Signal speeds (average horizontal ray velocities) are calculated on the basis of bathymetry and nine sound speed profiles (SSPs) for three great circle paths from shot detonation sites off Perth, Australia to receivers near Fernando de Noronha (14 552 km) and Bermuda (19 766 and 19 810 km). We use two signal speed algorithms, one a long-range ray-tracing code and the other an adiabatic invariant approximation (AIA) method, which assumes J = ∮ c−1 sin ϑ dz is constant for a given ray (where ∮ dz is an integral over one-ray cycle, c the local depth-dependent sound speed, ϑ the ray angle, and z the depth coordinate). Comparison with field measurements made 21 years ago shows that the ray-tracing and AIA methods, when applied to the same eigenrays, yield values averaging, respectively, 5.9±2.3 and 4.5±2.4 m/s below the observed values. The AIA method, however, yields values only 2.0±1.6 m/s below measurements when applied to rays vertexing just above the chief bathymetric obstructions near the Crozet and Kerguelen Islands. The specific reason for the offset is unclear, but this study shows AIA methods can be accurate for very general range-dependent SSPs.
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Shockley et al. (1982) studied this question.