Seismic shift on whale safety

03:02, Feb 25 2013
tdn survey stand
The survey vessel Voyager Explorer berthed at Port Taranaki before heading off to survey an area southwest of Taranaki.

Otago University associate professor of zoology Liz Slooten was not a happy woman last week.

She'd just learned that a pygmy sperm whale had beached itself on Ngarunui Beach, near Raglan, and that the Department of Conservation had shot it and then buried it without conducting any sort of autopsy.

That meant that no-one would ever know what had caused the 2.7-metre whale to run ashore, and it certainly meant Dr Slooten would never be able to confirm whether or not the mammal had become disoriented by seismic survey operations currently taking place in the sea between New Plymouth and Raglan.

Right now a big 93m-long survey vessel, the Western Monarch, is in the area, towing behind it 10 streamers that are up to 8km long. These streamers contain sensitive devices called hydrophones, and they record the acoustic energy of a burst of compressed air released from the vessel's stern every 25 metres it travels.

Most of this energy is directed downwards and into the sea floor, and just like an echo sounder or ultrasound imaging, the hydrophones measure what is reflected back from buried layers of rock beneath the seabed.

These measurements are, in turn, processed and interpreted by geologists to establish whether the sub-surface geology has the potential to contain oil or gas.


In this case the geophysical survey is being conducted by a company called WesternGeco, a subsidiary of major international oilfield servicing company Schlumberger.

WesternGeco is bound by a work commitment to the New Zealand Government to acquire 4000 square kilometres of such data during the first quarter of this year. The survey area extends from 25km north of New Plymouth to a point about 35km offshore Raglan Harbour.

It's not the only seismic survey taking place off the New Zealand coast. Further to the south another survey vessel, the smaller Voyager Explorer, is starting a survey over a 450sqkm area south- west of Taranaki for the joint venture owners of the Maari oilfield, including New Zealand company Todd Energy.

This vessel will be towing four of the streamers, and the information obtained will be via the same acoustic energy created by regular bursts of compressed air.

A feature of both survey operations is that they are going to what seem to be extraordinary lengths to have as little impact as possible on marine life. And with good reason. Not only are both areas on migration paths for various species of whales, but the northern area between New Plymouth and Raglan is officially recognised as the home of one of the world's rarest marine mammals, the Maui's dolphin.

Government regulations are already in place to ensure that such surveys are conducted responsibly and conform with government environmental protection measures.

In addition, last year the Department of Conservation established a voluntary code of conduct that exceeds these measures to further protect marine life and the environment.

WesternGeco's survey is going even further again. The company has hired five environmental consultants who have been certified by DOC under its code of conduct, and they are all out at sea observing for marine mammals and monitoring the project itself.

Four of them are on board the Western Monarch, and the fifth is on a support vessel that is steaming in front of the survey vessel to spot any whales or dolphins prior to the big vessel approaching.

Not only that, but WesternGeco is also using a Passive Acoustic Monitoring (PAM) system to detect any vocalisations by whales or dolphins, and to help the observers locate any that may be present at night.

In addition, there's another support vessel on hand at all times, capable of taking the survey vessel under tow should this be required.

If any marine mammal is spotted, the DOC code of comfort requires a seismic survey vessel to either stop or delay the recording if the mammal enters what are known as mitigation zones - these are a distance of 1 km for species of concern, and 1.5 km for species of concern with a calf.

The seismic survey industry says this is sufficient to protect marine life. Modelling carried out for WesternGeco by an independent research organisation showed that when the burst of compressed air is let off, while the acoustic energy is directed downwards, some residual energy travels horizontally.

But while the air has been compressed to as much as 2000 psi when the burst is let off, the pressure of the sound wave decreases rapidly so that by the time it has reached 20 metres away in a horizontal direction it will have reduced to around 40 psi which is about the same as a typical garden hose, says the industry.

Last week the Taranaki Daily News was able to tour the survey vessel Voyager Explorer while it was berthed at Port Taranaki preparing for its new assignment off southwestern Taranaki.

The ship's master, Ukranian man Khytryi Valerii confirmed that it will have two observers scanning for marine mammals at all times, and that the vessel also operates a PAM system. The Voyager Explorer will also have its own chase boat that will steam ahead of the survey vessel.

"If we spot anything, we will turn the compressor guns off," he promised.

As we visited an area known as the Gun Deck - an area right down by the waterline where the compressor guns are stored - we asked what the surface of the sea looks like when the pulse of compressed air is fired.

"It looks very much like the surface of a spa pool," said Todd geophysicist Rick Henderson. "Actually the seals and common dolphins love it - they hang around whenever the surveying is taking place."

But none of this is of any comfort to Dr Slooten. She remains angry that DOC shot the stranded whale and then promptly buried the animal, and she is concerned that the measures the survey boats have in place have been introduced without any research into the extent of whale and dolphin populations in the areas.

"This is like claiming to have found a cure for cancer, and then not providing any evidence to back it up," she said.

"If someone wants to go blasting a particular area with airguns for weeks at a time, surely there must be an associated responsibility to find out what the effect is. This all should have happened before the surveys started."

Dr Slooten said that obviously no-one will ever know what caused the little pygmy sperm whale to beach itself, so the finger can never be pointed at the seismic surveyors.

"But it is one potential cause - and that has to be a concern," she said.

Taranaki Daily News