Unsound Great Australian Bight seismic survey decision
Questions by Centre Alliance Senator Rex Patrick at Estimates have raised doubts as to the validity of the decision made by NOPSEMA to permit seismic surveying in the Great Australian Bight.
"I think the decision is unsound," said Rex.
"Marine life will be put at risk because, on the face of it, the surveyor has not used worst case oceanography to calculate the distance at which the sound from the seismic noise source will disturb or damage the hearing of marine life.
"Imagine walking slowly towards a jet engine, there will be some distance when the engine sound will disturb you and, at an even closer distance, when it will damage your ears.
"The same is true for marine life, except the distance at which the disturbance or the damage occurs varies dramatically depending on something referred to as the ‘sound velocity profile’."
The noise level of the seismic source that will be used in the Great Australian Bight is a very high 256 decibels (the loudest navy sonar is around 235 dB). By way of example:
- On a day when the sound velocity is strongly negative as the depth increases, a dolphin may need to get within about 2 km of the survey vessel before hearing damage occurs.
- On a day where the sound velocity is constant as the depth increases a dolphin will not suffer damage to hearing unless it approaches between about 4 and 6 km.
- On a day when the sound velocity profile results in a strong surface duct (which traps sound) a dolphin will need to be well beyond 100 kms from the survey vessel before it will be safe from having it hearing damaged.
Senator Patrick, himself a former submariner and trainer in acoustic theory, questioned NOPSEMA’s environmental director on its decision making process at Estimates on Thursday night.
On the face of it NOPSEMA has not properly considered worst case sound propagation in its decision making. Instead it has taken mean ‘sound velocity profiles’ which puts them into scientific error.
Page D-4 of the PGS submission shows four mean ‘sound velocity profiles’ for the months of May, September, October and November. Even though the surveying will take place from September to November this year, they used the May mean sound velocity profile because it was worst case.
"They only got it half right in picking the worse case mean," said Rex, "but this could prove fatal to sea life. Whales are known to become disoriented and beach themselves when subjected to high sound levels.
"What NOPSEMA should have done is looked at worst case on a day to day basis for the period over the proposed survey schedule. When acoustic conditions are worse than the mean (which will happen for about half of the month) sea life could be in danger."
Senator Patrick asked NOPSEMA if the survey company will be measuring the sound velocity profile every day, NOPSEMA took the question on notice. When he asked if the survey company intended to listen passively for the presence of whale and dolphin calls, NOPSEMA took the question on notice.
Senator Patrick has requested an urgent meeting with acoustics engineers at NOPSEMA to discuss the issue further.
"I want to discuss my concerns with them and let them convince me I’m wrong. If they can’t, I’ll be formally asking them to set aside the decision they have made until such time as safety of marine life can be guaranteed."