Summary: the effect of wind in the sub 0.1 Hz tilt band is very local (little coherence between seismometers 20m apart) and more than a factor of two greater in the HAM 2 and 5 seismometer locations then in the beer garden. We may be less sensitive to wind if the sensor correction seismometer(s) are located only in the beer garden. Also, because tilt is so local, real tilt meters, like Krishna’s at EX, should be as close as possible to the chambers.

Wind tilts our buildings, which produces spurious control signals from servo seismometers and can make it difficult to lock or maintain lock.  A previous log showed that there was almost no wind tilt at a location 40 m from the EY building, making it clear that wind tilt is a local effect (Link).  As a result, Hugh and I have been wondering if the HAM 5 seismometer location is better because it is down-wind for most storms or if the beer garden is better because it is furthest from walls. With Hugh’s help, I looked at chance coincidences between wind storms and seismometer huddles over the last few months as well as data with seismometers in the 3 locations. I think the answer is that the beer garden shows substantially less tilt than either the HAM 5 or 2 locations.

Figure 1 shows how local tilt is. The blue seismometer traces are for “huddled” seismometers (about 2m apart) in the beer garden and show high coherence below 0.1 Hz. But the red seismometer trace shows much lower coherence in this tilt band between the beer garden seismometer and the HAM5 seismometer, only about 20 m away. During high wind, I also found low coherence in the tilt band between the beer garden and the HAM2 seismometer locations.  The local nature of the tilt has implications for true tilt meters used to correct the tilt signal from seismometers. The tilt meter at EX is about 4 m from the chamber and, in Figure 1 we saw very little coherence at 20m. While it may not be enough of a return to move this one, it may be best to try and place the next one even closer, and, to the degree possible, engineer the BRS so that it can be as close as possible or even under the chamber.

Figure 2a and b show that tilt is very different at different locations in the LVEA and, of the 3 locations, the beer garden is the best. In both horizontal axes, the tilt in the beer garden is at least a factor of two better than the best of the HAM2 and HAM5 locations. It is about a factor of ten better than the worst of the HAM2 or 5 locations. I checked the 3 windstorms during the period when all 3 seismometers were working and, for each time that I examined, the beer garden seismometer was better. Figure 3 shows the two seismometers that were available during the windy period that caused locking problems last night: the tilt noise was half as much in the unused beer garden seismometer than in the HAM5 seismometer that was used for sensor correction. So, a sensor correction seismometer in the beer garden may be better than in the HAM2 or 5 locations in the frequency band dominated by tilt instead of real acceleration (roughly below 0.5 Hz). This morning Jim switched sensor correction to the beer garden seismometer.

Finally, when we have two STSs available, I think we should do a more detailed study of tilt-band coherence length, and attenuation with distance from the walls.

Robert, Hugh