Underwater Sound Propagation across the Hawaiian Arch Free

Recordings of sounds of 300 small explosive charges were made from SOFAR depth hydrophones off Midway and Wake Islands. The charges were detonated at various depths north of Hawaii. Underwater sound signals from selected shots were analyzed for signal duration, peak pressure level, and frequency content. The effect of the Hawaiian Arch on the signals received was measured by comparing data from events whose travel path to Wake crossed the Arch with those at Midway that traveled unobstructed paths. When travel paths to Wake crossed the Arch between the longitude of Oahu (157°30′W) and the longitude of Midway (176°W), the signals were 5–25 dB below those at Midway and 4–6 sec shorter than expected in duration. Complete blocking of the signal at Wake occurred when both of the following conditions were met: (1) when travel paths across the Arch made angles of <35° with the trend of the Arch and (2) when water depths less than 2000 fm were encountered. For angles >35°, diffraction of sound partially ensonified the shadow zones behind shoals atop the Arch. Atolls near the western end of the Arch (Kure, Midway) completely blocked the signal at Wake. Isolated seamounts caused little shadowing unless their minimum depth was near 850 fm or less.