The measurement of underwater noise radiated by dredging vessels during aggregate extraction operations

The total marine aggregate extracted from the seabed in UK waters can exceed 20 million tonnes each year, and there is a need to understand the noise generated during the extraction process in order to evaluate any potential impact on the marine environment. For aggregate extraction, the type of vessel used is a trailing suction hopper dredger, which lowers a drag head and suction pipe to the sea floor to extract the sand or gravel, depositing it in a hopper on the vessel, whilst returning unwanted material and water over the side of the vessel. There are a number of potential noise generation mechanisms during this type of dredging activity. This paper presents the results of underwater noise measurements for six different dredgers measured in three locations around the UK, with aggregate type varying from sand to coarse gravel. One vessel was measured in two different areas with different aggregate types. The methodology used to derive the source level for the dredgers is described, and the results of an investigation undertaken into the origin of the radiated noise is given. Measurements were made at frequencies up to 100 kHz, with limited data obtained up to 200 kHz. Noise levels are shown for the same dredger under different operational modes, illustrating that the noise output level is partially dependent upon the mode of operation and the aggregate type being extracted.