Marine Seismic Acquisition at Scott Reef, Western Australia: Minimising Environmental Impacts in a Sensitive and Remote Location

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 4m2 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.