At ~2350 UTC on June 2, the seismometers were turned off. After the granite slab was repositioned with the new lead, the Ranger was turned on, but not the Guralps.
Now, after ~24 hours, I have put the Guralps onto the granite and turned them on. During this off time, the input channels should be ADC noise limited (or perhaps limited by the INA134 differential receiver chips inside of the Sander Liu AA chassis). The following plot shows that this noise level is ~0.8 uV/rHz and then rising like ~1/sqrt(f) below 5 Hz:
I checked the slab again by whacking it. It still rings with a Q of several, so I think we need to make the lead flatter. There should barely be any room between the granite and the linoleum.
I guessed that it should be possible to make the slab-to-floor coupling better with flatter lead (Brian Lantz suggested to use lead sheets). So I removed my booties and jumped up and down on the granite several times. Because of my soft sole shoes, I was able to make an impulsive impact without shattering the granite. The effect of the stomping was pretty dramatic - the horizontal resonance frequency has gone up by a factor of 2. The red trace shows the new TF after the stomping:
And the resulting spectrum is here too. As you can see, there is no excess between the Ranger and the Guralps until ~50 Hz where the mechanical resonance in the short direction (non-MC dir) takes over.
So, the lesson for next time is to flatten the balls a little more. I leave it to Nancy to calculate the horizontal resonant frequencies of this lead/granite combo to see if it matches with our measurements.