Milton Garces, Claus Hetzer, and Mark Willis

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(No Transcript)
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Source modeling of microbarom signals generated
by nonlinear ocean surface wave interactions
Milton Garces, Claus Hetzer, and Mark
Willis University of Hawaii, Manoa
2003 Infrasound Technology Workshop, San Diego,
California
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What are microbaroms, and why we care?

• Microbaroms signals, like microseisms, are
  believed to be created by the nonlinear
  interaction of ocean surface waves
• The microbarom peak near 0.2 Hz is right on the
  detection frequency band for 1 kt explosions
• IMS arrays with large apertures (gt 1 km) were
  supposed to render microbaroms incoherent, but
  distinct coherent bursts may still be detected
• Microbaroms may be generated in open ocean or by
  reflections with coastline and islands, and are
  prominent on island stations
• Theoretical energy peak of microbarom radiation
  is near vertical, but this energy is lost
• Sufficient energy is radiated near the
  horizontal, where most microbarom arrivals are
  detected
• Study microbarom statistics at I59US and global
  distribution of microbarom signal levels

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Microbaroms 2002
N Swells, Aleutians
Trade and S Swells
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Microbaroms Year 2002
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Theory Arendt and Fritz, 2000

• Assumptions
• Wave height small relative to wavelength of ocean
  wave
• Distance greater than acoustic wavelength

• Solution
• For a prescribed surface wave displacement g(x,t)
  and vertical velocity uz(x,t), the acoustic
  pressure is

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Theory Interfering plane waves, w2 w1 0.2 Hz
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Theory

• Consider a ocean surface wave displacement
  spectrum A,

The radiated acoustic spectrum would be
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Theory

• Vertical acoustic wavenumber and a function of
  the ocean wave and acoustic wavenumbers

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Wave Watch 3 (WW3)

• WAVEWATCH III (Tolman 1997, 1999a) is a third
  generation wave model developed at NOAA/NCEP in
  the spirit of the WAM model (WAMDIG 1988, Komen
  et al. 1994). It is a further development of the
  model WAVEWATCH I, as developed at Delft
  University of Technology (Tolman 1989, 1991) and
  WAVEWATCH II, developed at NASA, Goddard Space
  Flight Center (e.g., Tolman 1992).
• WAVEWATCH III solves the spectral action density
  balance equation for wavenumber-direction
  spectra. The implicit assumption of these
  equations is that the medium (depth and current)
  as well as the wave field vary on time and space
  scales that are much larger than the
  corresponding scales of a single wave.
  Furthermore, the physics included in the model do
  not cover conditions where the waves are severely
  depth-limited. This implies that the model can
  generally by applied on spatial scales (grid
  increments) larger than 1 to 10 km, and outside
  the surf zone.

http//polar.ncep.noaa.gov/waves/wavewatch/
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Wave Watch 3 (WW3)

• Surface winds and dominant period

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Wave Watch 3 (WW3)

• Significant wave height

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Wave Watch 3 (WW3)

• Significant wave height

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Evaluating the Theoretical Model
The Wave Watch 3 model outputs the variance
density , F, of the surface wave field as a
function of frequency, f, and propagation
direction,q. The variance density can be
integrated over angle and frequency to provide
the total wave energy E,
The peak source pressure occurs when k -k, w
w
For preliminary amplitude estimates, we use
Whitakers relationship
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Microbaroms January 21-28, 2003, 90s window, ½
second consistency, 0.1-0.5 Hz
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Microbaroms January 21-28, 2003, Family Size
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Microbaroms February 22, 2003, 60s window, 1
second consistency, 0.1-0.7 Hz
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Sea State February 22, 2003
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Sea State February 22, 2003
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Infrasonic Source February 22, 2003
Geometric frequency steps (1.1f) from 0.08 to
0.8 Hz, dynamic range of 90 db
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REB Location February 21-23, 2003, no seismic
contributions
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Need to add climatological specifications
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Conclusions and future work

• All IMS infrasound arrays, and particularly those
  close to the ocean, are susceptible to
  microbaroms
• Microbaroms may be generated in the open ocean
• Obtained surface wave spectrum from Wave Watch 3
  global model
• Developed an algorithm to evaluate a theoretical
  source pressure field induced by the open ocean
  surface wave field
• Used simple relationship to estimate global
  infrasonic field
• Need to add atmospheric specifications,
  attenuation losses, and direction of arrival
  information
• Need to incorporate better propagation algorithms
  to provide time dependent and frequency dependent
  estimates of the propagating infrasonic field
• Need to add reflections with coastline and
  islands mesoscale problem, site dependent and
  not trivial
• Approach can be adapted to microseisms

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