Infrasonics and the USArray
Dr. Kristoffer T. Walker, University of California - San Diego
Atmospheric infrasound travels thousands of kilometers with little attenuation in atmospheric waveguides controlled by the temperature and wind structure. While Infrasonics shared a spot with Seismology during the proliferation of nuclear weapons in the early to mid 20th century, the ratification of the 1963 Limited Test Ban treaty pushed nuclear testing underground and interest for infrasound subsequently diminished. Recently, the adoption of the 1996 Comprehensive Test Ban Treaty has led to a renewed interest in Infrasonics and the development of a 45-station global infrasonic array network. Hundreds of infrasonic studies have been published since then, and the average number of infrasound-related presentations at the Fall AGU Meeting is at an all-time high of about 40. These studies show that infrasound is a tool for a number of academic and practical studies. Infrasound can be used to study hypotheses surrounding natural events such as earthquakes, meteors, volcanoes, ocean wave interaction, aurora processes, meteorological vortices, and mass-wasting processes. Infrasound can also be used to measure the degree of anthropogenic activity in cities, as well as locate explosions, rocket launches, and jet trajectories. Atmospheric processes such as turbulence and the interaction of gravity waves can also be constrained with infrasound. Our ability to predict temperature and wind as a function of altitude have progressed to the point that one can now use these models as starting models to invert for second-order structure, which is useful above the middle stratosphere where direct measurements of winds are difficult to obtain.
The global infrasonic array network has an intra-station spacing of 2200 km. Although each array comprises several microbarometers that permit one to pinpoint the direction from which infrasonic waves originate, the large intra-station spacing makes it impossible to study the finer details of how infrasonic ray paths move through atmospheric structure. The 70-km intra-station spacing of the USArray, either using seismometers or microphones, allows one to investigate these details in unprecedented spatial resolution.
In this presentation, I aim to briefly discuss infrasonic sources, propagation physics, and reception. I will then focus on recent UCSD studies of infrasound and gravity waves using USArray seismometers and microphones. I will briefly discuss the potential of using USArray seismometers and microphones in joint studies of sources of opposing oceanic waves. Lastly, I will conclude with a list of needed things to facilitate future infrasonics research and improve the ability of the infrasonics community to grow.
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