Download - LSC Aug. 14-16,2001G010280-00-L Mark Coles 1 Livingston Seismic Environment Mark Coles LLO
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Livingston Seismic Environment
Mark Coles
LLO
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Main Topics
• Environmental characterization of seismic channel properties
• Multi element array measurements of the seismic environment
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Geophysical measurements along the X arm
• Measure the propagation velocities of the of the shear and compressional wave signals
• Measure velocity dispersion• Measure the attenuation dependence on distance
and frequency• Use these measurements as guidance in determining
possible sources of seismic noise, and for estimating gravity gradient
Measurements made with the assistance of Kevin Tubbs – SURF student from Southern University
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Measurement Technique
• 5 Guralp CG40T seismometers and Reftek data loggers borrowed from:
IRIS PASSCAL Instrument CenterNew Mexico TechSocorro, New Mexico
http://www.passcal.nmt.edu/passcal.shtml
• Placed in linear array along X-arm at 500 meter intervals
• Impulsive source made from soda containers partially filled with liquid nitrogen submerged in erosion control pond. 8 foot depth provides reaction mass to couple sound wave into soil
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Rayleigh and Compressional Wave Propagation Modes Visible
• Differing propagation velocities due to different shear and Young’s modulus
• Rayleigh waves:– Propagate along surface– Combination of vertical shear wave and compressional wave
that satisfies boundary condition that vertical stress vanishes at surface.
– Amplitude is ellipsoidal and retrograde– Vertical/horizontal amplitude ratio:
1.3 -> 1.8 depending on Poisson’s ratio
Wave direction
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Intersection of arrival times points at pond boundary
Slope is velocity of wave packet:
cR = 368+/- 11 m/sec
cC = 1780+/- 80
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Time domain 4 pole High-pass Butterworth IIR filter
Fc=20 Hz, Fs = 250 Hz
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Amplitude Ratio vs Distance
1.00E-02
1.00E-01
1.00E+00
1.00E+01
0 200 400 600 800 1000 1200 1400 1600
Distance from source (meters)
Am
plit
ud
e r
atio
Compressional Wave Amplitude Ratio
Shear Wave Amplitude Ratio
Compressional Wave Amplitude Ratio *sqrt(distance ratio)
Shear Wave Amplitude Ratio * sqrt(distance ratio)
1/e lengths:
Compressional = 1540+/- 50 meters
Rayleigh = 300+/- 6 meters
We can couple to seismic sources that are kilometers away.
Some evidence that berm structure or underlying formation guides waves.
Disp ~ e-kr/sqrt(r)
Disp ~ e-kr
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Evidence of berm construction or geological substructure?
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More details of X arm channel characterization at
T010073-01-L
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Measured Microseismic Motion at Livingston and Hanford
From Alan Rohay 7/9
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Time trace for typical data goes here
Typical trigger vertical axis time trace
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What level of ground velocity causes lock problems?
OSEM force/amp total 4 OSEMS F/I = 0.08 nt/amp
Imax = 90 mA
Assume stack and pendulum add x5 to ground motion over 0-20 Hz interval
Mass = 10 Kg
Max velocity to hold lock is around 2 microns/sec
Look at events that are bigger than this to see what we can learn about them.
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Analysis method
• Time domain filter data using 4 pole Butterworth high pass filter (fc=1 Hz) IIR filter. Look for impulses where velocity > vthreshold
• Compute cross correlation function between elements of array using 2 second window.– 2 seconds is a typical autocorrelation time
• Least squares fit using both horizontal axes of all seismometers in array to determine propagation velocity and direction
• Compare horizontal and vertical amplitudes
Analysis done with Sean Hardesty, Caltech SURF student at LLO
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Angular distribution of impulsive events with ground velocity > 1u/sec 9am – 3 pm,
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daytime
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Night time vertical impulses
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Velocity distribution for vertical triggers
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nightime
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Vertical triggers during train
Note angular clustering
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Horizontal train signal
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bridge locations
Weyerhauser mill
Liv. Industrial Park
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East bound train
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West bound train
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There are some trajectories at other times also
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RMS ground vibration
• Analysis technique• Make spectrograms of 200 sec data with Hanning
window, 85% overlap• Least squares fit of horizontal displacements to both
horizontal axes of the array elements• Fit vertical displacement, ratio of vertical/horizontal
amplitudes around 1.6, velocity is around 600 m/sec• => Rayleigh waves
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1.6 Hz
200 sec spectrogram window with 85% overlap
600 m/s
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Samples at 6 hour intervals beginning at 6pm
on Wednesday
Select data with mean square error<40
50 mHz band at 1.6 Hz
Daytime rms noise from south east
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Pipeline
• A useful calibration point at 5 Hz and 10 Hz• Very monochromatic, few mHz, for long periods• V = 2 km/sec, compressional wave• Two distinct directions:• From the west, as expected, since pipeline runs NS• From the north. Pumping station?
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Pipeline signal comes from the west (0=wave headed east)
2 km/sec => compressional wave
Additional signal from the north (pumping station?)
Data with chisq<40/8df
daytime
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Conclusions
• Daytime noise at 2 Hz comes from the south, at 5 Hz from all over. Impulse noise comes broadly from the south, no single source.
• The angular distributions of the seismic disturbance due to the train appear to be coming from the trestle locations
• Local site construction noise was not identified in the array data
• Surface waves are consistent with Rayleigh waves• Seismic waves waves can travel a long way (multi
km) and still be of concern.