Abstract
Given conditions of “downward refraction,” low-frequency acoustic propagation in shallow water displays greater attenuation than it does in isovelocity water. Ray theory suggests this but is untrustworthy here. Since the main acoustic losses occur at the bottom, this attenuation should rise as does the intensity of the sound just at the bottom. When the bottom material acts practically like a fluid (Pekeris case), the increase of intensity is readily calculated by perturbation theory from the normal-mode solution for isovelocity water, provided at least a few modes are allowed. This increase depends on the fractional change in sound velocity, the detailed change with depth, and the square of H/λ (water depth/acoustic wavelength). Specific calculations for a linear velocity gradient and a bottom-limited sound channel show that the effect on attenuation can vary from negligible to large. Pertinent data are scanty and rough, but show the proper trend. The method is also applicable to “upward refraction,” which should lower the attenuation. A competing mechanism is the attenuative effect of a rough sea surface, which (relative to the isovelocity case) will play a larger or smaller role with upward or downward refraction, respectively.
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