Vuk Mandic

Marcel Grossmann 13, Stockholm07/06/12

Newtonian Noise in an Underground Environment: Seismometer Array at the

Homestake Mine

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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

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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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