I have taken a few days of data with the T240 standing on the ground (and not on the optical table). I expected horizontal particle trajectories to lose some of their ellipticity. And that's indeed the case (left ground, right table):
So the table is doing something to the displacement at high frequencies. Note that you cannot compare absolute displacements in these plots. They are individually normalized so that the maximum dispalcement during that time seen in any of the channels is equal to one. You can also compare H/V ratios (left ground, right optical table):
The 0.1Hz-1Hz peak in the table data is gone. My first thought was that this may be related to the table, but the peak height also varied during the table measurement and is getting stronger again right now. So maybe there is a table effect, probably not (too low frequencies). Moving the seismometer to the ground, the H/V ratio above 1Hz has dropped to something significantly below 1. In principle you can use this information to measure the Poisson ratio of the ground, but only by making a few assumptions: (a) the spectrum is determined by Rayleigh waves, (b) the ground is half-homogeneous. In this case, the dashed line in the following figure tells us what the Poissson ratio is:
The H/V ratio should be frequency independent if the two assumptions were true. This is not the case, so our assumptions are wrong. Only the 1Hz to 10Hz range seems ok. Below 0.1Hz we sort of know that the seismic displacement is not driven by Rayleigh waves, but local sources causing heavy tilts that cannot be associated with Rayleigh waves (which is quite a non-trivial statement). Maybe we should also not forget that it may be possible that temperature changes, air currents and pressure changes may also couple into the low-frequency data. This is always the problem at the surface. You need to spend a lot of money to make good seismic measurements above ground. Between 0.1Hz and 1Hz it is also not completely surprising that the model fails (the ratio is too close to 1) since large scale heterogeneities break the half-homogeneity of the model. The weird structure above 10Hz is mysterious, but again since seismicity is likely driven by local sources at these frequencies, one would not expect a simple picture to form.