Approaches to Ambient Noise Characterization at Tidal Energy Sites

Tidal hydrokinetic power generation is an emerging field within renewable energy with the potential to produce predictable, sustainable power from fast-moving tidal currents. The technology is at an early stage and little is known about its potential environmental impacts.  An identified concern is the effect of acoustics emissions (i.e., noise) from operating tidal turbines on aquatic species (principally fish and marine mammals). The effects of tidal energy projects on these species must be placed in the context of existing ambient noise.  Once ambient noise is characterized, these data can be used to determine the relative importance of an additional acoustic source in the area. 

For economic hydrokinetic power extraction in estuaries, strong currents associated with large tidal amplitudes and topographic constrictions are generally required. Such areas, specifically if near a population center, are likely to have high levels of anthropogenic noise from pre-existing activities.  In particular, shipping traffic may cause broadband sound pressure level fluctuations on the order of 30 dB over time scales as short as an hour.  In addition, analysis of ambient noise is complicated by the fact that turbulent pressure fluctuations associated with strong tidal currents, termed psuedosound, are measured as noise although they do not propagate.  Psuedosound significantly increases measured noise at low frequencies (up to approximately 500 Hz depending on the intensity of the tidal currents) and should not be included in a pre-installation ambient noise characterization. At higher frequencies (e.g., above 5 kHz), strong currents may also induce bedload transport, introducing an additional source of propagating noise at frequencies relevant to marine mammal communication. 

Different platforms are available to collect ambient noise data from tidal energy sites.  Data from three measurement platforms are presented for a proposed tidal energy site in Admiralty Inlet, Puget Sound, Washington State.  The first is an autonomous hydrophone deployed near the seabed (depth of 50-60 m) recording on a low duty cycle (i.e., 1%) for three-month deployments.  The second platform is a hydrophone cabled to a drifting research vessel deployed at various depths. The third is a drifting spar buoy equipped with a hydrophone recording sound within 1 m of the surface.  Results from measurements by the three platform types are compared followed by a qualitative assessment of their relative utility for characterizing ambient noise. Approaches to minimize the contribution of pseudosound at energetic locations are presented.