L086 Marine Geophysical survey North West Continental Shelf, 1968

Brief description

The Bureau of Mineral Resources has carried out two large scale reconnaissance marine geophysical surveys in the Joseph Bonaparte Gulf and Timor Sea areas in 1965 and 19670 It is proposed to continue this geophysical reconnaissance to the south over the northwest continental shelf from Ashmore reef to Barrow Island. 16 000 line miles should be covered in 3 months on mainly east-west lines 9 nautical miles apart. The boat speed will be about 10 knots. The survey will be done by Ray Geophysics (Australia) Pty Ltd under contract.

Lineage

Maintenance and Update Frequency: asNeeded

Statement: The emphasis in oil exploration in Australia has gradually shifted from land to offshore areas over the last few years ago. This trend has been accentuated by the proving of considerable oil and gas reserves in the offshore Gippsland Basin. The results to date indicate that the volume of prospective Palaeozoic sediments offshore is about equal to the volume on land, offshore, so the trend is expected to continue, and possibly accelerate, during the next decade.

The Bureau of Mineral Resources has now carried out two large scale marine geophysical surveys, both of them in the Joseph Bonaparte Gulf-Timor Sea area. They have been carried out by the Bureau to assist in the exploration and development of the continental shelf, and to introduce and test the feasibility of geophysical methods new to surveys in Australian.The 1965 survey (Smith 1966, GAl 1966) was mainly confined to the Joseph Bonaparte Gulf. Navigation was by the Toran hyperbolic system which, while accurate, is expenSive to install and operate, and over large distances (100 + miles) only operates satisfactorily during the daylight hours. The seismic reflection system used a sparkarray energy source of 14 000 joules. A La Coste and Romberg gimbalmounted surface gravity meter was installed -on the ship and gave a standard deviation of the difference at line intersections of 3 milligals.

As the seismic system proved successful, the source energy was increased to 21 000 joules aiming at greater depth of penetration in the 1967 survey which covered the Timor Sea area to the north of the previous survey. The Toran navigation system was replaced by a VLF system which, though of lower accuracy, was still adequate and enabled 24-hour-a-day operation to be maintained. The Askania marine gravity meter gave a slightly improved performance, a mean difference of 2.5 milligals at intersections being aChieved, equivalent to a standard deviation of 4.0 milligals despite the less accurate position and consequent velocity determination. Continuous magnetic profiling was added to the program.