[Debasmita, Anamaria]

We wanted to check if there is any microseismic coupling at LHO, similar to that of LLO. So far, for LLO our understanding is that the microseismic ground motion (0.1-0.5 Hz) is being amplified by a potential scatterer which has a transfer function as shown in the first image. This transfer function was modeled based on some other information and seems to explain the LLO noise quiet well (LLO aLOG 73845). 

For LHO, we amplified the ground motion by the same transfer function and simulated the scatter shelf. If we assume that the scattered light amplitude is same as LLO (4e-10), given the amount of microseismic ground motion at LHO the scatter shelf falls below the current DARM spectra of LHO (second image). Even if we double the light amplitude, the scatter shelf does not show up above the present noise background (third image) but it comes quiet close to the noise floor.

But, if we double the microseismic ground motion and then further amplify it by the model transfer function, the scatter shelf shows up above the present DARM spectra (fourth image). This shows that, the same source which is producing the microseismic scattering at LLO can produce similar noise at LHO, but it will only be visible if the IFO is locked during a somewhat high microseismic motion. Even if the amount of light is half of what it is at LLO, the scatter shelf would still be visible if the ground motion is higher (fifth image). 

I used the ground motion as it was during 2023-12-06 11:00:00 UTC at LHO. This is one of the high microseismic days of LHO, the median of rms ground motion in 0.1-0.3 Hz was 689.66 nm/s and in 0.3-1.0 Hz was 95.02 nm/s. 

In the last image, I plot all the simulated traces together for a better visualization of how the light amplitude and amount of motion can modify the scatter shelf.