Automated detection of underwater explosions by the IMS hydroacoustic network

Abstract

An important component of the CTBT monitoring system is the 11-station hydroacoustic network designed to monitor the oceans for unannounced underwater explosions. Explosions that do not breach the sea surface consist of a series of pulses due to gas bubble oscillations. The period of oscillation is a well-known function of the yield and detonation depth for explosives up to several thousand kilograms of TNT equivalent. The presence of a bubble pulse is a strong indicator of an explosive source and, therefore, is useful in event characterization. Examples of underwater explosions, which can be used to calibrate signal processing algorithms and propagation models, are rare. A set of explosive and other impulsive events recorded on hydrophones at Wake Island and Point Sur, CA is presented. These include the French nuclear tests, volcanic events, and chemical explosions. The set also includes pressure time-series simulated using a normal-mode propagation model. The automatic signal processor has been developed to search the real cepstrum for peaks using a noise-spectral equalization algorithm similar to those used in passive sonar. Examples illustrate complications due to range, depth, ambient noise, and waveguide distortion.

Observations and Bayesian location methodology of transient acoustic signals (likely blue whales) in the Indian Ocean, using a hydrophone triplet

“…The arrival times at various measurement instruments (such as the three members of the hydrophone triad) will be different depending on the distance and azimuth to the source. This method of detection and arrival determination and terminology is inspired from the detection methods used at the IDC on the waveform data from the IMS global seismic and hydroacoustic network (Laney et al, 1999;Hanson et al, 2001). The method of detection, with three absolute times picked on the trace, also gives us the differential times of the picks at the three hydrophones and, therefore, a means to determine a direction of propagation for a plane wave.…”