Abstract:

We compared TF from OMC geophone to OMC error signals around 100 Hz when exciting different shakers.
100Hz peak in TF can be seen only when exciting by ground shaker.
For further investigation, it might be better to put small shaker on the bellows legs.
Also, noise projection from this peak is important to confirm if this peak is problematic or not including consideration of OMC LSC modification.

Detail:

To investigate the new feature at 100Hz around OMC reported in JGW-G2516652-v1, we compared the TF from geophone to OMC error signals (fig1).
Blue and green lines show the TF from OMC geophone to OMC error signals when exciting from OMC ground shker and OMC duct shaker, respectively.
There is large difference around 100 Hz.
To check the linearlity of the TFs, I measured the same TF with different amplitude (brown line) but there seems no significant difference.

In the TF from ACC near shaker to geophone signals, there is no large peak at 100Hz even when exciting from OMC ground shaker (fig2).
There is several possibilities such as :
1. There are two (or more) pathes, which vibration passes through to the OMC, and ground shaker and duct shaker vibrations transferes to the OMC through different pathes.
2. Ground shaker somehow excites OMC effectively, for example by deforming the ground and making bellows legs tilted, which causes different motion to OMC compared with only translational motion are existing.
3. OMC duct shaker somehow doesn't excite OMC geophone effectively, for example because of the node of the stack.
So, for further investigation, it would be nice to have shaker on the one of (or more) OMC bellows to confirm it is not due to shaker itself.

In addition, it is important to project the OMC noise into DARM with obtained TF from OMC geophone to OMC LSC when exciting from ground shaker to estimate the upper limit of the noise.
If it is still less than our target sensitivity for O4c, we can ignore this problem though it would be problematic after O4.