Page 14 - January 2006

may be recorded by microphones as
distinct explosions or bursts16, jet noise,
or as a continuous vibration of the
atmosphere known as tremor.
Landslides and pyroclastic flows at volcanoes also produce a distinct acoustic signature that may be used for tracking the flow deposits. The forecasting potential17 and usefulness18 of volcano-acoustic monitoring has been well documented for explosive as well as effusive eruptions, and efforts are under- way to test the ability of infrasound to provide regional, low-
19
latency eruption warnings to the airline industry . The proof
of concept for this application can be readily found in the tomographic study of Reference 20, where persistent volcanic eruptions near New Caledonia have been used to infer the seasonal variability of the atmospheric wind structure. Of all the geophysical monitoring applications for infrasound, vol- cano surveillance is the most mature and the closest to the IMS aim of listening to explosions at large distances. However, the physics of volcanic eruptions involve pressur- ized, high-temperature multi-phase mixtures that may be moving at supersonic speeds through conduits of unknown geometry and stability. Infrasound usually measures the pressure at the vent, so inferring the eruptive source process- es at depth requires multidisciplinary observing systems and modeling.
Ocean swells
Microbaroms are coherent infrasonic signals in the 0.1 to
0.5 Hz frequency band that may be observed anywhere in the
world and are related to strong storm and ocean wave activity
(e.g., Reference 21). Reference 22 showed that these micro-
baroms signals could also depend strongly on the atmospher-
ic wind conditions during the year. Microbaroms are believed
to originate from the nonlinear interactions of ocean waves
traveling in nearly opposite directions with similar frequencies
(e.g., Reference 23). Theoretical acoustic models can be cou-
pled with global ocean wave spectra to estimate the acoustic
source pressure spectra induced by microbaroms near the
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ocean surface . The predicted acoustic source values exhibit
peaks in wake regions of marine surface lows, and show a large
number of weaker source regions at a distance from wave-gen-
erating storms. Comparison of microbarom observations with
surface weather, ocean wave charts, and predicted acoustic
sources suggests that microbarom source regions occur in
locations that contain opposing wave trains, instead of exclu-
sively from regions of marine storminess. In addition to allow-
ing the tracking of large swells in the open oceans, micro-
baroms may be used for passive acoustic tomography of the
24 atmosphere .
Infrasonic stations located near ocean shores routinely record infrasound from breaking waves. Reference 25 recog- nized a clear relationship between infrasonic amplitude in the 1-5 Hz range and breaker height, and postulated that a breaking wave may generate infrasound by barreling (plung- ing), slamming against a cliff, or by impacting against dry reef. Reference 26 corroborated the relationship between infrasonic and ocean wave amplitudes and located active surf regions along the coastline of Tahiti. Reference 27 reported surf infrasound propagating over 200 km inland under favor-
able wind and swell conditions. Recent experimental results28 suggest that low- frequency sound can be used to moni- tor the energetics, spatial distribution,
and temporal variability of different types of breaking ocean waves. These experiments confirmed that infrasound may be produced by plunging waves as well as by surf impinging against cliffs and exposed reefs, and demonstrated the possi- bility of extracting the height and period of breaking waves from single-sensor infrasound data. These new capabilities could allow more extensive oceanographic studies and mon- itoring of the surf zone.
Tsunamis
Infrasonic measurements of recent tsunamis29,30 strongly suggest that low-frequency atmospheric sound may be com- bined with other technologies as a discriminator for tsunami genesis. Infrasonic signatures associated with the December 26, 2004 Great Sumatran Earthquake were captured by the IMS station in Diego Garcia that recorded (1) seismic arrivals from the earthquake, (2) tertiary arrivals (T-waves), propa- gated along sound channels in the ocean and coupled back into the ground, (3) infrasonic arrivals associated with either the tsunami generation mechanism near the seismic source or the motion of the ground above sea level, and (4) deep infrasound (with a dominant frequency lower than 0.06 Hz) originating from the Bay of Bengal. A similar sequence was observed during the March 28, 2005 Nias earthquake and tsunami. These events off the coast of Sumatra were ~3000 km to the closest infrasound station in Diego Garcia. The large ranges, coupled with the fact that all infrasound stations used in those studies were transverse to the axis of Sumatra, caused uncertainty in the ability to discriminate between sounds potentially produced during tsunami genesis at the ocean surface and the sounds produced by the earthquake- induced vibration of mountains and islands. In contrast, IMS infrasound station IS30 in Japan (Fig. 1) is optimally situated to recognize the different source regions of infrasound asso- ciated with the Miyagi-Oki earthquake and tsunami. The magnitude 7.2 event occurred August 16, 2005 at 02:46:28 UTC (Fig. 3), and the epicenter was less than 400 km from the station. Therefore, all arrivals must be stratospheric except for the furthermost sources. The earthquake produced a minor tsunami. There was no deep infrasound component suggesting that very low infrasonic frequencies are produced only by the largest tsunamis. T-waves are also absent, possi- bly due to the shallow bathymetry at the epicenter. However, the infrasonic arrival sequence is similar to that observed during the Sumatra events, with infrasound originating from nearby mountains, the epicentral region in the ocean, and a shallow bay that may resonate in response to the water dis- placement. Note that shallowest parts of the bay also appeared to produce infrasound.
The aforementioned studies suggest that tsunami-associ- ated infrasound may be radiated from the ocean surface or be excited by the interaction of the ocean waves with the coast- line and bathymetry. The effective propagation speeds of tsunami (~50-200 m/s) and sound waves (~300 m/s) yield an advance warning time of at least 1.7 s/km. At 100 km, sound
12 Acoustics Today, January 2006
“Infrasound, A new frontier in monitoring the Earth”