EIMR 2012 poster - Swansea University

The Impact of Sediment and
Cobble Motion on Underwater
Ambient Noise in Ramsey
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.
HW + 5h, 1.4
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.
HW + 6h, 1.8
HW + 7h, 2 m.s-1
HW + 8h, 1.6 m.s-1
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.
1. Introduction
• These high frequency, high noise levels were not recorded for each entire drift –they were not apparent towards
the end of each drift survey.
The acoustic interaction of a Tidal Stream Turbine (TST) on marine life might be masked
by the background noise characteristics of the site itself.
• 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)).
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.
• Each sound file was listened to and compared with existing data from sediment and cobble surveys.
Background noise levels in sites subject to high currents are site-specific and tidallydependent [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.
•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.
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.
• 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.
2. Measurement and post processing
• 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.
• 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.
Courtesy of Tidal Energy
Figure 3: Google map showing typical recording tracks
high current speeds
Figure 2: Variation of centroid frequency with particle
diameter., Thorne (1986)
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
[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.
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.
[4]. P.D. Thorne, Laboratory and marine measurements on the acoustic detection of sediment transport, J. Acoust. Soc. Am (3), September 1986
Merin Broudic
[email protected]
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