Contact: Merin Broudic m.broudic@swansea.ac.uk 01792602326

1. The Impact of Sediment and Cobble Motion on Underwater Ambient Noise in Ramsey Sound Authors: Merin Broudic1, Miles Willis1, Nick Croft1, Sei-Him Cheong1, Ian Masters1 1 Marine Energy Research Group, Swansea University Keywords: underwater acoustics, tidal stream energy, background noise, sediment and cobble motion noise. Sediment/cobble noise • Summary • A series of acoustic drift surveys have been conducted in Ramsey Sound, Pembrokeshire to determine whether or not the frequency response could be used to discern areas of the seabed that were covered in sediment and cobbles. • Sediment/cobble noise frequency in Ramsey Sound ranged between 5kHz and about 20 kHz; over a range of tidal conditions. • The frequency range of sediment/cobble motion increases with increasing water speed. • Sediment/cobble motion noise emission reaches up to 120dBre1mParms @ 10/15m from the source and at 2ms-1 tidal flow. • According to Thorne’s correlation between particle size and frequency response, particle diameters transported in Ramsey Sound channel is around 5.10-2m (Thorne, 1986). • These ambient noise levels associated with particle movement have the potential to mask the noise associated with marine energy devices under maximum operation levels. • Using this acoustic method, areas of seabed were identified that contained sediment and cobbles. • Underwater video surveys of the seabed confirmed that this acoustic method was capable of identifying areas of seabed particles. • 1. Introduction • The acoustic interaction of a Tidal Stream Turbine (TST) on marine life might be masked by the background noise characteristics of the site itself. • For this reason the assessment of the acoustic disturbance of a tidal stream turbine on marine life must start with an investigation of underwater background noise. • Background noise levels in sites subject to high currents are site-specific and tidally-dependent [1]. Turbulence, surf, breaking waves, current flow, sediment motion and biological ambient noise all contribute to the overall background noise and are linked to the tide and current flow. • This poster presents the results of an investigation into the noise associated with sediment/cobble motion in Ramsey Sound, Pembrokeshire, West Wales at various states of the tide and locations within the Sound. • 2. Measurement and post processing • A hydrophone was deployed at a depth of 15m from a rigid inflatable boat. To avoid flow noise and cable strum [2], the hydrophone was released and allowed to drift with the current. A total of 20 drifts were performed throughout a tidal cycle. • 480,000 acoustic samples were taken per second. These were processed using a 65536 points Fast Fourier Transform (FFT). Resulting frequency distributions were smoothed according to Hann [3] with a 50% FFT overlap. The sound level values are displayed in root mean Square (RMS) terms. Figure 1: Spectrogram of Ramsey Sound background noise (2-22kHz) at high current speeds • 3. Spatial variation of sediment/cobble motion noise • Figure 1 highlights the background noise response at various times throughout the ebb tidal cycle. • The high frequency response seen in the top right of figures 1 (i-iv) varied between 80 and 105 dBre 1mParms2/Hz. • These high frequency, high noise levels were not recorded for each entire drift –they were not apparent towards the end of each drift survey. • Spectrograms were then filtered to reduce low frequency levels (<5kHz) and to filter noise associated with boat traffic in the area (refer to spikes in Figures 1(ii) and (iv)). • Each sound file was listened to and compared with existing data from sediment and cobble surveys. • From figure 2, according to Thorne’s theory (ref), the maximal diameter of transported sediment/cobbles in Ramsey Sound is around 5.10-2m. • The frequency band impacted by sediment/cobbles motion noise increase with water speed increasing . Thorne [4] shows that the frequency noise increases with decrease particle diameter. The diameter of particles transported in Ramsey Sound increase with increasing water speed. • Seabed sampling using an underwater camera attached to an ROV mapped those areas of seabed in the area that contained sediment/cobbles. Refer figure 3. It can be seen from the figure that the seabed was made up of cobbles, sand and shell fragments. • Comparison of the spectrograms in figure 1 with the particle map in figure 3 reveals that the high frequency response was recorded in areas associated with these sediment/cobble areas. Sand/cobble location Start ii i Figure 3: Google map showing typical recording tracks at high current speeds • 4.Temporal Variation of sand motion noise • The spectrograms from each drift survey were filtered once more so that just the noise associated with sediment/cobble movement were analysed in dBre1mParms. • The variation of specific frequencies (10 and 20kHz) throughout a tidal cycle were analysed in 1/3rd octaves • Figure 4 displays the temporal variation of 10 and 20kHz 1/3rd octave band levels. Included in the figure is the drift speed of the boat which is an indicator of the tidal current. • Particle noise motion is strongly affected by the current. As the tidal flow varies from 3.6 to 0.4 m.s-1 the 20kHz 1/3rd Octave drops from 112 down to 90 dBre1mParms. The 10kHz response also follow a similar trend. • As the tide ebbs and flows the sediment/cobble noise levels increase as the tidal flow increases. • Traditional sonar surveys in high tidal streams are limited to short periods of slack water due to the presence of bubbles/turbulence as the tidal flow increases. • This work has shown that it is possible to detect sediment/cobble locations in high tidal stream areas through a relatively inexpensive acoustic survey using hydrophones. • It is the intention that these background noise libraries are used for comparison purposes once tidal stream turbines are deployed in the area. Figure 4: Variation of the as a function of time Acknowledgement This work was undertaken as part of the Low Carbon Research Institute Marine Consortium (www.lcrimarine.org). The Authors wish to acknowledge the financial support of the Welsh Assembly Government, the Higher Education Funding Council for Wales, the Welsh European Funding Office, and the European Regional Development Fund Convergence Programme. References: [1]. M.R. Willis, M. Broudic, C. Haywood, I. Masters, S. Thomas, Measuring Underwater Background Noise in High Tidal Flow Environments, Journal of Renewable Energy (2012), doi:10.1016/j.renene.2012.01.020. [2]. Richardson WJ, Green Jnr C, Malme CI, Thomson DH. Marine mammals and noise. Virginia Press; 1995. 87e155. [3]. R. B. Blackman , J. Tukey. Particular Pairs of Windows, The Measurement of Power Spectra, From the Point of View of Communications Engineering", New York: Dover, 1959, pp. 98-99. [4]. P.D. Thorne, Laboratory and marine measurements on the acoustic detection of sediment transport, J. Acoust. Soc. Am (3), September 1986 0001-4966/86/090899. End Contact: Merin Broudic m.broudic@swansea.ac.uk 01792602326 iii iv HW + 5h, 1.4 m.s-1 HW + 6h, 1.8 m.s-1 HW + 7h, 2 m.s-1 HW + 8h, 1.6 m.s-1 Figure 2: Variation of centroid frequency with particle diameter., Thorne (1986) Courtesy of Tidal Energy Limited