Rod Whitaker

High‐altitude infrasound calibration experiments

At the 152nd Meeting of the Acoustical Society of America, Andre and Bass reported an infrasound experiment conducted at White Sands Missile Range during the 2005‐2006 time frame. The experiment consisted of exploding a 22.4 kg charge at altitudes from 31.3 km to 49.6 km then recording the waveforms at 30 infrasound arrays (not all at the same time) at distances up to 1200 km from the source. The analysis is not yet complete but some preliminary observations have been reported in the most recent issue of Acoustics Today. This talk will summarize the findings published in Acoustics Today and offer suggestions to others who might want to access and analyze the data.

Extracting changes in air temperature using acoustic coda phase delays

Blast waves produced by 60 high-explosive detonations were recorded at short distances (few hundreds of meters); the corresponding waveforms show charge-configuration independent coda-like features (i.e., similar shapes, amplitudes, and phases) lasting several seconds. These features are modeled as reflected and/or scattered waves by acoustic reflectors/scatters surrounding the explosions. Using explosion pairs, relative coda phase delays are extracted and modeled as changes in sound speed due to changes in air temperature. Measurements from nearby weather towers are used for validation.

Modeling the generation of infrasound from earthquakes

Earthquakes can generate complex seismo-acoustic wavefields, consisting of seismic waves, epicenter-coupled infrasound, and secondary infrasound. We report on the development of a numerical seismo-acoustic model for the generation of infrasound from earthquakes. We model the generation of seismic waves using a 3D finite difference algorithm that accounts for the earthquake moment tensor, source time function, depth, and local geology. The resultant acceleration-time histories (on a 2D grid at the surface) provide the initial conditions for modeling the near-field infrasonic pressure wave using the Rayleigh integral. Finally, we propagate the near-field source pressure through the Ground-to-Space atmospheric model using a time-domain parabolic equation technique. The modeling is applied to an earthquake of MW 4.6, that occurred on January 3, 2011 in Circleville, Utah; the ensuing predictions are in good agreement with observations made at the Utah network of infrasonic arrays, which are unique and indicate...

Global infrasound monitoring—Research issues

The International Monitoring System being installed to support monitoring compliance with the Comprehensive Nuclear Test Ban Treaty provides scientists with a unique opportunity for research. There are still a number of problems which limit the full exploitation of the system. These include limitations on signal‐to‐noise imposed by wind noise and the absence of well defined, internationally accepted calibration standards for sensors. But perhaps the major research challenges lie in the area of source characterization and definition. Most of the signals recorded at the few sites now operating come from unidentified sources. There has been some effort devoted to identifying local and regional sources but the unidentified category still exceeds 50% of all distinct events. There are a number of infrasound sources that occur naturally. These include volcanic eruptions, bolides, microbaroms, mountaintop/wind interactions, severe storms, and earthquakes. Manmade sources include most any energetic activity. After...