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     •   What is Hydrographic Surveying?  
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     •   Side Scan Sonar  
     •   Multibeam Echo Sounders  
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     •   Horizontal & Vertical Positioning  
     •   Dangers to Navigation: Notice to Mariners  

Multibeam Echo Sounders

NOAA hydrographic survey units use multibeam echo sounders to acquire water depth information in a survey area; to determine least water depths over critical items such as wrecks, obstructions, and dangers to navigation; and to detect objects in general.

Multibeam echo sounders, like other sonar systems, emit sound waves in the shape of a fan from directly beneath a ship's hull. These systems measure and record the time it takes for the acoustic signal to travel from the transmitter (transducer) to the seafloor (or object) and back to the receiver. In this way, multibeam sonars produce a “swath” of soundings (i.e., depths) for broad coverage of a survey area. The coverage area on the seafloor depends on the depth of the water, typically two to four times the water depth.

Digital Terain Model (DTM) created from multibeam sonar data, draped over nautical chart of Quicks Hole, Mass.

Graphic depicting multibeam sonar achieving full bottom coverage.

Many multibeam systems are capable of recording data on acoustic backscatter ‒ artifacts that may interfere with the accuracy of depth soundings. Multibeam backscatter is intensity data that can be processed to create low-resolution imagery for work products. Backscatter is co-registered with the bathymetry data and is often used to assist with bathymetric data interpretation and post-processing.

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