Microseisms and Infrasound

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Microseisms and Infrasound

• Jacques C. Richard, Samuel Bowen
• Chicago State University
• JC-Richard_at_CSU.edu
• S-Bowen_at_CSU.edu
• And
• Jay Mancini, Vassilius Fessitides, Benjamin
  Crooker
• Fordham University

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Overview

• Microseisms and infrasound
• Infrasonic wave, microseism detection
• Previous analyses
• Current models
• Different storms
• Conclusion and future work

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Microseisms and Infrasound

• Infrasonic noise generated by storms, etc.
• Microseismic signals due to storms pounding on
  the ocean and seabed.
• Just below hearing.

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Microseisms and Infrasound
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Variety of Infrasonic Signal Types Vs. Period
Range
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Wavelength Vs. Signal Period
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Spectral Intent of an Infrasonic Signal Vs. Time
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Microseisms and Infrasound

• Infrasonic noise generated by storms, etc.
• Microseismic signals due to storms pounding on
  the ocean and seabed.
• Just below hearing.

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Long Range Meteor Detection by Single Infrasonic
Observatory
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Micro-seisms and Infra-sound

• Fordham observed microseisms closely associated
  with cyclones passing over water
• Key storms cyclonic nature - no significant
  microseisms associated with linear waves

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Hurricane Bonnie
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Previous Work

• Longuet-Higgins (L-H) showed possible mechanisms
  for microseism generation using
• Bernoulli's principle,
• linearized hydrodynamic equations and
• Impact of waves moving across water surface
• Only linear wave structure that could generate
  microseisms was a standing wave

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Previous Work

• Standing waves provided spatially coherent and
  persistent, oscillating pressures for microseism
  generation
• Standing waves kinetic energy gives rise to
  microseisms with twice its frequency

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Previous Work

• Tabulevich suggested
• Wind waves before a moving storm and those
  following, perhaps moving in opposite directions,
  
• Then combining to form standing waves - even when
  these two winds are not present at the same point
  at the same time
• Others proposed complex non-linear higher order
  interactions as source of microseisms

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Our Model

• Linear theory based on Bernoulli's principle more
  deeply examined
• Wave forms, other than linear standing waves,
  give rise to microseisms

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Hurricane Bonnie
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Hurricane Bonnie
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Hurricane Bonnie
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Hurricane Bonnie
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Resonance Model

• Predicts frequency w g/v
• But frequency does not respond as such for low
  velocities
• Amplitude probably too small to be observed in
  seismometers

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Slowly Moving Hurricane Stress Flow Theory

• Follows principles of mass, momentum and energy
  conservation
• Shear balance between air and water
• Result

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Cylindrical Shear Flow Model and Bonnie
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Cylindrical Shear Flow Model and Bonnie
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Noreaster

• Cyclonic storm over Atlantic ocean near
  Northeastern coast of USA
• Microseisms from one such storm 1/98

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Noreaster Track Coincides Bonnies
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Cylindrical Shear Flow Model and a Noreaster
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Conclusion

• Predicted average frequencies of microseisms and
  their dependence on environmental variables
• Most microseism effects well characterized by
  linearized hydrodynamic approximation using
  Bernoulli principle

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Future Work

• Other peaks in microseism power spectra
• Fluctuations from mean motion
• Lifetime broadening of microseisms
• Microseisms with short wave trains less than ten
  cycles
• Infrasound near large bodies of water as storms
  pass over them