Deep Structure of the Eastern Malita Graben and Adjacent Areas in the Timor and Western Arafura Seas: Survey 118 Operational Report

The objectives of Survey 118 were twofold. The primary objective was the acquisition of high quality deep seismic data and other geophysical data over the eastern Malita Graben region (including the Calder Graben and western part of the Goulburn Graben) in the northern Timor and western Arafura Seas (Project 121.39). The secondary objective was to collect deep seismic data on several long regional traverses in the Petrel Sub-basin - Londonderry Arch - NE Browse Basin region. The cruise was part of a continuing major program by AGSO designed to establish the structural architecture of the northwest Australian continental margin, examine reactivation histories and develop models for basin evolution in the region. The program aims to assist petroleum exploration and development of offshore resources. The Rig Seismic left Darwin at 1700 hrs (local time) on Friday 30 April 1993 and transited to the Manta Graben survey area to the northwest. Seismic acquisition began at 0800 hrs on Monday 3 May. The survey progressed well and without major interruption. Survey operations were suspended on two occasions due to bad weather, total time lost was about one day. Seismic cable inspection and maintenance, including replacement of batteries in depth controllers, also resulted in some down-time. The survey ended in the NE Browse Basin at 0300 hrs Saturday 29 May. The ship transited back to Darwin, arriving at 0815 hrs on Monday 31 May. There was no significant damage to the streamer, and apart from 3 Norwegian buoys (from the gun arrays), there was no loss of equipment. When the seismic cable was retrieved at the end of the survey, it was found to be in very good condition and no sections had to be replaced. Breakdown of the ship's shaft generator and failure of the UPS (Uninterruptible Power Supply) early in the survey had little impact on productivity, apart from the loss of 6 hours survey time incurred in looping back on line. A total of 5 gun maintenance loops were required during the cruise; an additional loop was needed to free a tangle on one array between towing chain and gun bundle. Both the seismic and non-seismic (DAS) acquisition systems ran without problems and there were no system crashes. The onboard, real-time monitoring systems gave good QC on the data as they were collected. Navigational data were of very good quality, with differential GPS positioning being available at least 95% of the time. Data collected on Survey 118 comprised:- - Deep seismic reflection data, 48-fold, 16 second record-length. These were acquired with a 4800 m active length streamer and dual tuned airgun arrays (20 sleeve-guns of 50 litres total capacity). - Gravity data on all lines, i.e. on all seismic lines and all transit lines. - Magnetic data on 90% of seismic lines and most transits. - Bathymetry data on all lines. A total of 17 seismic lines were shot in the Malita - Goulburn Graben region, and 4 lines were shot in the Petrel Sub-basin - NE Browse Basin area. The lines were tied to 7 exploration wells. The total seismic production was 3602 line-km. Of this, 2858 km were collected in the Malita - Goulburn Graben region and the rest (744 km) were collected on the 4 regional lines in the Petrel Sub-basin - NE Browse Basin area. Survey 118 was a very successful operation with all lines in the planned core seismic program completed and a further two lines (on the Sahul Platform and across the western Goulburn Graben) added.