This image shows the survey ship launching one of two ROV units prior to layout the Z 3000 nodes on the seabed. (Images courtesy of Fairfield)

Look for 4-D, or time-lapse, applications to become more widespread as the technology evolves.

Currently, these types of surveys are most common in the North Sea. A high-profile example is the BP-operated Valhall field in Norway, where BP installed a permanently placed ocean-bottom cable (OBC) system in 2003.

A downside to installing — particularly trenching — an OBC system for 4-D purposes is the considerable up-front cost.

Towed streamers have proven quite successful for 4-D surveys in some instances. However, some folks question just how accurately streamers can go back and record over the same exact place.

Yet repeatability — accurate repetition of each successive survey — is key to successful 4-D application. Consequently, the industry is looking closely at ocean-bottom seismic (OBS) nodes as a potential “magic bullet.”

Autonomous self-contained nodes are deployed via remotely operated vehicles (ROVs), ensuring positional accuracy and repeatability, according to David Hays, vice president of the technology group at Fairfield Industries. They also can be placed essentially anywhere on the seafloor, even among dense infrastructure.

Deimos study

There’s been considerable industry buzz about the 2-D node repeatability study conducted in 2007 by Shell and Fairfield at the Shell-operated Deimos field in the Gulf of Mexico (GoM). The study was performed during the same time that the companies acquired an OBS node 3-D survey over the field in 3,300 ft (1,000 m) of water.

The repeatability study proved to be a giant step forward in documenting the ability to acquire 4-D seismic via nodes on the seabed.

“For us, at the start of the Deimos survey, there were two purposes,” noted Frans Smit, senior operations geophysicist at Shell E&P Co. “First was to do the repeatability test, and second was to actually see Fairfield’s deployment and retrieval methods in operation.

“Deploying 20 nodes ahead of the survey, retrieving them, and downloading the data from each with 100% success gave us a lot of confidence,” said Smit, who helped design the repeatability program at Deimos along with Fairfield.

Hays emphasized no one had conducted a full-scale time-lapse seismic experiment with nodes.

“We didn’t this time either,” he said. “But we wanted to get insight into how nodes would measure up compared with other methods by doing a limited study where we just acquired one swath within a 3-D survey.”

Prior to initiating production on the 3-D survey at Deimos using Fairfield’s Z3000 system, Hays said his company laid out 16 nodes on a single 2-D line in the normal positions they would occupy in the 3-D grid. Fairfield then shot a swath of seven dual-source sail lines into that one receiver line of nodes during what is referred to as “Day 1.”

Hays distinguished sail lines from shot lines, noting the boat has two gun sources on it, so there are two separate tracks of shots that are acquired in one pass of the boat. In the swath there are seven sail lines that produce 14 actual shot-point tracks.

In addition to the initial 16 nodes deployed, there were four extra nodes laid out side-by-side — or co-located — with four of the regular grid locations.

“The purpose was to give a side-by-side look to see, if you had identical shots and replaced a node almost exactly on top, just how repeatable the data would be,” Hays said.

“We did this as kind of a sideline experiment.”

After acquiring the swath of data into the one line of 16 nodes, those original 16 nodes were recovered via ROV, and the data were downloaded and set aside.

“On Day 6, it was time to begin production on the 3-D survey,” Hays said. “Prior to starting, we went down to the same line and redeployed 16 like nodes with the intent of getting back to the same location, knowing we wouldn’t precisely get to it but to the same nominal grid location.”

The instruction for this redeployment was to not search for the imprint of nodes on the seafloor from the first layout but to place them where the crew thought they should be.

“I later looked at the diving video,” Smit said, “and it showed that the ROV operators did not go out of their way searching for the imprint of the previous deployment. They instead redeployed the nodes where they determined they should be, as will be the case in a ‘real life’ 4-D survey.”

This second set of 16 nodes was subjected to a second acquisition via the seven sail lines, and the node set remained on the seafloor for the 60-day Deimos acquisition program.

On Day 45 the crew was back over this same location and acquired another set of seven sail lines — constituting the third experiment — as part of the production 3-D.
Following completion of the 3-D survey on Day 60, one last shooting occurred before the 16 nodes were recovered.

Processing the data

In a perfect world, the data acquired from a repeatability exercise such as Deimos could be processed to form a seismic image to hand off to the interpreter. All would be identical, and nodes could be declared absolutely repeatable.

Perfect world it’s not.

“They’re not decidedly identical and are different for various reasons,” Hays said. “But you can measure those differences, and there’s a statistic that’s calculated to come up with a hard number to quantify that difference.”

It’s called normalized root mean square (NRMS).

“When the NRMS number is low, the data are very repeatable, Hays said. “If it’s zero, you have identical sets of traces from the experiments.”

He noted differences encountered at Deimos. “Between Day 1 and Day 6 [data], we had rather typical statistics of 10% NRMS, which is good,” he said. “In the North Sea, streamer typically is 20% to 50%, and OBC is typically 15% to 25%.

“The positive news of the story is in the deepwater environment, seismic data acquired by nodes is very repeatable, more so than streamer and OBC,” Hays said. “They’re fairly easy to put back close to where they were originally [planted], which in this case was about 16.4 ft (5 m) — you could never control a 4.8-mile (8-km) streamer in varying currents and put every trace back with that precision.”

Comparing Day 1 and Day 60, the NMRS registered a still-respectable 20% but prompted the question, “What’s different this time from the first time?”

“In that 60 days, the conditions in the ocean changed, affecting the temperature and salinity of the water,” Hays noted. “One of the important things we learned is that to get a repeatable survey, processing has to comprehend the wave speed differences in the water layer itself. Water is more dynamic than the crust of the earth, so things can change there — it’s a complicating factor, but when comprehended properly, that repeatability statistic goes down.

“The hypothesis was that nodes will be a good tool for time lapse, and the conclusion is that’s right, based on this experiment,” Hays said.
Smit noted three pertinent aspects of the repeatability program:
• The survey showed the importance of subsea positioning — being able to reach the same location with new nodes;
• The ability to repeat source positions is important, especially where loop currents exist as in the GoM; and
• The shear-wave noise on the vertical geophone is very location-dependent and non-repeatable, so it’s quite critical that this noise be removed, and the Fairfield processing showed this can be done.

“This test did a lot to convince us at Shell that OBS node technology is suitable for time-lapse seismic,” Smit noted.

Hays predicts 4-D will become increasingly common in the GoM.

“It will catch on, especially in these high-dollar fields where the wells are so expensive,” he said. “They really want to exploit these fields with maximum efficiency.”