E11 Oceanic Acoustics and Optics

Topic

Ocean acoustics studies how sound propagates through the ocean—at a speed determined by water temperature, salinity, and pressure, typically ranging from 1,450 to 1,550 meters per second. It is precisely this dependence of sound speed on water properties that allows the sound itself, as it travels through the ocean, to reveal information about the temperature and salinity of the waters it traverses.

At depths between 600 and 1,200 meters, there is a layer where sound speed reaches its minimum value; this minimum speed causes refraction to trap the sound within the layer, creating a deep sound channel that allows sound waves to propagate over thousands of kilometers with minimal dissipation. This long-range propagation makes the channel useful for remote ocean monitoring, tracking marine mammals, and the acoustic tomography described below.

Ocean acoustic tomography utilizes this deep sound channel to measure the travel time of sound pulses between pairs of transmitters and receivers; this travel time allows for the inference of the average water temperature along the entire sound path, with a sensitivity of a ten-millisecond delay for every one-degree Celsius change in that average temperature. Indeed, this technique is the only method capable of measuring the average temperature of large ocean volumes with updates as frequent as every few hours.

Ocean current velocity, meanwhile, is measured acoustically using profilers that exploit the Doppler effect: these instruments emit an acoustic signal that is scattered back toward the instrument by particles suspended in the water, and the Doppler shift of this backscattered signal allows for the calculation of current velocity at various depths simultaneously along a single vertical profile. Ocean optics—paralleling the acoustics just described—studies how sunlight propagates within the ocean; this propagation determines the amount of energy available for photosynthesis in the sunlit zone of the sea. Light attenuates exponentially with depth according to the Beer-Lambert law, and the rate of this attenuation is determined by the concentration of chlorophyll, dissolved organic matter, and particles in the water. It is precisely this optical composition of the water that allows the ocean's color—when observed from a satellite—to be used to estimate chlorophyll concentration and, consequently, the primary productivity of the entire sea.

Thus, from the speed of sound mentioned at the beginning of this document to the ocean color discussed at its conclusion—and encompassing the deep sound channel, acoustic tomography, and Doppler current measurements—ocean acoustics and optics enable the ocean itself to reveal its temperature, currents, and biological productivity through the way it transmits the sound and light passing through it.

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