Pile driving of steel monopile foundations for offshore wind turbines is a recent, but very significant addition to anthropogenic noise sources in the ocean. The primary concern with respect to impact of pile driving noise is marine mammals, most notably small cetaceans, including the harbour porpoise (Phocoena phocoena).
A significant potential effect of the pile driving noise is temporary habitat loss due to avoidance of the noise source. Another potential effect of pile driving is direct injury due to the sound pressure, which is sufficiently high to be able to induce temporary or permanent hearing loss.
Mitigation of injury from loud noise can be accomplished by three different methods: reduce the generated noise energy, reduce the radiated noise energy and reduce the received noise energy. Deterrence, by means of powerful seal scarers, belongs to the third type and has been used extensively for pile driving during construction of offshore wind farms in the North Sea and continues to be a key element in mitigation of noise induced injury. The objective of the seal scarer is to deter porpoises out to a safe distance from a pile driving site, before the pile driving itself starts.
In order to provide better estimates of the deterrence distance a so-called controlled exposure study was conducted at Helgenæs, Denmark. Harbour porpoises were tracked from land by means of a theodolite placed at a high observation point, before, during and after emission of replica seal scarer signals at reduced levels from an underwater loudspeaker.
Experiments were conducted over a period of three weeks in June-July 2015. In total, 121 groups of porpoises were observed during nine days with usable weather conditions. In total 14 trials with sound exposure were conducted.
During control periods without playback of sound the porpoises were predominantly observed in a band approximately 500-800 m from the coast, approximately along the 10 m depth contour. During trials with sound on the number of observations was too low to detect any clear patterns in distribution, except for some tracks clearly leading away from the loudspeaker.
Trials were scored as either “response” or “no response” without distinction between strong and weak responses. Two groups of porpoises that were within 300 m of the loudspeaker when the sound was turned on both responded, but beyond 300 m some groups responded and others did not. One group at 600 m responded very strongly, whereas several other groups that were closer at onset didn’t respond.
Reaction distances to the reduced levels of the controlled exposure must be scaled up to be applied to a real seal scarer. Doing so results in a scaled-up deterrence distance of 3,100 m, somewhat higher, but of the same order of magnitude as previous estimates.