K. Venkateswara

I have attached ASD plots from 30k second-data from the BRS and the ground T240 seismometer x channel measured last night, when wind-speeds were fairly low. The first plot is in angle units and the third is in meters. I have also shown the tilt-subtracted ground super_sensor in red.

Both BRS and T240 look similar, but have no coherence. The amplitude of BRS looks lower between 8-20 mHz than the T240X, while it looks higher below that. Note that I have corrected for the transfer function of the T240 (double pole at 4.3 mHz).

The noise level between the two instruments also seems to vary with ground motion. Last Friday, Brian Latnz had suggested that it might be cross-coupling between different axes. This made me wonder if it was due to down-conversion from higher order modes of the beam-balance. I had seen similar issues with it in the lab at UW, which were mitigated when I added some vibration isolation in the form of rubber pads under the platform. So, I tried the same thing under the BRS platform on Monday morning. However, there was no change in the "noise" level as seen in the plots. It may still be cross-coupling in BRS or T240, but it doesn't seem to be the same type of down-conversion that I observed in the lab.