SEISMIC SURVEY GREENLAND 2014 Underwater sound propagation for North East Greenland offshore seismic survey

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1 TGS February 2014 SEISMIC SURVEY GREENLAND 2014 Underwater sound propagation for North East Greenland offshore seismic survey Appendix: NEG14 modelling results Mark Mikaelsen

2 PROJECT Seismic Survey Greenland 2014 Underwater sound propagation Project No Prepared by MAM Checked by SHKR Approved by ISA Summary This appendix is an addition to the report: Seismic Survey Greenland 2013 for North East Greenland from February Worst-case modelling was undertaken for 2 of the planned seismic survey lines closest to areas of marine mammal importance, in order to determine the worst case scenarios for the 2014 seismic survey. This appendix should not be viewed as standalone modelling for the North East Greenland Seismic survey 2014, but as an addition to the results for modelling undertaken in 2013, which are still considered representative for the 2014 survey. The modelling results are presented by the M-weighted cumulative sound exposure level, SEL C (M PW ) and SEL C (M LF ), in accordance with MLSA guideline requirement for multiple-pulse scenarios [Kyhn et al., 2011]. The method for modelling the SEL C is described in chapter 2, and the results are presented in chapter 3. The results are given as maximum and average distances from source to the species specific thresholds presented in Kyhn et al., (2011). Modelling was performed using NISIM, which uses a ray theory implementation called Bellhop. This implementation is range-, depth- and frequency-dependent, and takes the actual bathymetry, sound speed profiles, source beam pattern and seabed sediment type into account when calculating the sound propagation. All these parameters were included in the modelling. Modelling was performed using available data for all the above mentioned parameters. NIRAS Greenland A/S Reg. No Denmark P: D: Aaboulevarden 80 FRI, FIDIC F: M: Aarhus C, Denmark E: niras@niras.gl E: mam@niras.dk

3 1 Introduction Modelling method Seismic survey area Modelling approach Description of modelling parameter Sound exposure level (SEL) M-weighted sound exposure level (SEL(M PW ), SEL(M LF )) Results Distance to threshold Sound level maps

4 1 INTRODUCTION This appendix documents the additional underwater sound propagation modelling performed in connection with the environmental assessment for TGS proposed seismic activities in the North East Greenland waters One of the requirements for the assessment is to calculate the extent of underwater sound exposure, as described in Kyhn et al., (2011). The newest version of the guidelines, the 3 rd revision of December 2011, states: Each pulse should not exceed the peak pressure criterion discussed above, but in addition, the summed energy of all pulses the animal is exposed to should not exceed the limits suggested by Southall et al. (2007). These limits are: Cetaceans 198 db re 1 µpa 2 s (M-weighted) Pinnipeds 186 db re 1 µpa 2 s (M-weighted) 2 MODELLING METHOD This chapter provides an overview of the approach used to model the SEL C (M PW ) and SEL C (M LF ) towards marine mammal protection zones in the vicinity. 2.1 Seismic survey area The 2014 seismic survey area for North East Greenland is shown in Figure 1 by the red line. From a number of survey lines two lines of interest, especially in relation to the red Closed areas, Jun-Sep, were chosen for modelling by CMACS Ltd. The line numbers on the map refer to the results in chapter 3. Figure 1: Modelled seismic survey lines for North East Greenland 2014 TGS survey. 2

5 2.2 Modelling approach The modelling approach is similar to the one used for the 2013 modelling. This includes the environmental parameters and the reader is therefore referred to the main report for further information. The airgun array used, is the 3680 cu. inch. airgun array, modelled with a source beam pattern to reflect the non-uniform radiation pattern of the array, and thereby give the most accurate results. The software used to perform the modelling, is the NISIM tool, described in the main report. 2.3 Description of modelling parameter The modelling results are presented using the SEL C (M PW ) and SEL C (M LF ) metrics as required according to Kyhn et al., (2011) Sound exposure level (SEL) The SEL, also known as the sound exposure level is defined as the time-integral of the square pressure over a time window T covering the entire pulse duration, and is given by: ( ( ) ) In the case of impulsive sources like airgun array pulses, SEL describes the summation of energy for the entire impulse, and can be expanded to represent the summation of energy from multiple pulses. The latter is written SEL C denoting that it represents the cumulative sound exposure. The sound exposure level is often used in the assessment of marine mammal and fish behaviour over an extended duration of impulsive sources, or for multiple concurrent sources. To accurately model cumulative effects for seismic surveys, it is necessary to use a moving source that approximates the different positions of the airgun arrays throughout a 24 hour modelling period. The lines previously shown in figure 1, each represent 24 hours of seismic survey M-weighted sound exposure level (SEL(M PW ), SEL(M LF )) The M-weighted SEL adapts the SEL modelling to reflect the hearing of a certain species or group of species with similar hearing ability. M-weighting functions can be thought of as the waters counterpart to the A-weighting function which is often used to represent the hearing of humans in air. These weighting functions take into account the nonlinear hearing of the species by a set of correction coefficients at each frequency. Thus, the results represent what the species will actually hear when exposed to a certain noise. The M- 3

6 weighting functions are therefore very useful when determining the behavioural responses of marine mammals to any noise. It was chosen by CMACS Ltd. to use the M-weighted SEL C metric to present the results, for low-frequency cetaceans, SEL C (M LF ), and for pinnipeds in water, SEL C (M PW ). The results will reflect the distances to the thresholds set in Kyhn et al., (2011). 3 RESULTS The NISIM modelling results are presented by maximum-over-depth M-weighted SEL C maps using colour coding to represent the maximum sound level at any depth. Results from the underwater noise modelling, are given in a combination of tables and maximum-over-depth M-weighted SEL C maps. 3.1 Distance to threshold Distances in Table 1 are given in meters from source location. R max indicates the maximum distance at which the sound level can be present in any direction from the source. R mean indicates the average distance from source at which the sound level can be present. If R max for e.g. SEL C (M PW ) = 186 db re. 1 µpa is 250 m, it means that sound levels in excess of 186 db will only occur within 250 m of the source, and that beyond that distance, noise levels will be below 186 db. The distances reflect the entire 24 hour survey line. Cumulative SEL modelling results are given in Table 1. Threshold Distance to threshold Survey line 1 Survey line 2 R max (m) R mean (m) R max (m) R mean (m) M-weighted Sound Exposure Level - SEL(M LF ) [db re. 1 µpa 2 s] 198 db M-weighted Sound Exposure Level - SEL(M PW ) [db re. 1 µpa 2 s] 186 db Table 1: Maximum and average distance from seismic survey line to thresholds by Kyhn. et. al., (2011). 3.2 Sound level maps The sound level maps only show part of the full 24 hour survey line, as the thresholds typically occur within 10 km of the seismic survey line. A map of the full 24 hour survey would therefore not give enough attention to the sound levels of interest, but rather focus too much on lower sound levels. Sound level maps therefore only show part of the survey line, while the threshold distance results given in Table 1, reflect the entire 24 hour survey line. 4

7 Figure 2: Sound level map for survey line 1, showing the maximum-over-depth SEL C(M PW). The threshold sound level is at 186 db, and is shown on the figure by the extents of the dark red color. Figure 3: Sound level map for survey line 1, showing the maximum-over-depth SEL C(M LF). The threshold sound level is at 198 db, and is shown on the figure by the extents of the dark red color. 5

8 Figure 4: Sound level map for survey line 2, showing the maximum-over-depth SEL C(M PW). The threshold sound level is at 186 db, and is shown on the figure by the extents of the dark red color. Figure 5: Sound level map for survey line 1, showing the maximum-over-depth SEL C(M LF). The threshold sound level is at 198 db, and is shown on the figure by the extents of the dark red color. 6

9 References Kyhn, L.A., Boertmann, D., Tougaard, J., Johansen, K., Mosbech, A., (2011): Guidelines to environmental impact assessment of seismic activities in Greenland Waters, 3 rd revised edition, Danish Center for Environment and Energy, Dec