Page 15 - January 2006

Fig. 3. Estimated infrasonic source locations associated with ground vibration, tsunami genesis, and the interaction of the tsunami with the coastline. The squares represent stratospheric arrivals with a celerity of 0.3 km/s. The diamonds are also stratospheric arrivals but with the celerity of 0.32 km/s predicted for that azimuth. The circles are thermospheric arrivals with a celerity of 0.27 km/s. The triangles are stratospheric arrivals with a celerity of 0.3 km/s, but with an addi- tional delay time of ~250 s. The color of the symbols indicates the arrival time in seconds (~700–1450s, purple to red) since the earthquake’s origin time. The topography is from the National Oceanic and Atmospheric Administration (NOAA) ETOPO2 data.
(Fig. 4) reverses at the stratosphere when the temperature begins to increase. These variations give rise to a propagation duct that can trap sound. As sound moves in this duct, its amplitude decreases as 1/r, characteristic of cylindrical spread- ing rather than 1/r2 characteristic of spherical spreading. Since the ducts that are formed are not rigid, sound leaks out of the duct back to the ground and sound propagating from the sur- face of the Earth can be diverted back to the surface by the duct boundaries. As a result, where rays can propagate upward to the duct and are reflected back to the ground, shadow zones and hot spots are formed. At greater altitudes there is another duct at the thermosphere that is always present regardless of winds. That duct is so high that high-frequency sound is absorbed prior to reflecting from the upper part of that duct. Only very low-frequency sound can make it back to the ground so signals received after reflection from the thermos- phere are very low in frequency content.
The speed at which sound propagates is also affected by the winds. If sound is propagating against prevailing winds, the increase in sound speed above the stratosphere can be negated by winds destroying the ducted propagation in a direction against the wind while retaining ducted propaga- tion with the wind.
Non-linear effects arise as sound propagates over long distances and at high altitudes. As a sound wave propagates upward, the amplitude decreases as 1/r or 1/r2 modified by
leads the tsunami by at least 170 s, but some of this time would be taken up by signal processing and identification, leaving less than one minute to issue an alert. However, infra- sound may offer substantial advance warnings to areas greater than a few hundred kilometers from the tsunami source region.
Effects of the atmosphere
As infrasound propagates from source to receiver, the atmosphere has a dramatic effect on the amplitude and fre- quency content. In general terms, as sound propagates, the amplitude decreases exponentially with an absorption coeffi- cient α that is proportional to the ratio of the frequency, f, squared divided by the ambient pressure, P. Absorption also depends on relative humidity. At sea-level, a signal at 100 Hz experiences absorption near 300 dB/1000 km while a signal at 1 Hz is absorbed at a rate of 0.03 dB/1000 km—a huge dif- ference favoring low-frequency propagation. At an altitude of 30 km, characteristic of the stratosphere, the atmospheric pressure is typically about 1/100th that at sea level that would give rise to absorption of 3dB/1000 km at 1 Hz. At 120 km, in the thermosphere, the absorption increases to near a 1000 dB/1000 km. For all practical purposes, a 1 Hz signal cannot
31 get to the thermosphere and back .
The other atmospheric factor that influences infrasound propagation is the variation of wind and temperature with alti- tude. Under typical conditions, as the altitude increases from sea level, the speed of sound decreases due to decreasing atmospheric temperature (the adiabatic lapse). This trend
Fig. 4. Annual mean sound speed in the northern hemisphere from the equator to 80° North. These curves are based on an empirical model of atmospheric tem- perature provided by the Committee on Space Research (COSPAR) in 1986 (CIRA-86).
Infrasound 13