The Ocean's Acoustic Waveguide
The SOFAR channel is one of nature's most remarkable waveguides. Discovered during World War II by Maurice Ewing and Joe Worzel, it arises from the competing effects of temperature and pressure on sound speed. Temperature decreases with depth (lowering sound speed) while pressure increases (raising it). The resulting sound speed minimum, typically between 600 and 1200 meters depth, acts as a lens that continuously refracts acoustic energy back toward the axis.
Sound Speed Profile
The Mackenzie equation calculates sound speed from temperature, salinity, and depth. Near the surface, warm water yields high sound speed (~1520 m/s in the tropics). Through the thermocline, speed drops rapidly. Below, in the cold isothermal deep ocean, increasing pressure steadily raises speed again. The minimum — the SOFAR axis — is where temperature's downward pull exactly balances pressure's upward push.
Trapping and Propagation
Sound rays launched near the SOFAR axis oscillate up and down about the minimum, never reaching the surface or bottom. This eliminates reflection losses and reduces geometric spreading from spherical (1/r²) to cylindrical (1/r). Combined with the ocean's extremely low absorption at frequencies below 100 Hz, this allows signals to traverse entire ocean basins — a phenomenon exploited by blue whales whose calls can be heard across the Pacific.
Modern Applications
Acoustic thermometry uses the SOFAR channel to measure basin-averaged ocean temperatures to millidegree precision by timing signals across thousands of kilometers. Changes in travel time reveal warming trends with unprecedented sensitivity. The same channel supports submarine communication, earthquake detection via T-phase arrivals, and autonomous underwater vehicle navigation using acoustic positioning networks.