Deep Structure of the Offshore Canning Basin, North West Shelf : SNOWS-3 (Survey 120) Post-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. This program is part of a wider program that is acquiring a regional grid of deep-seismic data along the length of the North West Shelf. In particular, the program aims to: Determine the broad regional structural framework of the southern North West Shelf byexamining 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 fieldsand plays, and in particular the structural and depositional effects resulting from reactivation 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 h_Torth Eest Shelf; AGSO Survey 101) 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 recordedand processed; these data frequently show basin structure down to a depth of at least 10 s two-way time (TWT). The second survey, SNOWS-2 (AGSO Survey 110), acquired morethan 2800 km 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 lineswere tied to 21 exploration wells, of which 3 were also tied during SNOWS-I, and again show reflections down to 12 s TWT. The third survey in the program, SNOWS-3 (AGSO Survey 120), took place in July-August 1993, and acquired a total of 4052.5 km of high-quality deep-seismic data along 14 lines, tiedto 6 exploration wells (of which 2 were also tied on SNOWS-2) and Ocean Drilling Program Site 765 in the offshore Canning Basin, northeast extremity of the Carnarvon Basin, andO^Argo Abyssal Plain. SNOWS-3 ties in with the SNOWS-2 survey to the southwest and withthe Browse Basin deep-seismic survey (AGSO Survey 119, acquired in June, 1993) to the northeast. The seismic data were recorded from a 4800 m streamer, configured with 192 x 25 m active groups. The record length was 16 seconds and the sample interval 2 msec. The seismicsource consisted of tuned airgun arrays with a total volume of 49 litres (3000 cu in). Shots were fired every 50 m at an average ship speed of 5 knots, providing 48-fold coverage.Streamer noise levels were low, with the average noise levels on acceptable channels generally being less than 1.5 rnicrobars. Navigation for the survey was provided by two differential Global Positioning Systems (DGPS), using shore reference stations in Dampier and Broome, adjacent to the survey area.Full differential coverage was achieved for all but a few minutes of the survey. Positionalaccuracy is estimated to be probably better than +I- 10 metres. The offshore Canning Basin is a complex amalgam of a WNW-ESE trending EarlyPalaeozoic intra-cratonic basin overlain by a NE-SW trending Late Palaeozoic to Mesozoicpassive margin basin (the Westralian Super-basin)