Use of a low power, airgun sound source to accurately determine sound. Transmission Loss characteristics at the proposed Robin Rigg. Windfarm site.

Transcription

1 Submitted to: Submitted by: The Scottish Executive On behalf of: Mr M J Swanwick Mr S J Parvin EON-UK Subacoustech Ltd Westwood Way Chase Mill Westwood Business Park Winchester Road Coventry Bishop s Waltham CV4 8LG SO32 1AH Tel: + 44 (0) Tel: +44 (0) Fax: + 44 (0) Fax: +44 (0) matthew.swanwick@eon-uk.com steve.parvin@subacoustech.com website: Use of a low power, airgun sound source to accurately determine sound Transmission Loss characteristics at the proposed Robin Rigg Windfarm site. Document Reference: 663 R 0102 by S. J. Parvin 23/05/05 Approved for release: Dr J R Nedwell

2 Contents Section Page 1 Introduction Objective Test Plan Date Duration Location Measurement Technique Sound Source Number of Signals Environmental impact of a low volume, low pressure, airgun Mitigation for use of a low volume, low pressure airgun source Risk / Benefit Risk Physical Injury Behavioural Response Benefit References...7 Appendix: Definitions and symbols used in this document...8 Subacoustech Ltd -i-

3 1 Introduction. This document has been prepared by Subacoustech Ltd., and is submitted to the Scottish Executive on behalf of E.ON UK Renewables Developments Ltd. It outlines a proposal to use a single low volume, low power, air gun as a sound source to measure the underwater sound Transmission Loss characteristics at the proposed Robin Rigg windfarm site. This will allow more accurate prediction of the range at which the proposed piling operation could have an impact on significant marine species in the region. The measurements are being conducted as part of the mitigation for underwater noise impact prior to the piling operation [1]. Transmission of sound in the underwater environment is highly variable from region to region and as a consequence, considerable errors can occur if local factors that can influence sound propagation are not taken into account. Acoustical models attempt to quantify these variables, but whenever possible the Transmission Loss characteristics for the region should be measured. The ideal approach is to back up initial acoustic modelling with a series of sound Transmission Loss measurements for the region, and then use this data to refine the model. Transmission Loss is measured by undertaking a series of sound and range measurements at incremental range from a consistent sound source, and then determining the sound level decay with range. Where the noise being modelled is impulsive (as is the case for piling) then it is preferable if the sound source used to replicate the noise is a similar, but lower level impulsive source. The impact of underwater sound on marine species will vary depending upon their ability to perceive different frequency components of the noise. As underwater noise propagates, different frequency components decay at different rates depending upon the local environment. For example, the propagation of low frequency sound is inefficient in shallow water since it typically travels in combined modes involving the simultaneous motion of the water and the seabed, in which losses can readily occur. By comparison, high frequency sound may propagate in the same environment with relatively low losses. Consequently, at range a species with good high frequency hearing (typically marine mammals) will perceive the same incident sound at a very much high level than a species with low frequency hearing sensitivity (typically fish). From the sound and range measurements the species dependent (db ht (species)) technique [2] will be used to confirm, or more accurately predict, the behavioural impact range for significant marine species in the Solway Firth region. This metric presents noise and range data in a biologically significant format, and may be used to estimate behavioural response. Where an activity is likely to generate high levels of underwater noise, and may therefore have a behavioural impact over long range, the Transmission Loss characteristics of the local environment can have a considerable influence on the species impact range. In these cases, Subacoustech believe it is best practice to measure the overall and species specific Transmission Loss characteristics and thereby accurately predict the impact range for species of significance. Subacoustech Ltd -1-

4 2 Objective. The purpose of this study is to measure the overall and species specific sound Transmission Loss characteristics at the proposed Robin Rigg windfarm site in the Solway Firth. This will provide data to confirm, or more accurately predict, the behavioural impact range for significant marine species from the underwater noise generated during the piling that is required to construct the wind turbine site. Subacoustech Ltd -2-

5 3 Test Plan 3.1 Date The measurements will be undertaken on a single day in late May or early June depending on boat and personnel availability, and local tide and weather conditions. Date to be advised. 3.2 Duration The measurements will be undertaken over a period of up to 6 hours during hours of daylight. 3.3 Location The sound source will be lowered beneath a vessel, at a fixed location, at a point within the proposed Robin Rigg windfarm site, Solway Firth. The site is bounded by the following points (WGS 84 datum); N, W N, W N, W N, W N, W N, W N, W N, W 3.4 Measurement Technique The measurements will be taken along transect lines which extend radially from the noise source. The measurements will extend to a sufficient range that the noise has fallen below the level of background noise, or to sufficient range that the Transmission Loss can be accurately determined. Suggested transect lines and approximate locations for the measurements will be determined based on local conditions. As underwater noise varies significantly with distance, measurements will be made at regular incremental ranges, typically, 100 m, 200 m, 400 m, 800 m, 1600 m and so on. Measurements will be taken at two depths below the surface to elucidate any variations with depth in the water column. These will be selected based upon the depth of the water in which the activity is being undertaken. 3.5 Sound Source The sound source will be a bolt sleeve air gun, fitted with a low volume (20 cu. inch.) fire chamber. The airgun will be operated at low pressure (50 bar) to reduce the peak pressure and sound spectrum levels produced by the source. Recent tests in a freshwater lake with this particular airgun have indicated that when operated in this mode the airgun produces a peak Source Sound Pressure Level of 222 db re. 1 1 m. The sound level measurements for the airgun are summarised in Table 3-1. Subacoustech Ltd -3-

6 Range from Source (m) Sound Pressure Level (db re. 1 µpa) Predicted Source (1 m) 222 Table 3-1 Peak Sound Pressure Level measurements and predicted Source Level (1 m) for the bolt sleeve airgun operated on low volume (20 cu. inch) and low pressure (50 bar) mode. 3.6 Number of Signals The airgun will be operated for a maximum of 200 discharges. 4 Environmental impact of a low volume, low pressure, airgun. Use of the airgun will introduce a loud anthropogenic (man-made) noise source into the marine environment. Underwater noise at high levels can cause physical injury to marine species and a behavioural response (Species avoiding feeding grounds or being displaced from migration routes or breeding grounds) over long range and large volumes of waterspace. The author is familiar with the levels of Peak Pressure and Impulse that can cause physical injury in marine species, and has prepared documents reviewing this data for the UK MOD [3] and US Navy [4]. The Source Level Peak Pressure of 222 db re. 1 µpa (126, 000 Pa or 126 kpa) is in the region where the injury potential is summarised as unlikely to cause injury. At ranges greater than 1 m from the source the potential for any injury to marine species diminishes rapidly. It is therefore anticipated that use of this sound source will not cause physical injury to marine animals. Airguns typically produce underwater sound energy over a low frequency range from 1 Hz to 500 Hz. Peak hearing sensitivity for marine mammal species is at very much higher frequencies than this (typically above 10 khz), however, what little evidence exists [4] indicates that marine mammals have some hearing capability at low frequency. By comparison, hearing in fish occurs almost exclusively at low frequency and it would be expected that low frequency impulse noise would be audible to fish species over a considerable range. Table 3-1 presents a species dependent analysis [2] of the low volume airgun noise in terms of the range from the airgun source. For ranges to 10 m the data indicates that the airgun noise will produce a strong avoidance response (based on levels exceeding 90 db ht (species)) for both marine mammals and fish. The data for the typical marine mammal (the common seal) indicates that an avoidance response in some individuals is likely to occur to a range of 50 m (based on the range at which the airgun noise falls below 75 db ht (species)). Fish, being low frequency hearers [4], perceive the airgun noise at a much higher level and are likely to have a strong avoidance reaction to the airgun noise at ranges beyond the 40 m range for which the data is presented in Subacoustech Ltd -4-

7 Figure 3-1. By extrapolating this data, it is likely that fish species are likely have a strong avoidance reaction to the low volume airgun at ranges to approximately 100 m, with some individuals avoiding the airgun to ranges of 500 m. Range (m) SPL (db re. 1 µpa) Common Seal db ht (Phoca vitulina) Cod db ht (Gadus morhua) Herring db ht (Clupea harengus) Source Table 3-1 Summary of measured and predicted (Source) perceived loudness levels for typical marine species from the low volume airgun. 4.1 Mitigation for use of a low volume, low pressure airgun source. A soft start procedure will be used during the initial three airgun discharges of each series of measurements. The peak level being built up over a period of one minute. The company possesses and regularly deploys Acoustic Harassment Devices to remove marine mammals from areas where high power acoustic sources are to be used. In view of the comparatively low level sound being generated in this study, it is not proposed that these devices are required. 5 Risk / Benefit 5.1 Risk Physical Injury There is a small risk of physical injury to marine species that approach within 1 m of the airgun during discharge. Considering the small volume of waterspace within which this effect can occur it is considered extremely unlikely that any marine animals will be injured as a consequence of this study Behavioural Response The analysis conducted in section 4 of this document has indicated that the low volume airgun noise is likely to cause a behavioural avoidance to marine mammals over a range of approximately 50 m and in fish to a range of 500 m. The airgun is being used in open water and is therefore unlikely to restrict marine mammal or fish movement to feeding or breeding grounds. The source is being used for a limited period of 6 hours after which it is anticipated that any displaced animals would return to the area. 5.2 Benefit The source level noise from piling at the Robin Rigg site has been predicted [1] to be at a Sound Pressure Level of 262 db re. 1 1 m. The low volume airgun source proposed for these measurements produces Peak Sound Pressure Levels at 222 db re. 1 µpa. As the decibel scale is logarithmic sound energy from the airgun is substantially lower than the sound energy from the proposed piling operation. The data that is obtained, and the small numbers of marine animals effected during this study, will allow more accurate predictions of site specific sound power transmission loss. This will provide confirmation of the transmission loss assumed for the Robin Subacoustech Ltd -5-

8 Rigg site to date and will provide greater confidence in the mitigation measures proposed to be implemented during the Robin Rigg piling operation. It is considered that the benefit of conducting this study in terms of improved protection to marine animals during the Robin Rigg piling operation far outweighs the minimal risk to small numbers of marine animals during this study. Subacoustech Ltd -6-

9 6 References 1. The impact of construction noise from the Robin Rigg Offshore Windfarm Development on the Marine Mammal populations of the Solway Firth. Centre for Marine and Coastal Studies report CMACS/J3018v1.1, April Nedwell, J R and Turnpenny, A. W. H. The use of a generic weighted frequency scale in estimating environmental effect. Proceedings of the Workshop on Seismics and Marine Mammals, 23rd-25th June 1998, London. UK, Parvin S J and Cudahy E A. The effects of underwater blast on divers. QinetiQ report QINETIQ/CHS/PPD/TR010054/1.0, December Cudahy E A and Parvin S J. The effects of underwater blast on divers. Naval Submarine Medical Research Laboratory report 1218, February Nedwell, J. R., Edwards B., Turnpenny A. W. H., Gordon J. Fish and Marine Mammal Audiograms: A summary of available information. Subacoustech Report ref: 534R0214, Subacoustech Ltd -7-

10 Appendix: Definitions and symbols used in this document Units of sound measurement. The fundamental unit of sound pressure is the Newton per square metre, or Pascal. However, in quantifying underwater acoustic phenomena it is convenient to express the sound pressure (either peak or Root Mean Square (RMS)) as a Sound Pressure Level (SPL) through the use of a logarithmic scale. There are three reasons for this: 1. There is a very wide range of sound pressures measured underwater, from around Pascal in quiet sea to say Pascal for an explosive blast. The use of a logarithmic scale compresses the range so that it can be easily described (in this example, from 0 db to 260 db re 1 µpa). 2. Many of the mechanisms affecting sound underwater cause loss of sound at a constant rate when it is expressed on the db scale. 3. The effects of noise tend to increase in proportion to the SPL rather than the linear level. For instance, a given increase in effect will occur each time the sound is doubled, rather than each time it increases by a given unit of pressure. The Sound Pressure Level, or SPL, is defined as P SPL = 20log P ref where P is the sound pressure to be expressed on the scale and P ref is the reference pressure, which for underwater applications is 1 µpa. dbht(species) weighted level The dbht(species) weighted level is the calculated Sound Pressure Level after the pressure time history has been passed through a filter which has a similar sensitivity to the species' auditory function. A level of above 75 dbht may cause behavioural response and above 90 dbht there is the potential for a strong avoidance reaction. More information on this can be found on the Subacoustech website [ Measurement of sound using electronic recording equipment provides an overall linear level of that sound. The level that is obtained depends upon the recording bandwidth and sensitivity of the equipment used. This, however, does not provide an indication of the impact that the sound will have upon a particular fish or marine mammal species. This is of fundamental importance when considering the behavioural impact of underwater sound, as this is associated with the perceived loudness of the sound by the species. Therefore, the same underwater sound will effect marine species in a different manner depending upon the hearing sensitivity of that species. The perceived species sound level (db ht ) incorporates the concept of loudness for a species. The metric incorporates hearing ability by referencing the sound to the species hearing threshold, and hence evaluates the level of sound a species can perceive. Experimental evidence indicates that the scale provides an objective rating of the effects of underwater noise on marine animals Since any given sound will be perceived differently by different species (since they have differing hearing abilities) the species name must be appended when specifying a level. For instance, the same sound might have a level of 70 db ht (Gaddus morhua) for a cod and 110 db ht (Phoca vitulina) for a seal. The perceived noise levels of sources measured in db ht (species) are usually much lower than the unweighted (linear) levels, both because the sound will contain frequency components that the species cannot detect, and also because most marine species have high thresholds of perception to (are relatively insensitive to) sound. If the level of sound is sufficiently high on the db ht (species) scale it is likely that an avoidance reaction will occur. The response from a species will be probabilistic in nature (e.g. at 75 db ht (species) one individual from a species may react, whereas another individual may not), and may also vary depending upon the type of signal. For unusual, man-made noise a response may occur with a level as low as 30 db ht (species). A level of 0 db ht (species) represents a sound that is at the hearing threshold Subacoustech Ltd -8-

11 for that species and is therefore at a level at which sound will start to be heard. At this, and lower perceived sound levels no response occurs as the receptor cannot hear the sound. Currently, on the basis of measurements of fish avoidance of noise it is proposed that levels of 90 db ht (species) and above will cause significant avoidance reaction by most individuals, with nearly 100% avoidance at 100 db ht (species). Milder avoidance reaction occurs in a majority of individuals at levels above 75 db ht (species). The Transect method; Source Level and Transmission Loss. To assess the impact with range from an underwater sound source there are two critical parameters that need to be determined, the Source Level and the Transmission Loss. These factors are defined as: 1. The Source Level (SL); this is a measure of the level of sound generated at the noise source, and may be considered to be the sound level at a nominal 1 metre from the source. 2. The Transmission Loss (TL): this is the rate at which sound from the source is attenuated as it propagates through the water. Both of these factors can be determined by undertaking a series of underwater sound measurements using the Transect method. This method requires many repetitive measurements to be taken at a range of distances from a noise source, allowing the variation of the sound level with distance to be assessed. If a given sound can be represented in terms of these two parameters it allows the sound level at all distances to be specified. Both are specified using a decibel scale, which can for instance be the peak to peak Sound Pressure Level, the Impulse Level, or the db ht level. It should be noted, however, that the values of SL and TL will depend on which unit is being modelled. Usually the decrease in Sound Pressure Level (SPL) with range is due to geometric losses (i.e. the sound mainly reduces as a result of being spread over an increasing area). Under these circumstances the SPL is modelled as: SPL = SL N log R, where SL is the noise Source Level at 1 metre, in db re. 1 µpa, N is the attenuation constant and R is the distance from the source in metres. In shallow coastal waters N is typically between 20 and 25. Where high levels of bottom and surface reflection occur the value of N may be lower than this indicating that sound is able to propagate to longer range before falling below background levels. Subacoustech Ltd -9-