Deep Structure of the Southern North West Shelf : Offshore Canning Basin (SNOWS-3) Pre-cruise Report

In mid-1991, the Bureau of Mineral Resources (BMR; now the Australian Geological Survey Organisation, AGSO) commenced a program of deep-seismic acquisition on the southern North West Shelf with the intention of providing a regional framework data set for explorersin this highly prospective segment of Australia's continental margin. In particular, the program aims to: Determine the broad regional structural framework of the southern North West Shelf by examining the boundaries between the major structural elements; Determine the deep crustal structure of the region; Assess the control of deep structure on the development of the major hydrocarbon fields and plays, and in particular the structural and depositional effects resulting fromreactivation of these structures; and Acquire a set of high-quality seismic tie lines linking the deeper exploration wells throughout the region, to allow regional seismic correlations. To address these aims, a multi-cruise program was devised during which deep-seismic data are being recorded. The first survey, SNOWS-1 (for Southern North West Shelf; AGSOSurvey 101; Stagg, Brassil, & others,1991) was concentrated in the Barrow and Dampier Sub-basins and inner Exmouth Plateau. 1654 km of good-quality seismic data tied to 20 exploration wells were recorded and processed; these data frequently show basin structuredown to a depth of at least 10 s two-way time (TWT). The second survey, SNOWS-2 (AGSO Survey 110; Stagg & Survey 110 Shipboard Party, 1992), acquired more than 2800km of high-quality deep seismic data along 13 lines in the Beagle, Dampier, and Barrow Sub-basins, and over the full width of the Exmouth Plateau in mid-1992. These lines were tied to21 exploration wells, of which 3 were also tied during SNOWS-I, and again show reflections down to 12 s TWT. In the second half of 1993, the Marine Geosciences and Petroleum Geology Program (MGPG) at AGSO will continue this program with the third survey in the area, to be knownas SNOWS-3. The survey will be concentrated in the offshore Canning Basin, and will tie in with the SNOWS-2 survey to the southwest in the Beagle Sub-basin, and the 1993 Browse Basin survey to the northeast. The survey will tie the principal exploration wells in the regionand is expected to acquire at least 3000 km of data along long regional lines; the program allows for up to 3500 km of data to be recorded. As part of the MGPG program of acquiringdeep crustal transects across the margin adjacent to the North West Shelf, the opportunity will be taken to record at least one margin transect from near the coast out to Ocean DrillingProgram Site 765 on the southeast Argo Abyssal Plain. Recording parameters for the survey will include a 4800 m streamer, configured with 192 x25 m active groups; the record length will be 16 seconds, and the sample interval 2 msec. The seismic source will be provided by tuned airgun arrays of total capacity 49 litres, firedevery 50 metres to give 48-fold CDP coverage. These parameters are the same as were used on the SNOWS-1 and SNOWS-2 surveys. Navigation for the survey will be provided by differential Global Positioning System (DGPS),using shore reference stations in Western Australia. Full differential coverage should beachieved for the entire survey and it is estimated that positional accuracy should be betterthan +/- 10 metres.