Echo sounding
Hydroacoustic assessments have traditionally employed mobile surveys from boats to evaluate fish biomass and spatial distributions.Though a fully operational échosondeur (sonar) was not ready for use in wartime, there were successful trials both off Toulon and in the English Channel as early as 1920, and French patents taken for civilian uses.Oceanographic ships and French high-sea fishing assistance vessels were equipped with Langevin-Florisson and Langevin Marti recording sonars as early as the mid/late 1920s.[5] Distance is measured by multiplying half the time from the signal's outgoing pulse to its return by the speed of sound in water, which is approximately 1.5 kilometres per second.For high accuracy depths, usually restricted to special purpose or scientific surveys, a sensor may be lowered to measure the temperature, pressure and salinity.As well as an aid to navigation (most larger vessels will have at least a simple depth sounder), echo sounding is commonly used for fishing.Therefore, it is especially important when sounding in deep water, as the resulting footprint of the acoustic pulse can be very large once it reaches a distant sea floor.In order to meet these standards, the surveyor must consider not only the vertical and horizontal accuracy of the echo sounder and transducer, but the survey system as a whole.
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