ConocoPhillips Australia SH1 Pty Limited (‘ConocoPhillips Australia’) and 3D Oil T49P Pty Limited (‘3D Oil’) are proposing to undertake the Sequoia three-dimensional (3D) marine seismic survey (MSS) within Exploration Permit T/49P in the Otway Basin, west of King Island.
The purpose of the Sequoia 3DMSS is to acquire geophysical data to provide a 3D image of the subsurface geology within the T/49P permit area in order to identify prospective commercially viable gas reservoirs for future development.
The proposed Sequoia 3DMSS comprises two areas, these being the:
- ‘Acquisition area’ – the physical area in which the seismic source will operate (i.e., acquire data), covering an area of 2,840 km2. The acquisition area measures 129 km long (north-south orientation) at its longest and 25 km wide (west-east orientation) at its widest.
- ‘Operational area’ – the physical area in which operations ancillary to achieving survey coverage will take place. This includes vessel approach, vessel line turns (16 km allowance), ‘soft starts’ of the seismic source, run-ins and run-outs of the seismic source and miscellaneous maintenance operations. The operational area measures 153 km long (north-south orientation) and 26.5 km wide (west-east orientation), covering an area of 4,089 km2.
The acquisition and operational areas combined are referred to as the ‘survey area’. Water depths in the survey area range from 70 m to 1,000 m, with 91% of the survey area being in water depths of less than 150 m. At its nearest points, the survey area is located 23.5 km from the west coast of King Island and 26 km from the Victorian coast.
The Sequoia 3DMSS is scheduled to take place during the window of 1st August to 31st October 2021. This window of time is considered most suitable for survey acquisition because:
- Displacement of commercial fishing vessels is kept to a minimum;
- The Victorian southern rock lobster and giant crab fishery are closed;
- The Tasmanian southern rock lobster fishery is closed;
- There is low catch in the Tasmanian giant crab fishery;
- There is a low likelihood of presence of the pygmy blue whale, southern right whale or humpback whales in the survey area.
The survey is expected to take approximately 60 days, though the vessel may be present on location for longer than this (but not acquire outside the months of August and October) depending on factors such as sea state conditions, whale-instigated shutdowns and technical issues. The actual number of days of active seismic acquisition will be approximately 31 days.
The survey vessel will acquire the seismic data by towing three acoustic source sub-arrays operating alternatively, one discharging as the others recompress. The lateral distance between each of the sources will be 25 m to 37.5 m. The source volume will be a maximum of 3,480 cubic inches (cui) with an operating pressure of 2,000 pounds per square inch (psi). There will be 12 to 18 hydrophone ‘streamer’ cables approximately 6,100 m long.
The vessel will sail back and forth across the acquisition area along 48 sail lines (based on a 14 streamer configuration) that will vary in separation from 500 m to 900 m, primarily influenced on the sea conditions at the time of acquisition. The survey will use ConocoPhillips’ Compressive Seismic Imaging (CSI) technology, which means that while the overall width of the streamer configuration remains consistent, the internal streamer separation will vary between 25 m and 100 m with a non-uniform interval distribution. Using the CSI technology means that the survey can sail fewer line kilometres and use fewer streamers and acquire the seismic data quicker than a conventional seismic survey.