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Vuk Mandic
Marcel Grossmann 13, Stockholm07/06/12
Newtonian Noise in an Underground Environment: Seismometer Array at the
Homestake Mine
1
Third-Generation Science
● Primordial GW background.» Might be sensitive enough to probe
inflationary models.» 0.1-10 Hz band appears to be free
of unresolvable foregrounds.● Equation of state:
» Binary coalescences reveal luminosity distance: standard sirens.
» Measure 1000s of binary mergers for which the source (and redshift) can be identified (e.g. GRBs).
» Fit to measure cosmological parameters, such as w.
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● Progenitors of short GRBs: believed to be NS-NS or NS-BH mergers.
● Neutron star observations: how large ellipticities can they support?
● Origin of intermediate mass black holes (102 – 104 solar masses) – binary merger or accretion?
● Understanding gravitational collapse and supernovae.
● Are massive objects at galactic nuclei really black holes?
Cosmology Astrophysics
3
Many astrophysical models of stochastic background have been proposed.
Almost all of them are significant above 1 Hz or below 0.1 Hz.– May have access to the
primordial background around 1 Hz.
Exception: binary coalescences.– Will have to identify and
subtract all binary signals in this band.
– Non-trivial, but appears to be doable.• Cutler and Harms, PRD73,
042001 (2006). 3
Primordial Stochastic Background?
Accessing Low Frequency
● How low in frequency can we go? Can we build GW detectors at 1 Hz?
● Several noise sources to address:– Radiation pressure noise,
~P1/2 .– Thermal noise.– Seismic noise (mechanical
coupling).– Newtonian noise.
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Newtonian Noise
● Or, gravity gradient noise.● Fluctuations in the local gravitational
field.» Seismic motion.» Atmospheric fluctuations.» Human factor (traffic etc).
● Theoretical estimates:» At 1 Hz, equivalent strain of ~10-20 Hz-1/2.» Seismic and atmospheric
contributions similar.● We need ~103x suppression at 1 Hz.
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Why Underground?
● Atmospheric fluctuations are irrelevant.
● Local disturbances much more controlled:» Limited access, no passers-by... » Controlled use of machinery.» Ventilation well understood and
controllable.● Much more stable environment.
» Temperature, pressure, humidity...● Seismic noise is reduced
exponentially.» At 1 Hz expect ~10x suppression.» Depends on depth and rock
structure.
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Why Underground?
● Need another 100x suppression of Newtonian noise at 1Hz:» Measure the seismic motion, subtract its effects.
● In ideal conditions (homogeneous, infinite spherical medium) one seismometer is enough.
» Local sources, fault lines, surface of the earth (finite medium), rock uniformity: require an array of seismometers.
» J. Harms et al, PRD80, 122001 (2009).» J. Harms et al, arXiv:0910.2774.
● Reality may be different (geology, finite medium…):» Need measurements!
DUGL at the Homestake Mine
● Deep Underground Gravity Lab: Measure the seismic noise underground.
● Many collaborating institutions: Caltech, Carleton, Columbia, Eotvos U, NIKHEF, Florida, Minnesota.
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Array of Seismic Stations
Developing an array of seismometer stations underground at Homestake.
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Initial Station Design
● Substantial effort to optimize the measurements.● Nested polyurethane huts to minimize environmental effects
(temperature and pressure fluctuations).● Granite tiles supporting seismometers, minimize tilts etc.
● Included thermometers, barometers etc to monitor environment.● Data acquired locally, using PC's, connected to the surface
machine via optical links.● Stations synchronized using NTP network protocols, to ~0.2 ms.● Surface machine to package the data into frame format.
Environment Stability
Relative Humidity
Pressure
Temperature
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Pressure
300ft Station
2000ft Station
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First Results● Good measurement of the seismic
noise level at several depths.» ~10x quieter than LIGO sites at
1 Hz.● Occasionally matches (or even
surpasses) the Peterson 1993 low-noise model.» Very quiet seismic environment.» Also of interest in geophysics
and seismology.● First results published:
» J. Harms et al, Class. Quantum Grav. 27, 225011 (2010).
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First Results
● Observe various surface effects at 300ft:» E.g. Ventilation maintenance, not observed at 2000ft.
● Observe blasting, at Homestake and elsewhere.
Transients
● Deployed standard tools used by GW community to look for transients (Kleinewelle triggers).
● Observed rate ~ SNR-4.● Deeper levels consistent (when
taking into account relative background noise levels at 1 Hz).
● At the 300ft level observe more transients, many due to surface effects.
● For example, 300ft level seismic noise correlated with wind speed on the surface. No such correlation exists at 2000ft or 4100ft.
Array Upgrades
● Learned much about working in the harsh underground environment.
● Humidity.● Limited access.● Loss of network/power.
● Custom-made DAQ system from LIGO Lab (V. Dergachev).
● 24-bit, enough dynamic range to measure both background levels and earthquake signals.
● Small CPU units (Dreamplugs) to minimize humidity failure.
● Optical timing system (Columbia).● UPS, with remote network switch.
Array Upgrades
● Added infrasound microphones, built by the Eotvos group, to monitor sub-audible air currents etc.
● Oriented all seismometers using Octans instrument borrowed from the USGS.
● Can't use a compass or GPS underground!
● Octans is a (very expensive!) gyrocompass and motion sensor, based on fiber optics gyroscope technology.
● Most important improvement: true 3D configuration.
Array of Seismic Stations
New station built at 4100ft, considering others at 4850ft.
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New Data
● Have been acquiring data since January 2012.
● Five operating stations.● Some power/network interruptions,
but mostly stable.● Currently in the process of
calibrating data, correcting for orientation etc.
● Will produce data in both frame and miniSEED formats (used by the geophysics community).
● We have much to learn from geophysicists!
800ft
2000ft
800ft
4100ft
Future Plans
● Shifting the focus toward data analysis.
● Take advantage of the 3D configuration.
● How well can we estimate the seismic field with these few measurements?
● Surface topographic effects? Surface wave generation and suppression with depth?
● Long term: proposing to build a 23-element array at Homestake.
● Probing 1.5 km cube.● Close collaboration with geophysics
community.
Conclusion
● Detailed measurements of the seismic field underground can inform us about how well we can suppress the Newtonian noise due to seismic motion.
● Can do better than what nature gives us by suppressing surface waves with depth!
● Learned much about working underground.● New data from the true 3D array configuration should be ready
soon.● Hope for a substantial expansion of the array in the coming
years.● Close collaboration with geophysics community is crucial for
this effort.
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Coherence Between Levels
Microseismic peak appears coherent, especially in horizontal DOF.
Elsewhere the coherence is strongly reduced:» Reliable range is: 50 mHz –
0.7 Hz.» Effect of local disturbances at
300 ft?» Effect of the rock structure
between 300ft and 2000ft? Of course, we really care about
coherence at a given level.» Hopefully soon...
Horizontal
Vertical
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