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Introduction The Torosa gas field forms part of the proposed Browse LNG Development, a project to commercialise three gas and condensate fields located on the outer continental shelf of northwest Australia, approximately 400km north of Broome. Two of these fields (the Brecknock and Calliance fields) are located in deep water and have been fully imaged by modern 3D towed streamer seismic surveys. However, the Torosa field partly underlies Scott Reef, which consists of two coral atolls separated by a deep channel. The only permanently emergent land at Scott Reef is a small sand cay (Sandy Islet - Figure 1), although the reef crests of both atolls are exposed at low tide. Outside the reefs the seafloor drops away rapidly, with water depths of about 350m to the east, increasing to more than 1000m to the west. South Scott Reef lagoon is open to the north, with water depths increasing to about 50m before deepening abruptly into the channel between the two reefs. North Scott Reef lagoon is shallower - generally less than 25m - and is connected to the ocean by two narrow channels. Semi-diurnal tides with a range of up to 4.6m produce strong tidal currents in and near these channels. Small, steep-sided coral heads, or 'bommies', are common throughout the lagoons, especially in water less than 25m deep. Since 2005 Woodside Energy Ltd has acquired five seismic surveys over the Torosa field using a range of acquisition techniques. These surveys were important steps towards acquiring full seismic coverage over the Torosa gas field (Figure 1). The first of these surveys, the Torosa 3D marine seismic survey (MSS), was a conventional deep water 3D towed streamer marine seismic survey acquired in late 2005, which covered the deep water portion of the Torosa Field located northeast of Scott Reef. Four further seismic surveys have extended the area of 3D seismic coverage into the south Scott Reef lagoon and across north Scott Reef. Prior to commencing the seismic programme over the shallow-water parts of the Torosa Field, Woodside carried out an extensive airborne bathymetric survey of the entire Scott Reef system in early 2006. Additional bathymetric surveying was undertaken to fill some data gaps within the airborne bathymetry survey, resulting in a comprehensive, highly-detailed data set on a 4m grid with vertical resolution of 0.1m covering both north and south Scott Reef to depths of about 50m. Woodside also undertook extensive metocean work including both tide and current modelling and model verification. These datasets were critical for the planning of the subsequent seismic surveys.
Abstract The Alaska North Slope, within the Arctic Circle, is a challenging environment to operate seismic crews. The company has operated land and marine seismic programs on the North Slope since 1979. We present a review of the summer 2012 Simpson Lagoon seismic survey, outlining the safety, operational and technical challenges faced in an arctic transition zone environment. Ocean Bottom Seismic (OBS) data on the North Slope is essential for the effective development of the hydrocarbon fields, providing improved and accurate subsurface imaging. The arctic open-water season provides a short window of opportunity to collect OBS data, bounded by the seasonal break-up and freeze-up of the sea ice and subsistence whale hunting. These factors constrain the quantity and influence the quality of the data collected, however sufficient resource allocated to pre-season planning is critical to the success of the seismic project. We demonstrate the planning and decisions made were influenced by the safety of the seismic crew, minimizing impact to the environment, and the collection of quality seamless subsurface seismic data. Key successes of the survey include: 24 hour operations completed within 45 days; completion of the seismic objectives equating to 88 square miles of data; enhanced imaging below the lagoon barrier islands carried out by safe and efficient helicopter operations utilizing autonomous land nodes; and innovative techniques for burying receiver sensors into the surf zone which significantly improved data quality.
The Atlantis ocean bottom seismic (OBS) node project applied the technology of deep water node seismic acquisition in an effort to achieve high-quality subsalt imaging. As the first large-scale, deep water seismic acquisition survey to obtain data from autonomous nodes, the survey fostered innovations in node manufacturing, node deployment and survey design. Deployment equipment included a back deck designed to minimize human intervention, two remotely operated vehicles (ROVs), specially designed ROV skids and an advanced navigation system. Intensive modeling provided an assessment of optimal node spacing, node layout grid and source offsets. Further considerations of operational and investment costs, geophysical criteria, and processing requirements led to a final survey design of approximately 6 nodes/km2 in a hexagonal sampling pattern, divided into two patches. The Atlantis OBS node project successfully took a complex technology directly from inception to deep water application, bypassing the gradual progression from shallow to deep water implementation that has been standard for most deep water technologies.