Statoil proposes to undertake a geophysical and geochemical survey (GGS) in petroleum exploration permit WA-506-P which lies in deep offshore waters of the Northern Carnarvon Basin. Data will be collected using the dedicated geophysical survey vessel, the dynamically positioned MVFugro Supporter. The MVFugro Supporter is 75 m long with a crew and passenger capacity of approximately 47 people. Mobilisation and demobilisation of the vessel, equipment and personnel is to be conducted from Dampier, Western Australia. The objective of survey is to provide adequate information regarding the seabed bathymetry, geomorphology and sediment geochemistry to guide further exploration activities in WA-506-P.
 
The GGS will be completed in two phases; geophysical survey and geochemical and geophysical survey, as described below, with the final design of the second survey phase dependent on the results from the first survey phase.
Phase 1 Geophysical Survey
The first phase of the activities will comprise geophysical surveys; a bathymetric survey using a multi-beam echo sounder (MBES) and sub-bottom profiling (SBP) to characterise shallow sub-seabed geology and geohazards. The survey will be undertaken over an area of 10,000 km2. All of the equipment will be keel mounted (i.e. no trailing equipment). This phase is expected to take approximately 15 days.
 
The Kongsberg EM 122 deep water MBES proposed for the survey has 12 kHz frequency and an estimated sound pressure level at source of 242 db re 1 µPa rms at 1 m. The Kongsberg EM 122 is capable of achieving full bathymetry coverage in excess of the water depths likely to be encountered on this project and will be tuned to suit the depths in the operational area.
 
For accurate mapping of water depth using a MBES a sea water CTD / Sound Velocity (SV) measurement will typically be required every 24 hours during the survey. A Lockheed Martin (Sippican) XSV-02 expendable probe system will be used to acquire sound velocity data, thus negating the requirement to frequently stop the vessel to conduct an SV measurement using a conventional probe.
 
SBP data for the purpose of selecting coring and heat flow target locations will be acquired using the Fugro Supporter’s hull mounted Edgetech 3300 chirp system. Being hull mounted as opposed to towed will result in an increase in overall survey speed and a decrease in line turn times, thus increasing overall efficiency.
 
The Edgetech 3300-HM SBP is a 4 × 4 chirp system operating on a frequency range of 1.5 KHz–12 KHz. The system features user selectable bandwidth options that can be selected to suit the sediments encountered. The Edgetech 3300-HM SBP has a power output of 4 KW with a beam width 24 degrees at 4.5 KHz centre frequency.
Phase 2 Geochemical / Geophysical Survey
The second phase of the activities will comprise geochemical surveys; collecting seabed samples using a gravity piston-corer and measuring heat flow in surface sediments using a heatflow probe. The exact locations of geochemical sampling will be determined following on-board analysis of the geophysical data collected during the first phase of work. Approximately 50 locations will be sampled using the piston-corer. At six of these locations, heat flow will also be measured. This phase is expected to take approximately 10 days.
Seabed coring will use a 6 m long gravity piston-corer lowered from the A-frame of the survey vessel to within 6 m of the seabed and then allowed to free fall the remaining distance to the seabed. Cores up to 6 m long will be collected and returned to the vessel for sampling and storage. During coring, the survey vessel will use dynamic positioning to remain on site for up to three hours.
The heatflow probe is approximately 6 m long and will be allowed to free fall into the seabed penetrating the soft sediments up to 6 m. After a period of equilibration, a heat pulse (equivalent to about 20 °C in air) is fired and it heats up the sensor tube and the surrounding sediment. The probe remains in the sediment for 20 minutes to measure the thermal decay of the heat, which allows the depth-dependent determination of the sediment´s thermal conductivity and thermal diffusivity. It is then recovered to the vessel.
Location
All of the survey activities will occur within WA-506-P which lies approximately 350 km north-west of Dampier.