Sound propagation in a surface duct: Can the deep water profile be neglected?

Abstract

In calculating normal‐mode acoustic propagation in an ocean surface duct, the sound‐speed profile is often represented by the simple bilinear profile. The question arises as to whether the positive gradient in the sound‐speed profile that occurs beneath the SOFAR axis has a significant effect on surface duct propagation. Calculations have been made for typical oceanic profiles and it has been found that the bilinear profile and the complete profile yield the same result as a function of range (even though the individual normal modes are different) until the convergence zone effect becomes significant. The threshold range at which this effect becomes significant decreases as frequency is decreased below the surface duct's cutoff frequency (since the energy propagated within the duct decreases while the width of the convergence zone increases). For a duct that has a cutoff frequency of 200 Hz for example, the threshold range decreases from 30 to 15 km as the frequency decreases from 200 to 100 Hz.