T-Phase Observations from the May 1999 Ascension Island Experiment
The Comprehensive Nuclear-Test-Ban Treaty (CTBT) specifies that an International Monitoring System (IMS) will be used to detect and locate disturbances that could be related to nuclear testing. In order to monitor disturbances in and near the world's oceans, the IMS will rely on a network of 11 hydroacoustic stations. This hydroacoustic network will be composed of 6 hydrophone stations and 5 T-phase seismic stations. The hydrophone stations will record pressure variations in the ocean. The T-phase stations will record the seismic waves in the solid earth that are excited when an hydroacoustic wave strikes an island or continental margin. The coupling of hydroacoustic-to-seismic energy for the purpose of CTBT monitoring is currently an active area of research. We report observations of hydroacoustic waves and their conversion to seismic waves (T-waves) at the volcanic edifice of Ascension Island. Earthquakes to the south of Ascension Island were recorded by International Monitoring System hydrophones and temporary seismic stations deployed on Ascension Island by LLNL. The hydrophone recordings are rich in high frequency (10-40 Hz) energy. However, the converted seismic waves (T-waves) are dominated by much lower frequencies (2-8 Hz). The T-waves demonstrate amplitude and travel time variations on the Island. We performed two-dimensional finite difference simulations to determine if the amplitude variability is the result of conversion along the complex bathymetry of the island and subsequent propagation to the station or a site effect. These observations suggest that the T-phase stations of the IMS may be useful for detecting and locating events in support of the CTBT provided that T-phase travel times are appropriately calibrated. However on volcanic islands, event identification based on T-phase data, which relies on high frequencies, may be inhibited by strong attenuation and low signal-to-noise.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15013205
- Report Number(s):
- UCRL-JC-138996; TRN: US200802%%1173
- Resource Relation:
- Conference: 22nd Annual Department of Defense/Department of Energy Seismic Research Symposium, New Orleans, LA, Sep 12 - Sep 15, 2000
- Country of Publication:
- United States
- Language:
- English
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