In addition to the papers listed at [http://ilog.ligo-wa.caltech.edu:7285/advligo/Seismic_Isolation the AdvLIGO wiki], I found [http://www.iop.org/EJ/abstract/0264-9381/15/11/004/ this VIRGO paper], which contains a section "Effect of the apparatus infrastructure".

[http://www.ligo.caltech.edu/docs/P/P040032-00.pdf Rana's thesis]

[http://www.ligo.caltech.edu/docs/G/G040557-00.pdf Rana's presentation with scale diagram of BSC]

I attached a plot of the GG noise from the BSC vibration, calculated from real accelerometer data (from early this morning) and a generous estimate of the gravity coefficient (7.3e-9 (m/s^2)/m). It doesn't come close to the total noise curve, so unless another model says something very different, we might not have to worry about the BSC at all.

Okay, here's the worst case noise curve. I computed it by assuming

• Radius: 1.5 m
• Height: 5 m
• Mass: 8000 kg
• Test mass at center
• Each point of the BSC moves in the direction that maximizes the gravity gradient along the beam axis (not a physically reasonable vibration mode, but a limiting case).

This results in a gravity coefficient of 1.0e-7 (m/s^2)/m. It uses accelerometer data from the noisiest BSC at a noisy time of day, and also uses a real harmonic oscillator transfer function instead of a free mass TF (although the difference is negligible above 2 or 3 Hz).

Moving the test mass away from the center decreases the GG noise.

Next I'm going to model the piers, but since they're both less massive and farther away from the test mass, I don't think they'll be a problem either. -- KeenanPepper 2007-06-28 01:38:54