D5 Underwater Acoustics

Topic

Underwater acoustics exploits sound propagation within the ocean to measure properties remotely or within volumes of water inaccessible to conventional sensors. The speed at which sound travels through seawater depends on three simultaneous factors—temperature, salinity, and pressure—as described by the Mackenzie formula: this speed increases with temperature, albeit with diminishing intensity as temperatures rise; it also increases with salinity above a reference value of approximately thirty-five Practical Salinity Units; and, finally, it increases with depth as water pressure rises. Furthermore, at depths between seven hundred and twelve hundred meters, there exists a layer where the speed of sound reaches a minimum; it is precisely this minimum that traps the sound within the layer, allowing it to propagate over thousands of kilometers with negligible energy loss.

Ocean acoustic tomography capitalizes on this long-range propagation: it measures the time a sound pulse takes to travel between a transmitter and a receiver and uses this travel time to infer the average water temperature along the sound's entire path. The technique offers a travel-time precision of just ten milliseconds—equivalent to a distance of about fifteen meters—which corresponds to a temperature precision of one-hundredth of a degree for the entire water column traversed; this level of precision enables ocean acoustic tomography to detect temperature changes too subtle for other methods to register at that scale. In contrast, the long-range Acoustic Doppler Current Profiler (ADCP) uses much lower-frequency pulses—ranging from 75 to 300 kilohertz—to profile currents at depths of 300 to 800 meters; it is precisely this lower frequency that gives it a much greater range than a conventional ADCP. Consequently, this instrument is the standard choice for both large-scale global hydrographic cruises and long-term monitoring moorings.

Multibeam sonar, on the other hand, measures neither temperature nor currents but rather the shape of the seafloor itself: it emits a fan-shaped array of acoustic beams downward and captures the echoes returned by the seabed, thereby generating bathymetric maps with resolutions ranging from meters to tens of meters and a swath width of several kilometers per pass. It is the standard method for seafloor mapping and enables the detection of geological structures and hydrothermal plumes; furthermore, by analyzing the intensity of the returning echo, it is possible to classify the type of seafloor that produced it.

Finally, passive listening with hydrophones involves no sound emission whatsoever; the system simply listens to the ocean. This capability is sufficient to detect the presence of marine mammals, earthquakes, and underwater nuclear explosions, allowing a simple, silent hydrophone to monitor the ocean just as closely as active instruments that emit sound. Thus, combining the sound that underwater acoustics systems both emit and receive—such as tomography for measuring temperature, profilers for measuring currents, and sonar for mapping the seabed—with the sound simply listened to without emission, underwater acoustics emerges as the only way to see, hear, and measure an ocean that no other technique can fully penetrate.

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