Seismic surveying in the 21st century

Increased environmental awareness and technical evolution drive today’s approach to surveying on the North Slope

Alan Bailey

Petroleum News Staff Writer

Seismic surveying on Alaska’s North Slope has changed dramatically in the past quarter century. Precision 3-D surveying has largely replaced 2-D techniques. And the days of tolerating damage to the delicate Arctic tundra are long gone.

During early exploration of the North Slope environmental protection was not a high priority when conducting seismic surveys — scars are still visible on the tundra from those surveys, Michael Faust, exploration manager and chief geophysicist for ConocoPhillips Alaska Inc, told an audience at the Alaska Forum on the Environment recently.

“Today we do absolutely everything we can to ensure that that’s not the case,” Faust said. “If you go out in the summer time after our seismic crew is on the tundra, the first summer you will see where we’ve been green up first … if you back the second year or the third year you will not be able to tell where we were.”

Improvements began the early 1980s, when people started looking at environmental issues and weighing these against production targets, Faust said. Since then environmental protection has grown in importance and has now become the prime consideration. Zero lasting impact on the environment has now become the standard.

“(Up to the mid-‘90s) you were trying to get as much production as you could without causing too much of a problem,” Faust said. “… now the most important thing is to make sure that we don’t cause a problem and we get what we get in terms of production.”

Faust attributes much of what’s been achieved to the efforts of seismic contractors. Health, safety and the environment, referred to as HSE, have driven competition for contracts, he said.

“We’ve seen a massive amount of competition between seismic contractors to have the very best HSE performance,” Faust said.

ConocoPhillips has encouraged this trend by only allowing contractors with an acceptable HSE record to bid for work. When we’re bidding a $20 million to $30 million contract “the very first thing that’s looked at is the HSE performance of that operator and that seismic crew both in Alaska and worldwide,” Faust said.

And the company expects contractors to deliver on stringent HSE standards — in 2001 we terminated a contractor for disturbing about half-an-acre of tundra, Faust said.

Evolution to 3-D surveys

Faust said that from the 1960s to the 1980s people only shot 2-D seismic surveys on the North Slope. A 2-D survey involves shooting seismic along a line, to produce a two-dimensional cross-section of what’s beneath the surface of the ground.

“From the 1980s and into the mid-‘90s that changed significantly and we started to shoot what are called 3-D seismic surveys,” Faust said. “…everything being shot on the North Slope now is 3-D”

In a 3-D survey the survey crew positions seismic recording devices called geophones across an area of land. A seismic sound source moves around a set pattern across the same land area. The resulting seismic recordings can generate a three-dimensional image of the subsurface. A three-dimensional image gives more complete and precise insights into the subsurface structure than the two dimension cross-section from a 2-D survey.

And a better image of the subsurface leads to more precise targeting of wells, thus enabling extended reach drilling and an increased success rate with exploration wells, Faust said. That all translates to fewer wells, fewer ice roads and fewer drilling pads.

“Overall we believe that 3-D seismic, especially with the current practices, has reduced the overall cumulative impact of oil development on the North Slope,” Faust said.

Using vibrators

“We’ve not used dynamite (on the slope) in 20 years or more,” Faust said.

Instead, a massive vibrator truck transmits high frequency sound energy into the ground through a vibrator pad pressed into the snow. The vibrator pad does not damage the ground in any way. You can hear the vibrator operating if you’re standing next to it but if you lie down on the ground next to the pad you can’t feel the vibration, Faust said.

But there’s a whole science to designing the positions of the vibration points and of the geophones that detect the seismic echoes from subsurface structures. Heavy vehicles called jug trucks lay out the geophones and their associated cabling — unsuitable survey designs can route the vibrator trucks and jug trucks in ways that result in some tundra damage. Tight turns by a skid-steered vehicle, for example, can cause scraping of the tundra.

ConocoPhillips and its contractors have spent several years perfecting seismic survey designs and have now standardized on a design called a broad cross-line swath. This design only requires wide radius vehicle turns and makes it possible to conduct a survey with no long-term environmental impact.

Vehicles and personnel

The crew working on the survey lives in a mobile camp that needs to accommodate 80 to 100 people.

The camp units sit on skis. And tow vehicles drag the camp along behind the seismic survey as the survey progresses across the ground. We now require that the seismic crews move camp every three or four days, Faust said. A camp that remains in one place for too long tends to have an impact on the ground, he said.

Vehicle evolution

But increased environmental awareness has caused a rapid evolution in vehicle design and usage. Seismic contactors have invested heavily in improved vehicles. Faust said that the first phase of evolution brought in the use of rubber wheels and articulated steering.

“We tried that in ’98 and it worked beautifully and we fully implemented that in the ’99 season,” Faust said.

In 2000 a successful trial of the use of rubber tracks on some of the lighter vehicles led to the fitting of rubber tracks on the jug trucks in the following year. This innovative design involved articulated steering and a pair of independent rubber track systems front and back. By spreading the vehicle load across the ground and avoiding the problems associated with skid steering, rubber track designs have now become the norm for all vehicles.

“At this point every piece of equipment on the seismic crew is on rubber tracks,” Faust said.

The massive 65,000 to 90,000 pound vibrator trucks perhaps best illustrate the success the new designs. The old steel track design used skid steering and exerted a ground pressure of 16 pounds per square inch. The rubber track design uses articulated steering and exerts a pressure of about 6 psi.

It all adds up to obtaining ever better information about the subsurface without causing environmental damage.