Modeling infrasonic propagation through a spherical atmospheric layer—Analysis of the stratospheric pair

Abstract

Methods are developed to calculate acoustic propagation paths through an atmospheric layer surrounding a spherical globe in order to more accurately model the propagation of infrasonic signals, which are often observed after propagating hundreds or thousands of kilometers. A generalized curvilinear coordinate system is used to define the ray tracing equations from the eikonal equation for a moving, inhomogeneous atmosphere, and the specific case of spherical coordinates is applied to obtain a system of coupled equations describing geometric propagation paths in an atmospheric layer surrounding a globe. Comparison with propagation predictions using a Cartesian geometry shows that even for relatively short infrasonic propagation distances of a few hundred kilometers, differences introduced by the change in geometry are significant. Characteristics of the stratospheric pair are considered, and it is found that differences in the upward and downward legs of the propagation paths corresponding to the fast and slow stratospheric arrivals are changed such that spherical coordinate geometry predicts a decrease in the relative arrival time between the two. Analysis of regional infrasonic signals produced by a series of surface explosions shows that predictions obtained using spherical geometry are more accurate in cases where the tropospheric and stratospheric waveguides are accurately characterized.

A Bird’s‐Eye View on Ambient Infrasonic Soundscapes

“…This model can be used to approximate propagation losses in atmospheres with a dual (stratospheric-thermospheric) duct and neglects tropospheric ducting. A different approach could involve using a 3-D ray-theory model cast in spherical coordinates (Smets, 2018;Blom, 2019), to quantify propagation losses. The use of formal propagation models requires atmospheric specifications from the ground to the upper atmosphere.…”

Identification of Acoustic Wave Signatures in the Ionosphere From Conventional Surface Explosions Using MF/HF Doppler Sounding

“…The integrated path of an AW can be approximated using the ray tracing methods provided by the InfraGA/GeoAc package provided by Los Alamos National Laboratory for the purpose of modeling long range infrasound propagation. The software can be downloaded at https://github.com/LANL-Seismoacoustics/infraGA, and further details can be found in (P. Blom & Waxler, 2012; P. Blom & Waxler, 2017; P. Blom, 2019; P. Blom & Waxler, 2021). InfraGA is capable of 2D, 3D‐cartesian and 3D‐spherical calculations of infrasound ray tracing and waveform evolution well into the thermosphere.…”

“…Furthermore, the presence of turbulence and other atmospheric waves produce a complicated medium for infrasound propagation. These effects, along with a varying wind profile can severely modify the arrival time of infrasound at a given altitude (P. Blom, 2019; Evers & Haak, 2009; Sabatini et al., 2019).…”

“…The software can be downloaded at https://github.com/LANL-Seismoacoustics/infraGA, and further details can be found in (P Blom & Waxler, 2012;P. Blom & Waxler, 2017;P. Blom, 2019;.…”

Identification of the infrasound signals emitted by explosive eruption of Mt. Shinmoedake by three-dimensional ray tracing

“…The formulation discussed here follows that presented by Landau and Lifshitz (1959) and is analogous to calculating the motion of a particle using Hamilton equations [see also Blom (2019), Jones andBedard (2018), andLighthill (1978)]. Analogous Hamilton equations for a wave of angular frequency x and wavevector k are given by where r denotes the position vector.…”