Fig. 1

Illustration of how acoustic signals propagate in isothermal water with signal loss through three-dimensional spreading (a), and how acoustic signals propagate in thermally stratified water with signal loss through three-dimensional spreading that is amplified by refraction when the source is above the thermocline (b) and below thermocline in c. In a the attenuation of acoustic signal is just a function of the distance from the source, whereas in b an acoustic “shadow zone” of reduced sound intensity forms below the thermocline, while in c an acoustic “shadow zone” forms above the thermocline. The green circle represents the acoustic source, the gradients of blue shading represent speed-of-sound gradients, and red shading represent attenuation of sound intensity. In all sketches, the dashed gray line represents a threshold value of sound intensity below which detection efficiency is poor