The Atlantis OBS Project: OBS Node Positioning and Management

Abstract The Atlantis ocean bottom seismic (OBS) project collected wide-azimuth seismic data from the deepwater Gulf of Mexico using over 900 autonomous nodes which were planted on and retrieved from the sea floor by remotely operated vehicles (ROVs). Rugged topography posed challenges, with the Sigsbee Escarpment, which crossed the area, having depths that varied from 1,200 m to 2,300 m. The project required precise and highly repeatable navigational data to position the nodes correctly and retrieve them quickly. The navigation technique employed a dual Hydrostatic Acoustic Aided Navigation (HAIN) system for node positioning and retrieval, the first time a HAIN system has been used in a stop-and-go configuration with two ROVs running simultaneously. Detailed bathymetric data provided data needed to position the nodes in relatively flat terrain (a maximum slope of 10 degrees). Additional tools included real-time 3D visualization software to gather ROV position data and GIS seabed gradient maps, contour maps, and shaded relief maps to review node locations. This navigation system proved to be successful in yielding a close relationship between node deployment and recovery positions. Introduction Offshore hydrocarbon reservoirs in areas of salt deposits when mapped with traditional seismic techniques from the surface result in a ?blurred' image at best. Although the general location and geological features of the reservoirs are known the information is still insufficient to delineate the reservoir in detail. To address this issue a new development in seismic technology is being employed by BP and Fairfield Industries in the Atlantis Field (Figure 1) in the Gulf of Mexico that would allow petroleum engineers and geologists to better understand the size and shape of these subsea reservoirs. This paper describes the navigation and data management techniques employed for the Atlantis ocean bottom seismic (OBS) project. The project is a pioneering effort in which autonomous nodes designed to collect wide-azimuth data for subsalt imaging are deployed onto and retrieved from the deepwater seabed by remotely operated vehicles (ROVs). Other authors1-3 provide details of the goals and background of the project. Openshaw and Beaudoin4present an account of the sea trials used to test the OBS system. Mitchell and Grisham3discuss how the OBS nodes were manufactured, and Thompson, Fraser, and Fyffe5 explain the system used to deploy the OBS nodes. Figure 1: 3D Multibeam-derived view of the Atlantis Field and the Sigsbee Escarpment, Gulf of Mexico. (available in fullpaper) Background Fairfield Industries subcontracted Canyon Offshore, a deepwater-ROV company, and Geo Century, a specialist in deepwater surveying and positioning, to deliver a new survey systems approach for a reliable, accurate, and highly repeatable position and data management solution. The project (currently ongoing in January 2006) started offshore in mid-October 2005 onboard Canyon's new build ROVSV UT745, the M/V Northern Canyon. The offshore program involved an innovative approach for the acquisition of 360-degree azimuth seismic data through the use of Fairfield's newly designed seabed seismic node acquisition system.