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In 1960 sound from explosive charges detonated off Perth, Australia was detected at Bermuda in the North
Atlantic. The yellow path is the geodesic on a sphere, the white path is a geodesic on the earth's elipsoid
(WGS84). Because of horizontal refraction, however, the direct path is more complicated than that. Munk,
O'Reilly and Reid (1988) calculated the ray refracted along the sound channel axis, a 2-D calculation,
and found that the ray seemed to intersect the African continent. The magenta path has a slight kink at
Cape Agulhas, as it bends around the African continent and continues on to Bermuda. This path was
conjectured by Munk et al as the most likely path taken by the sound from the explosives. Heaney, Kuperman
and McDonald (1991) used a more accurate approach to show that acoustic modes do indeed have direct,
unobstructed paths from Perth to Bermuda - with one path near the geodesic path and another path near the
spherical path (See figure). The colors in the figure show ocean depth: blue (4-6+ km), green/yellow (2-3 km), orange/red (2 km and
shallower), and land masses in blue.
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