North West Margin Transects : Survey 128 Post-cruise Report

The North West Margin Transects Cruise (Survey 128) forms part of a programbeing undertaken by AGSO to determine the structural architecture of the north-western margin of Australia and the influence of structuring on the location,migration and trapping of hydrocarbons in the region. An important aspect of thisprogram is the acquisition of a series of full margin 'dip transects extending fromunrifted cratonic basement to beyond the continent-ocean boundary, and at leasttwo 'strike' transects that will extend along the full length of the shelf from theeastern Timor Sea to North West Cape. The major objective of Survey 128 was toextend previous AGSO deep-seismic surveys across the Browse (Survey 119),offshore Canning (SNOWS-3/Survey 120) and northern Camarvon basins (SNOWS-1/Survey 101, SNOWS-2/Survey 110) to complete these margin transects. The survey vessel Rig Seismic left Port Hedland on 30 May 1994. A weather buoy0^was deployed for the Bureau of Meteorology at the beginning of the survey. Seismic acquisition commenced on 4 June 1994. The survey was completed on 25 June0^and finished in Fremantle on 30 June 1994. During the survey, 11 seismic lineswere completed for a total of 3403.05 km at an average of 155 km per day (based on 22 days of acquisition). Transits between lines amounted to more than 800 km. The survey ties into the 1991 SNOWS-1 (101), 1992 SNOWS-2 (110), 1993 Browse Basin (119) and SNOWS-3 (120) deep-seismic surveys, and the 1990 standardrecord length Triassic Reef Survey (95). The data were recorded from a 4800 m streamer, configured with 192 x 25 m active groups. Data were recorded with a 16 s record length and a 2 ms sample interval.The seismic source consisted of dual sleeve gun arrays with a total capacity of 50 litres, and was fired every 50 metres to give 48-fold CDP coverage. Navigation for the survey was provided by differential Global Positioning System (dGPS), using shore reference stations in Dampier, Broome, Perth and Darwin. Both the seismic and non-seismic acquisition systems ran without major problems. Navigational data were of good quality, with differential GPS being available for more than 95% of the survey.