I n an ever changing and evolving seismic acquisition market, recent technological advances have made access to multi-component, full-azimuth high-density data much more viable and practical than

ever before. Four component (4C) ocean bottom node (OBN) technology has become a major player in the seismic acquisition field, not only for exploratory data acquisition in challenging environments such as obstructed, high traffic or congested waters, but also as a tool for life of field reservoir management to optimise a reservoir’s full potential.

Karla Keeton-Page and Changjun Zhang, Seabed Geosolutions, USA, investigate developments in ocean-bottom seismic node technology.

| Oilfield Technology Reprinted from February 2015

New developmentsOne advance in this technology is Seabed Geosolutions’ Manta™ node system, the evolution of seabed seismic technology. Manta is a flexible seismic ocean bottom solution that can be tailored to almost any survey requirement and available resource.

The system provides a single ocean bottom node solution for all water depths ranging from 0 - 3000 m (Figure 1). It can overcome challenging environments with complex geologies and deliver improved data clarity through versatility in survey design and flexibility of deployment. The system was designed to deliver improved geophysical illumination with varied options for dense source grid, full-azimuth and long offset surveys. These capabilities, together with the low ambient noise environment generally found on the seabed, have made OBN surveys particularly appealing for high fidelity imaging required for detailed reservoir characterisation.

Fault definition and fluid detection are critical factors in reaching higher extraction rates in a producing reservoir asset. The system records PP and PS seismic attributes which enable better delineation of hydrocarbon zones and subsurface lithologies. The shear waves that are recorded can be used for imaging under gas clouds and for lithology characterisation. Shear wave data is particularly advantageous when imaging

through gas clouds, as shear waves are largely insensitive to pore fluid content. The shear waves add information that helps distinguish between lithology, fluid and pore pressure effects. In addition, shear wave splitting is a robust analysis that helps characterise fractures and tectonic-effective stress variations.

Seabed Geosolutions’ research and development team has combined contemporary micro-electronic components with recent advances in rechargeable power-dense lithium-ion battery technology to create a compact, lightweight node suitable for almost all survey locations and designs. The Manta node weighs 20.5 kg (45 lb) in air, and measures 350 mm by 350 mm by 130 mm high (1.15 ft x 1.15 ft x 0.43 ft high). The stackable node with its reduced size helps economise deck space, allowing for increased flexibility and optimisation of node counts (Figure 2). The battery capacity offers a minimum of 74 days of continuous recording time. The recording system records on 4 channels at a 2 msec. sample rate, with a capacity of 64 Gb over 120 days at a 24 bit recording resolution.

The node is comprised of a four component (4C) sensor incorporating three geophones mounted in a Galperin orientation and one hydrophone. The integral inclinometer continually records the orientation of the node once it is positioned on the seafloor. This component configuration, which

Figure 1. The Manta system offers single node ocean bottom technology for all water depths from 0 - 3000 m. The system has the flexibility to be deployed via a traditional dense receiver configuration or by remotely operated underwater vehicle (ROV).

Reprinted from February 2015 Oilfield Technology |

includes omni-directional geophones, provides a uniform azimuthal response capturing the full wavefield including converted PS waves. Combining the geophone and hydrophone recording will eliminate receiver ghost notches on the signal spectrum due to the pressure and velocity sensor’s complementary receiver ghosts. This combination will then expand the effective frequency band of the recorded seismic signal. With properly designed acquisition geometry, an OBN survey can generally acquire data with full azimuth and a wide frequency band. Specifically, the low frequency component is imperative for high-resolution imaging and full wave inversion velocity model building.

Health and safetyManta nodes have been designed with maximum safety in mind. This unique technical offering has been developed in unison with proprietary designed robotics to create a ‘no touch’ automated handling and rapid continuous deployment system, thereby reducing HSE risks while achieving operational efficiency. The customised ‘no touch’ handling system delivers the Manta node in either a traditional dense receiver configuration or by remotely operated underwater vehicle (ROV). The entire concept of the ‘no touch’ deployment and recovery system with highly automated node storage, data download and battery charging, is founded on a transportable modular solution using locally available state-of-the-art dynamic positioning (DP) vessels. Its ease of use makes the technology ideal during the production phase for understanding, monitoring and managing the reservoir across the life of the field.

FlexibilityThe flexibility of the Manta system means it can go where traditional streamer seismic cannot. With the variety of deployment methods the system offers, the highest resolution subsurface data can be acquired in challenging obstructed areas and from shallow transition zones to deepwater. As well as operating in water depths up to 3000 m, the Manta system can be safely and precisely positioned in heavily obstructed fields close to infrastructure, providing the ability to collect high quality full azimuthal and full offset datasets in areas where it is impossible to navigate with large streamer spreads. Moreover, the flexibility to combine ‘node on a rope’ variable in-line receiver spacing and ROV deployment in one survey makes Manta even more efficient for a larger survey with platform or field facility obstructions. The receiver location can be accurately repeated in future monitor surveys, making the system well suited for 4D life of field management.

The node also can be deployed in an ad-hoc ‘node on demand’ basis for a small, densely shot area whereby the target needs special focus on high quality data, but where a large-scale ocean bottom seismic survey is not necessarily needed. Node on demand is designed for a small survey using up to 400 nodes and is characterised by its short rigging and de-rigging time with minimal equipment and crew. This type of survey can be achieved with

vessels of opportunity or by using in-field resources already in the survey zone. The equipment is shipped to the site in containers by road, sea or air as required.

Node on demand using Manta nodes offers operational advantages over a large full-scale survey as well. These include mitigation of tidal interference, a fast response when weather windows open, and it can avoid many of the issues involved with timeshare when other surveys are being acquired nearby, or when the field is in the process of development and installations are being built. It greatly reduces many of the operational risks inherent in standard streamer-based survey designs. This node on demand solution complements streamer data for undershoots, ensuring data with no infill (compared to streamer seismic), a higher proportion of near offsets and full azimuthal coverage.

Another benefit of node on demand utilising the Manta system is that quality control (QC) is done using laptops, so a large QC system rig up is not required. Minimal onboard quality control is performed to validate data integrity before further processing.

Data can easily be co-processed with streamer data when it is used in conjunction with a streamer survey. It can also be used for stand-alone acquisition over smaller areas for a more focused approach or for intermediate 4D monitoring. Node on demand is a proven concept that has been effectively applied by Seabed Geosolutions with good results in the North Sea and offshore West Africa.

ConclusionAs oil companies work to improve the efficiency of currently producing fields, and as they look further and further afield for future discoveries, advances in seismic acquisition technology are increasingly important to keep up with the demand for creative and versatile ways to leverage oilfield resources. Crowded fields, geophysically challenging areas and a tightening market all affect today’s seismic survey design and acquisition planning. Flexible and state-of-the-art technologies are necessary to help navigate this new acquisition landscape, allowing for high quality data collection while simplifying operational and safety requirements. Seabed Geosolutions’ research and development team has created the Manta node system to address these new geophysical mandates, offering simplified access to full azimuth, wide broadband 4C data in almost any water depth and with comprehensive versatility of survey design and safe and efficient deployment. 

Figure 2. The Manta node weighs 20.5 kg in air, and measures 350 mm by 350 mm by 130 mm high. Its stackable shape helps economise deck space and allows for a variety of modular transport and deployment methods. Its flexibility of use makes it a versatile and comprehensive ocean bottom node solution.