Perth–Bermuda sound propagation (1960): Adiabatic mode interpretation

Abstract

A 1960 experiment is examined in which sound from three underwater explosions near Perth, Australia, was detected near Bermuda. A recent attempt [Munk e t a l., J. Phys. Ocean.1 8, 1876 (1988)] to calculate propagation paths for this event included rotational flattening of the Earth and horizontal refraction determined from the vertical sound speed minimum. That calculation left Bermuda in a shadow zone. The current work invokes adiabatic mode theory to include refraction due to horizontal variations in the vertical mode structure. These results include separate horizontal rays for each of the first few vertical modes, using an archival data set of 230 ocean sound profiles to generate the modes numerically. Where appropriate, interaction with bathymetry is included. This solution possesses two eigenray groups: Group A passes just south of the Cape of Good Hope, at which point group B is almost 1000 km to the south. Intermediate rays are blocked by islands. Group A proceeds unimpeded to Bermuda for a total time of flight of 13 354±5 s, while group B interacts slightly with bathymetry off Brazil, arriving at 13 403±9 s, and suffering roughly 7–12 dB more bottom attenuation. The spread in these arrivals overlaps satisfactorily with experimental data (main arrival at 13 364±5 s; pulse train half‐width 15 s; second arrival 30±5 s later roughly 10 dB below the first arrival).