A variety of techniques are used for dredging. Mechanical dredging involves the use of a crane-operated grab or bucket to remove seabed materials. Hydraulic (suction) dredging works by sucking a mixture of dredged material and water from the seabed through a suction pipe that may or may not have a tool at its end (“drag head”) to disturb and/or break up bottom materials.Underwater measurements of noise generated by dredging vessels are limited. The main processes that contribute to noise associated with dredging are collection noise, pump noise, transport noise (material being lifted from the seafloor to the dredger), deposition noise (placing material within a barge or hopper), and ship/machinery noise from the dredging vessel itself. In general, dredging produces continuous, broadband sound concentrated at or below 1 kHz. Sound pressure levels can vary widely depending on dredger type, operational stage, or environmental conditions. Noise levels also depend on the materials being extracted, with harder sediment extraction generating higher noise levels than loose or soft sediment extraction. Estimated source levels for dredging range between 168 and 186 underwater dB at 1 m.
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- US Army Corps of Engineers, "About Dredging." (Link)
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