LIGO Livingston SURF presentation
31 July 2002
supported by CIRE (NSF)
Outline |
II. Description of gravity gradient
III. Description of ground waves
IV. Previous data and estimates for LLO
V. Description of model
VI. Model results to date
theoretical | experimental |
---|---|
1. characterize Rayleigh waves
2. model ground motion from waves 3. model gravity gradient from waves 4. relate to LIGO noise spectrum |
1. review equipment and software
2. prepare seismometers 3. deploy seismometers 4. gather data |
combined |
---|
1. use model to find gravity gradient from observed ground motion
2. characterize gravity gradient noise for LIGO-Livingston |
Supervisor: Mark Coles
Team members: William Quarles, Michael Cheung
Gravity gradient--any nonuniform component of the gravitational field.
On the Earth, both the magnitude and direction of gravitational acceleration varies due to:
There is also an apparent gradient due to the Earth's rotation for observers in the rotating
frame of reference.
Gravitational wave observations can be affected by time-dependent gravity gradients from:
This is a concern for interferometers more than for resonant bar detectors.
Ground waves from seismic noise (Hughes and Thorne, 1998, gr-qc/9806018) | |
People walking near test masses (Thorne and Winstein, 1999, gr-qc/9810016) | |
Tumbleweeds blown into side of buildings (Creighton, 2001, gr-qc/0007050) |
body waves | P waves (pressure, primary, longitudinal) | |
S waves (shear, secondary, transverse) | ||
surface waves | Rayleigh waves (vertical and longitudinal) | |
Love waves (transverse horizontal) | ||
interface waves | Stonley waves | |
free oscillations |
torsional oscillations
spheroidal oscillations |
(wave illustrations from Fowler, The Solid Earth, 1990, 1992)
Rayleigh waves are the greatest concern in terms of producing gravity
gradient noise, since they fall off more slowly with distance than body waves and
represent most seismic energy from localized surface sources.
source | depth (m) | Poisson's ratio | C_{P} (m/s) | C_{S} (m/s) | C_{H} (m/s) |
---|---|---|---|---|---|
LIGO survey (1995) |
2 5 15 | 0.4 |
213 247 293 | ||
Hughes & Thorne (model, 1998) |
0-5 5-105 105-905 905-3005 |
0.33 0.47 0.40 0.31 |
440 1660 1700 1900 |
220 400 700 1000 |
205 360 660 930 |
Coles (2001) | shallow | 0.48 | 1780 | 368-379 | 335 |
Saulson, 1994
Hughes and Thorne, 1998, gr-qc/98061018
Elements of current model:
Expression for gravity gradient:
Strain from simulated gravity gradient (acceleration) amplitude:
where L = arm and B = transfer function
© 2002 by Wm. Robert Johnston.
Last modified 3 October 2002.
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