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Ho Jung Paik University of Maryland GW Astronomy, Korea August, 2016

Mitigation of Newtonian Noise Using Superconducting Gravity Gradiometer. Ho Jung Paik University of Maryland GW Astronomy, Korea August, 2016. Newtonian gravity noise. Seismic and atmospheric density modulations cause Newtonian gravity gradient noise (NN) , which cannot be shielded.

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Ho Jung Paik University of Maryland GW Astronomy, Korea August, 2016

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  1. Mitigation of Newtonian Noise Using Superconducting Gravity Gradiometer Ho Jung PaikUniversity of MarylandGW Astronomy, KoreaAugust, 2016

  2. Newtonian gravity noise • Seismic and atmospheric density modulations cause Newtonian gravity gradientnoise (NN), which cannot be shielded. • Advanced laser interferometers will be limited by the NN due to Rayleigh waves below 10 Hz. NN dominated by Rayleigh waves • On way to reduce the NN is bygoing underground. At z = - 200 m, NN is reduced by a factor of 36 at 10 Hz and 3 at 3 Hz. • KAGRA: 200 m depth, ET: proposed to be at 100-200 m depth. Paik

  3. Mitigation of NN for surface detectors • Seismic motion and atmospheric density modulations are measured by using seismometers and microphones. • Apply coherent noise cancellation by Wiener filtering. Data from reference channels are used to provide a coherent estimate of the NN. Residual from Wiener noise cancellation • Inhomogeneity and a change of spatial correlation due to scattering and local sources may produce systematic errors. Paik

  4. Could SGG be used to mitigate NN? • 13- and 23-comp SGGs could be used to measure and remove X() and Y() precisely without relying on external seismometers. • Worthy mitigation goal: x 5 improvement to 2  1023 Hz 1/2 at 10 Hz. • At 1-10 Hz, NN is uncorrelatedbetween interferometer test masses. One SGG must be co-located with each test mass. SGG Paik

  5. Sensitivity requirement Paik

  6. Correlation requirement • Mitigation factor S is limited by correlation CSNbetween interferometer test mass and NN sensor: Beker et al., GRG 43, 623 (2011) • SGG with  < 0.8 m must be brought to within 0.8 m to the test mass. • Such a small SGG would not be sensitive enough and cannot be brought to such proximity to the test mass. • Is there a way out? Paik

  7. Bypassing correlation requirement • Rayleigh waves are surface waves with no phase shift along z. • CSN = 1 for SGG of any as long as its test masses occupy the same (x, y) with interferometer test mass. • Solution: Locate an SGG with only vertical arm under each test mass. SGG is sufficiently well isolated from seismic noise by pendulum suspension. Interferometer test mass SGG test masses Paik

  8. SGG with 4-m arm • SGG with only vertical arm (= 4 m, M = 1.5 ton, T = 4.2 K) is located under each interferometer test mass. • SQUIDs are further cooled to 0.1 K to reach 10 noise level. Has been demonstrated using two-stage SQUID. • Seismic noise is rejected to one part in 109by CM rejection. • Scattering of Rayleigh waves off underground cavity and NN from local sources must be examined. NN mitigation by using SGG appears to be feasible! Paik

  9. Use of co-located tilt meters • Test mass displacement due to Rayleigh waves: • A tilt meter under the test mass measures Completely correlated with the test mass displacement even in the presence of multiple waves. • Solution: Locate a sensitive tilt meter under each test mass. • Technically, the tilt meter approach seems to be more straightforward. • What are the pros and cons of the two approaches? Further analyses are needed. Harms and Venkateswara (2016) Interferometer test mass Tilt meter Paik

  10. P-Wave S-Wave S-P time What is Earthquake Early Warning ? ability to provide a few to tens of seconds of warning before damaging seismic waves arrive San Andreas Fault

  11. Blind zones of EEWS Blind zone size in California (Kuyuk and Allen, 2013) From presentation by P. Ampuero (Caltech Seismolab) To reduce the blind zone, can we use gravity signals that travel at c, much faster than seismic waves? GRACE and GOCE missions have measured static gravity changes after vs before large earthquakes. Can dynamic gravity signals following fault rupture be measured quickly? Paik

  12. Expected dynamic gravity signal Ampuero et al., Prompt detection of fault rupture for earthquake early warning (preprint) SNR after 5 s Gravity signal following a rupture Epicentral distance = 70 km Next stage: h = 1015 Hz1/2, MANGO: h = 1020Hz1/2 SNR after 10 s Paik

  13. SEED(Superconducting Earthquake Early Detector) at 70 km QD SQUID 120 SQUID By levitating two Nb test masses (M = 10 kg, L = 50 cm) separated along z axis, h13andh23are measured. To reject the seismic noise to below the intrinsic noise, CMRR = 109is achieved. Sensitive axes must be aligned to  105 rad. Test masses are cooled to 1.5 K and coupled to 120 SQUIDsvia a capacitor bridge transducer. Paik

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