I think it's possible that we're closer to the Newtonian gravitational noise limit than I had thought. This is on the list of "things we knew were coming, but are perhaps here sooner than I thought they would be".
The punch line is that we may be limited by Newtonian noise between 16-20 Hz. Not a wide band, but reasonably consistent with the expectations from papers such as P1200017.
In the attached plot, the blue trace is the calibrated DARM spectrum (CAL-DELTAL_EXTERNAL_DQ) that we show on the wall, taken yesterday. The green trace is my estimate of the Newtonian noise.
For the Newtonian noise, I have taken the Z-axis STS-2 seismometer data from the sensors on the ground near each test mass. (There is one seismometer at each end station, and one in the vertex near the ITMs - I use the same seismometer data for each ITM). The seismometers are in velocity units (I believe Jim said it's nm/s), so I pwelch to get velocity/rtHz, then apply the calibration zpk([],0, 1.6e-10) to get to meters/rtHz. I then translate to acceleration due to Newtonian noise using eq 1 from T1100237. Finaly, I add the 4 acceleration contributions (one from each test mass) incoherently and get to displacement by dividing the spectrum by (2*pi*f)^2.
The Newtonian noise is touching the DARM spectrum between about 16 - 20 Hz. We're within about an order of magnitude in the band 10 - 30 Hz. Evan will shortly re-run his noise budget code using this "measured" Newtonian noise to see if it helps explain some of the discrepancy between the measured and expected DARM spectra (this spectra is higher than the GWINC curve that is currently used in the noise budget).
Notably, this estimate of seismically-induced Newtonian noise is somewhat larger than what we've quoted in P1200017 and T1100237. If I use only the ETMY spectrum as an estimate for all 4 test masses, I get an answer more consistent with our past estimates. However, using the actual seismic signals from each test mass, I'm getting this slightly higher estimate.
The script to generate this plot is attached, as is the exported-from-DTT text file of the calibrated DARM spectrum.
Adding SEI tag, so people see it.
Is the STS signal calibrated correctly above 30 Hz or are you just assuming its a flat velocity sensor?
If its really close, you should be able to add the seismic data streams with the right signs and then take the coherence between this pseudo-channel and DARM and see something more than we expect by just the seismic model estimates.
Hmmm, good point Rana. I should have thought of that - it looks like the STS calibration doesn't compensate for the roll-off, so I'll put that in, and redo the traces.
Jenne, The estimate you're getting in the 15-20 Hz region is an order of magnitude or more higher than the estimate made by Jan Harms for L1, found in T1500284.
Can you post the ground noise spectra you are using so we can compare with what Jan used for L1?
The originally posted NN estimate spectra is totally wrong. I forgot to take the sqrt of the seismic spectra after pwelching, before calibrating to meters.
This corrected plot is much more consistent with Jan's estimates from T1500284.
EDIT, 3:15pm: Calibration was missing a factor of 2*pi. Plot has been updated.
Great. I like consistent results. Another remark; seismic displacement measured at a test mass has vanishing correlation with its NN. This is true at least for seismic surface fields. So if you want to proceed with correlation measurements, then the pseudo-channel needs to be constructed from an array of seismometers, with non of these seismometers being located at the test mass.
Here I include a version of the Newtonian noise estimate plot, with the GWINC estimate of aLIGO's sensitivity, in addition to the current LHO sensitivity.
The trace "GWINC with NN term" is just the regular output of Gwinc, assuming no Newtonian noise cancellation. The trace "GWINC no NN term" is all terms in gwinc except for the Newtonian noise. In particular, recall that the Gwinc NN term is not identical to the NN estimate I plot here.
The point here is to show that, although at our current sensitivity we are not limited by Newtonian noise, if we can eliminate the LSC and ASC control noise terms from our latest noise budget (aLog 21162), we likely will be.
EDIT: a further version of this plot now includes the GWINC NN curve.