T. M. Georges

Infrasound from convective storms. III. Propagation to the ionosphere

We model mathematically the spectral features of infrasound observed in the ionosphere and believed to be radiated by severe thunderstorms. We explain the dominant 2–5‐min wave period as an effect of atmospheric filtering; shorter periods are excessively attenuated by absorption in transit to the ionosphere, and longer periods are attenuated in portions of the atmosphere where the waves are evanescent because their frequencies are below the acoustic cutoff. An observed spectral ’’fine structure’’ within the 2–5‐min band is explained in terms of resonant interactions between the waves and the atmospheric temperature structure. Accurate quantitative modeling of all these details of the storm‐to‐ionosphere transmission coefficient requires numerical integration of the acoustic‐gravity wave equation, including the effects of ground reflection, absorption, and partial reflections in the atmosphere.Subject Classification: [43]28.30.

Nonperturbative modal tomography inversion. Part I. Theory

The mathematical basis for a method to invert modal ocean acoustic tomography measurements without explicitly assuming an initial sound‐speed profile is described. The method is based on determining the group and phase travel times for each vertical slice through the tomographic region. The group travel times are determined directly as the measurements of modal pulse travel time. The phase travel times are determined by resolving the cycle ambiguities in the phase measurements with constraints connecting the group and phase travel times. Standard tomographic techniques then determine the modal group and phase speeds within the tomographic region, and Abel transforms can be used to determine the symmetric part (the difference between the upper and lower profiles) of the sound channel. As with ray tomography, modal tomographic measurements supply no information about the antisymmetric part of the sound channel (the average of the upper and lower profiles), but determining the lower part of the sound channel...

NONPERTURBATIVE OCEAN ACOUSTIC TOMOGRAPHY INVERSION OF 1000-KM PULSE PROPAGATION IN THE PACIFIC OCEAN

A nonperturbative inversion was performed of acoustic tomography measurements made in the northeastern Pacific Ocean in July 1989, in which acoustic transmissions from a 250‐Hz broadband source located near the sound‐channel axis were recorded at a long vertical array of hydrophones 1000 km away. In contrast with a conventional inversion, this nonperturbative inversion does not assume that travel times are linearly related to the sound‐speed deviations from a background sound‐speed model. The inversion process involved three steps: (1) Measured pulse travel times and the source and receiver locations were used to determine the range average of the equivalent symmetric sound‐slowness profile. That part of the inversion used only curve fitting and Abel transforms, and required independent (nontomographic) information only to help identify the pulse arrivals. (2) Under the assumption that the range dependence of sound speed was small, we used the reciprocal of the range‐averaged sound‐slowness profile to app...

Nonperturbative ocean acoustic tomography inversion

A method for estimating range‐averaged sound‐speed and sound‐slowness profiles from single‐slice tomographic travel‐time measurements is demonstrated. The method directly yields the range average of the equivalent symmetric profile and the asymmetry of the sound channel at the source and receiver. In the absence of independent information, the measurements themselves indicate whether they are consistent with a range‐independent sound channel. The inversion method is applied to a simulated pulse arrival sequence (generated by ray tracing), and the recovered sound speed agrees with that used for the simulation. Using climatology (or other independent information) for the sound speed below the sound‐channel axis would allow an estimate of the range average of the profile above the sound‐channel axis. The method yields the range average of sound slowness without linearization and gives the range average of the sound speed to first order.

Test of nonperturbative ocean acoustic tomography inversion

This work is part of an effort to compare various ocean acoustic tomography inversion methods given the same hypothetical sound‐speed field (specified in a 1000‐ by 1000‐ by 5‐km region). To test the WPL nonperturbative inversion method [Jones et al., NOAA Tech. Memo. ERL WPL‐217 (1991)], the authors simulated tomographic data using ray tracing, performed an inversion, and compared the results with the sound‐speed field provided. The simulation sampled the region in several vertical slices, and the inversion yielded the range average of the symmetric part of the sound‐slowness profile for each slice.

Propagation of severe‐storm infrasound to ionospheric heights: a narrow‐band atmospheric filter

We set out to model theoretically the spectral features of infrasound observed in the ionosphere and believed to be radiated by severe thunderstorms. We explain the dominant 2–5‐min wave period as the effect of atmospheric filtering; shorter periods are excessively attenuated by absorption in transit to the ionosphere, and longer periods are attenuated in portions of the atmosphere where the waves are evanescent because their frequencies are below the acoustic cutoff. An observed spectral “fine structure” within the 2–5‐min band is explained in terms of resonant interactions between the waves and the atmospheric temperature structure. Accurate quantitative modeling of all these details of the storm‐to‐ionosphere transmission coefficient requires numerical integration of the acoustic‐gravity wave equation, including the effects of ground reflection and partial reflections in atmosphere.