We have noticed during the transition to the earthquake state for the seismic systems that a lot of IFO signals get worse at microseismic frequencies. I've done a little modelling of this and I think that this can be improved by changing the filter used for the common mode subtration. The earthquake mode uses the average of the seismometers for all the VEAs to as a common ground signal, this average is then low passed and subtracted from the local ground sensor to get a local differential sensor correction signal.

I've been modeling the effects of different low passes by reconstructing the local differential sensor correction signals and seeing how that differential control signal subtracts the local ground. I forgot to attach the plot where I compare my calculated performance with the original filter, but it does a very good job up to about 1 hz. The original low pass filter was just a .3hz pole, so it's pretty simple, but using this model, I've come up with an elliptical filter that I think is a little better.  The first attached plot compares the calculated performance of my new filter to the actual performance of the original .3hz pole during an earthquake on March 28. During this earthquake, SEI_CONF was in EARTHQUAKE. The dotted blue line is the ITMY Y ground, the gold and dashed darker blue (purple?) lines are St1 motions seen by the ITMY and ETMY St1 T240s, the red line is the calculated performance given by my new eq mode low pass. You can see that both ITMY and ETMY are getting no isolation relative to the local ground motion up to about .3 hz. The new low pass gives some isolation starting at .1hz, so this should be better, and the motion isn't really worse below .1hz (the region we are winning by using the common ground subtraction). The thin green (?) line is the performance given by using the normal sensor correction path and filter. The suppression of the microseism is good, but the amplification of the low frequency  ( < .1hz ) motion is pretty bad.

The second plot is a repeat of the first, but for an earthquake on March 30th, where SEI_CONF was left in the nominal, WINDY configuration. Comparing the red line to the dashed dark blue and gold lines shows that the new eq mode low pass should give similar microseismic performance to the nominal seismic configuration. Note also that the .030 - .070 hz motion should be better than the motion seen with the nominal configuration. The largely good agreement between the thin green line and gold and dashed blue line shows this is a relatively good model to predict st1 motion. The gap from .1-.3 hz is because I'm not including St1 RX tilt.

The third plot shows bode plot of some of the low passes I've looked at. The red line is the original .3hz plot low pass. The blue line is the new ellptical filter that I think we should try. The green line is just as .1hz pole, one of the first improvements I looked at, but I think the ellipse is better. I've installed the ellipse and turned it on, we'll see if it works any better for the next couple earthquakes.