Environmental Impact Study for the PGS Marine Vibrator Andrew Long, Chief Scientist Imaging & Engineering | Geoscience & Engineering, 5 Oct 2016

A 61 page report titled “A modelling comparison between received sound levels produced by a marine vibrator array and those from an airgun array for some typical seismic survey scenarios” was recently written by the Centre for Marine Science and Technology (CMST), Curtin University (Perth) for the International Fund for Animal Welfare (Australia) Pty Ltd.

Two configurations of a vibrator array were simulated, as shown in Figure 1. As discussed below, careful attention to the layout of sources in a marine vibrator array is crucial to minimising its environmental footprint. The concept marine vibrator array layout has four low-frequency ‘Subtone’ elements operating 6-20 Hz at 25 m depth and eight high-frequency ‘Triton’ elements operating 20-100 Hz at 8 m depth. The sources are all simultaneously electrically driven by a swept frequency sinusoidal signal covering the appropriate frequency band. The frequency changes linearly with time over the signal's duration of 5 seconds, and the amplitude of the signal is tapered at the beginning and end to avoid high-frequency components that would arise if it started or ended suddenly.


Figure 1. Plan view of the original PGS 16 x 16 m concept marine vibrator array (left) and the revised Mk2’ version (right). Large squares are low frequency elements at 25 m depth, small squares are high frequency elements at 8 m depth. In-line direction is up.
Click here to view image enlargement.

Three modelling scenarios were chosen to be representative of seismic surveys in shallow continental shelf waters, deep ocean waters, and over the continental slope, each being located off the southern margins of Western Australia. The shallow/coastal and deep water scenarios used perfectly flat seabeds for both short and long range modelling whereas the continental slope scenario utilised a flat seabed for short range modelling and the bathymetry for this area for long range modelling.

Figure 2 demonstrates why the Mk2 vibrator was chosen for comparison to an airgun array, and shows the long range scatter plots for maximum received SEL [click here to view article] for both vibrator and airgun sources in the shallow/coastal scenario. The differences are explained below.


Fig. 2. (left) Long range scatter plots showing maximum received SEL at any depth as a function of range for all azimuths in the shallow/coastal scenario for the original marine vibrator array (red) and the Mk 2 array (green). Note the reduced SEL values for the Mk2 array. (right) Long range scatter plots showing maximum received SEL at any depth as a function of range for all azimuths for the Mk 2 marine vibrator array (green) and the airgun array (blue). Note the reduced vibrator SEL values at long range.
Click here to view image enlargement.

The CMST report presents the results of an investigation of the underwater sound levels likely to be produced by a realistic marine vibrator array, and how these compare to those produced by an airgun array with similar effectiveness for seismic exploration. The results conclude that the principle advantages of marine vibrator seismic sources would be a reduction in peak pressure and a reduction in signal bandwidth, the latter being of particular benefit to high frequency cetaceans. It also concludes that sound exposure levels (SELs) produced by the marine vibrator array reduced more rapidly with range than those produced by the airgun array. This effect was most pronounced for the shallow water scenario (100 m water depth) where it resulted in SELs at 100 km range produced by the marine vibrator being approximately 8 dB lower than those produced by the airgun array (refer to the right side of Figure 2). In this scenario the differences started to become apparent at a range of 1 km whereas in the deep water scenario (4000 m water depth), there were similar differences at 100 km range, but no difference was observed at ranges less than 10 km.

The peak to peak sound pressure level (SPL) comparison shows that the marine vibrator produces levels as much as 20 dB lower than those produced by the airgun array. This is an expected result and is caused by the marine vibrator signal containing the same acoustic energy as the airgun signal, but spread over a much longer time period. Signal propagation effects that include interactions with both the seabed and the sea-surface in shallow/coastal scenarios combine to remove more and more low frequency energy from the received signal as the range increases, leaving the bulk of the energy in the high frequencies. This favours the marine vibrator which emits much less high frequency energy than the airgun array (refer also to Figure 3). The situation for deeper water means that sound leaving the source at steep angles does not undergo as many seabed reflections and therefore does not attenuate as rapidly, which has the effect of preserving low frequency energy to longer ranges, and therefore increasing the range beyond which the marine vibrator has a clear advantage.


Fig. 3. Shallow/coastal modelled received spectra at the ranges specified in the legend in the cross-line direction averaged over depths of 20 m to 80 m. (left) Mk 2 marine vibrator array, (right) airgun array. Note the differences above 100 Hz.
Click here to view image enlargement. 

References

Duncan, A., 2015, A modelling comparison between received sound levels produced by a marine vibrator array and those from an airgun array for some typical seismic survey scenarios. PROJECT CMST 1274, REPORT 2014-48, 61 p.

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