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  SOund SUrveillance System (SOSUS): Acquisition System Description
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The original data acquisition system ("PHONE"), installed on August 29, 1991, accessed individual hydrophones from arrays throughout the North Pacific and provided omni-directional coverage of low-level seismicity (mb > 2.4) in the entire North Pacific basin. Two channels were allotted for each array, collecting individual hydrophone elements from the two ends of each array. The omni-directional nature of the hydrophones allow detection of sources at any bearing, and the availability of the pair of signals allows calculation of the arrival bearing. By combining arrival time and arrival bearing on several arrays, common events can be associated automatically within software.

By combining all of the hydrophone element signals with predefined phase lags for each element based on their relative locations and local sound speed, an acoustic "beam" can be formed, oriented in any direction. The "BEAM" system, installed June 22, 1993, accessed formed-beams from six hydrophone arrays trained on the northeast Pacific seafloor spreading system from the Mendocino to the Sovanco Fracture Zones. The signal gain obtained through beam forming allows the detection of seismic events with source magnitudes as low as 1.8. This system was continuously analyzed to provide immediate detection of significant seismo/acoustic events of potential interest, including volcanic dike injections and eruptions, and was the basis of several successful event response efforts.

On October 12, 2000, changes in the Navy's electronics configuration resulted in the loss of beamformed outputs to dual users. At that time, the two NOAA/PMEL data acquisition systems were combined into a single system ("WHIDBEY") that collects multiple hydrophone elements at 256 Hz sampling rate. The data are transferred, digitally beamformed, and displayed at PMEL in Newport. Although more effort, the new configuration allows complete flexibility in beamforming. In performing this upgrade, PMEL and University of Washington Applied Physics Laboratory engineers made accommodations to allow future dual users to easily assess all hydrophone elements from all systems.

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