Looking at data from a couiple of days ago, there is evidence of some transient bumps at multiples of 11.6 Hz. Those are visible in the summary pages too around hour 12 of this plot.
Taking a spectrogram of data starting at GPS 1410607604, one can see at least two times where there is excess noise at low frequency. This is easier to see in a spectrogram whitened to the median. Comparing the DARM spectra in a period with and without this noise, one can identify the bumps at roughly multiples of 11.6 Hz.
Maybe somebody from DetChar can run LASSO on the BLRMS between 20 and 30 Hz to find if this noise is correlated to some environmental of other changes.
I took a look at this noise, and I have some slides attached to this comment. I will try to roughly summarize what I found.
I first started by taking some 20-30 hz BLRMS around the noise. Unfortunately, the noise is pretty quiet, so I don't think lasso will be super useful here. Even taking a BLRMS for a longer period around the noise didn't produce much. I can re-visit this (maybe take a narrower BLRMS?), but as a separate check I looked at spectra of the ISI, HPI, SUS, and PEM channels to see if there was excess noise anywhere in particular. I figured maybe this could at least narrow down a station where there is more noise at these frequencies.
What I found was:
- Didn't see excess noise in the EY or EX channels at ~11.6 Hz or at the second/third harmonics.
- Many CS channels had some excess noise around 11.6 hz, less at the second/third harmonics.
- However, of the CS channels that DID have excess noise around 11.6 Hz and 23.2 Hz, HAM8 area popped up the most. Specifically these channels: H1:PEM-FCES_ACC_BEAMTUBE_FCTUBE_X_DQ, H1:ISI-HAM8_BLND_GS13Z_IN1_DQ, H1:ISI-HAM8_BLND_GS13X_IN1_DQ.
- HAM3 also popped up, and the Hveto results for this day had some glitches witnessed by H1:HPI-HAM3_BLND_L4C_RZ_IN1_DQ.
- Potential scatter areas: something near either HAM8 or HAM3?
I was able to run lasso on a narrower strain blrms (suggested by Gabriele) which made the noise more obvious. Specifically, I used a 21 Hz - 25 Hz blrms of auxiliary channels (CS/EX/EY HPI,ISI,PEM & SUS channels) to try and model a strain blrms of the same frequency via lasso. In the pdf attached, the first slide shows the fit from running lasso. The r^2 value was pretty low, but the lasso fit does pick up some peaks in the auxiliary channels that do line up with the strain noise. In the following slides, I made time series plots of the channels that lasso found to be contributing the most to the re-creation of the strain. The results are a bit hard to interpret though. There seems to be roughly 5 peaks in the aux channel blrms, but only 2 major ones in the strain blrms. The top contributing aux channels are also not really from one area, so I can't say that this narrowed down a potential location. However, two HAM8 channels were among the top contributors (H1:ISI_HAM8_BLND_GS_X/Y). It is hard to say if that is significant or not, since I am only looking at about an hours worth of data.
I did a rough check on the summary pages to see if this noise happened on more than one day, but at this moment I didn't find other days with this behavior. If I do come across it happening again (or if someone else notices it), I can run lasso again.
I find that the noise bursts are temporally correlated with vibrational transients seen in H1:PEM-CS_ACC_IOT2_IMC_Y_DQ. Attached are some slides which show (1) scattered light noise in H1:GDS-CALIB_STRAIN_CLEAN from 1000-1400 on Septmeber 17, (2) and (3) the scattered light incidents compared to a timeseries of the accelerometer, and (4) a spectrogram of the accelerometer data.