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Properties of seismic noise at Virgo
Irene FioriINFN Firenze
for the Virgo Collaboration
5th Amaldi Conference on Gravitational Waves Tirrenia (Italy) 7-11 July 2003
Seismic Monitoring Network
Low Frequency Accelerometers Tri-axial, 0.2-200 Hz(Kinemetrics - “Episensors”)
High Frequency Accelerometers Vertical Piezo, 10-10000 Hz(Bruel&Kjaer)
3km
3km
Virgo Central building
MC End b.
West End b.
North End b.
Also:• Microphones• Magnetometers• Pressure probes• Humidity • Temperature
.
..
ADC
ADC
MAIN FRAME
BUILDER
MAIN FRAME
BUILDER
ETHERNET LINK
C1 C2 DISK…
Anti-aliasingfilters
Other chs.
Seism General Properties
Seism amplitudes (m)• 10-5 @ 0.1 Hz • 10-7 @ 1.0 Hz• 10-9 @ 10. Hz
Displacements (m/√Hz)
10-7
f 2(m/√Hz)d ≈
~
Acceleration spectral amplitudesTri-axial accelerometer
• 0-200 Hz
• On concrete floor of Virgo Central B.
x-y-z directions
Approx. flat acc. spectrum in 3D
DAY vs. NIGHT seism accelerations
• Human & Naturalseismic sourcesbelow ≈ 10 Hz
Daily Seismic Variations
NIGHT vs. WINDY NIGHT• Wind & Sea Wavesseismic sourcesbelow ≈ 1 Hz
0.4Hz0.8Hz
Monitor of RMS Seismic Motion
in 3 frequency bands: 0.2-1 Hz ; 1-4 Hz ; 4-10 Hz
• 1 - 4 Hz: day-night-weekend variations à local traffic ? • 0.2 - 1 Hz: slow motion à correlated with wind & sea-waves ?
• 4 - 10 Hz: spikes between 10AM-6PM à human activities on-site.
mic
rons
/s2
RMS = ( |a(f)|2df )½∫ ~
f1
f2
Time (DAYS) since July 3rd 2002 00:00 (local)
On-line seismic monitor
• 1 week data at 10 minutes steps
• Continuously running on 50Hz re-sampled data
http://wwwcascina.virgo.infn.it/commissioning/Monitoring_50Hz
Earthquake shakes
31 Oct. 2002, Magn. 5.9, 540 km SE from Virgo 2 Nov. 2002, Magn. 4.2, 270 km E from Virgo
Two shakes recorded at Virgo site :
3
SEISMIC ARRAY ANALYSIS
5 tri-axial velocimeters(Lennartz 3D/5 sec)
Ø Coherent spectral peak at ≈ 0.3 Hzà signals are band-pass filtered (0.1÷0.5 Hz) à
Up-Down
N-S
E-W
Investigating the 0.2-1 Hz seism: Oceanic MicroSeism ?
SEA 15 km
Oceanic MicroSeism at Virgo
Ø Consistent with OMS typical properties:• Peak freq. is double the sea waves freq. [R.A.Haubric et al.] • Peak freq. changes slightly with weather conditions [T.Braun et al.]
≈ East àWest
• Plane wave-front• peak frequency ≈ 0.3 Hz• speed 750-800 m/s
0 200 400 600 Time (seconds)
LAG
(sec
onds
)
+6
+4
+2
0
-2
-4
-6
Vi1Vi2
Vi4Vi3
Vi5Travel time between each pair of stations :
Investigating the 1-4 Hz seism:Local Traffic ?
ü “bursts” at ≈ 3 Hz
ü approx. 20 s long
ü ≈ 1 / minute during daytime
ü Minimum at 2AM (local)
ü Very reduced during week-ends
Fre
quen
cy [
Hz]
Hypothesis:Trucks exciting oscillations of nearby bridges
Road bridges around VirgoØ 1-3 km far from Virgo arms ends
Bridges Oscillations 1/3
Ø Resonant frequencies: 2, 6, 11,… Hz
Ground vibrations under bridge:
Road bridges around VirgoØ 1-3 km far from Virgo arms ends
Bridges Oscillations 2/3
Correlation measurement(preliminary):
x-correlation
@Virgo
@bridgeFiltered 1÷ 4 Hz
Road bridges around VirgoØ 1-3 km far from Virgo arms ends
Bridges Oscillations 3/3
A(s,ω) = A0 exp(- )ω s2vQ
Ø A(6Hz)/A(2Hz) < 0.1 (@1km)
à Only 2 Hz component survives at 1 kmand seen by Virgo seismometers
Ø A(3km)/A(1km) < 0.1 (@2Hz)
à “bursts” are not coherent among seismic stations at Virgo arms ends
Sedimentary soil at Virgo :
Q = 2-10 , v ≈ 1 km/s
Which seismic sources are relevant for Virgo?
Seismic isolation at Virgo
8m
Seism
GROUND
LASER BEAMMirror
Inverted Pendulum
ØActively controlled todamp horizontal SAresonances below 2. Hz( “Inertial damping” )
• Multiple mechanical pendulum= I.P. + 5 filters + marionetta + mirror
• Inverted Pendulum
Super Attenuator: (SA)
= Ultra-low frequency oscillator
Ground seism attenuation: ≥ 1013 @ 4 Hz
Low pass filter in all d.o.f.
Seismic isolation at Virgo
8m
Seism
GROUND
LASER BEAMMirror
Inverted Pendulum
GOAL: seismic noise below thermal noise above ≈2 Hz
“Seismic wall”
Crossing point
• Seismic noise accounted for all of the DF noise below 1.6 Hz (à mainly Oceanic MicroSeism)
• Entered exciting IP Vertical resonances :à improvements for final Virgo from implementing IP vertical control
2
SA vertical resonances
Seismic noise to Dark FringeCITF commissioning, eng. Run E3
Dark FringeWESNVerticalTOTAL
Two recent results: a good one, … and a not-so-good one
Other seismic sourcesCoherence DF vs. seism (CITF - Run E4)
Frequency (Hz)
Ø Coupling mechanism still under investigationü NOT through SA towers
Importance of seism coherence analyses !
43.7 Hz 46.89 Hz
ONOFFDAQ ROOM AIR-COND SYS
Ground vibrations due tomotor devicesin Virgo CB
10÷100 Hz
SA performance during an earthquake shake
June 30th 2003 - Magnitude 4.1 Richter – 20 km West of Tirrenia
0.03
0.015
0
0.015
-0.03
acce
lera
tions
(m/s
2)
2 4 6 8 10
TIME(minutes)
X d
ispl
acem
ent (
mic
rons
)
Y displacement (microns)
(mic
rons
)
(mic
rons
)
TIME TIME
• ID control was active on NI tower
• ID control was active on NI tower
• Tower top stage displacement ≈ 400 µm peak-to-peak
• Tower top stage displacement ≈ 400 µm peak-to-peak
Ø Control was not lost !! Ø Control was not lost !!
Ø Usual RMS (≈ 0.3 µm) recovered in 3-4 minutes
Ø Usual RMS (≈ 0.3 µm) recovered in 3-4 minutes
SA performance during an earthquake shake
Summary
• Main seismic sources below 10 Hz: - oceanic micro seism, peaking at 0.3-0.4 Hz- local traffic within 1- 4 Hz
• Ground motion at Virgo site agrees with empirical law:A / f 2 with A = 10 -7 m/√Hz
• Seism daily variations due to human&natural sourcesoccur mainly below 10 Hz
• At Virgo CITF- seismic noise contributed to Dark Fringe noise below ≈1.5 Hz- mainly due to vertical seism,
that was not actively damped at low frequencies• Other (potentially dangerous) sources: motor devices
(AC, pumps, rack fans …)