On the Analysis of the Spatial and Temporal Structure of Natural Infrasound Signals
ALASKA UNIV FAIRBANKS GEOPHYSICAL INST
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The principal contaminant of the infrasound spectrum in the band between 0.2 - 0.5 hertz is the acoustic radiation associated with marine storms termed microbaroms. These signals appear worldwide and, during winter months, can be the dominant signal during quiet periods. They tend to persist as collections of wavelets that last for hours and days. It is primarily against the spectral clutter presented by microbaroms that the detection of low-yield, man-made explosions must be made. We have made the hypothesis that the spatial coherence of microbarom wavelets is limited to scales of approximately 1km. This is based upon previous experience operating arrays of various sizes at the Geophysical Institute, University of Alaska Fairbanks. In order to test this hypothesis we are conducting two experiments. In the first experiment we have begun to operate a single microphone at various distances from the permanent, four-microphone infrasound array located on the campus of the University of Alaska. By performing simple coherence tests as a function of the spacing of the microphone from the array we intend to make estimates of the mean coherence length. In the second experiment we are estimating the coherence length of microbarom wavelets in the time-series recorded by the microphones in the permanent array using Hilbert transform techniques. Preliminary estimates indicate that the microbarom has a spatial coherence of only a few wavelengths. Studies of historical data sets at Fairbanks indicate an average periods in the range of 3 7 seconds indicating a spatial coherence of the order of 5km. The properties of the microbaroms and other coherent infrasound signals can be easily extracted from the broadband spectrum of incoherent signals through the use of the Pure-State filter. This is a coherence based filter that, as distinct from other coherence-based analysis techniques, is not biased by the detector array geometry.
- Acoustic Detection and Detectors