Krishna

As described before, we are testing to see if the seismometer mounted on BRS-Y platform sees better coherence with it as compared to the ground seismometer. Latest measurements suggest this may finally be happening. The first attachment shows the coherence between BRS-Y and the ground seismometer (red) followed by that between BRS-Y and the platform seismometer (blue). For the first time, blue is slightly better than red in a few places. The next page shows the spectra. Note the increased motion at 10-70 Hz, as mentioned previously.

The next attachment shows the coherence/spectra that was commonly seen before we bolted the seismometer feet to the table/platform. Once again, note the spectra at higher frequencies. It looks like we may have reduced higher frequency motion by bolting the feet to the platform.

The improved coherence after we bolted the seismometer to the platform supports two possible explanations: a) high frequency motion was causing the seismometer to move/slip on the table, which would cause low-frequency noise or 2) high frequency motion was down-converting through other mechanisms in the seismometer (flexure non-linearities).

In any case, we can try to improve/restore the thermal shielding and see if we can get even better coherence in order to do better tilt-subtraction.

Edit: The pink line is the current tilt-subtracted ground super-sensor, which is the ground seismometer minus tilt measured by BRS-Y.