Robert Schofield, Thomas Vo

While looking at ITMY spectra from HIFO-Y of the optical levers, we saw that the first pier resonance occurred at ~31Hz, which was caused by the transmitter.  The real question was whether or not this peak is due to rigid body motion or if it's from flexing/vibrations in the pier.  This is important because the piers are not yet grouted and if the resonances are from rigid body motion, then grouting will help but it wouldn't help the vibrations and flexing of the pier as much.  So we set up a little experiment to see if grouting will help this particularly troublesome 31Hz peak.

First we used an accelerometer and took spectra as is, with only three bolts holding up the pylon, then we added 4 machine jacks to help support the pylon, much like grout would do.  Attached are pictures of the two setups, and spectra showing that the 31Hz peak shifted up 50 Hz after adding the machine jacks, yay.  Of course, we'll repeat a similar measurement when grout is in place.