Observations and Modeling of Infrasound Produced by Ocean Waves

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Observations and Modeling of Infrasound Produced
by Ocean Waves

• By Mark Willis
• Masters Student/Dept of Meteorology

Acknowledgements Milton Garces (HIGP/ISLA),
Claus Hetzer (ISLA), Steven Businger (UH Dept of
Meteorology) and Paul Wittmann (FNMOC)
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Introduction to Infrasound
Surf Breaking 2-5 Hz
Microbaroms 0.1-0.5 Hz
Infrasound - low frequency sound waves below the
20 Hz hearing threshold of the human ear.
Infrasonic waves can propagate thousands of
kilometers due to low atmospheric absorption at
low frequencies. Natural sources - severe
weather, volcanoes, bolides, earthquakes, surf,
mountain waves, open ocean wave-wave
interactions.
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Man-made sources of Infrasound also exist

• Nuclear explosions, space debris entering the
  Earths atmosphere, and supersonic aircraft are a
  few examples of manmade infrasound sources
  (Bedard and Georges 2000).
• Infrasound detectors were widely used in the
  early 1950s during the Cold War to monitor the
  Globe for nuclear explosions. Satellite based
  nuclear detection systems diminished the need for
  infrasonic detectors in the 1960s.
• The Comprehensive Nuclear Test Ban Treaty (CTBT)
  was adopted by the United Nations General
  Assembly (Sept. 24, 1996) and now includes 170
  countries.
• CTBT prohibits nuclear testing of any kind.

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Infrasound portion of the International
Monitoring System
The International Monitoring System (IMS) was
organized to ensure compliance of the CTBT IMS
provides global monitoring of nuclear testing in
all of the Earths environments underwater,
underground, and in the atmosphere
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Geophysical Infrasound Sources can Mask Nuclear
Signals

• At Oceanic Infrasound Stations, most of the
  background noise is related to a nearly
  continuous pressure oscillation occurring in the
  0.1 to 0.5 Hz frequency range.
• These signals are known as microbaroms and are
  believed to originate from open ocean wave-wave
  interactions.

Microbarom range
1 kiloton nuclear explosion results in infrasound
in the microbarom range.
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History of Microbaroms

• Microbaroms were first discovered by Benioff and
  Gutenberg (1939) on an electromagnetic
  microbarograph who concluded the signals were the
  result of offshore low pressure systems.
• Further studies confirmed microbaroms originated
  from severe weather in the ocean (Saxer, 1954,
  Daniels, 1962, Donn and Posmentier, 1967, Rind,
  1980).
• These studies related microbarom arrivals to
  major surface weather patterns (esp. cold fronts
  and low pressure centers) and the associated high
  ocean surface waves.
• Microbaroms were shown to have similar
  characteristics to their underground counterpart
  microseisms - by Donn and Posmentier (1967),
  Donn and Naini (1973), Rind (1980).

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Source Mechanism of Microbaroms and Microseisms

• Longuet-Higgins (1950) described a microseism
  source involving the interaction of standing
  ocean waves with the sea floor.
• Posmentier (1967) - When two ocean waves of
  opposite direction but nearly identical
  frequencies meet, the associated standing wave
  creates a nonlinear pressure perturbation that
  can travel at very high phase velocities. The
  corresponding acoustic wave will gain properties
  of the interfering wave trains (amplitude is
  proportional to ocean wave height, frequency
  twice that of individual ocean waves).
• Arendt and Fritts (2000) presented a microbarom
  source pressure formulation based on the
  Longuet-Higgins/Posmentier approaches. They
  found that frequency-doubling nonlinear
  interactions of pairs of ocean waves traveling in
  equal but opposite directions produces
  propagating acoustic waves of an isotropic
  nature.

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Research Plan

Garces (2003) developed an algorithm, based on
the Arendt and Fritts model, to compute
microbarom source pressure fields from ocean wave
spectra provided by the NOAA Wavewatch III (WW3)
model. WW3 outputs ocean wave energy densities
in 24 directional and 25 frequency bands. For
thesis research, I am using WW3 output to
characterize microbarom generation regions and
comparing to data received at our infrasound
array located near Kona, Hawaii (IS59).
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Data and Methods- Infrasound
IS59 Consists of 4 Chaparral 5 microphones. 3
are organized in a triangle with 1 in the
center. Data recorded by 24-bit digitizers and
sent in real time via radio telemetry to
Infrasound Lab at Keahole Point. PMCC algorithm
of Cansi et al. (1995) is used to detect coherent
infrasonic energy across the array. This allows
us to extract speed, arrival angle, and amplitude
of detected arrivals.
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CTBT IMS ARRAY IS59 KONA, HAWAII
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Data and Methods WW3

• WW3 (Tolman, 1999) driven by NOGAPS 10m surface
  winds and global ice concentration values used to
  produce realistic wave spectra on a global 1
  degree grid. WW3 outputs wave energy values
  (m2/DegHz) in 24 directional and 25 frequency
  bins this is used to calculated microbarom
  source fields.

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Microbarom Observations
NW
S
E
Microbarom Arrivals appear to show annual cycle
associated with known weather patterns in the
Pacific Basin.
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Case Study January 1-7, 2003
Microbarom Arrival Azimuth appears to correspond
with low pressure location.
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Case Study Jan. 4, 2003 18Z
Very active surface weather pattern, 7 low
pressure centers evident in/near the Pacific
including the intense cyclone just NNW of the
Islands that created massive surf for exposed
beaches on the 5th (gt12m breakers observed)
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WW3 Wave Spectra Jan 4, 2003 18Z
Wave spectra in region of benign surface pressure
pattern
Wave spectra upstream of 950 mb storm location
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Preliminary Source Modeling Results
(Jan. 4, 18Z 2003 Case)
Log Base 10 of Acoustic Source Pressure (Pa m3)
Acoustic Source Pressure for 0.197 Hz shown,
corresponds to ocean waves of 10s interacting
Peaks in wake regions of low pressure centers,
also several scattered peaks throughout the
Pacific
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Summary - Observations

• Yearly microbarom arrivals show an annual cycle.
• During boreal summer, arrivals concentrated from
  E and S directions. East arrivals expected to
  originate in the wake of tropical cyclones in the
  EPAC or from island reflection of trade wind
  swells. South arrivals likely originate in the
  wake of mid-lat storms in the SPAC.
• During boreal winter, arrivals concentrated from
  W, NW, N directions. These arrivals likely
  originate in the wake of surface lows in NPAC
  mid-lat and from WPAC Tropical Cyclones.
• Atmospheric conditions and attenuation of
  microbarom signal along propagation path have
  large effect on arrivals.

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Summary Microbarom Source Modeling

• Past research related microbaroms to low pressure
  centers, cold fronts, and associated regions of
  high significant wave heights.
• Our source modeling results show that microbaroms
  are generated in regions where wave trains are
  moving in opposite directions with similar
  frequencies, such as in the wake regions of low
  pressure centers.
• However, interfering wave trains may occur at a
  substantial distance from the wave producing
  winds, and thus microbarom production can be
  widespread.

The End
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