Refraction of Sound by Islands and Seamounts

Abstract

We consider the propagation into shallow water of low-frequency and low-order acoustic modes trapped in the sound channel. The phase velocity slightly decreases and then increases with decreasing depth. This leads to an unusual pattern of wave refraction. Waves are “attracted” by islands and seamounts (rays turn toward shore) in “almost deep” water, and then are strongly “repelled” in shallow water. We examine the relative intensity of scattered arrivals in very long-range ocean transmissions. Forward scatter by islands and seamounts can give significant (order −10 dB) scattered arrivals, as can large-angle scatter (including backscatter) by large islands and by islands near the source or receiver. However, transmission of acoustic energy into the sea floor leads to loss of the scattered energy in the sound channel. Total reflection (no loss) is favored by glancing incidence on steep slopes of islands and seamounts with large compressional seismic velocities. We suggest a complementary relation between the intensities from an underwater explosion as recorded on axial hydrophones versus nearby land seismometers, with conditions favorable to reflection favoring hydrophones and vice versa. A preliminary attempt is made to compare measured and computed scattered intensities in the sound channel for operation WIGWAM (a deep-water nuclear explosion in 1955), operation CHASE, and the Perth to Bermuda transmission of 1960. Computed intensities are too low.