Activities in the Homestake mine 07-20-2008/09-19-2008     Angelo Sajeva Jan Harms Riccardo De Salvo Vuk Mandic LIGO-G080475-00-R

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)

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

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)

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.

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

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?

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

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

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

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

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

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

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

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)

300 L

300ft Station

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?

800 L

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!

2000 L

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

2000 ft installation

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

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

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

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

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