Krishna

The attached pdf file shows the comparison between BRS-Y, the ground seismometer (an STS-2) and the platform seismometer (a T240), called a PEM signal in this case (since we are using spare PEM channels for it). Also shown are the online tilt-subtracted super-sensor output (online residual) and a second tilt-subtracted signal between the T240 and BRS-Y (PEM residual). Page 2 shows the coherences between various signals and Page 3 shows the wind-speeds. As I showed before, the coherence between BRS-Y and the platform seismometer (T240) is better than that between BRS-Y and the ground seismometer (STS-2).

Note that in this plot I have used the normal calibration factor for BRS-Y and STS-2. For the T240, I matched its output to the STS-2 at the secondary microseism. You'll notice that while the STS-2 and BRS-Y are coherent below 0.1 Hz, their outputs differ by 20 percent. Initially some had suspected we got our calibration wrong, but this is now known to be due to differential floor tilt. For the online tilt-subtraction we use a factor of 0.78 for BRS-Y to correct for this. However, the platform seismometer agrees very well with the raw BRS-Y calibration (we did get our calibration right!). This confirms that the two instruments are now seeing the same tilt.

The tilt-subtracted T240 signal (PEM residual) is now marginally better than the tilt-subtracted STS-2 signal in the 15-50 mHz range. It is possible that this is still limited by temperature noise on the T240, since the tilt-subtracted noise looks similar at lower wind speeds too (not shown). Hugh will attempt to improve the thermal insulation when possible.

The most important test is to now see if the two residual curves diverge even more when wind speeds pickup above 20 mph. If so, this would confirm the differential floor tilt hypothesis. If not, it would suggest some other noise/nonlinearity limits the tilt-subtraction. Unfortunately, it looks like we'll have to wait till next week for higher wind speeds.