seismic noise characterization at virgo

Irene Fiori – XXXVIIIth Rencontres De Moriond, March 22-29, 2003 - 1 -

Seismic noise characterization at VirgoIrene Fiori

(Universita’ di Urbino – INFN Firenze)

for the Virgo Collaboration

• Seismic isolation at Virgo• General features of seismic noise• Sources (preliminary)• Seism from soil to dark fringe at Virgo CITF

• Transfer function• E4: unexpected seismic noise sneaked in…

Outline:


Seismic isolation at Virgo

Mirror

∼ 10 -7 f -2 m Hz -1/2Pre-isolator stage

• Actively controlled todamp horizontal SAresonances below 2 Hz

( inertial damping )

8m

• Multiple mechanical pendulum(5 filters + marionetta + mirror)

• Inverted Pendulum stage (IP)Seism

Super Attenuator (SA):

Ultra-low frequency oscillator (30mHz)

Attenuation performance:

• 10^-13 (horizontal) @ 4 Hz

• 10^-9 (vertical) @ 4 Hz


Goal for seismic noise attenuation

“Seismic Wall”

Crossing point

<<Thermal Noise @ 4.1 Hz :

Residual Seismic Noise Upper Limit@ 4.1 Hz :

Measurement of horizontal seismic noise transmission:

4 × 10-18 m Hz-1/2

9 × 10-17 m Hz-1/2

“Residual seismic mirror vibrations below the thermal noise floorstarting from a few Hz”


Seismic noise: general characteristics

Seism amplitudes (m)

• 10-5 @ 0.1 Hz • 10-7 @ 1. Hz• 10-9 @ 10. Hz

10-6

10-5

10-4

10-1

1 10 102

frequency(Hz)

(m/s

2 )/sq

rt(H

z) Acceleration spectral amplitudes (m/s2/√Hz) x-y-z directions

Tri-axial accelerometers(Kinemetrics EpiSensor)

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-1

1 10 102

frequency(Hz)

m/s

qrt(

Hz)

Displacements (m/√Hz)

• [0.2, 200.] Hz• 7.48×10-7 m/s2

• On concrete floor near BS tower

10-7

f2 (m/√Hz)d ≈~


10-6

10-5

10-4

10-1

1 10 102

frequency(Hz)

(m/s2 )/s

qrt(H

z)

10-6

10-5

10-4

10-1

1 10 102

frequency(Hz)

(m/s2 )/s

qrt(H

z)

Daily seismic variationsDAY .vs. NIGHT seism accelerations

NIGHT .vs. WINDY NIGHT


Monitor of seism RMS in frequency bands

0

10

20

30

40

50

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Time (DAYS) since July 3rd 00:00

micro

ns/s2

+20.

+5.

Wed

.

Thu.

Fri.

Sat.

Sun.

Mon.

Tue.

Wed

.

Thu.

Fri.

Sat.

Sun.

Mon.

Tue.

3 frequency bands: 0.2-1.Hz ; 1.-4.Hz ; 4.-10.Hz

• 1.0-4.0 Hz: day-night-weekend variations !bridges oscillations excited by heavy trucks ?

• 0.2-1.0 Hz: slow motion ! correlated with wind & waves activity ?

• 4.0-10. Hz: spikes between 10AM-6PM ! human activities on-site


0

10

20

30

40

50

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Time (DAYS) since July 3rd 00:00

Arb

itrar

y U

nits

RMS seism in [0.7, 0.9] Hz frequency bandWind speed (data from weather station in Pisa)

Observed correlation with wind speed

RMS seism in [0.7, 0.9] HzWind speed


On-line RMS monitor

Earthquakes detection !!

• Nov. 2, Mgn. 4.2, 270 km

• Nov. 1, Mgn. 5.8, 540 km

• Oct.31, Mgn. 5.9, 540 km

• 1 week data at 10 minutes steps

• Work in progress to improve earthquake sensitivity

• Preliminary test version continuously runninghttp://wwwcascina.virgo.infn.it/commissioning/Monitoring_50Hz


Sources: ocean microseism

3

SEISMIC ARRAY ANALISYS

5 tri-axial seismometers(Lennartz 3D/5 sec)

24h of data[0.2, 25.] Hz

• Coherent spectral features at 0.3-0.4 and 0.8 Hz


≈ East !West

speed 750-800 m/s

Plane wave-frontpeak frequency ~ 0.3 Hz

Cross correlation analysis

• Consistent with oceanic microseism:• peak freq. is double the sea wave freq. [R.A.Haubric et al.]• peak freq. position related to weather conditions [T.Braun et al.]


Sources: local traffic1-4 Hz frequency band:

Signal filtered between 1. and 4. Hz

•“Bumps” approx. 30 sec long

• One per min. during daytime

• Minimum at 2AM (local)

• Very reduced during week end

Hypothesis:TRUCKS crossing nearby bridges


Measured ground vibrations under Fi-Pi-Li expressway

Bridge oscillations

(~ 1 km from Virgo site)• Resonant frequencies:

2, 6, 11, … Hz

• Work in progress:coincidence measurement

• Hypothesis:only lowest frequency survive at s = 1 km distance

• General feature of bridges

• Impulses approx. 30 sec long

assuming: Q = 5, v =1000 m/sA(2.Hz)/A(6.Hz) = 0.1

A(s,ω) = A0 exp(- )ω s2vQ


From soil to dark fringe (I)

• Seism contributes to DF noise only below 1.5 Hz, • contribution is mainly from vertical vibrations

10−2 10−1 100 101

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

frequency (Hz)

Coherence of dark fringe signal with seism (E3 e-run)

VERTICALNSEW

0.01 0.1 1. 10 0 0.5 1. 1.5 2. 2.5

seismometeris blind here!

ZOOM: f < 2.5 Hz

Frequency (Hz)Frequency (Hz)


From soil to dark fringe (II)Transfer Function (E3)

VERTNSEW

SA resonances

• Decreasing phase indicates a finite travel time


From soil to dark fringe (III)

• Travel time: of the order of few seconds (~3s) • Ringing lasts for up to 200 seconds (!!)

0 100 200 300 400 500 600 700 800 900−40

−20

0

20

40Vo

lts

0 50 100 150 200 250−40

−20

0

20

40

Volts

0 5 10 15 20 25 30 35 40−40

−20

0

20

40

Volts

time (s)

DF response to impulsive seismic noise (inverse of TF)


• Seismic noise is responsible of the DF noise from 0.4 to 1.6 Hz (!ocean microseism)

• Vertical noise dominates: vertical Inertial damping not implemented (yet)

Seismic noise contribution to Dark Fringe

WESNVerticalTOTALDark Fringe

2


E4 seismic investigation• Same coherence analysis repeated with E4 data

showed many unexpected coherent lines above 10Hz !!Seism .vs. E4 Dark Fringe COHERENCE

Frequency (Hz)1001010.2 200


Frequency (Hz)

CoherenceDF.vs.seism

Dark fringe

Soil accelerations

11.4 13.3 18.6 23.524.6


Sources ?

• Same lines also seen by acoustic channels

• Identified as vibrations of motor devices in CITF building:

• air conditioning, • air cleaning, • vacuum pumps …

11.4

13.3

16.5 19.2

19.9

" seism propagates through air

11.4 18.6 23.524.6


Noise enters from Injection System…

• Laser frequency is locked on MC length⇒ Seismic noise is somehow transmitted

to suspended I.S. mirrors and converted into laser frequency noise in the MC cavity

Same seism coherent lines in laser frequency noisefeedback loop

I.S. (laser + 140m MC cavity) first used in E4

CITF Same seism coherent lines in laser beam

from I.S. to CITF

MC


Which path ?

• Injection System control loops:- Local control on suspended mirrors- Automatic alignment of laser beam

• These loops are fed by cameras and photodiodes placed on soil

Solutions• Short term: steeper low pass filtering in I.S. feedback loops• Virgo 3000: laser frequency stabilization using Dark Fringe signal

coherent seismic lines in feedback error signals

• Seism above 10Hz can be re-injected if feedback loops have not null gain at those frequencies

Investigation still in progress…


Conclusions

• Main sources below 10 Hz: - seismic wave field from the sea at 0.3-0.4 Hz- local traffic 1.- 4. Hz (preliminary results)

• Discussed general characteristics of seism at Virgo site• Seism daily variations occur mainly below 10 Hz

• All dark fringe noise below 1.5 Hz is of seismic origin,- mainly due to vertical seism, currently not actively damped at low frequencies

- above 1.5 Hz seism is efficiently damped by SA• Lesson from engineering runs:seism may have other ways to enter the interferometer(feedback loops).


1

10 -10

10 -20

SA Transfer Function

1 10 1000.1Freq (Hz)

10 -13

4

HOR.VERT. x 10^-4

f -2N

Super Attenuator expected performances

0.01


Which path ?


• These loops are fed by cameras and photodiodes placed on soil


coherent seismic lines in feedback error signals

• Seism above 10Hz can be re-injected if feedback loops have not null gain at those frequencies

shaking of IS optic bench mirrors back and forth in front of the beamcould produce frequency noise (optic bench is not seismically isolated)

• Doppler EffectInvestigation still in progress…


Which path ?• Suspensions: (…unlikely)

MC mirrors are suspended by short SA towers• short towers should attenuate seism down two

at least 3 orders of magnitude below observed values(according to design performance: vert. T.F. = 10^-7 @ 10Hz)



• These loops are fed by cameras and photodiodes placed on soil • Seism above 10Hz can be re-injected if feedback loops

have not null gain at those frequencies

Investigation still in progress…

seismic noise characterization at virgo

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