Jay J. Pulli, Ted Farrell, & Rob Gibson
GTE/BBN Technologies, 1300 N 17th St, Suite 1200, Arlington, VA 22209

Sponsored by U.S. Defense Threat Reduction Agency

Contract No. DSWA01-97-C-0164


The objective of this research is to establish an understanding of the hydroacoustic signals that reflect from continents and bathymetric features so that these signals may be utilized as part of the hydroacoustic component of CTBT monitoring. This understanding is important because long-range propagation modeling predicts that some direct source-receiver paths are either blocked by bathymetric features or do not exist because of horizontal refraction. In these cases, reflections may be the only observable signal from a source, and may provide important information for localization and source type estimation.

In our research we have identified, acquired, and analyzed numerous historical and contemporary databases of hydroacoustic signals for sources with known origins. These include the Chase ship scuttling exercises in the 1960's and 70's; French nuclear tests in the southwest Pacific; the Japanese crustal exploration explosions in 1996; and signals generated during the May 1999 Ascension Island experiment. Results to date indicate that hydroacoustic reflections can be consistently seen at specific receivers for repeated events in a given source area. For example, the Chase explosions off the US eastern seaboard produced signals that reflected off the Guianna Plateau off the coast of South America and were recorded at Ascension Island. The locations of these reflecting features can be determined either from their arrival times at multiple receivers or by beamforming hydroacoustic array data (when available) and combining the back azimuth estimation with the time difference between the direct and reflected signals. Reflections that are 30 dB lower in power than the direct arrivals can be seen from the Japanese exploration explosions recorded at WAKE, but only for the largest events. For the smaller events with lower SNR, these reflections are below the ambient noise and hence not observed.

Information about the source also appears to be preserved in the reflected signals. For example, both the direct and reflected signals from the Chase 22 explosion recorded at Ascension show spectral scalloping in the band 5-30 Hz, unlike the signals from the nearby Chase 21 explosion, which have smoother spectra.

Key Words:

Hydroacoustics, reflections, long-range propagation, bearing estimation

21st Seismic Research Symposium: Technologies for Monitoring the Comprehensive Nuclear-Test-Ban Treaty
21-24 September 1999, MGM Grand Hotel and Conference Center in Las Vegas, NV