Abstract
Traditional background noise spectra were measured preceding the construction of a 108 MW wind farm power-generating facility 8 -18 km off the Dutch coast of Egmond aan zee. The analysis of the measurements showed that the ambient noise levels were mainly determined by ship-noise and matched the Wenz reference category “heavy ship traffic”, which was mainly pronounced in the frequency range of 40 to 200 Hz. The higher ambient levels around 100 Hz were attributed to sand dredging operations of three large trailing suction hopper dredgers in the vicinity of the measurement locations. From two of the three vessels the underwater sound signatures were measured at close range. In a time period of 2 to 3 hours dynamic ambient signature changes of 10 dB occurred, which were related to coastal traffic lanes of coasters, 220 kW licensed fishing vessels, tug boats and suppliers. Ambient noise levels in the northern locations could have been affected by pump noise from the gas transport rig Q8-B and its transport network through the area.
By coincidence very high impulsive sound pressure levels of underwater detonations were captured on two of the three missions. Out of 12 cases seven events were recorded and analysed. In all cases the exact firing location could be retrieved with the distance to the source varying between 29 and 35 km. The sound pressure levels at this range were in the range of 155 to 165 dB re 1 μPa Vrms, which exceed the internationally adapted guideline category “maximum exposed zone” for harbour porpoise at a large distance from the source. The results further suggest that mitigation measures for harbour porpoise and seals on these controlled events are urgently needed.
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