Infrasonic Imaging with the USArray

Infrasonic Imaging with the USArray The detector function (upper left) images the source in time; there is a peak at the launch time (thick red line) with a signal-to-noise ratio of 22 dB. Other colored lines indicate known regional and teleseismic earthquake times. The map shows the seismic energy migrated back to the source, where it constructively interferes, imaging the source in space at the source time. The record section to the right shows that the migrated energy moves out at ~300 m/s, indicating that rocket infrasound is observed out to ~1500 km. Horizontally aligned signals are earthquakes.
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The USArray directly measures ground motion, which can mostly be attributed to ocean waves, earthquakes, volcanoes, and weather systems that load the Earth’s surface. Another source of ground motion is the transfer of atmospheric acoustic energy into seismic energy at the Earth’s surface. Infrasound (low frequency sound below ~20 Hz) can travel great distances unattenuated in atmospheric ducts created by layers of slow sound speed. The infrasound wave field is rich due to a variety of anthropogenic and geophysical phenomena including earthquakes, volcanoes, landslides, meteors, lightning and sprites, auroras, and oceanic and atmospheric processes. Globally spaced microbarometer arrays with apertures of 100 m to 2 km are typically used to study these sources. However, these arrays are separated by thousands of kilometers, which places considerable limits on what they can teach us about infrasound source physics. The USArray is in a position to study infrasound sources in unprecedented detail. Array processing methods, such as reverse-time migration (RTM), may work well in automated detection and location of infrasound sources registered by the USArray (see also Hedlin et al. contribution). Hundreds of sources have been detected thus far using this technique. For example, below is a USArray “infrasonic image” of a Vandenberg Air Force Base rocket launch.</p>

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