Seismic Ocean Botttom Nodes: A Versatile Field Development Tool

Abstract

Abstract In deepwater such as Santos basin pre-salt environment enhanced imaging can be achieved using wide azimuth acquisition geometry (WAZ). There are two options available for seismic imaging: towed streamers and seabed receivers. During field development, surveys usually have a limited size (<300km2). WATS (wide azimuth towed streamers) have operational restrictions due to surface obstructions such as FPSOs, offloading buoys and drilling rigs. Undershooting with multiple vessels is a solution with significant data quality limitation, especially in the context of seismic monitoring (4D). Ocean Bottom Cables (OBC) also have operational issues due to water depth and due to seabed obstructions such as pipelines, and anchor chains. Ocean Bottom Nodes (OBN) provide excellent Full-Azimuth (FAZ) Multi-component (4C) seismic data. OBN can be safely used in areas with both surface and seabed obstructions. Deployed by Remotely Operated Vehicle (ROVs) the nodes provide high repeatability for 4D seismic monitoring. Some OBNs can be mobilized on a vessel of opportunity and can be deployed by a dynamically positioned vessel already working in the field, thus increasing the safety and reducing the cost of the node survey. The technology is ready for commercial applications and spectacular results have been already achieved. The industry is now developing specific business models and operational models for combining nodes with surface towed streamers and with permanent reservoir monitoring OBC. Ocean Bottom Nodes are emerging as a new acquisition method Unlike surface towed streamers and Ocean Bottom Cables (OBC) the Ocean Bottom Nodes (OBN) are autonomous recording systems (Fig 1) and Remotely Operated Vehicles (ROV) (Fig 2) can deploy them safely on the seabed. Each node has seismic sensors, an analog to digital convertor, a clock, memory and battery, they are able to provide continuous records for weeks and month. They also record their tilt, voltage, temperature, and humidity. Because each node has its own clock and keeps its time, Clock drift and synchronization should be handled carefully and may causes delays in data processing but has never caused loss of data. The seismic sensors are a hydrophone and three geophones for four components (4C). The seismic sensors record both primary P waves and secondary Shear waves. The combination of the hydrophones and the geophones enables separation of the P waves to up-going and down-going wavefields. This separation is important for imaging with both primaries and multiples. Ocean bottom nodes are not a new idea. Oceanographers have used them for decades, but recent computing- battery- and ROV- technologies are making them more and more practical and affordable. In addition to the technology push, there is an increasing market pull for nodes for exploration of hard to find oil and for reservoir development and production monitoring.