How Do Scientists Measure the Depth of the Ocean?
Historically, before scientists developed new techniques, the depth of the ocean was actually measured by sending a rope with marks on it from a ship to the bottom of the ocean and then counting how many marks had gone into the water.
Now day’s scientists measure the oceans’ depths with special instruments called echo sounders. An echo sounder is mounted on a ship. Sound waves are sent to the ocean bottom and the echo sounder measures the time it takes for the echo of the sound waves to return to the surface.
These soundings are made continuously and are recorded on a moving strip of paper that shows the varying depths of the ocean beneath the ship. So far as anyone knows, the deepest spot is in the Pacific Ocean, near the Mariana Islands. Here the water is nearly seven miles (11 kilometers) deep.
On a ship, apart from using the SONAR, the depth is also measured whenever an instrument called CTD (conductivity, temperature, depth) is sent straight down to the ocean floor. This instrument measures temperature and salinity and can also collect water samples while it’s coming back to the surface using big bottles.
Another way to measure depth is to use satellite imagery. This has been used to calculate the mean depth (and also the volume) of the entire ocean. The satellites that orbit in space can be used for this. First, the scientists have measured the height of the satellite above the Earth’s surface (assuming that it’s perfectly flat, even if it is not).
Then, the satellite measures the height above the ocean surface (also in this case by sending out a pulse and measuring how long it takes for it to come back). Finally, the difference between these two gives you the height of the ocean surface. These calculations are adjusted based on observations and measurements of tides and waves.