Antipodal acoustic propagation (Australia‐Bermuda) in the high‐resolution global ocean state estimate.

Abstract

In 1960, the sound signals from a sequence of 300‐lb TNT shots deployed off Perth, Australia were recorded at Bermuda. The acoustic propagation of these signals are examined using ECCO2 1/6 degree ocean state estimates [1992–2006; www.ecco2.org] and Smith‐Sandwell 1’ global topography. Only the first acoustic mode need be considered. Intense, small‐scale features (e.g., Agulhas rings, the Antarctic circumpolar current) greatly influence the acoustic paths, giving them a scintillating and time‐dependent nature. Previous analyzes found, in the absence of the influence of the sea floor, Bermuda to be in the acoustic shadow of the African continent. The oceanic features of ECCO2 are sharper, hence more refractive, than the smoothed ocean atlases previously employed, but the additional refraction still does not give “direct” arrivals. The “direct” arrivals miss Bermuda by only 135±52 km, however. Scattering from Kerguelen and the Crozets, and reflection from the eastern tip of South America are sufficient to account for the 1960 measured arrivals. Intensity considerations show that because Bermuda was shadowed, the recorded signal levels of the explosive shots (15 dB RE noise) were greatly reduced compared to theoretically expected signal levels (30 dB RE noise).