Operational processing of hydroacoustics at the Prototype International Data Center

The Prototype International Data Center (PIDC) has designed and implemented a system to process data from the International Monitoring System's hydroa coustic network. The automatic system detects and measures various signal c haracteristics that are then used to classify the signal into one of three categories. The detected signals are combined with the seismic and infrason ic detections to automatically form event hypotheses. The automatic results are reviewed by human analysts to form the Reviewed Event Bulletin (REB). Continuous processing of hydroacoustic data has been in place since May 199 7 and during that rime a large database of hydroacoustic signals has been a ccumulated. For a two-year period, the REB contains 13,582 T phases that ar e associated to 8,437 events. This is roughly 25% of REB events after takin g station downtime into account. Predicted travel times used in locations a re based on the arrival time of the peak amplitude mode calculated from a n ormal mode propagation model. Global sound velocity and bathymetry database s are used to obtain reliable 2-D, seasonally dependent, travel-time tables for each hydroacoustic station in the PIDC. A limited number of ground-tru th observations indicate that the predicted travel times are good to within 5 seconds for paths extending to over 7,000 km - corresponding to a relati ve error of less than 0.1%. The ground truth indicates that the random erro rs in measuring arrival times for impulsive signals are between 1 and 6 sec onds. This paper describes and evaluates the automatic hydroacoustic proces sing compared to the analyst reviewed results. In addition, special studies help characterize the overall performance of the hydroacoustic network.