SOFAR Channel

The SOFAR (Sound Fixing And Ranging) channel is the deep-ocean acoustic waveguide centered on the sound-speed minimum, typically near 1000 m depth at mid-latitudes. Sound speed falls with cooling temperature in the upper layer and rises with pressure below, so a minimum forms at the channel axis; rays launched near the axis refract back toward it and propagate with low loss for thousands of kilometers. Low-frequency signals stay trapped, which is why a small charge fired on the axis can be heard across an ocean basin. The channel carries SOFAR-float tracking, acoustic tomography, and CTBTO hydroacoustic monitoring.