1. 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

2. Newtonian gravity noise
Seismic and atmospheric density modulations cause Newtonian gravity gradient noise (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 by going underground.
At z = 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 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 uncorrelated between 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 CSN between 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 NN mitigation by using SGG appears to be feasible!
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 109 by CM rejection.
Scattering of Rayleigh waves off underground cavity and NN from local sources must be examined.
Parameter
SGG
Each test mass M
1.5  103 kg
Arm-length 
4 m
Antenna temperature T
4.2 K
SQUID temperature TSQ
0.1 K
DM quality factor QD
107
Amplifier noise number n 10
Detector noise Sh1/2(f )
2  1020 Hz1/2
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. What is Earthquake Early Warning ?
P-Wave
S-Wave
ability to provide a few to tens of seconds of warning before damaging seismic waves arrive
San Andreas Fault
S-P time

11. Blind zone size in California (Kuyuk and Allen, 2013)
Blind zones of EEWS
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?
Blind zone size in California (Kuyuk and Allen, 2013)
From presentation by P. Ampuero (Caltech Seismolab)
Paik

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

13. SEED (Superconducting Earthquake Early Detector)
By levitating two Nb test masses (M = 10 kg, L = 50 cm) separated along z axis, h13 and h23 are measured.
To reject the seismic noise to below the intrinsic noise, CMRR = 109 is achieved.
Sensitive axes must be aligned to  105 rad.
Test masses are cooled to 1.5 K and coupled to 120 SQUIDs via a capacitor bridge transducer.
at 70 km
QD SQUID
120 SQUID
Paik