Abstract
A key step towards streamlined permitting of marine renewable energy (MRE) activities is a better understanding of potential environmental effects. In particular, sound generated by marine energy installations in the ocean environment remains a particular concern despite the limited evidence showing low levels of MRE sounds relative to other anthropogenic sounds. In an effort to increase understanding of potential environmental effects of marine energy projects and help reduce barriers to marine energy deployments, a new acoustic monitoring technology, the NoiseSpotter®, was developed and recently demonstrated around CalWave’s operational scaled xWaveTM wave energy converter (WEC). The technology improves upon traditional acoustic sensing techniques by use of a cost-effective, compact array of acoustic particle velocity sensors that characterizes, classifies, and provides accurate location information for anthropogenic and natural sounds in real time. The NoiseSpotter®’s ability to make direct measurements of acoustic particle velocity also helps address growing concerns on the effects of the kinetic elements of sound on fishes and invertebrates.
We present results from co-deployments of NoiseSpotter® with the operational CalWave WEC that were conducted over a 9-day period in fall 2021 offshore of Scripps Research Pier in San Diego, California. The NoiseSpotter® was deployed over durations ranging from hours to days, at distances of 100 m and 200 m from the CalWave WEC, and at the four cardinal directions from the WEC. Acoustic particle velocity and pressure measurements were obtained at 35 cm, 50 cm, and 75 cm above the seabed, in the frequency band 50 Hz to 3 kHz. Each vector measurement of particle velocity inherently provides directional information (acoustic bearing) regarding the source of sound, allowing the measurements to spatially discriminate sounds emitted by the WEC from other ambient sounds.
Results from a multi-day deployment of the NoiseSpotter® at 100 m from the CalWave WEC revealed a rich library of sounds that include:
- Low-level (~95 dB re 1 µPa relative to an ambient noise floor between 80-90 dB re 1 µPa) sounds from the WEC associated with the deliberate actuation of mechanical components,
- Sounds from a hovering helicopter,
- Marine mammal vocalizations, and
- Small boat engines.
Sound levels from the WEC were placed within the context of ambient sounds, and reveal little deviation from the ambient soundscape. The azimuthal anisotropy of WEC sound was investigated via deployments along four cardinal directions around the WEC. While a noticeable increase was observed along the north-south orientation, the sound levels along all directions still showed little deviation relative to the ambient noise floor. Analysis of low-level WEC sounds demonstrate the utility of a directional acoustic sensing technology like the NoiseSpotter® in distinguishing marine energy sounds from the myriad other sounds in the surrounding ocean environment.