Path in the wilderness

‘Tabasco’ Database Charts Seismic Terrain

Eric Bathellier
Eric Bathellier

Petroleum industry activity continues to move ahead in the often-frigid Arctic, where a widespread database to mitigate exploration risk and encourage successful exploration is sorely lacking.

Bracing themselves against the elements, CGG workers ventured forth a couple of years ago to acquire a land-based high-resolution 3-D seismic survey on Alaska’s North Slope.

Dubbed “Tabasco,” the multi-client data set covers 133 square miles.

It was the first multi-client survey performed in the area in more than a decade, according to Eric Bathellier, technical sales manager at CGG. The company has been engaged for more than 30 years in both onshore and offshore Arctic acquisition.

“Tabasco was designed for optimum imaging of the faulting and onlaps in the Cretaceous and Jurassic hydrocarbon-bearing sequences,” Bathellier said. “It was acquired in separate northern and southern parts, both of which were processed independently and saw the first use of many high-end acquisition techniques in the region.”

“The high productivity techniques that were employed for Tabasco enabled the high-density survey to be carried out in 60 days, from February to April, 2012,” he noted.

The North Slope sits on a coastal plain abutting the Beaufort Sea. Besides being home to the famed Prudhoe Bay oil field, it contains both the National Petroleum Reserve and the Arctic National Wildlife Refuge.

To call this region “environmentally sensitive” would be a gross understatement.

“Onshore exploration is permitted only in a short season during the winter, when the snow cover reaches 15 centimeters and the ground is sufficiently frozen for vehicles to be taken over the snow without damaging the tundra,” Bathellier said, “December at the earliest to May at the latest.”

Bearing the Challenges

Ensuring minimal impact on the fragile locale is fundamental to the seismic crews. In addition to painstaking vehicular management and control, long-term camp areas are iced over manually or else set up on frozen pools to minimize effects on the ground.

Bathellier noted that the 150-person Tabasco camp was comprised of sled-mounted units that were moved one or two miles every few days with the acquisition spread, leaving absolutely nothing behind in accordance with the company’s strict “no spills” policy.

Program participants faced the potential of having to deal with a somewhat unusual problem: bears.

“The crews used integrated GPS tracking units to map and enforce exclusion zones … such as government-supplied locations of polar bear or grizzly bear dens,” Bathellier said. “Approaching closer than one mile to a known bear den causes an alarm to be triggered in the recorder truck, as well as in the offices of the recording crew manager.”

Because grizzlies tend to snooze their time away during the winter and polar bears hang out on or near the sea ice away from land, the likelihood of an encounter was slim. Yet crew members likely kept a wary eye – not only for bears, but wolves as well.

And you thought the south Louisiana swamps were a tough tour of duty.

Rough conditions aside, the activity moved right along.

“Tabasco was a first for the North Slope in that it was a high-productivity, extended slip-sweep vibroseis program,” Bathellier commented. “It broke previously established records and set a new standard at 5,000 records gathered in a 24-hour period.”

For the geophysically challenged, slip-sweep entails starting up a vibroseis fleet while the previous one is still shaking – in other words, one vibroseis cycle slips over the top of another. Simultaneous shooting is critical to expedite high-density applications in the limited time windows available (in this particular region), while keeping equipment and manpower to a manageable assemblage, according to Bathellier.

Acquisition using slip-sweep is commonplace in numerous parts of the world. But the strict regulations in the Arctic tundra are designed to maintain tight control on vibroseis applications. In fact, the vibrator operators are required to check the machines every hour to avoid any fluid spills.

The acquisition program was improved via use of the proprietary EmphaSeis low-dwell sweep.

“This target spectrum sweep starts at lower frequency, which was 4 Hz in this case compared to conventional linear sweeps starting at 8 Hz,��� Bathellier explained.

“The sweep has a short, low-dwell component that lasts a little more than a second,” he said. “This is designed to fit the vibrator mechanical constraints, which in this case (was) mostly the pump flow for the type of vibrator used.”

A New Norm?

Processing of Arctic data includes resolving image problems related to permafrost and dealing with the intrusive noise from ice breaks.

The dense sampling and high fold of the Tabasco survey ensured that ice break noise did not pose a severe problem for processing, according to Bathellier.

He emphasized that the project represents the first production application of high-density, high-productivity broadband vibroseis in Alaska.

“The broadband data, with added low-frequency content, penetrates to deeper targets, improving illumination and thin beds separation through the inversion process,” Bathellier said.

“We believe that with ever-improving seismic technology, tighter geometries and efficient seismic acquisition should become the norm on the North Slope,” he predicted.

For sure, the CGG team proved with Tabasco that it’s possible to acquire high-density broad bandwidth data in remote areas such as this, despite the tight time window.

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