A primary tool used to map the seafloor from a ship is sonar, which stands for SOund NAvigation and Ranging. The full name gives you a clue as to how sonar works: Sonar systems send sound waves from the bottom of the ship to the seafloor. These sound waves bounce off the seafloor and back to the ship.
Single beam and multibeam echosounders measure the length of time it takes for sound to travel from the ship to the seafloor and back to the ship as an echo. Scientists use the time measurements to determine how deep the water is (and thus the distance to the seafloor).
As their names imply, single beam sonar collects data over a single point on the seafloor, while multibeam sonar collects data over a fan-shaped “swath” of many points.
By putting the individual sound returns, or pings, together, you can build a picture of the seafloor, creating a bathymetric map that displays the depths and shapes of the underwater terrain.
A survey ship uses multibeam sonar to measure the depth of the seafloor. These depth measurements are then used to created rainbow-colored maps. In general, red is used to show shallow depths and blue or purple for deeper depths.