F. Matichard, K. Venkateswara

On Tuesday (12/16/2014), Fabrice and I got an opportunity to test the ETMX ISI in different configurations. The wind speed was in the 7-12 mph range at EX. The configurations I tested were:

A) ISI Damped

B) ISI isolated with High (T750) blends. Inertial isolation happens only at frequencies above ~750 mHz.

C) Nominal LLO blend configuration. Inertial isolation happens above ~45 mHz.

D)  (C) + sensor correction using LLO filter.

E)  (C) + sensor correction using Rich's Z sensor correction filter and the tilt-subtracted super-sensor.

The first page of the first pdf shows the ground seismometer (RED) and the tilt-subtracted super-sensor output (BLUE) in displacement units. It shows factors of ~5 subtraction below ~50 mHz. The second page shows the T240 on Stage 1 under the different configurations. The following page shows the CPS and the final page shows the T240 RY.

Looking at Page 2, Configuration E appears to improve over D, roughly by factors of 2-4 below 0.5 Hz, while showing no excess amplification at low frequencies, as measured by the CPS. This is in agreement with modeling predictions described here. Unfortunately, I couldn't make a measurement in (E) but with BRS off for comparison, because someone walked into EX VEA and drove up the BRS amplitude.

Measuring this improvement interferometrically has not been possible using the ALS (green laser system). It is likely that ALS is noise limited in this frequency range as described here. Measuring it using the IR laser needs much more dedicated time which is not easy to come by.

So far, we have not tried using different blends for the inertial sensors. Some improvement may be possible by blending higher in X on Stage 1. But so far, it looks like BRS can improve ISI performance by factors of 2-5 in the 50-500 mHz range. Improving performance at the microseism might significantly improve detector robustness as seen during ER6 at LLO.

The other two attachments show the coherence between various sensors when the ISI was damped and when ISI was in the (E) state.