A Novel PiezoelectricCable Infrasound Sensor Initial Results

1
A Novel Piezoelectric-Cable Infrasound Sensor -
Initial Results

• F. Kern, S. Africk, R. Chaves and P. Cataldi
• Physical Sciences Inc.
• 20 New England Business Center
• Andover, MA 01810
• M. Hedlin and C. Coon,
• Laboratory for Atmospheric Acoustics, Scripps
  Institution of Oceanography, University of
  California, San Diego, CA
• October 2003

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Captain Dirk Barker
2
What is Piezocable?

• Pressure produces strain between shield and
  center core
• PVDF is radially poled to produce electrical
  response to strain
• Coax with PVDF copolymer piezoelectric
  dielectric
• Coax with PVDF spiral wrap piezoelectric
  "dielectric"

3
Advantages of Piezocable

• Direct electronic transduction "speed of light"
  averaging over array length
• Eliminates mechanical systems(manifold and
  microbarograph)
• Enables novel enhanced performance options
• signal processing options
• noise reduction strategies
• infrasound station geometries
• Environmentally robust

4
All-Electronic Reception and Local Processing
5
Comparison with Standard Systems
6
Background Piezocable Noiseand Calibration
Measurements

• Background noise in steel tube to minimize
  thermal, pressure, and EMI excitations
• Calibrations in short and long plastic chambers
  with modified compressor
• Calibrations in foam tube with compressor also
  made

7
Piezocable Amplifiers

• Signal Amplification (up to 60 dB)
• Impedance transition
• Very low noise
• Calibration signals added

Piezocable amplifiers, calibration signal
generator and batteries
8
Amplifier Noise with 20 nF Capacitor Cable
Simulator

• Amplifier noise -110.7 dB re 1 V2 at 1 Hz w/ 58
  dB amp gain
• This meets the required spec

9
Outdoor Testing at PSI
10
Outside PSI Data Sample
Uncoiled cables in thermally insulating foam
under snow cover Wind speed 14 mph
Power Spectrum
Coherence

• Conclusions
• Good coherence on two sensor/amplifier pairs
• Sensitivity consistent to less than 2 dB

11
Outdoor Testing (In Massachusetts)

• Conclusions Cables will have to be buried for
  operation in windy environments. Tests in quiet
  environment needed to compare to current arrays
  (e.g. Piñon Flat).

12
Piñon Flat
New 100m Dirt Trench
New section, including old 30m trench, looking
West
New section looking East
13
Piezocable Infrasound Sensor Test StationPiñon
Flat Observatory
14
Piñon Flat Testing
15
Typical Acoustic Signal Observed by Piezocable
SensorsPiñon Flat Observatory

• Signal arrives from South East
• Signal velocity 370 m/s (not seismic)
• Data filtered w/ 0.7-2.0 Hz bandpass to remove
  wind noise
• Signal is observed on three piezocable sensors,
  microbarograph, and permanent pipe array.

16
Acoustic Event Observed on Three Piezocable
Sensors and USCD MicrobarographPiñon Flat
Observatory, DOY 268
17
Quiet Periods Piñon Flat Observatory
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Directivity Implied from Acoustic Event Detected
by Perpendicular Piezocable Sensors

• Signal observed on all three sensors
• Sensitivity of PVDF/Amplifiers 10 dB less than
  microbarograph as expected
• 100 m sensor has very little energy above 3 Hz,
  the cutoff for 100 m
• 30 m sensor does not filter 4-6 Hz, cutoff at 11
  Hz
• Spatial filter of 100 m sensor implies
  North/South signal incidence

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Possible Wind Noise Event
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Wind Noise Geometry
From Sensors
21
Microbarom Event - DOY 268 Observed on Three
Piezocables and Reference Microbarograph
22
Continuing Efforts

• Characterize signal reception and environmental
  noise sensitivities and statistics to generate
  receiver characteristics.
• Absolute signal sensitivity when in T.I. tube and
  when buried
• Noise statistics - variability with
• Time
• Wind speed and direction
• Frequency
• Sensor length
• Optimize packaging and deployment
• Thermal insulating hose
• Electronic shielding
• Depth of burial and medium (gravel, dirt)
• Evaluate potential effectiveness of multiple line
  arrays for noise control and novel signal
  processing
• Evaluate magnitude of environmental
  electromagnetic noise with coax cable and
  possible use as a signal-free reference for
  electromagnetic noise cancellation

23
Summary and Conclusions

• Piezocable sensors are an inexpensive means to
  measure infrasound
• Over large apertures
• With flexible geometries
• At "speed-of-light"
• Electronically-formed arrays of lines can provide
  directivity, signal enhancement and noise
  rejection. On-site or remote reconfiguration is
  possible to adjust to changing site conditions
• Piezocable sensor/amplifier systems meet the CTBT
  noise specification
• Signal events identified on permanent arrays have
  been detected on piezocable sensors with
  anticipated sensitivity

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Summary and Conclusions (continued)

• Microbaroms clearly identifiable in piezocable
  output.
• Length averaging (spatial filtering) has been
  observed.
• A means to infer signal incidence direction using
  perpendicular lines has been demonstrated.
• Six piezocable sensors and two RG174A coax
  reference cables operating 24/7 at Piñon Flat and
  data analysis is ongoing.