To measure ocean water depths before the development of echosounders, a heavy line was lowered to the seafloor. The length of line that it took to reach the bottom was measured and recorded. This was called "sounding". The H.M.S. Challenger is shown in the figure to the right with a line over the side. The Challenger expedition which set sail in 1872 and lasted four years, was considered the first purely oceanographic investigation. During the expedition 492 soundings were made using a heavy line.
Soundings are now made by sending a sound pulse into the water. To use sound to measure water depth we need to know how sound travels through the ocean. The speed of sound depends on the temperature of the water, its salinity, and the pressure (which is equivalent to depth below the sea surface). The speed of sound ranges between 1400 and 1570 m/sec (4593 and 5151 ft/sec). This is roughly 1.5 km/sec (just under 1 mile/sec) or about 4 times faster that sound travel through air. The graph below shows how sound speed varies as a function of water depth.
Since World War II echosounders have been used to determine water depth in the oceans. Echosounders are attached to the hull of a ship or a towed vehicle. An echosounder sends an outgoing sound pulse into the water. The sound energy travels through the water to the ocean bottom where it is reflected back towards the source, received, and recorded.
The time that it takes for sound to make the round trip to the seafloor and back is
accurately measured. Water depth is determined from the travel time and the speed of sound
Water depth can be estimated simply by using an average sound speed and the following relationship:
Distance = speed multiplied by time/2
The time is divided by 2 to account for the two-way trip to the sea floor and back.
If a sound pulse takes 2 seconds to travel to the seafloor and return to the ship, what is the depth to the seafloor? (Use 1500 m/sec as the speed of sound).
Check your answer below to see if you are right.