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AbstractBP initiated the Atlantis ocean bottom seismic (OBS) project to acquire high-quality wide-azimuth seismic data from deepwater subsalt reservoirs. The project, focusing on the Atlantis field in the Gulf of Mexico, represents the first largescale commercial use of autonomous nodes as receivers. Prior to implementing this large-scale project, BP sponsored several sea trials. Conducting the sea trials accelerated the application of the technology to deepwater E&P, avoiding the long initial period of shallow-water implementation that has been the rule for much of today's deepwater technology. The primary goals of the sea trial were to validate hardware designs and operating method, develop budgetary production survey cost estimates, and record and assess the quality of the seismic data in the various seabed conditions. The critical decision in the planning for the sea trials was the selection of the node technology. A node from Fairfield Industries that combined the sensors and a recorder in a single pressure vessel was chosen for the sea trials. Investigations of deployment methods followed, with the requirement that the method would need to deploy the nodes efficiently and safely in the rugged topography of the Sigsbee Escarpment.With limited time available, the sea trials were able to perform both seismic tests and tests of the methods. Seismic tests included the effect of soil shear strength on seismic quality, a comparison of OBS node seismic data to existing towed streamer seismic, vector fidelity, PZ summation, and the quality of hydrophone data gathered at the sea floor. Tests of the method included node deployment and retrieval and some deck operations.From the sea trials, we concluded that the single pressurevessel node is a successful design for collecting OBS seismic data, and that new generation navigational techniques can provide accurate positioning, well within the requirements of the survey. The lessons learned from the sea trials directly benefit at-scale projects using OBS node technology, providing insights not only into improving the methodology, but also into project management techniques for large complex projects.
Shipborne bathymetric surveys south of Santa Barbara, CA indicate that massive slope failures have occurred along the northern flank of the Santa Barbara Basin. This region is seismically active and has the highest sedimentation rates along the California coast due to rapid erosion of the Transverse Ranges. A set of geodetic monitoring systems is being developed to study the character and deformation of the ocean bottom in this region, particularly that of a prominent east-west trending crack aligned with the head scarp of one of the underwater landslides. One component is a high-resolution repeat bathymetric mapping system composed of a multibeam sonar mounted inside a Bluefin 21 AUV. Navigation to the decimeter level is provided by an inertial navigation system and accurate depth sensor, and by a long baseline (LBL) acoustic transponder system previously developed to monitor tectonic plate motions. The quality of the data, particularly that from the high-precision LBL system, is enhanced significantly by the decrease in the radiated acoustic and vibration noise of the AUV to levels at, or below, ocean background noise levels through modifications to the AUV propulsion system. [Work sponsored by British Petroleum and the Office of Naval Research.]
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