Showing papers by "Michael P. Hickey published in 2019"

Numerical Modeling of the Propagation of Infrasonic Acoustic Waves Through the Turbulent Field Generated by the Breaking of Mountain Gravity Waves

Abstract: The nonlinear propagation of low-frequency acoustic waves through the turbulent fluctuations induced by breaking mountain gravity waves is investigated via 2-D numerical simulations of the Navier-Stokes equations, to understand the effects of atmospheric dynamics on ground-based infrasound measurements. Emphasis is placed on acoustic signals of frequency around 0.1 Hz, traveling through tens-of-kilometers-scale gravity waves and subkilometer-scale turbulence. The sensitivity of the infrasonic phases to small-scale fluctuations is found to depend on the altitudes through which they are refracted toward the Earth. For the considered cases, the dynamics in the stratosphere impact the refracting acoustic waves to a greater extent than those in the thermosphere. This work clearly demonstrates the need for accurate descriptions of the effects of atmospheric dynamics on acoustic propagation, such as here captured by the full set of fluid dynamic equations, as well as of the subsequent effects on measured signals. Plain Language Summary Infrasound is the low-frequency part of the acoustic spectrum, with periods ranging from around 0.05 s to about 300 s. Infrasonic waves are generated by a large variety of natural and artificial sources (such as earthquakes, volcanic eruptions, auroras, thunderstorms, explosions, rocket launches, and sonic booms) and can propagate up to the thermosphere and over thousands of kilometers on the ground. The ground recordings are currently employed in numerous applications, including the detection and localization of specific sources (such as clandestine nuclear tests) as well as the research in the domain of the atmospheric dynamics. Mountain gravity waves are buoyant motions associated with the gravitational acceleration and are excited as air flows over mountain ranges. They can have horizontal wavelengths as small as 10 km and as high as 100 km and, if sufficiently strong, can break and lead to smaller-scale turbulence. The subsequent turbulent fluctuations can severely influence the infrasonic waves. This paper reports the results of numerical simulations of the atmospheric propagation of infrasonic signals through the inhomogeneities induced by the breaking of mountain waves. The present analysis provides new insights into the understanding of the effects of the aforementioned turbulent perturbations on the infrasonic signals potentially recorded on the Earth's surface.

An analysis of the atmospheric propagation of underground-explosion-generated infrasonic waves based on the equations of fluid dynamics: Ground recordings.

Abstract: An investigation on the propagation of underground-explosion-generated infrasonic waves is carried out via numerical simulations of the equations of fluid dynamics. More specifically, the continuity, momentum, and energy conservation equations are solved along with the Herzfeld-Rice equations in order to take into account the effects of vibrational relaxation phenomena. The radiation of acoustic energy by the ground motion caused by underground explosions is initiated by enforcing the equality, at ground level, between the component of the air velocity normal to the Earth's surface and the normal velocity of the ground layer. The velocity of the ground layer is defined semi-empirically as a function of the depth of burial and of the yield. The effects of the depth and of the source energy on the signals recorded in the epicentral zone are first discussed. The tropospheric and stratospheric infrasonic phases traveling at a long-range are then analyzed and explained. Synthesized ground waveforms are finally discussed and compared to those recorded at the I45RU station of the International Monitoring System after the 2013 North-Korean test. Good agreement is found between numerical results and experimental data, which motivates the use of infrasound technologies alongside seismic techniques for the characterization of underground explosions.