Multibeam echosounder

Echosounders measure the distance to the seafloor during bathymetric surveys. They record the two-way-travel-time (TWT) of acoustic signals from the transducer to the seabed and back to the receiver. This TWT combined with the speed of sound in water allows to measure the distance from the echosounder to the seabed. An individual acoustic pulse is called a ping and the resulting depth measurement is called a sounding. 

Two main types of echosounders exist: 

1) Singlebeam echosounders (SBES) that record individual depths directly below the instrument sending out one individual ping 

2) Multibeam echosounders (MBES) are more sophisticated and can simultaneously record depths at angles off-nadir, producing a line of soundings per ping, also known as a swath. This swath is recorded continuously as the ship moves, resulting in a swath of mapped seafloor. The width of this swath and the spacing of the soundings is a function of the water depth. Swath width is usually expressed in multiples of water depths. Under perfect conditions, state-of-the-art MBESs can map swaths as wide as six times the water depth. In addition to depth information, MBES can also collect backscatter, which contains information on seabed sediment composition (e.g., soft sediment, rock, etc.). 

Polarstern has a hull-mounted Teledyne Atlas HYDROSWEEP DS III (HSDS3) full ocean-depth MBES permanently installed. It operates with frequencies between 13kHz and 17kHz and can survey depths of up to 11000m. With each measurement, it produces up to 960 soundings and achieves a depth accuracy of approximately 0.2% of the water depth.