Centre for Marine Science and Technology

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1 Centre for Marine Science and Technology Prediction of underwater noise associated with the operation of a drilling rig in the Great Australian Bight Prepared for: BP Developments Australia Pty Ltd Prepared by: Iain Parnum, Daniel Wilkes, and Alec Duncan PROJECT CMST 1103 REPORT nd May 2013

2 2 Executive Summary BP Developments Australia Pty Ltd is planning to operate a drilling rig in the Great Australian Bight, and have contracted the Centre for Marine Science and Technology (CMST) to quantitatively model the likely underwater sound levels caused by the drilling rig up to 300 km from the source. This report considers underwater noise produced while the drilling rig is holding station using its eight azimuth thrusters, to represent the worst-case noise produced by this facility. The source spectrum was modelled using previously recorded measurements for a similar noise source. The source level of the drilling rig was estimated to be db re 1 Pa 1 m (over a frequency range of 10 Hz to 2 khz). The result of the source modelling was combined with the results of an acoustic propagation model to predict the received levels as a function of depth and range due to a drilling rig at several representative locations. Predicted received levels at long range were higher in the winter than in the summer due to the presence of a surface duct during the winter months. The fact that the surface duct dominated propagation also meant that, in winter, the results were similar for all three source locations and there was little dependence on direction. For all three points, the SPL during winter dropped below 160 db re 1 μpa (RMS) within 100 m, and below 120 db re 1 μpa (RMS) between 10 km and 40 km from the location of the drilling rig. To the east, as in other directions, the predicted SPL was below 106 db re 1 μpa (RMS) at the maximum modelled range of 300 km and would be expected to continue to decrease with further increases in range. With the rig at the most northern position, the noise level was predicted to be less than 115 db re 1 μpa (RMS) at the 200 m depth contour and below 106 db re 1 μpa (RMS) at the most northerly extremity of the Great Australian Bight. These levels are well within the range of measured ambient noise levels at these two locations. Received levels calculated using the modelled source locations should be representative of those that would be produced by the drilling rig at other locations with similar water depths during the winter, and are typical of the levels that would be expected in calm weather. Received levels would be expected to reduce much quicker with range in other seasons in which the surface duct is not present, or in periods of rough weather during

3 3 winter when scattering of sound from the rough sea surface will markedly increase the rate of attenuation of sound within the duct. Modelling of a vertical seismic profiling (VSP) source in a worst-case orientation at the most northerly source location predicted that received sound exposure levels (SELs) would drop below 120 db re 1μPa 2.s at the shelf edge, after which the sound would attenuate relatively slowly as it propagated across the broad, flat shelf, with levels dropping below 110 db re 1μPa 2.s just off the coast. The shallower source depth and lower frequency spectral peak of the VSP source resulted in the surface duct having a much smaller effect on the received levels from this source than it did in the case of the azimuth thrusters.

4 4 Table of Contents 1 Introduction Methods Source modelling: Azimuth thrusters Source modelling: vertical seismic profiling (VSP) array Propagation modelling Bathymetry Seabed composition Water column sound speed profile Received level calculation Results Azimuth thruster noise: effect of season Azimuth thrusters: spatial variation of noise levels VSP seismic source Conclusions Appendix A Appendix B...36

5 List of Figures Figure 1. Geographic location of modelled source locations...7 Figure 2. Noise source spectra...9 Figure 3. Top and end views of the modelled 1200 in 3 VSP source with elements centred at a depth of 10m Figure 5. Far field array signals and spectra in the vertically downward direction Figure 6. Horizontal plane azimuth-dependent spectral level for model source, excluding surface reflection...13 Figure 7. Selected vertical plane elevation-dependent spectral level for model source, excluding surface reflection...14 Figure 8. Sound velocity profiles (SVPs) used for modelling Figure 9. Maximum SPL at any depth versus range for P1 at an azimuth of Figure 10. Received SPL as a function of range (r Tx ) and depth (z) from P1 on an azimuth of Figure 11. Maximum SPL at any depth versus range for P1 at an azimuth of Figure 12. Received SPL (db re 1 μpa RMS) as a function of range (r Tx ) and depth (z) from P1 on an azimuth of Figure 13. Maximum SPL db re 1 μpa RMS (at any depth) for P1 over bathymetric contours for the winter season...23 Figure 14. Maximum SPL db re 1 μpa RMS (at any depth) for P2 over bathymetric contours for the winter season...24 Figure 15. Maximum SPL db re 1 μpa RMS (at any depth) for P3 over bathymetric contours for the winter season...25 Figure 16. Maximum SPL db re 1 μpa RMS (at any depth) versus range for P1 (blue dots), P2 (green dots) and P3 (red dots) for the winter season Figure 17. Normalised histograms of measured ambient noise levels...27 Figure 18. Maximum SEL db re 1 Pa 2 s (at any depth) over bathymetric contours for the VSP seismic source at P Figure 19. Maximum SEL (db re 1 Pa 2 s) at any depth versus range for the VSP seismic source at P1 along specified azimuths...30 Figure 20. SEL in a vertical plane in the cross-line direction...31 Figure 21. SEL in a vertical plane for an azimuth of Figure 22: Diesel noise as a Function of Engine Power and Speed (taken from: Ross, 1987) List of Tables Table 1. Drilling rig locations used in noise modelling....6 Table 2. Characteristics of the Pacific Ariki and the resulting estimates of the source level from the drilling rig facility...10 Table 3. Seabed composition used in propagation modelling in water less than 150 m Table 4. Seabed composition used in propagation modelling in water greater than 150 m Table 5. Ambient noise measurement locations and statistics and predicted noise at these locations from a drilling rig at P1...27

6 6 1 Introduction BP Developments Australia Pty Ltd, henceforth referred to as the Client, is planning to operate a drilling rig in the Great Australian Bight. The Client has contracted the Centre for Marine Science and Technology (CMST) to quantitatively model the likely underwater sound levels caused by the drilling rig up to 300 km from the source. This report presents the results of this modelling at three sites in the Great Australian Bight (GAB). The locations of these sites (P1-P3) used in the modelling are reported in Table 1 and shown geographically in Figure 1. Sites P1-P3 are representative of the general extent of the locations and water depths in which the drilling rig will be located. This report considers underwater noise produced while the drilling rig is holding station using its eight azimuth thrusters, to represent the worst-case noise produced by this facility during normal operations. The source spectrum was modelled using previously recorded measurements for a similar noise source. Vertical seismic profiling (VSP) is also likely to take place occasionally at the rig site. This type of source was also modelled, but only for the most northerly site, which was considered worst-case in terms of potential environmental impacts. The results of the source modelling were then combined with the results of an acoustic propagation model to predict the received levels as a function of depth and range, up to 300 km from the source locations. Table 1. Drilling rig locations used in noise modelling. Modelling Location Latitude (South) Longitude (East) Depth (m) P ' " ' " 1010 P ' " ' " 1541 P ' " ' " 3238

7 Figure 1. Geographic location of modelled source locations: top panel shows the location of the bottom panel as a white rectangle; bottom panel shows a close up of the three modelled source locations (P1-P3). The locations of the ambient noise loggers (BP-SL-01, BP-SL-02) are also indicated in the figure. 7

8 2 Methods Received noise levels were calculated as a function of range, depth and azimuth from each source location (P1-P3) for the drilling rig. Additional modelling was carried out to predict the received sound exposure levels due to a VSP source at location P1. In both cases this required three main tasks: 1. Source modelling 2. Propagation modelling 3. Receive level calculations Details of these steps are given in this section. In summary, the first step determines the source spectrum for each type of source; then a propagation model is used to predict what happens to sound energy transmitted into the environment (i.e. how much sound energy is lost); finally the results of the first two steps are combined to calculate the received level. The method of source modelling for the azimuth thrusters is presented in Section 2.1 and that for the VSP seismic source is presented in Section 2.2. The remaining sections describing the modelling methods are identical for both types of source with the exception of the received level calculations described in Section 2.7. The method of calculating the received levels by combining the source level and transmission loss calculations is essentially the same for both types of source but the received levels for the continuous thruster sources are specified as sound pressure level (db re 1 μpa root mean square) while those for the impulsive VSP source is given as sound exposure level (db re 1 Pa 2 s). Because the units are different, these two sets of results are not directly comparable. 2.1 Source modelling: Azimuth thrusters The highest underwater noise levels produced during the operation of this facility are expected to occur during the simultaneous operation of its eight azimuth thrusters as they hold the rig on station. In the absence of any measurements of underwater noise from the actual rig planned to be used by the Client, acoustic source spectra were based on generic source spectra derived by CMST from measurements made of underwater noise from azimuth thrusters of a known power. The source model used in this report is based on measurements made by CMST of underwater sound levels produced by a rig tender (Pacific Ariki) while using its propulsion system to remain stationary near an offshore platform (McCauley 1998). 8

9 9 The characteristics of the Pacific Ariki are given in Table 2, and the measured, one-third octave source spectrum is shown in Figure 2. This Pacific Ariki spectrum is based on measurements made in a single direction relative to the vessel, so no source directionality data are available. However, given the nature of cavitation noise, and the fact that the thrusters are located at different positions on the vessel, and in many cases can be rotated in azimuth, it is reasonable to assume that it is omni-directional. The noise levels produced by the Pacific Ariki have been extrapolated to those to be expected for the thruster power in operation on the proposed drilling rig. This assumed that a constant proportion of the mechanical power is converted to acoustic power. This relationship has been found to hold reasonably well for surface vessels operating at their normal cruising speed (Ross, 1987) 1. The resulting source spectra are also shown in Figure 2, and peak in the frequency range 100 to 400 Hz. The corresponding broadband source level over 10 Hz to 2 khz was calculated to be db re 1 Pa 1 m for the drilling rig facility, db re 1 Pa 2 1m Pacific Ariki Drilling rig Frequency (Hz) Figure 2. Noise source spectra: measured for Pacific Ariki (blue line) using its propulsion system to remain stationary near an offshore platform, which was used to predict the source spectrum for the drilling rig facility thrusters (red line). 1 An extract from Ross (1987) is given in Appendix A.

10 Table 2. Characteristics of the Pacific Ariki and the resulting estimates of the source level from the drilling rig facility. Vessel Pacific Ariki Drilling rig facility Installed thruster 4.8 MW 8 x 4.2 MW power (tunnel thrusters plus 2 of 4 (all thrusters in use) Installed thruster power relative to Pacific Ariki main engines) 1 7 Source level correction 0 db db Equivalent broadband source level (10 Hz to 2kHz) db re 1 Pa 1 m db re 1 Pa 1 m Assumed source depth 6 m 28 m Source modelling: vertical seismic profiling (VSP) array The Schlumberger Magnum 1200 in 3 array was identified by the client as being representative of the largest VSP array likely to be used at the site. Information about the array was extracted from a Schlumberger brochure containing some details of the array (air gun volumes, far field signal and spectral density profiles). Detailed gun position information was not given, so the array configuration was inferred from images in the brochure and is indicated in Figure 3 for the proposed maximum deployment depth of 10m. Based on the configuration data, model acoustic fields for the array elements and for the array, have been synthesised using CMST s coupled-element array model. The CMST model output was calibrated, and its performance checked, by comparing the predicted far field array signal in the vertically downward direction with the example waveform given in the Schlumberger brochure. This was done by scaling the model signal so that the integrated squared pressures of the two waveforms matched. The results of this process are plotted in Figure 4.

11 Figure 3. Plan (upper plot) and end elevation (lower plot) views of the modelled 1200 in 3 VSP source with elements centred at a depth of 10m. The cylinder sizes are much bigger than the actual airgun sizes but their volumes are proportional to the individual airgun volumes with the larger being the 250 in 3 guns and the smaller being 150 in 3 guns. 11

12 12 Figure 4. Far field array signals (top) and spectra (bottom) in the vertically downward direction comparing CMST-simulated signals (red) with the example waveform provided in the Sclumberger brochure (blue). Spectral characteristics of the source, without the ghost, are required as inputs to the acoustic propagation models. These were computed using the CMST model, and are plotted in Figure 5 and Figure 6. Despite the compact size of the source its horizontal-plane beam pattern, shown in Figure 5, has an azimuthal dependance. To provide a worst-case scenario the VSP seismic

13 13 source was therefore modelled at an orientation of 100 azimuth so that the largest source levels occurred in the direction of Head of Bight. The vertical-plane beam patterns shown in Figure 6 show only a weak angular dependence for elevations near horizontal and so the horizontal plane source characteristics were used as input to the propagation models. Solely for the purpose of providing a single number that can be used to compare different arrays, an equivalent source level was calculated by integrating the horizontal plane source spectrum over frequency and averaging over azimuth. This procedure gave a source sound exposure level of 218 1m. Note that this value was not used for any of the subsequent received level calculations, which were carried out using the procedure described in Section 2.7. Figure 5. Horizontal plane azimuth-dependent spectral level for model source, excluding surface reflection.

14 14 Figure 6. Selected vertical plane elevation-dependent spectral level for model source, excluding surface reflection. An elevation angle of 0 corresponds to the vertially downward direction. The left image is for 0 azimuth and the right image is for 90 azimuth 2.3 Propagation modelling The three source locations used for this work, P1-P3, were selected to be representative of the different propagation conditions found in the survey area (Figure 1). For each point, propagation tracks were defined over absolute azimuths of 0-360, every 10. The acoustic propagation model RAMGeo was used to calculate transmission loss as a function of range and depth along these tracks for one-third octave spaced frequencies from 8 to 794 Hz. RAMGeo is a well-tested parabolic equation model suitable for rangedependent fluid seabeds written by Michael Collins from the US Naval Research laboratory (for more details see Jensen et al., 2011). RAMGeo takes into account the effect of bathymetry, seabed composition and water column sound velocity on transmission loss; the selection of these data is described below.

15 Bathymetry Bathymetry data used in the range-dependent propagation modelling was extracted from the Geoscience Australia Bathymetry (0.15 ) database along the tracks from each point. The bathymetry was adjusted from mean sea level to mean high water spring tide in the area by the addition of 0.75 m. 2.5 Seabed composition Using the results of a 3D seismic survey supplied by the client, geo-acoustic seabed models for shallow (< 150 m) and deep water (> 150 m) have been determined. As the RAMGeo model was designed for fluid seabeds, the seabed compositions have been adapted to suit this type of model through the use of fluid equivalent seabeds. The geoacoustic model used in shallow water is show in Table 3, here there was a thin layer of sand (1.5 m thick) over a calcarenite material that increases in hardness, and hence sound speed, with depth below the seafloor. The geo-acoustic seabed model used in deep water is shown in Table 4; here the same type of calcarenite material was present but under 350 m of a sand-silt layer. Table 3. Seabed composition used in propagation modelling in water less than 150 m. Layer name Sand Calcarenite equivalent fluid Basement Sedimentary rock Depth below the seafloor (m) [0 1.5] [ ] 1000 Compressional [ ] [ ] 1200 wave speed (m/s) Density (kg/m3) [ ] [ ] 2400 Shear speed (m/s) [0 0] [0 0] 0 Compressional wave absorption Shear wave [ ] [9 9] 9 [0 0] [0 0] 0

16 16 Table 4. Seabed composition used in propagation modelling in water greater than 150 m. Layer Sand-silt Calcarenite Basement equivalent fluid Depth below the seafloor (m) [0 350] [ ] 1000 Compressional wave speed (m/s) [ ] [ ] 1200 Density (kg/m3) [ ] [ ] 2400 Shear speed (m/s) [0 0] [0 0] 0 Compressional wave absorption [ ] [9 9] 9 Shear wave [0 0] [0 0] Water column sound speed profile The water-column sound speed profile was taken from the nearest grid point of the World Ocean Atlas 2005 (NOAA, 2005) for the shallow (33 South, 131 East) and deep water (34 South, 131 East) sites for both summer and winter seasons. The major difference between the seasons is that the summer profile shows a very thin mixed layer just below the sea surface above a steep, downward refracting thermocline, whereas the winter profile shows a thick mixed layer with a sound speed that increases with depth, resulting in upward refraction that would tend to trap sound energy near the surface. Received sound levels would therefore be expected to be higher in winter than in summer. The autumn and spring sound velocity profiles (not shown) are intermediary to the summer and winter seasons, i.e. their effects would be somewhere in between.

17 17 0 SVP - Shallow water (< 150 m) Depth (m) Summer Winter Compressional sound speed (m/s) SVP - Deep water (> 150 m) Summer Winter 2000 Depth (m) Compressional sound speed (m/s) 0 SVP - Deep water (> 150 m) Depth (m) Summer Winter Compressional sound speed (m/s) Figure 7. Sound velocity profiles (SVPs) used for modelling: (top panel) shallow water (< 150 m); (middle panel) deep water (> 150 m); (bottom panel) first 200 m of the deep water SVP.

18 Received level calculation For the azimuth thrusters, the received spectra along each track was calculated as a function of range and depth by subtracting the appropriate transmission loss from the source spectra shown in Figure 2. The received spectra were then integrated over frequency (in the pressure squared domain), and converted to db to obtain the received sound pressure level (SPL) in db re 1 μpa root mean square. The received SPLs are presented in the results section in three formats, as the: 1. Maximum received level at any depth for each range, plotted as a scatter plot. 2. Received level as a function of depth and range for specific azimuths. 3. Maximum received level at any depth for each range, plotted over the bathymetry contours. This is also provided electronically, the format of which is explained in Appendix B. Presenting the results like this helps identify maximum received levels for each point at each range. Similarly for the VSP seismic source, frequency-dependent source levels were obtained by integrating the source spectrum for the appropriate azimuth over each frequency band. The source level and transmission loss were then combined to compute the received sound exposure level (SEL) in db re 1 Pa 2 s as a function of range, depth and frequency. The received SEL results are presented graphically using the same formats as described above for the azimuth thruster SPL results. 3 Results 3.1 Azimuth thruster noise: effect of season The effect of the season on the SPLs was only seen at long ranges, where on average SPLs were found to be higher for the Australian winter compared to the summer. This was seen most dramatically in noise travelling up the shelf towards the coast. The maximum SPL from P1 along an azimuth of 0 (i.e. north) was very similar for summer and winter conditions at ranges less than 10 km (Figure 8). At ranges greater than 10 km, the downward refracting profile in the surface waters of the summer result in more interactions with the seafloor and so the SPLs start to drop much more rapidly than seen

19 19 in the winter season, during which the sound energy is being channelled by a strong surface duct (Figure 9) SPL, db re 1 Pa (RMS) r, km Figure 8. Maximum SPL at any depth versus range for P1 at an azimuth of 0 for the winter (blue dots) and summer (red crosses). The black dotted line represents the expected levels from spherical spreading losses.

20 20 Figure 9. Received SPL as a function of range (r Tx ) and depth (z) from P1 on an azimuth of 0 (i.e. north) during summer (top panel) and winter (bottom panel). For noise travelling away from the coast, there is less difference seen in the SPLs between the seasons. For instance, for P1 at an azimuth of 180 (i.e. south) the maximum SPL was very similar up to ranges of 100 km (Figure 10). At ranges greater than 100 km the maximum SPL during winter was higher than during summer (typically 3-6 db), but the difference was not as large as for noise travelling towards the coast (Figure 8). Examination of the cross section for this azimuth, shows that sound energy is coupled in the deep sound channel in both seasons, but the surface duct present in the winter leads to sound being propagated further in winter than for summer (Figure 11). Based on these findings, the remaining results section will focus on SPLs for the winter season with the knowledge that levels will be lower during all other seasons.

21 SPL, db re 1 Pa (RMS) r, km Figure 10. Maximum SPL at any depth versus range for P1 at an azimuth of 180 for the winter (blue dots) and summer (red crosses). The black dotted line represents the expected levels from spherical spreading losses.

22 22 Figure 11. Received SPL (db re 1 μpa RMS) as a function of range (r Tx ) and depth (z) from P1 on an azimuth of 180 (i.e. south) during summer (top panel), winter (middle panel) and winter for depths m (bottom panel). 3.2 Azimuth thrusters: spatial variation of noise levels The maximum SPL (at any depth) for the winter season overlayed on bathymetric contours for P1 is shown in Figure 12, for P2 in Figure 13, and for P3 in Figure 14. The maximum SPL (at any depth) versus range as a scatter plot for all directions from P1-P3 during winter are shown in Figure 15. The noise from the drilling rig was found to attenuate at a similar rate from all three source locations and in all directions. This is a surprising result given the steep bathymetry of the continental shelf, but is due to the winter sound speed profile producing a strong surface duct that allows sound to travel close to the sea surface, away from the influence of the seabed, with little attenuation.

23 23 Sound travelling in this duct is trapped by a combination of upward refraction within the water column and downward reflection from the sea surface (Figure 9 and Figure 11). In practice there will be some loss of energy due to scattering when the sea surface is rough, however this will be highly weather dependent and has not been included in the modelling. The results presented here are thus representative of very calm weather, during which the highest sound levels are likely to occur. Figure 12. Maximum SPL db re 1 μpa RMS (at any depth) for P1 over bathymetric contours for the winter season. Commonwealth marine reserves in the area are indicated by the magenta lines. The locations of the ambient noise loggers (BP-SL-01, BP-SL-02) are also indicated in the figure.

24 Figure 13. Maximum SPL db re 1 μpa RMS (at any depth) for P2 over bathymetric contours for the winter season. Commonwealth marine reserves in the area are indicated by the magenta lines. The locations of the ambient noise loggers (BP-SL-01, BP-SL-02) are also indicated in the figure. 24

25 25 Figure 14. Maximum SPL db re 1 μpa RMS (at any depth) for P3 over bathymetric contours for the winter season. Commonwealth marine reserves in the area are indicated by the magenta lines. The locations of the ambient noise loggers (BP-SL-01, BP-SL-02) are also indicated in the figure. For all points, the SPL dropped below 160 db re 1 μpa (RMS) within 100 m, and below 120 db re 1 μpa (RMS) between 10 and 40 km (Figure 15). In all cases the predicted SPL is below 106 db re 1 μpa (RMS) at the maximum modelled range of 300 km and would be expected to continue to decrease with further increases in range. With the rig at the most northern point (P1), the noise level is predicted to be less than 115 db re 1 μpa (RMS) at the 200 m depth contour and below 106 db re 1 μpa (RMS) at the most northerly extremity of the Great Australian Bight (Figure 12). When it is at the

26 26 most southern point (P3), the noise level is predicted to be less than 120 db re 1 μpa (RMS) at the 4000 m depth contour. Figure 15. Maximum SPL db re 1 μpa RMS (at any depth) versus range for P1 (blue dots), P2 (green dots) and P3 (red dots) for the winter season. The black dotted line represents the expected levels from spherical spreading loss. To put these results in context, Figure 16 plots measured ambient noise for two locations north of P1, one (BP-SL-01) at Head of Bight in 50 m of water and the other (BP-SL-02) at the shelf-break in 190 m of water (McCauley et al 2012). Statistics of the measured ambient noise are listed in Table 5. Obvious anthropogenic noise sources, such as seismic survey signals, have been removed from the plotted and tabulated data. The ambient noise levels at the more southerly location are much higher than those at the more northerly location, which reflects the origin of the dominant noise sources which are in the Southern Ocean and and at the edge of the Antarctic continent. A comparison of the data presented in Table 5 indicates that, at both these locations, the predicted noise levels

27 27 due to the azimuth thrusters with the drill rig at its most northerly location are well within the corresponding ranges of measured ambient noise. At BP-SL-01 the predicted drill rig noise exceeds the median ambient noise by just over one standard deviation, whereas for at BP-SL-02 it exceedts the median ambient noise by less than one standard deviation. Figure 16. Normalised histograms of measured ambient noise levels (using 1/3 octaves from 8 Hz to 2.5kHz ) in 50 m of water at BP-SL-01 (top plot) and in 190 m of water at BP-SL-02 (bottom plot) for the period November 2011 to June Table 5. Ambient noise measurement locations and statistics and predicted noise at these locations from a drilling rig at P1 Designation BP-SL-01 BP-SL-02 Position ' S, ' E ' S, ' E Water depth 50 m 190 m Measured ambient noise level (db re 1 μpa rms) Minimum Maximum Mean Standard deviation Median Maximum predicted level due to azimuth thruster noise with drilling rig at P1 (db re 1 μpa rms)

28 VSP seismic source The geographical distribution of SEL up to a radius of 300km from the VSP source at source location P1 is plotted in Figure 16. A scatter plot of the maximum SELs along the 10 (inshore), 110 (perpendicular to the slope of the continental shelf) and 190 (offshore) azimuths for the VSP source at location P1 is shown in Figure 17. For comparison, a dashed line indicates spherical spreading for an equivalent source level of 218 db re 1 Pa 2 s. Figure 18 and Figure 19 show corresponding range slices of the SEL along the specified azimuths plotted in Figure 17.

29 Figure 17. Maximum SEL db re 1 Pa 2 s (at any depth) over bathymetric contours for the VSP seismic source at P1. Commonwealth marine reserves in the area are indicated by the magenta lines. 29

30 Figure 18. Maximum SEL (db re 1 Pa 2 s) at any depth versus range for the VSP seismic source at P1 along specified azimuths. The azimuths are 10 (blue dots), 110 (red dots) and 190 (green dots). The black dotted line represents the expected levels from spherical spreading loss. 30

31 31 Figure 19. SEL in a vertical plane in the cross-line direction. The left half of the image corresponds to an azimuth of 10 (inshore direction) while the right half corresponds to an azimuth of 190 (offshore direction). The VSP seismic source is located at 0km range. Figure 20. SEL in a vertical plane for an azimuth of 110 which corresponds to a direction perpendicular to the slope of the continental shelf at the P1 source location.

32 32 Received SELs from the VSP source at P1 are predicted to drop below 120 db re 1μPa 2.s at the shelf edge, after which the sound attenuates relatively slowly as it propagates across the broad, flat shelf, with levels dropping below 110 db re 1μPa 2.s just off the coast. The vertical plane VSP SEL predictions shown in Figure 18 and Figure 19 do show evidence of a surface duct but its effect on the horizontal plane distribution of received levels is much less than for the azimuth thrusters. There are two reasons for this: 1. The VSP source is at a much shallower depth (10 m) than the thrusters (28 m). For a shallow source, destructive interference between the direct and surfacereflected paths filters out energy that would otherwise propagate close to the horizontal and end up trapped in the surface duct. 2. The interference effect described in 1. is frequency dependent, with low frequencies being attenuated more than high frequencies, and the VSP source spectrum peaks at a lower frequency than the azimuth thruster spectrum (compare Figure 2 with Figure 4). The combined effect is that a much smaller proportioin of the VSP source energy is able to be trapped in the surface duct than is the case for the azimuth thrusters. It is not meaningful to carry out a simple comparison between sound exposure levels (SELs) for an impulsive source like the VSP source and measurements of continuous ambient noise such as those presented in Figure 16, so this has not been attempted. 4 Conclusions Underwater sound levels from a drilling rig for three locations in the GAB have been modelled. The proposed drilling rig is expected to utilise eight azimuth thrusters (each with a power rating of 4.2 MW) to hold the rig on station. The source level of the drilling rig was estimated to be db re 1 Pa 1 m (over the frequency range 10 Hz to 2 khz). Predicted received levels at long range were higher in the winter than in the summer due to the presence of a surface duct during the winter months. The surface duct also resulted in the noise from the drilling rig, during winter months, attenuating at a similar rate from all three source locations and in all directions.

33 33 For all points, the azimuth thruster SPL during winter dropped below 160 db re 1 μpa (RMS) within 100 m, and below 120 db re 1 μpa (RMS) between 10 and 40 km from the location of the drilling rig. To the east, as in other directions, the predicted SPL was below 106 db re 1 μpa (RMS) at the maximum modelled range of 300 km and would be expected to continue to decrease with further increases in range. With the rig at the most northern position, the noise level was predicted to be less than 115 db re 1 μpa (RMS) at the 200 m depth contour and below 106 db re 1 μpa (RMS) at the most northerly extremity of the Great Australian Bight. These levels are well within the range of measured ambient noise levels at these two locations. Received levels calculated using the modelled source locations should be representative of those that would be produced by the drilling rig at other locations with similar water depths during the winter, and are typical of the levels that would be expected in calm weather. Received levels due to the azimuth thrusters would be expected to reduce much quicker with range in other seasons in which the surface duct is not present, or in periods of rough weather during winter when scattering of sound from the rough sea surface will markedly increase the rate of attenuation of sound within the duct. Modelling of a vertical seismic profiling (VSP) source in a worst-case orientation at the most northerly source location predicted that received SELs would drop below 120 db re 1μPa 2.s at the shelf edge, after which the sound would attenuate relatively slowly as it propagated across the broad, flat shelf, with levels dropping below 110 db re 1μPa 2.s just off the coast. The shallower source depth and lower frequency spectral peak of the VSP source resulted in the surface duct having a much smaller effect on the received levels from this source than it did in the case of the azimuth thrusters.

34 References Jensen, F. B., Kuperman, W. A., Porter, M. B., Schmidt, H., Computational Ocean Acoustics, 2 nd Ed., Springer, 2011, ISBN McCauley, R. D., "Radiated Underwater Noise Measured from the Drilling Rig Ocean General, Rig Tenders Pacific Ariki and Pacific Frontier, Fishing Vessel Reef Venture, and Natural Sources in the Timor Sea, Northern Australia". Centre for Marine Science and Technology Report C98-20, McCauley, R. D, Duncan, A. J., Gavrilov, A., "Air gun signal transmission, ambient noise, whale and fish signals recorded during and after the Ceduna seismic survey in the great Australian bight, November 2011 to June 2012", Centre for Marine Science and Technology Report , September NOAA (2005), World Ocean Atlas, National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service 61-62, U.S. Government Printing Office, Washington, D.C., 182 pp., CD- ROM Ross, D., Mechanics of Underwater Noise, Peninsula Publishing, 1987, ISBN

35 35 5 Appendix A Figure 20 shows the relationship between mechanical power and noise level presented by Ross (1987). Figure 21: Diesel noise as a Function of Engine Power and Speed (taken from: Ross, 1987).

36 6 Appendix B The maximum received level at any depth for each range, plotted over the bathymetry contours for each point during Australian winter has been provided electronically in a text (*.txt) format. There is a separate for file for each point (P1-P3), file names are: SPLmax_GAB_Drilling_AustralianWinter_P1.txt ; SPLmax_GAB_Drilling_AustralianWinter_P2.txt ; and, SPLmax_GAB_Drilling_AustralianWinter_P3.txt Please note, there is: 36 No header. Five columns (starting left to right): Longitude (in decimal degrees), Latitude (in decimal degrees), Easting (UTM Zone 52 H), Northing (UTM Zone 52 H), Received sound pressure level (SPL) in db re 1 μpa root mean square.