Abstract
Chickerell BioAcoustics was contracted by the European Marine Energy Centre (EMEC) to carry out an acoustic characterisation of the underwater ambient noise field at their Scapa Flow nursery wave energy test site. The project was funded by the Scottish Government.
The scope of the project was to assess the methodology and equipment specified for measuring acoustic noise at EMEC’s main grid-connected wave test site and advise on its suitability for use at the nursery wave test site, and to train EMEC staff in the use of the recommended methodology and equipment for collecting acoustic data at the site. The project called for three surveys to be carried out using the selected methodology and equipment. Data collected from the surveys were analysed and used to characterise the ambient acoustic baseline for the site. The results of these analyses will be made available to developers testing wave energy converter devices at the Scapa Flow test site. This will provide a better understanding of ambient noise at the site, to assist with the acoustic characterisation of installed devices. The characterisation of noise from specific devices operating at the test site was out-with the scope of this project.
The work involved carrying out a number of surveys through autumn and winter 2011-2012, with each survey covering a range of tidal and weather conditions. Acoustic data were gathered using a self-contained hydrophone and recorder package deployed on the seabed. One unit was available for use in the surveys. Three surveys were carried out in September and December 2011, and March 2012.
The collected data were analysed to establish the acoustic levels under quiet conditions and this shows that the background noise levels were in line with that which could be expected for this type of shallow water site. Contributions over and above these conditions were then identified, with the major contribution being the natural sounds from wind/waves and precipitation. The major anthropogenic source was shipping noise from distant static and mobile sources. Local shipping traffic also contributed to the sound field, although this was only present for around 7% of the time. Other sounds identified included a thunderstorm, aircraft and various biological sources. The identification of marine mammal vocalisations was out-with the scope of this work; however it can be noted that although seals and harbour porpoise were present in the area at the times of the surveys, no calls from either species were identified on any recordings.
The characterisation was limited by the three survey periods not covering a full year and a full characterisation will need at least two more survey periods. Also, the close proximity of the hydrophone to the recorder canister seems to affect the spatial response of the hydrophone, introducing errors of up to 2.5 dB at some frequencies. It is therefore recommended that the mounting arrangement for the hydrophone be improved prior to undertaking any further surveys. The use of additional mobile hydrophone systems together with concentrated study periods using land-based visual observations would assist the analysis of data obtained from future surveys.