Bermuda shadow

Abstract

Abstract In a 1960 experiment, detonations in the sound channel off Perth, Australia, were recorded at Bermuda, almost halfway around the earth. In the original analysis it was assumed that the sound waves travelled along the great circle route. A subsequent analysis allowing for earth ellipticity and lateral refraction at the sound axis places Bermuda in the shadow of the “refracted geodesies” cast by the Cape of Good Hope. We summarize various processes for scattering sound energy into the shadow. Among these hypotheses only the diffusion of the acoustic beam by internal wave inhomogeneities appears plausible. We next examine the implications of a modal structure of the transmitted acoustic energy. The previous analysis applied strictly to the limit of low modes of high frequency with the energy sharply concentrated at the sound axis. Higher modes of lower frequency extend well above and beneath the axial sound channel, and their (horizontally) refracted paths can be significantly separated from that previously considered, and thus reach Bermuda. Global transmission between fixed points has to allow for the multiplicity of paths associated with different modes and frequencies.