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Understanding Gravity Gradient Noise in Underground Detectors

Explore the direct coupling between mass density fluctuations and gravitational field variations leading to gravity gradient noise. Learn why underground detectors are crucial, with a detailed modelization and estimation of noise reduction. Discover how GGN reduction varies with depth, and strategies for efficient noise subtraction using accelerometers in interferometers.

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Understanding Gravity Gradient Noise in Underground Detectors

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  1. G. CellaINFN Pisa GravityGradientNoise Considerations

  2. Gravity Gradient Noise Direct coupling between environmental fluctuation of mass density and test masses • Mass density fluctuates..... • ..... and generates fluctuations of gravitational field

  3. Gravity Gradient Noise Direct coupling between environmental fluctuation of mass density and test masses • Mass density fluctuates..... • ..... and generates fluctuations of gravitational field

  4. Why underground detectors? • GGN is averaged to zero on a scale ¸/(2 ¼) • Surface contributions are damped • Volume contributions come from a O(¸) layer around the test mass • Surface waves should be dominant

  5. Gravity Gradient Noise: modelization and estimation Bulk contribution + Surface contribution - Cavity contribution + Cav. surface contr. = GGN

  6. Reduction with the depth • Simple model • Homogeneous medium • Surface waves only • Stationarity • No strong local sources • Validation • Prediction about seismic correlations Surface -10 m -50 m -100 m -150 m ET-C ET-C ET-B M.G. Beker, G.C., R. DeSalvo, M. Doets, H. Grote, J.Harms, E. Hennes, V. Mandic, D.S. Rabeling, J.F.G. Van den Brand, C.M. Van Leeuwen, “Mitigating noise in future GW observatories in the 1-10 Hz band”, accepted by GRG ET-B

  7. Reduction with the depth • Simple model • Homogeneous medium • Surface waves only • Stationarity • No strong local sources • Validation • Prediction about seismic correlations Surface -10 m -50 m -100 m -150 m ET-C ET-C ET-B M.G. Beker, G.C., R. DeSalvo, M. Doets, H. Grote, J.Harms, E. Hennes, V. Mandic, D.S. Rabeling, J.F.G. Van den Brand, C.M. Van Leeuwen, “Mitigating noise in future GW observatories in the 1-10 Hz band”, accepted by GRG ET-B

  8. Speed of sound and GGN reduction • Reduction increases at low sound speeds From cL=200 m/s to cL=2000 m/s

  9. Subtraction • Measure quantities correlated with GGN • Displacements (accelerometers) • Compressions • Subtract from the interferometer signal in such a way to maximize SNR: Spectral correlation among sensors Signal power spectrum Spectral correlation between signal and sensors

  10. Subtraction strategies • Optimization.For a given # of sensors, what are their best • Positions • Orientations 3 accelerometers

  11. Subtraction strategies 512 accelerometers Simple model with a frequency dependent correlation length Test mass here

  12. Subtraction efficiency • Efficiency estimate • Several coherences • Regular grid • Optimal arrangement of sensors • Pessimistic assumtions • Decoherence effects probably overestimated

  13. Subtraction efficiency • Optimal arrangement of sensor is frequency dependent • More robust with an higher number of sensors • Coherence improves the subtraction efficiency

  14. Conclusions • Earth-bound interferometers are limited by GGN below 1 Hz • Good perspectives for beating GGN • Quiet site • Going underground • Subtraction feasibility must be tested in realistic scenarios: • Coherence • Non stationarity

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