Which instrument measures ocean depth by sending sound pulses to the seafloor?

The instrument that measures ocean depth by sending sound pulses to the seafloor is an echo sounder.

An echo sounder sends an acoustic pulse downward and measures how long the returning echo takes to arrive. Because sound travels through seawater at roughly 1.5 kilometers per second, the instrument can estimate depth by multiplying the travel time by the sound speed and dividing by two for the outgoing and returning journey.

Modern echo sounders are specialized sonar systems. Single-beam instruments measure depth beneath one transducer, while multibeam systems send several angled beams to map a wider strip of seabed. Sound speed changes with temperature, salinity, and pressure, so high-precision surveys account for those factors.

Echo sounding transformed hydrography because it is much faster than lowering a weighted line by hand. It is used for nautical charts, navigation, underwater mapping, and fisheries research. A fathometer is a related name derived from the fathom, an older unit of water depth. A seismograph, by contrast, records ground motion from earthquakes rather than routinely measuring seafloor depth.