1

The Windmills Project

• Motivations and description• Measurements and analysis plan • An example: sea microseism at Virgo

In this talk :

I Fiori – ILIAS WG1 meeting – Cascina – June 14, 2005

2

Motivations

• A wind power plant is going to be installed soon (<1yr) a few (5-6) kilometers away from Virgo, and we are concerned about the effects that the generated seismic disturbance may produce on the interferometer

• A similar wind power plant already exists near the GEO600 inteferometer

A study of characteristics of seismic signal produced by the GEO windmills and of possible effects on the interferometer

is a good subject on which we can exchange data and experience, interesting and useful for both experiments

3

The Windmills project

1. Characterization of the seismic wave-field produced by the GEO windmills (frequency content, speed, direction of propagation of the persistent wave-field, its dependency on wind conditions, its attenuation btw the windmills and the interferometer )

2. Investigate for effects in the GEO data (environmental sensors, mirror suspensions, ITF signals)

3. Build a physics model and extrapolate to predict size of effects that VIRGO may have to face

Reference work by LIGO: R.Schofield, “Seismic Measurements at the Stateline Wind Project” (T020104-00-Z)

4

The team:

Emanuele Marchetti (Geophysics Dept. U. of Firenze)

Gillian A. Mayer (GEO600 - U. of Konstanz)

Lara Giordano (VIRGO - INFN Napoli)

Federico Paoletti (EGO - INFN Pisa)

Harald Lück (GEO600 – U. of Hannover)

Hartmut Grote (GEO600 – U. of Hannover)

Stefan Hild (GEO600 – U. of Hannover)

I. F. (VIRGO - U. of Pisa)

Giovanni Losurdo (VIRGO - INFN Firenze)

5

3-bladed turbine• Blades = 90m diameter• Tower = 110m height• Rotor speed = 9.6-16.9 rpm• Wind speed : 10-90 km/h• Pitch control of blades for optimal functioning regime rotor speed (…)

Nordex S90/2300kW:The GEO windmills

• Seismic peaks in correspondance of resonances of tower structure : between 0.1 Hz and few Hz (steady)

• and peaks in correspondance of the rotation rate of turbine and 3 multiples (may change with wind speed)

We expect :

• Power generator 740-1310 rpm

6

3

3

3

2

2

2

1

1

1. Nordex S77 (2 windmills, recent installation)2. Enron Wind EW1.5s EW1.5 (2+1 windmills) 3. Nordex N90 (3 windmills, recent installation)

Wind Project Shliekum/Niedersachsen:

Frequency of seismic peaks can be slightly different for each type of windmill

7

Seismic peaks may accidentally match resonances of our mirror suspensions

• Low frequency surface seismic wave-field (Rayleigh type) propagating through area between windmills and GEO600• Persistent, but frequency content and intensity may change with wind conditions• Most intense seismic peaks 0.1– 5. Hz (plus harmonics).

We expect :

Potential danger for the interferometers :

• Lowest freq. components (less attenuated) are likely to reach GEO600 structures with sizable intensity

8

Seismic survey of GEO600 site July 24-29 th by a VIRGO-GEO team : Emanuele, Gillian, Lara, Federico, Irene

• 2 portable seismic stations (from Dept. Astronomy U. Firenze, M. Mazzoni)

- tri-axial low frequency (0.2-100 Hz) velocimeters (Lennartz 3D/5 sec)- A/D converter 24-bits (Guralp CMG-DM24) - portable hard disk (storage capacity of few weeks) - GPS antenna receiver

Equipement :

• 1 microphone LF (>1 Hz) plus preAmplifier card (from U. of Naples) which will be plugged into GEO data acquisition system (LIGO measurements indicate that propagation occurs in part through air)

• 1 piezo accelerometer (>1Hz) for easy measurement of resonances of each windmill

• 3 GEO seismometers STS2 Streckheisen (>10mHz) for correlation studies

• 1 GEO anemometer (wind speed, direction)

9

10

Reference Seismic Station

Movable Seismic Station

One station kept in a fixed position (reference) in continous acquisition

One moved to 10-20 different sites between windmills array and GEO

Measurements layout :

(1-2 hrs each)

11

• Long-term data acquistion: 2 seismic stations + microphone will be left at GEO (approx. 1-2 months) to allow repeat measurements in different wind conditions (Gillian)

Analysis of seismic data (plan)• Characterize frequency content of seismic field from analysis of peaks in single spectra: identify major structural resonances of each tower or blades and look for possible changes of amplitude/freq. with wind speed

• Identify and characterize the persistent seismic wave-field (i.e. its velocity and direction of propagation, polarization) by correlation analysis of pairs of simoultaneous recordings from the two stations (also by principal components analysis)

• Measure seismic attenuation of soil between windmills and GEO600, identify the propagation path of the surface wave: does it occurs through ground or partially through air ? Eventually, we want to build a model which describe the seismic wave attenuation which can be used to extrapolate results to VIRGO

• Search for effects in permanent GEO seismic stations (3 buildings) and eventually on other ITF channels: coherency analysis

12

Needs for data exchange

• Open access to GEO600 data across firewalls for some channels: seismometers, anemometer, B&K microphone… other interesting signals, - restricted to ILIAS-WG1 team - for the duration of windmills seismic studies: i.e. at least starting from July 24 - ftp server ?

• Data of seismic survey (approx. 500Mb in binary format) will be made available to both VIRGO and GEO, easily converted into matlab format eventually these data will be converted to frame format to allow easy correlation studies with channels of GEO interferometer

13

An example: the Microseismic wave-field at Virgo

14

Seismic Array for the detection of low freq. wave-field

• 5 tri-axial velocimeters Lennartz 3D/5sec

SEA 15 km

• Few days data taking.

• Fixed positions with 1-2km spaceing, along VIRGO arms

(by E.Marchetti, M.Mazzoni)

15

Vel

ocit

y [ m

/ s/s

qrt(

Hz)

] Amplitude spectra at 4 stations (2hrs data):

E-W

high coherence 0.1 - 0.4 Hz

Coherence :

Coherent and persistent spectral peak at 0.4 Hz

16

East West

speed =750-800 m/s

0 200 400 600 800

Time [s]

LA

G [

s]

+6

+4

+2

0

-2

-4

-6

Vi1

Vi2Vi4

Vi3

Vi5

• 15min data from 4 stations are band-pass filtered around 0.4 Hz• for each pair of stations, we compute correlation as function of time• high correlation region measures the travel time of seismic wave btw each pair of stations

The seismic field associated to the 0.4 Hz peak

• Determine the direction of propagation and speed of the plane surface wave at 0.3Hz which best fit all travel times

Delay time= +3.8 s

17

• The frequency content of the microseismic activity changes with weather and sea conditions and is seen to move to lower frequency ( 150mHz) during the decreasing phase of a swell

• During quiet conditions the microseismic peak detected at Virgo is at 0.4 Hz

0.1 0.2 0.3 0.4 0.5 1. [Hz]

• Observed a seasonal effect: the 150mHz peak is often excited in winter-time

Evolution of microseismic peaks during a swell

18

• First vertical resonance of Virgo SA lays between 130-170mHz and can be excited by the 150mHz microseismic peak

• Quiet time on Dec. 2 • “150mHz” microseism excited on Dec 7

Effects of the 150 mHz microseism on suspensions :

• The 150mHz microseism was often excited in past winter months (November – March) disturbing locking of recycled interferometer

Data from C5 run :

7

143mHz 151mHz

• April : SA vertical damping improved with increased gain at 150mHz … wait next winter to effectly test it !!

Most affected are the PR (150mHz) and NI (140mHz) towers

3

19

C5 data, VIRGO recombined

And …. some effect on the Dark Fringe :

Accelerometer WE top stage Dark Fringe

20

C1 : the large microseismic activity caused a large motion of the inverted pendulum stage of mirror suspensions

Another improvement related to microseism: Inertial Damping of IP stage of mirror suspensions

C2 : improved robustness of Inertial Damping feedback control, the effect substantially reduced

Accelerometer NI top stageSeismometer on ground

21

Monitor of motion of SA top stages : RMS btween 0.2 and 1 Hz

Horizontal E-W Horizontal N-S VERTICAL

22

Concluding remarks : • The WindMills project aims to characterize seismic wave-field produced by the wind power plan near GEO600, investigate for possible effects produced on GEO600 and predict effects produced by a similar plant to be installed few km from VIRGO.

• Gillian M. has spent few days with us at VIRGO, together we have practiced installing one seismic station and reviewed tools and tecniques for the data analysis

• July 24-29 : will perform seismic survey at the GEO site, then seismic stations and microphone will be left at GEO for longer term meas.

• Data will be made available to both sites for analysis, also necessary to access GEO600 data for correlation studies: need to open an ftp access across firewalls

• She will continue measurements with one Geophone, useful to characterize frequency content of seismic signal from each windmill