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How does sonar measure the depth of the seafloor?

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World map highlighting major features of the ocean basins, including trenches, ridges, basins, and plateaus labeled throughout the Pacific, Atlantic, Indian, Southern, and Arctic Oceans. Continents are shown with shaded terrain, and prominent undersea features such as the Mid-Atlantic Ridge, Mariana Trench, and Kerguelen Plateau are marked across the map. — ocean basin Major features of the ocean basins.

Sonar measures the depth of the seafloor by utilizing the principles of echo sounding. A transmitter on a ship generates an acoustic pulse—often using a piezoelectric transducer—that travels through the water column to the seafloor, reflects off the bottom, and returns to the vessel as an echo.12

The depth is calculated by multiplying one-half of the total elapsed time between the transmission of the pulse and the reception of its echo by the speed of sound in seawater, which is approximately 1,500 metres per second. Because the speed of sound in water can vary based on temperature, salinity, and pressure, modern systems must account for these factors to maintain accuracy.234

By operating these systems repetitively, researchers can record thousands of soundings per hour to create a continuous profile of the ocean basin, allowing for the mapping of complex underwater features like seamounts and deep-sea trenches.123