• For our current environment (often windy, low microseism), I think that we should change our nominal BSC to the 250mhz blend, 80mhz sensor correction configuration detailed below. Currently called the Windy state in the SEI_CONF guardian.

• This configuration does a better job of rejecting wind induced tilt and may perform better against microseism than 90mhz blend, .5hz sensor correction that was our previous nominal state.

• This answer may change when the microseism comes up in the winter. We will have to keep this in mind for O2.

Now for the excessive detail.

Before the last couple of months work, we thought that for the end station ISI's, we should use the Quite_90 blends and the Mitt_SC broadband sensor correction. A couple of months ago, Krishna pointed out that the Mitt_SC filter was probably insufficiently rolled off at low frequency, where the BRS actually makes the STS/BRS supersensor worse. There have been a number of iterations on sensor correction filters, but I think we now have a good filter. The first attached images shows a few of the filters we have tried, I won't go through them all, but the dark blue is the Mitt SC filter of old, and the cyan is what we are using currently. The other two are filters that we've tried and discarded, for various reasons,and there are many more in the foton files, but I'm tired of plotting them.

Around the same time, I realized that using the Quite_90 blend may not roll off the ISI T240's quickly fast enough at low frequency to avoid injecting platform tilt and that we should use a higher blend, at least while the microseism is low. Currently we're using the Quite_250 blends for a high blend, but there may be room for optimization there.

The next plot is the estimated supprestion of the "old" BRS configuration (the Mitt_SC filter, Quite_90 blend), the "new" SC/blend configuration (Warn_SC_v3, Quite_250 blend), and a couple relevant CPS blend filters (which are a good first approximation to the low frequency performance, without sensor correction). JeffK has a log (594) in the seismic log that explains this estimation of the expected sensor correction performance, for those interested.

We know from experience the 45mhz blends are hopeless in 10+ mph winds but suppress the microseism well, and the Quite_90 are okay in winds to 20 mph, but are no good in high (~>.5 micron BLRMS) microseism. I've tried using the Quite_250 blends alone with low microseism, but I think the motion around the first TM modes was too high (the spots on the AS port were moving around a lot at something like .5hz). The new configuration has allowed us to lock in 30-40mph winds (and lower), but microseism is very low. Based on the the simple model used in the second plot, it should do better than the Quite_90 blends alone in high microseism. It even looks like it should do better that the 45mhz blends at the secondary microseism (.150 hz), but the gain peaking is right on top of the primary microseism (~30-80mhz), which we don't see often.

The gain peaking of the new configuration looks like it would be worse than either the 45mhz blends or the Quite_90s, but this model doesn't include platform tilt, which is filtered out by using a higher blend on the ISI and a tilt-subtracted STS ( at the ETM's) or a low tilt STS ( ITMY)  for sensor correction. I think Krishna showed that this "new" configuration is in practice better, in his alog 27735.