MODELING HYDROACOUSTIC WAVEFORM ENVELOPES:
IMPLICATIONS FOR TEST BAN TREATY MONITORING
Jay J. Pulli, Robert Bieri, and Eugene Dorfman†
GTE/BBN Technologies, 1300 N 17th St, Suite 1200, Arlington, VA 22209
†GTE/BBN Technologies, 70 Fawcett St, Cambridge, MA 02140
Sponsored by U.S. Department of Energy
Office of Nonproliferation and National Security
Office of Research and Development
Subcontract Number B344846
Test ban treaty monitoring in the world's oceans is aided by the fact that the ocean environment is known to a degree of detail that is much finer than in the solid earth. Raypaths, traveltimes, and amplitudes can thus be accurately predicted. But additional information about the source and path may be contained in the received waveform envelope. This modeling effort has been undertaken to determine whether or not it is practical to extract information such as source depth and type from the waveform envelope, and how this may be used in a CTBT context.
To compute the waveform envelopes, a truncated set of normal modes is summed over a finite bandwidth to obtain an approximate time series. The normal mode code KRAKEN is used to compute modal shapes and wavenumbers for a set of frequencies, and the time series is then recovered using the Fast Fourier Transform. The model has been used to illustrate the factors that contribute to the shape of the received hydroacoustic waveform envelope, including the locations of the source and receiver with respect to the sound channel axis, the bottom conditions, and the bathymetry.
As expected, the largest amplitude is produced when both the source and receiver are in the middle of the sound channel axis. In this situation, path bathymetry determines the number of trapped (low loss) modes. As expected for typical SOFAR sound speed profiles, a buildup of signal energy with time is predicted, culminating in the large mode 1 arrival (this signal shape is often called "classic SOFAR"). Higher order modes are increasingly important as the source and/or receiver are moved away from the SOFAR axis. Bottom conditions are only an issue for the case of a bottom mounted hydrophone.
Modeling results are presented for a number of events, including the Chase21 ship scuttling explosion on June 25, 1970 off the New Jersey coast; nuclear explosions on Mururoa Atoll; and Japanese exploration explosions.
Hydroacoustics, long-range propagation, modeling, normal modes