this vibration problem even on a perfect flat because of the variation in Bernoulli’s forces associated with gusts. It may be that a LEMI signal is generated by the wind because of slight motions of the magnetometers in the earth’s huge DC magnetic field. We buried the LEMIs about 18 inches deep (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=29096). I think we might be able to mitigate the noise some by going much deeper. Once we have the vault seismometer working, it would be a good project to test the wind vibration hypothesis by comparing the LEMI and seismic signals.
There also seem to be some transients, some long and some short, possibly self inflicted by our system. It would be good to look into which transients would be a problem, and for those, details such as whether they are correlated with time of day, the average time between transients, etc., in order to help determine their source.
Finally, I would like to get the full system calibrated by comparing to a battery powered fluxgate magnetometer.
[Pat Meyers, Andrew Matas] We attach a few additional plots studying the Schumann resonances. Figures 1,2 show spectrograms using 16 hours of data from April 18, where the Schumann resonances are clearly visible. There are also a few glitches. We also show coherence (Figure 3) and cross power (Figure 4) between the Hanford and Livingston LEMIs. The first two Schumann resonances at about 8 Hz and 14 Hz are coherent between the sites.
We disabled the vault power on April 20th to upgrade the power supply, it will remain down until the this afternoon.