Marius Comper

Marine Physics & Geophysics

The Deep Ocean Highway

One thousand metres below the surface, the ocean turns into a colossal acoustic lens. Sound does not drift away into silence; instead, continuous refraction locks it into an endless horizontal corridor, crossing entire hemispheres without ever striking the rocky seabed or the surface air.

18,200 km
Underwater distance traversed by an acoustic pulse between Antarctica and Bermuda in January 1991.
3 h 24 min
Exact travel time of the wave propagating at an average speed of 1,485 metres per second.
0.001 dB/km
Chemical absorption loss in seawater for a low-frequency tone at 50 hertz.

SOFAR Waveguide Refraction Simulator

Select an ocean basin and adjust the transducer source depth to see how Snell's law bends sound rays around the acoustic minimum axis.

SOFAR Axis
1000 m
Min Velocity
1481 m/s
Trapped Energy
100%
Cycle Period
~42 km

Classic deep SOFAR channel at 1,000 m depth. Warm surface waters drop rapidly through the thermocline, before hydrostatic pressure dominates below 1,000 m.

Acoustic Waveguide Dispersion
Steeper rays arrive first at higher frequencies, followed by the concentrated convergence of axial rays in a sharp final crescendo.

Why does sound bend in water?

The speed of sound in seawater is governed by two opposing physical forces: temperature and hydrostatic pressure. Near the surface, solar radiation warms the upper ocean, speeding up sound propagation (around 1,530 to 1,540 metres per second in warm latitudes). As you descend through the first several hundred metres, temperature plummets rapidly across the thermocline, slowing the speed of sound down to a low of approximately 1,480 metres per second.

Below one thousand metres, temperature stabilizes at near-freezing levels (2–4°C), but the enormous weight of the water column exerts crushing pressure, adding one atmosphere every ten metres. This compression increases the elasticity of water molecules, accelerating sound back up above 1,550 metres per second near the abyssal plain.

Between the warmth above and the pressure below lies an acoustic trough of minimum velocity. Under Snell’s law of refraction, any acoustic wave trying to escape upward or downward is constantly curved back toward this slowest layer.

The resulting phenomenon is a natural oceanic waveguide: the SOFAR channel (Sound Fixing and Ranging). Instead of scattering against rough surface waves or dissipating against jagged seafloor rock, sound waves oscillate smoothly in cyclical loops spanning thousands of kilometres.

The 1991 Heard Island Feasibility Test

In January 1991, an international research team led by oceanographer Walter Munk lowered a heavy acoustic transducer to a depth of 175 metres near Heard Island in the sub-Antarctic Indian Ocean, broadcasting a continuous 57 hertz tone.

Three hours and twenty-four minutes later, hydrophone arrays operated by the US Navy in Bermuda—on the other side of the planet, 18,200 kilometres away—recorded the transmission with remarkable clarity. Simultaneously, the signal was detected across the Pacific Rim, in South Africa, Canada, and New Zealand.

Heard Island 1991 Global Hydrophone Network

Explore receiving stations across the planet and the exact arrival times of the transmission from the Southern Ocean.

Bermuda 18,200 km
Path navigated around South Africa (Agulhas Basin) and the full length of the Atlantic.
Travel time: 3 h 24 min
US West Coast 17,800 km
Diagonal great circle arc across the entire Pacific Basin past New Zealand.
Travel time: 3 h 20 min
Halifax (Canada) 16,500 km
Recorded on deep-sea hydrophone arrays in the North Atlantic.
Travel time: 3 h 05 min
Ascension Island 9,200 km
Direct line through the center of the South Atlantic acoustic corridor.
Travel time: 1 h 43 min
New Zealand 7,100 km
Measured at the flank of the Chatham Rise submarine plateau.
Travel time: 1 h 20 min
Cape Town 4,800 km
First major oceanic checkpoint entering the Atlantic basin from the Indian Ocean.
Travel time: 54 min

Attenuation Geometry: From Spheres to Cylinders

In open space, sound radiates outwards equally in all directions, dispersing its energy across the surface of an expanding sphere. Intensity drops with the square of distance (losing 6 decibels with every doubling of distance).

Inside the SOFAR channel, refraction prevents vertical dissipation. The wave is confined to spread solely in two dimensions as a shallow expanding cylinder. As a consequence, intensity drops only inversely with linear distance (losing just 3 decibels per doubling). Over a distance of 16,000 kilometres, this geometric wave-trapping preserves the signal over fifteen million times more intensely than free spherical radiation.

Blue whales and fin whales have exploited this acoustic physics for millions of years. Their deep infrasonic vocalizations (between 15 and 25 hertz) could travel across entire ocean basins in pre-industrial times, allowing solitary animals to communicate across thousands of kilometres of open ocean.

Methodological Note on Data and Physics

Sound speed profiles are calculated using K. V. Mackenzie's nine-term formulation (1981) from empirical temperature and salinity distributions in the World Ocean Atlas (NOAA / NCEI). Ray paths are obtained through numerical integration of Snell's refraction law in continuously stratified media. Travel times, station depths, and propagation distances are drawn from official records of the Heard Island Feasibility Test (Journal of the Acoustical Society of America, 1994).