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

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 ship’s 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. Unfortunately, since the hydrophones are located near the ocean bottom, the recording is contaminated by reflections and scattered energy, making a reliable deconvolution impossible. Instead, peak-to-peak amplitudes were used at the dominant source energy to determine clip levels and calibration factors (in pascals at 10 Hz) for each of the three hydrophones. Consistency was confirmed using background hydroacoustic noise. Imploding sphere sources were tested as a potential method to couple hydroacoustic energy directly into the Sound Fixing and Ranging (SOFAR) channel (at a nominal depth of 700 meters) without the use of explosives. Tests near Ascension Island and off the Pacific coast of California have demonstrated that imploding spheres can be made to fail at prescribed depths and that the signals are similar in amplitude and frequency content to about 1 lb. of high explosive. Although the source has promise as an alternative to small explosions, like explosives, most of the acoustic energy is at frequencies above that of the hydroacoustic monitoring band for nuclear explosions (1–50 Hz). The use of towed airguns for the near-field precision location and amplitude calibration of hydrophones is ideal. The precision of the source location and timing, the ease of obtaining a planned source shot geometry and numerous shots, and the relatively low-frequency content of the source (i.e., in the CTBT monitoring band) cannot be equaled with air-dropped explosives or most other methods. If the hydrophones are located on the ocean bottom, in situ calibration is very difficult, ideally requiring a co-located, calibrated hydrophone at each hydrophone needing calibration. This will not be necessary with the new floated hydrophone stations since a “pure” recording of the source will be possible. Imploding spheres and small explosive sources in the Sound Fixing and Ranging (SOFAR) channel need further evaluation as regional calibration sources due to the relatively high-frequency content of the source. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE SEP 2000 2. REPORT TYPE 3. DATES COVERED 00-00-2000 to 00-00-2000 4. TITLE AND SUBTITLE Calibration Of Hydrophone Stations: Lessons Learned From The Ascension Island Experiment 5a. CONTRACT NUMBER