Effects of the Antarctic Circumpolar Current on hydroacoustic propagation and implications for Nuclear Explosion Monitoring

A series of small depth charges was detonated along a transect from New Zealand to Antarctica over a period of three days in late December of 2006. The hydroacoustic signals were recorded by a hydrophone deployed near the source, and at a sparse network of permanent hydrophone stations operated by the International Monitoring System (IMS), at distances up to 9600 km. Our purpose was to determine how well signal characteristics could be predicted by the World Ocean Atlas 2005 (WOA05) climatological database for sources within the Antarctic Circumpolar Current (ACC). Waveforms were examined in the 1-100 Hz frequency band, and it was found that, for clear transmission paths, the shot signals exceeded the noise only at frequencies above 20-30 Hz. Comparisons of signal spectra for recordings near the source and at the IMS stations show that transmission loss is nearly uniform as a function of frequency. Where recorded signal to noise ratios are high, observed and predicted travel times and signal dispersion agree to within two seconds under the assumption that propagation is adiabatic and follows a geodesic path. The deflection of the transmission path by abrupt spatial variations in sound speed at the northern ACC boundary is predicted to decrease travel times to the IMS stations by several seconds, depending on the path. Acoustic velocities within the ACC are predicted to vary monthly, hence the accuracy of source location estimates based only on arrival times at IMS stations depends on the monthly or seasonal database used to predict travel times, and on whether we account for path deflection. However, estimates of source locations within the ACC that are based only on observed waveforms at IMS hydrophones are highly dependent on the configuration of the IMS array; a set of shots observed only at an IMS station in the Indian Ocean and another in the South Pacific was located to within 10 km in longitude, but was unconstrained in latitude. Several sets of shots observed only at IMS hydrophones in the Indian Ocean were constrained to within 55 km in latitude but were unconstrained in longitude.