Ever since researchers first embraced the concept of multicomponent seismic, acquiring shear- as well as compressional-wave data, the industry has been at odds over how best to gather this information, particularly offshore. Since shear waves don’t travel through water, the choice was to place sensors on the ocean bottom or measure “converted” shear waves with towed streamers. Purists preferred the first method. But the cost of deploying and recovering these sensors was cost-prohibitive.

An interim solution was found with ocean-bottom cables (OBC), which could be lain on the

Figure 1. Artist’s view of seabed multi-component seismic operations. (Images courtesy of SeaBird Exploration)
seafloor and recovered after the survey was complete. The deployment was expensive, and while the data were generally of better quality than stremer data, complete coupling to the seafloor could not be guaranteed. OBC equipment could also be subject to damage from fishing vessels and other ship traffic, and it wasn’t much easier than streamers to deploy around infrastructure.

An increase in commodity prices coupled with the industry’s desperate need to learn more about its assets has propelled a new technology forward — the ocean-bottom node. Nodes are placed in a grid using remotely operated vehicles (ROVs), left in place for several days while the survey is being shot and recovered afterwards for data download. While these surveys are quite expensive, they are sometimes the only method available in situations such as heavy infrastructure, environmentally delicate areas or ultradeep water, where working with a cable is difficult.

Getting a start
Only a few players currently offer node systems. One of these is SeaBird Exploration. SeaBird bought a Norwegian company called Seabed Geophysical in 2006 to add node technology to its portfolio of seismic services.

Seabed was the brainchild of Eivind Berg, who was awarded for his work by the Society of Exploration Geophysicists in 1999 with the Virgil Kauffman Gold Medal. In the write-up honoring Berg, his colleague Professor Martin Landrø noted that Berg was so enthusiastic about the concept of placing sensors on the seabed for multicomponent measurements that he convinced his employer, Statoil, to invest a considerable amount of money in the research. “In retrospect, four-component seabed seismic would be way behind its current level without this enlightened management decision,” Landrø wrote.

Early work revolved around imaging through gas clouds in various North Sea fields. But by 2003 Seabed had been spun off and was ready for a major field trial. The declining Cantarell field offshore Mexico provided the perfect test ground.

Operator Pemex knew it had a deeper horizon in the Sihil field underlying the giant Akal
Figure 2. 400 m by 400 m node layout over Cantarell field, Bay of Campeche, Mexico, with installations and pipelines indicated in green.
field. But imaging difficulties were numerous — in addition to being a fractured carbonate reservoir with salt and an overthrust structure, Cantarell is also a producing field, with considerable infrastructure to work around.

To cover the area of interest, more than 1,500 receiver units were deployed into seven swaths in a 1,300-ft by 1,300-ft (400-m by 400-m) grid, covering a total of almost 90 sq miles (230 sq km). While the Seabed system is unique in that the sensors are actually burrowed into the seafloor, Cantarell posed a challenge here as well.

“The softness of the seabed was an interesting problem,” said Brian Anderson, vice president of sales for SeaBird. “It’s difficult to tell where the seabed begins and the water ends.” ROVs were not totally successful in deploying the nodes, but a crane set-up eventually proved useful in the shallowwater field.

Anderson said that the nodes performed at 99% efficiency. In a couple of instances the nodes listed in the soft sediment, but acoustic links to the surface alerted the survey crew, and the problems were fixed.

To date this is the largest seabed node survey ever undertaken.

The technology
SeaBird, through its daughter company Seabed Geophysical, offers two node systems, CASE
Figure 3. Comparison of OBC (left) and node data from Cantarell field, with frequency spectrum plot. Time processed data from Vazquez et. al, SEG 2005. Data provided courtesy of PEMEX.
(Cableless Seismic System) and CASE Abyss, for deepwater applications. Both systems have a CPU, 24-bit resolution, 200 days worth of 2 ms seismic data storage (75 days for CASE Abyss), a quality-control data link, more than 60 days of battery life, three geophones and a hydrophone, and individually calibrated sensors.

The value case for this technology reads like a geophysicist’s wish list:
• full-azimuth, wide-offset distribution mapping;
• no holes, infill or data degradation;
• compressional and shear wave responses that are insensitive to water depth or seabed topography;
• accurate, repeatable receiver positioning and orientation;
• repeatable, low-distortion seismic vector response; and
• reliable receiver ghost and water-layer reverberation suppression.

These types of node systems offer several advantages over other forms of seismic acquisition. In a simple 2-D exploration mode, they can be useful in reducing uncertainty and dry holes in expensive deepwater environments. Anderson said that even with the sticky seafloor at Cantarell, the nodes were able to acquire excellent shear wave data. “That opens up multiple ways of looking at the same seismic data, and you can start looking at the stratigraphic traps,” he said.

In 3-D surveys, the high-quality data acquired can help geoscientists improve their definition of reservoir volume, lithology, fluid content, porosity, permeability, fracturing, principle stress direction and azimuthal anisotropy. This information, in turn, can lead to better well placement, fewer appraisal wells and increased recovery.

Uses for time-lapse surveys are obvious — placing nodes on the seafloor means that repeatability becomes much easier. “Our radius of error in terms of positioning is well within the requirements for time lapse [4-D] work,” Anderson said. “That’s something that makes us a great choice for a T-0 and subsequent time-lapse surveys.” The only significant repeatability variable is the location of the energy source, he said.

He added that companies that offer node surveys are well served by focusing their efforts on
Figure 4. CASE Abyss node design. Operational to 9,850-ft (3,000-m) water depths. Records 2 mSec continuous data for 75+ days with quality control via acoustic communication link.
the 4-D market. “We’re still slow enough on the acquisition side, and we’re still more expensive than even a multi-vessel wide azimuth towed-streamer survey,” he said. “That makes our primary focus reservoir, production and development geophysics, but with efficiency gains even some targeted exploration surveys will happen.

“The nice part about that is that development and production work is the part of geophysics that seems to weather the storm when we overbuild in the industry.”

Additional 4-D benefits include lower installation costs than with the trenching required for OBC and other permanent installations, easy relocation during any stage of the field’s life, no system degradation over time, and the ability to easily incorporate system improvements and evolution.

In the future Anderson said that SeaBird and Seabed will examine the possibility of using the nodes for passive seismic work, in which they’re left on the seafloor to listen to naturally occurring noises and hydrofrac-induced seismic energy. He’s also anticipating a more routine processing workflow for multicomponent data. “One factor hampering the widespread acceptance of our science is getting from where processing multicomponent data is like a handmade pair of shoes to where there’s some standardization,” he said.

Despite the obstacles, nodes are clearly past the “if only” stage. As Berg commented in an article for “CIS Oil & Gas,” “This market segment is expected to grow rapidly in the next 2-3 years.”

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