Robert, Jess, Nutsinee
Back in September Robert and control room folks did an injection of "5 people jumping in sync" (alog21180). A followup showed that the injection indeed coupled into DARM (alog21191) from ~10-400Hz (an omega scan of a jump can be found here). So I continue to followup the coupling mechanism hoping it would shed some light on how ground motion coupled into DARM (we suffer a slight range drop daily during high traffic time). Using Robert's Acoustic Coupling Functions (alog22797 figure 9) I was able to pin down the coupling mechanism above 100Hz. Figure1 shows the calibrated DARM and the PSL periscope accelerometer spectrum at 100-400 Hz.
- At 210Hz periscope amplitude measured 4e-4 m/s^2/sqrt(Hz), multiply by the coefficient of 3e-16 the predicted DARM becomes 12e-20 m/sqrt(Hz). The actual DARM amplitude measured 10e-20 m/sqrt(Hz). Off by a factor of 1.2.
- At 120Hz periscope amplitude measured 2e-4 m/s^2/sqrt(Hz), multiply by the coefficient of 3e-16 the predicted DARM becomes 6e-20 m/sqrt(Hz). The actual DARM amplitude measured 7e-20 m/sqrt(Hz). Off by a factor of 1.2.
- At 360Hz periscope amplitude measured 1.2e-4 m/s^2/sqrt(Hz), multiply by the coefficient of 3e-15 the predicted DARM becomes 3.6e-19 m/sqrt(Hz). The actual DARM amplitude measured 1.7e-19 m/sqrt(Hz). Off by a factor of 2.
The predicted DARM value is expected to agree with the actual measurement within a factor of 2. Thus it is reasonable to blame PSL periscope for ground-to-DARM coupling at above 100Hz. However, at f < 100Hz the coupling mechanism is still a mystery (figure4). Robert wasn't able to make periscope motion couple into DARM at low frequency (refer to figure 7 of alog22797). However, we were able to rule out HAM2 and HAM6 as coupling sites based on the upper limit (alog22797 figure 3 and 6). Figure 2 and 3 attached show that both HAM2 and HAM6 GS13 rise about a magnitude above the noise floor, while the upper limit from Robert's alog is about 1.5-2 order of magnitude. Meaning, shaking HAM2 and HAM6 GS13 at 2 order of magnitudes above its noise floor at low frequency didn't couple into DARM.
Whatever the coupling mechanism is at low frequency, we believe the same mechanism is responsible for HVAC coupling into DARM. Figure5 shows DARM spectrum during the HVAC injection time (alog22532). HVAC noise shows up in DARM at 20-100Hz.
