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 et al., J. Phys. Ocean. 18, 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 q-5 s, while group B interacts slightly with bathymetry off Brazil, arriving at 13 403 q-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 q-5 s; pulse train half-width 15 s; second arrival 30 q-5 s later roughly 10 dB below the first arrival).

Deep seafloor arrivals: An unexplained set of arrivals in long-range ocean acoustic propagation

“…Time fronts corresponding to weak surface-reflected bottom-reflected ͑SRBR͒ paths, which often attenuate very rapidly with range and are often unnecessary to successfully predict long-range, deep-water, propagation ͑de Groot- Hedlin et al, 2009;Heaney et al, 1991;Van Uffelen et al, 2009;Wage et al, 2003͒, can be seen as more horizontal time fronts at mid-water depths between 1076.5 and 1077.5 s and at shallow depths between 1078 and 1080 s.…”

“…The first strategy was compressional wave modeling without bottom interaction ͑keep the bottom properties the same as the water above it but add strong attenuation so that no energy is returned from the seafloor or sub-seafloor͒. This strategy, without including bottom interaction, has successfully predicted long-range, ocean acoustic propagation in the past Dushaw et al, 1999;Heaney et al, 1991;Van Uffelen et al, 2009;Wage et al, 2005;Xu, 2007͒. This seemed like a good initial strategy since bathymetry along the whole 3200 km long geodetic is everywhere deeper than 4400 m and for most of the propagation path is deeper than 5000 m ͑Fig.…”

Bermuda shadow

“…However, diffusion by internal waves and refraction by mesoscale eddies and the sloping bottom around Cape Agulhas may be contributing factors. Heaney et al (1991) have now constructed refracted non-axial geodesies for the measured global field of sound speed C (x,y,z). There are indeed a number of unimpeded eigenrays (horizontal multipaths) between Perth and Bermuda.…”

Modeling and Forecasting Ocean Acoustic Conditions

“…Modal tomography is thus another procedure to use acoustic information to constrain conditions in large volumes of ocean instantaneously and repeatedly. Note that horizontal refraction of rays or modes can be disregarded on many situations: the references (Heaney et al 1991;Dushaw and Menemenlis 2014) pertain to sound that travels one-half way around Earth, a special situation that must consider horizontal refraction, and the strongly bottom-interacting effects illustrated in Section 3 can be avoided in tomographic exercises.…”

“…This ray model can be extended to take into account the finite volumes surrounding rays to which sound is coherently sensitive (Skarsoulis et al 2009), a more detailed treatment than considering sound to travel along infinitesimal rays. Ocean waveguide acoustic normal modes speeds are sometimes used for tomographic purposes (Heaney et al 1991;Dushaw and Menemenlis 2014). In this procedure, ocean dynamical models (or climatological models) are used to compute horizontal modal ray paths and modal travel times along the paths, with the paths showing refraction from gradients in the ocean conditions.…”