Working the waterline

CARRYING out a transition zone seismic survey in Port Denison in Western Australia’s Perth Basin sounds tough enough.
Working the waterline Working the waterline Working the waterline Working the waterline Working the waterline

Add exposed coral reefs, a national park banning all vehicles, horrendous weather and peak crayfishing season and you have the challenge Roc Oil handed seismic specialist Grant Geophysical.

The Houston-based international geophysical contractor provides land, shallow water and transition zone seismic data acquisition services to resources clients around the world.

Grant says rather than restricting itself to a single technology base, it chooses the most appropriate up-to-date technology packages for each operating environment. It then develops advanced crossover capability between them.

Contracted by Roc in late 2004, Grant deployed a crew to Port Denison, 360km north of Perth, to conduct a small but complex transition zone survey.

Asia Pacific regional manager Greg Dunlop said the 40-man team faced a range of hurdles.

“It was a land and transition zone project, which meant our lines wound from on the beach out through the surf zone and out across the reef areas off Dongara in the Perth Basin,” he said.

“All the lines that went onshore were in national park, so it was very restricted with regard to what we could do with mechanical drilling and things like that.”

This restriction meant the team had no mechanical help for onshore operations – they had to do everything by hand.

“We cut the line through the dunes by hand,” Dunlop said.

“There was no vehicle access whatsoever, so we carried everything in by hand. While we do that in other parts of the world, a full man-portable crew is not something that’s seen very often in Australia.”

He said shooting for the survey also took place in summer, which in that part of WA is the most difficult season.

“Working at this time was necessary because of the window between numerous environmental restrictions and to make use of the good weather that usually lasts until about noon when the prevailing winds increase to 30 knots plus,” Dunlop said.

“The crew started work at 4.30am each day and continued as long as it was safe to be on the water.”

Dunlop said many of the lines crossed coral reefs that were exposed at low tide and when combined with bad weather, the two can create a dangerous working environment. On top of that, the survey was carried out in the heart of crayfishing season.

“A very large commercial fishing fleet operates out of Port Denison,” Dunlop said.

“Liaison with local fishermen and recreational divers was paramount in ensuring that no one was disadvantaged by the presence of the crew.”

Despite crayfish, weather and access problems, the project was completed on time and on budget, thanks largely to preparation, according to Dunlop.

With varying water depths across the survey area, the team decided that dual sensor receivers would be used on some liners and some sections of marine lines.

Dunlop said this required serious pre-planning to ensure the exact equipment in the correct quantities was where it had to be, precisely when it was needed.

“You have to be able to look ahead, look at the weather patterns, get up-to-date weather information, and plan and prepare for every problem you can get onshore and every problem you can get offshore.

“You really do need to plan well in advance – making sure everything is ready before the equipment and personnel arrive onsite and utilise every moment of good weather you have during the course of the survey.”

Dunlop said the company’s fleet – which includes containerised, shallow draft and seaworthy vessels – was crucial to Grant being able to economically and rapidly deploy their crews anywhere in the world at short notice.

In addition, technology must be tweaked to suit the survey area’s environment and other project-specific factors.

“Successful transition zone seismic acquisition requires accurate planning and adaptability with a little innovation to get around those inevitable project-specific challenges,” he said.

“It’s those little developments that we’ve made on existing technology, I think, which truly makes the difference.”