K. Venkateswara

Friday's 50 mph winds produced impressive amounts of ground tilt, especially at ETMY. The attached pdf shows the ASD of 5k seconds of data around the time winds were at their highest in the plot John posted. It shows the outputs of seismometers at EX, Corner station (HAM2 as the representative) and EY all along X direction, and the BRS located at EX. Note that the while the seismometers see both tilt and horizontal acceleration, BRS records only tilt. I've also shown Y direction at EX to check for cross-couplings. Seismometer displacement has been converted to tilt (radians) by multiplying by w^2/g.

Some interesting features:

1. Looking at the data, it is clear that X along EY showed particularly remarkable levels of tilt corresponding to ~100 nrad/rt(Hz) (at 0.1 Hz!) which is at least 1000 times the tilt during quiet times. This was enough to submerge the ~1 micron/rt(Hz) microseism!

2. The wind-induced tilt has a smooth spectral shape in the ~10 mHz to ~few Hz range (as noted by Brian earlier in 602). The dip in the BRS_RY_OUT near 10 mHz is due to an incorrect inversion filter (see 638).

3. Note the crossover in the EY_X spectrum at ~0.5 Hz. The acceleration spectrum seems to go as f^2 above 0.5 Hz (~constant displacement). This suggests that the coupling from wind to seismometer output is through tilt at frequencies below 0.5 Hz and through real displacement above that (again refer to 602). This perhaps suggests an interesting length scale of ~ g/(2pif)^2 ~ 1 m (thickness of the foundation/height of seismometer from point of rotation?).

4. There is good coherence between BRS and EX_X as expected. There is no coherence between BRS and any other sensor indicating that at least wind-induced tilt is very local for each test mass (also expected).

5. The corner station tilt is lower than that at ETMX by factor of 2-3. This was expected due to the much larger mass and size of the corner station foundation slab.