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Calibration of Hydrophone Stations: Lessons Learned from the Ascension Island Experiment

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Calibration of hydroacoustic stations for nuclear explosion monitoring is important for increasing monitoring capability and confidence from newly installed stations and from existing stations. Calibration of hydroacoustic stations is herein defined as the near-field precision location of the hydrophones and determination of the amplitude response and the regional-scale calibration of acoustic travel times, bathymetric shadowing, diffraction, and reflection as recorded at a particular station. An important type of calibration not considered here is ocean-basin- scale calibration of a hydroacoustic monitoring system. To understand how to best conduct hydroacoustic station calibrations, an experiment was conducted in May 1999 at Ascension Island in the South Atlantic Ocean. The experiment made use of a British oceanographic research vessel towing an airgun array and collected data over three Missile Impact Location System MILS hydrophones that were in use by the National Data Center NDC and the prototype International Data Centre pIDC. From the towed airgun data, we were able to determine the location for each of the three hydrophones to accuracy better than 100 meters in latitude, longitude, and depth. The agreement with the nominal locations was excellent in depth and to within 1 kilometer in latitude and longitude. The depths determined for the hydrophones and the ocean bottom depths determined from the ships sonar system force the conclusion that all three hydrophones are located at or near the ocean bottom. Amplitude frequency response of the hydrophones was also calibrated using a calibrated, temporarily deployed hydrophone to determine the airgun source function. With the source function known, the amplitude and phase response of the hydrophones could be deconvolved from the recorded waveforms provided a pure source waveform arrival is identified on the recording.

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  • Physical and Dynamic Oceanography
  • Acoustic Detection and Detectors
  • Acoustics

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