New Technologies Add to Knowledge of the Sea Floor

Deep-sea research got a boost with the invention of sonar in 1914. Developed to detect icebergs at night or in fog, sonar quickly proved to be an excellent depth-finder, much faster than the old sounding line. Sonar is based on the principle that sound travels through water at a rapid and fairly constant rate. A “pinger” mounted underwater on a ship's hull sends out periodic bursts of sound. The sound waves bounce off obstacles and get reflected back to the ship. By measuring how long it takes for the reflected waves to return to the ship, the distance to an obstacle can be calculated.

During World War I (1914–1918), Britain, France, and the United States refined sonar for use against German submarines. By the mid-1920's, most of the world's navies were equipped with sonar, and by the 1930's oceanographers were using it to map the sea floor. With experience, researchers learned that the speed of sound through seawater can be affected by changes in the water's temperature, pressure, and salinity (amount of salt), resulting in erroneous readings, so they worked out methods to correct these distortions. This refinement allowed oceanographers to accurately map portions of the sea floor. But because the ocean bottom is vast, only a small percentage of it could be mapped in this way.

In the 1930's, oceanographers also began adopting a technique called seismic reflection profiling, developed originally for use in oil exploration. In seismic reflection profiling, a large burst of sound is generated at the surface, and the sound waves that reflect from the ocean floor are picked up by an array of floating receivers. Originally, an explosive charge was used to create the sonic blast, but today a powerful air gun is used instead. Because the sound wave it produces is so strong, seismic reflection profiling not only reaches greater depths than sonar, it also penetrates sediments to give a true picture of the underlying bedrock.