1. Activities in the Homestake mine/
   
Angelo Sajeva
Jan Harms
Riccardo De Salvo
Vuk Mandic
LIGO-G R

2. Why Underground? Stochastic background signal intensity
Third generation GW detectors will seek the 1Hz frequency band
Strong scientific motivation (stochastic background, pulsars, massive inspirals).
Stochastic background signal intensity
ATNF Pulsar Catalogue,
The Astronomical Journal 129, 1993 (2005)‏
Maximum BH inspiral frequency
f ~ 4.4 Hz (103 Msun/M)

3. Why Underground? Advanced detectors: Gravity gradient limited
below 10 Hz.
Third generation detectors:
Possible solution to go to lower frequency is an
underground detector.

4. Gravity Gradient Newtonian Noise
fluctuations of Earth’s gravitational field acting the mirror test masses caused by masses in seismic motion
Rock surface movements and rock density fluctuations are the two main causes of NN
We study the seismicity in order to understand the Newtonian Noise (NN)

5. Newtonian Noise Reduced in depth Cannot be shielded
Can be subtracted from data flow
To what extent can NN be subtracted?
Depends of wave coherence, propagation, diffusion, etc.

6. What’s Homestake mine? former gold mine for 125 years,
Hosted Ray Davis exp.
2.4 km deep
Few miles across

7. What is DUSEL?
DUSEL is an underground laboratory space providing infrastructure for science and engineering research.
The primary motivation has been for fundamental physics research, exploiting the shielding from cosmic rays.
Can it host a GW observatory?

8. Work project: We want to establish:
a matrix of coherent sensor stations to measure seismic wave propagation
From a vertical array we expect to measure attenuation and coherence as function of z
From a horizontal arrays we will study wave propagation coherence and diffusion

9. Order of magnitude vpressure =8 km/s f=1 Hz l=8km
Shear =5 km/s, Superficie 0.3km/s
Waves can see only defects comparable or greater then l /2*p
Short distance rock inhomogeneities are irrelevant, but can scatter the waves and spoil coherence
Need measurements to understand at what depth on gets sufficient coherence for subtraction

10. What we’ve done: We set up three seismic stations:
300ft, 800ft, 2000ft
We got some preliminary data

11. Our Stations: Structure of the stations’ system: instruments hut
computer hut
Stations are connected through fibers in order to have synchronized clocks

12. Our dream spot Blind tunnel
Large pad of concrete well attached to the bedrock
No acoustic or human/artificial seismic noise
800 feet pretty good

13. How did we design a seismic station and why?
Mechanical Connection to Ground
Quiet location
Acoustic insulation
Power
Data acquisition
Connectivity
Environmental Monitoring

14. Internet Connectivity
Antennae as directional radio bridge down the shaft
Fibers on each level
private network
remote desktop connection to control local computers and sensors from surface
ftp to transfer data

15. Timing issue
We want synchronized clocks, consistent with the absolute time.
10 msec Indetermination in knowledge of absolute time
.2 ms max. relative error between surface and 300 ft.
Eventually timing with laser pulses and fibers (which will be with ns errors)

17. 300 L

18. 300ft Station

19. First data acquired on the 300 feet site:
This figure is a periodogram where each value is averaged with the 100 following values.
Multiple peaks from
Different oceans?

20. 800 L

21. 800ft Station Building seismic stations underground is not trivial!
800ft station completed, but did not have power or network access.
Required significant construction.
Building seismic stations underground is not trivial!

22. 2000 L

23. 2000ft Station Has power, fiber cable laid out.
2000ft station completed.
Has power, fiber cable laid out.
Expect network access this week.
We run it with two Nanometrics T240 seismometers and an STS-2 side-by-side.
Read out by local Nanometric data acquisition, will transition to ours soon.
Preliminary results are very encouraging.
Our station is within a factor of 3 from the quietest measured on Earth!
May get even better with time, as the instruments settle and we better shield from tunnel noise

24. 2000 ft installation

25. First results Even at 2000 feet
Except for microseismic peak at low frequency the ground is quiet enough to be close (~10 dB) to the sensitivity of the best available sensors
Close to quietest places on Earth
E-W
N-S
Vert

26. First result
At 2000 feet
At intermediate frequencies ground is an order of magnitude above sensor noise
Will gain faster with depth

27. How much NN can be subtracted
Rock motion already close to sensor noise
To subtract NN to, say, 10-3 level
Need to measure rock motion and density variation to 10-3 precision
Inertial sensors insufficient
Optical bars connected to rock to measure density fluctuations and movements
Coating noise
10-1
10-3
10-2

28. R&D Directions: Strainmeters
Rock density and
Motion of the cavern wall surface
dominant sources of Newtonian noise.
Measure the relative motion of wall surfaces
Mount mirrors on the walls.
Interferometers as optical strainmeters.
Probe different directions.
Extract fluctuating density.
Study correlations between different points.
Study caverns of different sizes.
Interferometer sensitivity
<10-10 m /arm length 28 28

29. Conclusion
DUSEL offers very interesting opportunity for gravitational physics
an exceptionally quiet Newtonian force environment.
Need to understand the levels of seismic noise and gravity gradient noise as a function of depth.
Planning a collection of measurements to characterize the site.
Interesting connections with geology/geophysics/seismology.
The results would inform future experiments for:
Underground 10km-scale gravitational-wave interferometers. 29 29