How Is Sonar Used to Determine Seafloor Depth?

Active Sonar

• Active sonar uses a mounted transducer, located on the bottom of a ship, often attached to a ship's keel. The transducer emits a short acoustic pulse, which travels away from the ship until it hits something solid. The transducer listens for the returned pulse, referred to as the echo. A computer measures the time a pulse takes to strike a solid surface and return. The distance from the transducer to the solid surface is measured by the formula, "Range = (speed of sound) x (time between pulse and echo) / 2." Computers measure a series of pulses, measuring the change in distance as the change in depth at various points in the water.
  
  

Low Frequency

• Low frequency pulses range from 50 kilohertz to 100 kilohertz. Scientists use low frequency pulses to measure the seafloor depth in areas where the expected depth is very large. Low frequency pulses travel greater distances in water than high frequency pulses; however, the resulting resolution from these pulses is of a lower quality. Despite the problems with resolution quality, low frequency pulses are effective at measuring seafloor depth in areas where the ocean is very deep.
  
  

High Frequency

• High frequency pulses are as much as one megahertz. High frequency pulses can only travel a short distance, but the resolution quality is very high. Scientists use high frequency sonar to measure seafloor depth in areas where the expected depth is shallow.
  
  

Side-scan Sonar

• Side-scan sonar uses a transducer, mounted under water, on the side of the ship. Scientists angle the transducer slightly downward, providing a wide-angle view of the ocean floor as a ship travels. This technique allows users to gather information about the seafloor construction from larger areas. Side-scan sonar can use either high frequency pulses or low frequency pulses, depending on the expected depth users intend to measure. Side-scan sonar relies on multibeam sonar to determine depth information.
  
  

Multibeam Sonar

• Multibeam sonar uses small mobile transducers, dragged behind a ship, to gather depth information for the side-scan sonar information. Multibeam sonar focuses on the length of time between pulse and echo, establishing the seafloor depth as a ship passes.