Ocean-bottom seismic (OBS) data acquisition is becoming increasingly synonymous with ocean-bottom nodes (OBNs) as the majority of recent seabed seismic surveys have been or are being acquired with nodes, either ROV-deployed or node-on-a-rope/node-on-a-wire (NOAW).

The quality of OBN data is exemplary not only in deep water, where many examples of the superior imaging achieved using full-azimuth sparse receiver long-offset OBN geometries in the Gulf of Mexico (GoM) and offshore West Africa have been published in recent years, but also in shallower waters such as the North Sea (Figure 1).

This North Sea data example shows the significant improvement in imaging the Top Dornoch Formation in the Central North Sea, a notoriously challenging target. It is worth noting that these impressive results were achieved using a very sparse receiver geometry—346 m by 300 m (1,135 ft by 984 ft)—considering the water depth in the survey area was only 120 m (394 ft).

The reasons for the superior imaging delivered by OBS data compared to towed streamer are well known:

  • Wider bandwidth, especially at the low frequencies;
  • Better signal-to-noise ratio;
  • Longer offsets;
  • Full azimuth;
  • Greater 4-D repeatability; and
  • Ability to acquire complete data in obstructed areas.

These benefits are the reason OBNs have become the technology of choice for appraisal and development for subsalt producing areas in the deepwater GoM. This expansion of OBN usage is set to continue as the use of high node-count survey geometries and increased speed of node deployment using both ROVs and automated NOAW handling systems encourage the application of simultaneous or blended sources, which has the double benefit of reduced survey duration/cost and increased data quality through denser shot intervals.

Geographic distribution

For the first five years of OBN usage all of the surveys were undertaken in the GoM, almost exclusively in deep water, but their application has widened geographically recently, with the first 3-D OBN surveys in Southeast Asia and the Middle East being acquired in 2016. Three large OBN surveys will be acquired offshore Brazil in the next year, while the recent downturn in OBN activity in the GoM is likely to be reversed in late 2017-2018. High node-count simultaneous- source OBN surveys will be acquired in Trinidad and Indonesia this year, while node crews will return to the North Sea in 2017 after a hiatus in 2016.

The challenge

It is generally accepted in the industry that OBS data quality is superior to marine streamer data quality, but it costs too much for larger scale adoption.

How can OBS costs be reduced?

  • The first step—the elimination of the traditional sources of OBS technical downtime such as connectors, terminations, power distribution and data telemetry—already is accomplished by adopting OBN rather than ocean-bottom cable technology;
  • Smaller, lighter and lower cost nodes result in smaller back deck footprints, which in turn permit larger node inventories per vessel and faster rigging/de-rigging of vessels of opportunity;
  • Using the same node across the full range of operating water depths allows reduced manufacturing costs and standardization of support and maintenance;
  • Improved node deployment and recovery rates reduce the waiting time between start of node deployment and first shot/last shot and completion of node recovery;
  • Automation of node handling, ROV and NOAW operations results in faster, more accurate node placement/recovery while also reducing QHSE risk;
  • Simultaneous or blended sources provide shorter source duration without reduced source effort;
  • Containerized sources, designed without compromising geophysical performance or operational capability, allow vessels of opportunity to be employed;
  • Single vessel operations, where the node handling vessel also is equipped with full volume seismic source capability, can (especially in remote locations) provide a more cost-effective solution; and
  • Surveys can be designed to eliminate costly waiting time on both the source and the receiver sides by ensuring the source and receiver efforts are properly balanced.

Operational performance

For ROV-deployed nodes with the advent of third-generation node technology, where the physical size and weight of the nodes have been greatly reduced, the latest generation of increased-power ROVs and automation of the ROV operations themselves have resulted in greatly improved node deployment and recovery speeds. Historically, single ROV node deployment rates of 40 to 50 nodes/day in deep water were typically achieved. However, this can be increased by a factor of four or more today. This, in turn, allows the use of simultaneous or blended sources. Surveys that were heavily source-bound, requiring several months of source effort, can be shot much more rapidly, reducing costs and potentially allowing OBN technology to be applied for exploration objectives.

Automated NOAW systems are set to deliver similar improvements in deployment/recovery speed, allowing similar efficiencies in dense receiver survey operations to be realized.

Figure 2 illustrates the relative project costs that can be achieved using the combination of the survey efficiency-enhancing approaches described above. The blue shaded area represents the OBN performance that can be achieved using NOAW or ROV deployment. It is not always intuitive which deployment method will result in the optimum, so an unbiased approach is recommended. This entails rigorous time and motion modeling of the two OBN survey methodologies based on acquiring data with the same geophysical parameters— bin size, fold, offset, azimuth and record length, to name a few. These are not the only criteria, of course. Once the survey objectives are understood, the environmental constraints are examined and the resulting potential survey designs are generated. The comparative technology solutions are then evaluated from both operational and financial perspectives before a final design and methodology are proposed.

The adoption of OBN data acquisition is increasing both geographically and in application beyond its traditional role in appraisal and development. The latest generation of nodes and associated deployment technology will, when coupled with the increasing use of simultaneous sources, expand adoption further. In the future, when AUV technology removes the need for either ROVs or deployment wires/ropes, the industry can expect to see a step change in receiver handling speeds that results in unprecedented survey performance. This will have a dramatic impact on the use of OBS for exploration 3-D. And once deblending algorithms are shown to be 4-D-friendly, even in “stiff” rock reservoirs, the combination of AUV and simultaneous source will expand into this arena too.

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