Automated Infrasound Event Detection and Location Using a Mesh of Arrays

Abstract:

We have developed a novel method to detect and locate geophysical events that makes use of any sufficiently dense sensor network. The algorithm applies Delaunay triangulation to divide a dense sensor network into a mesh of 3-element arrays, called triads. In this study, we use infrasonic sensors in the USArray Transportable Array (TA), which comprises 400 stations deployed in a Cartesian grid with an average interstation spacing of 70 km. At this density, infrasound waveforms are incoherent between the sensors within each triad so the data are transformed into envelopes, which are cross-correlated to find signals consistent with infrasound propagation. The propagation azimuth, phase velocity and signal arrival time are computed for each signal. Triads with signals that are consistent with a single source are grouped as a cluster. The ensemble of arrival times and azimuths of detected signals within a cluster are used to locate a common source in space and time. In this study a total of 513 infrasonic stations that were active for part or all of 2013 were divided into over 2,000 triads. Low (.5-2 Hz) and high (2-8 Hz) catalogs of infrasonic events were created for the eastern United States. The low-frequency catalog includes over 900 events and reveals several highly active source areas on land corresponding to coal mining regions. The high-frequency catalog includes over 2,000 events, with most occurring offshore. Most events are clearly anthropogenic as they occur during regular working hours each week. The regions to which the TA is most sensitive vary seasonally, with the direction of reception dependent on the direction of zonal winds. The catalog reveals large acoustic events that may provide useful insight into the nature of long-range sound propagation in the atmosphere.