Implosion Source Development and Diego Garcia Reflections
Calibration of hydroacoustic stations for nuclear explosion monitoring is important for increasing monitoring capability and confidence from newly installed stations and from existing stations. Past work at Ascension Island has shown that ship-towed airguns can be effectively used for local calibrations such as sensor location, amplitude and phase response, and T-phase coupling in the case of T-phase stations. At regional and ocean-basin distances from a station, the calibration focus is on acoustic travel time, transmission loss, bathymetric shadowing, diffraction, and reflection as recorded at a particular station. Such station calibrations will lead to an overall network calibration that seeks to maximize detection, location, and discrimination capability of events with acoustic signatures. Active-source calibration of hydroacoustic stations at regional and ocean-basin scales has not been attempted to date, but we have made significant headway addressing how such calibrations could be accomplished. We have developed an imploding sphere source that can be used instead of explosives on research and commercial vessels without restriction. The imploding sphere has been modeled using the Lawrence Livermore National Laboratory hydrodynamic code CALE and shown to agree with field data. The need for boosted energy in the monitoring band (2-100 Hz) has led us to develop a 5-sphere implosion device that was tested in the Pacific Ocean earlier this year. Boosting the energy in the monitoring band can be accomplished by a combination of increasing the implosion volume (i.e. the 5-sphere device) and imploding at shallower depths. Although active source calibrations will be necessary at particular locations and for particular objectives, the newly installed Diego Garcia station in the Indian Ocean has shown that earthquakes can be used to help understand regional blockages and the locations responsible for observed hydroacoustic reflections. We have analyzed several events with a back-azimuth from Diego Garcia between 100 and 140 degrees. The Diego Garcia records show a pronounced reflection that correlates in travel time and back-azimuth (calculated using the waveform cross-correlation of the tri-partite array elements to determine lag time across the array) with a reflector at the Saya de Malha Bank, on the Seychelles-Mauritius Plateau. We also show that to accurately predict blockage and reflection regions, it is essential to have detailed bathymetry in relatively small but critical areas.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15013224
- Report Number(s):
- UCRL-JC-143750; TRN: US200604%%176
- Resource Relation:
- Conference: 23rd Department of Defense/Department of Energy Seismic Research Review, Jackson Hole, WY, Oct 02 - Oct 05, 2001
- Country of Publication:
- United States
- Language:
- English
Similar Records
MODEL-BASED HYDROACOUSTIC BLOCKAGE ASSESSMENT AND DEVELOPMENT OF AN EXPLOSIVE SOURCE DATABASE
Use of Imploding Spheres: An Alternative to Explosives as Acoustic Sources at Mid-Latitude SOFAR Channel Depths