Echo Sounder is a type of SONAR (originally an acronym for SOund Navigation And Ranging) used to determine the depth of water by transmitting sound pulses into water. The time interval between emission and return of a pulse is recorded, which is used to determine the depth of water along with the speed of sound in water at the time.
After the disaster of the Titanic in 1912, the German physicist Alexander Behm conducted some research to find a way to detect icebergs. He discovered the technique of echo sounding which turned out to be inefficient in spotting icebergs, but a great tool to measure the depth of the sea. Behm had his invention patented in 1913. While the first serious attempts to quantify fish biomass were conducted in the 1960’s, major advances in equipment and techniques took place at hydropower dams in the 1980’s. Some evaluations monitored fish passage 24 hours a day for over a year, producing estimates of fish entrainment rates, fish sizes, and spatial and temporal distributions.
Application of echo-sounding principles to submarine detection during World War II resulted in the development of equipment to sound all ocean depths.
In the 1970’s, the dual-beam technique was invented, permitting direct estimation of fish size in-situ via its target strength. The first portable split-beam hydroacoustic system was developed by HTI (Hydroacoustic Technology, Incorporated) in 1991, and provided more accurate and less variable estimates of fish target strength than the dual-beam method. It also permitted tracking of fish in 3D, giving each fish’s swimming path and absolute direction of movement. This feature proved important for evaluations of entrained fish in water diversions as well as for studies of migratory fish in rivers. In the last 35 years, tens of thousands of mobile and fixed-location hydro acoustic evaluations have been conducted worldwide.
What is principle of Echo Sounder operation?
- Echo sounder measures the depth of water by measuring the time for a pulse of energy to travel to the sea bed and back and work on the principle of reflection of acoustic energy.
- Short pulse of sound energy is transmitted vertically down from the ship.
- This pulse having been reflected from the sea bottom returns to the ship in the form of an echo.
- Travel time (t) taken for its return, depends upon the depth of water (d), and on the velocity of sound (v) through the water.
- Time taken (t ) = (2 x d) / v or d =(v x t)/2
- Performance of a sonar system depends on the accuracy with which the velocity of source propagation is measured.
The velocity of propagation is a function of temperature of water, pressure and salinity. The velocity is usually taken as 1500 meters/second for sea water at 13°C and atmospheric pressure.