Detection and interpretation of infrasonic signals observed in Hawaii

Various array processing and signal detection algorithms have been tested and evaluated at International Monitoring System infrasound array I59US, also known as KONA, located on the western side of the Big Island of Hawaii. The array consists of four elements deployed as a triangle with a central element, with a baseline of 2 km. Initial tests were made by defining multiple beams and running STA/LTA, coherence, and F‐statistic detectors to estimate the apparent horizontal phase velocity and incidence azimuth of arrivals in frequency–wavenumber space. Such methods generally proved effective only for infrasonic signals with signal‐to‐noise ratios (S/N) greater than ∼4, and their performance varied with signal frequency. The Progressive Multi‐Channel Correlation (PMCC) detector is a time–domain detector that uses the correlation between various groupings of three sensors to obtain an estimate of the consistency of specified closure relations. If the consistency is below a certain threshold, a detection is registered. This detector has performed well for S/N ratios that are close to unity and for all frequencies. Examples of infrasonic detections in Hawaii for small (Leonids 2001) and large bolides (23 April 2001), blasting activity, surf, thunder, severe storms, and low‐frequency signals of unknown origin are presented.