GEOPHYSICAL EXPLORATION IN TORRES STRAIT

Abstract

Torres Strait is the narrow waterway on the Continental Shelf which connects the Arafura Sea to the Coral Sea and separates the Cape York Peninsula from South-western Papua. Within Torres Strait and the Arafura Sea the marine seismic survey area specifically discussed here covers the north-western part of Authority to Prospect 104P held by Marathon Petroleum Australia Ltd. Except for a few small islands this area is covered by a shallow tropical sea not more than 160 feet deep which is studded with coral reefs.

Four regional structural features influence the area. These are the Cape York-Oriomo Ridge to the east, the Carpentaria Basin to the south and the Morehead and Papuan Basins to the north. These features are indicated by regional geology and have been confirmed by the combined interpretations of reconnaissance geophysical surveys conducted by various exploration organisations during the past 30 years.

The A.T.P. 104P (West) Marine Seismic Survey, consisting of 618 miles of subsurface coverage, was carried out for Marathon between November 8 and November 28, 1964.

Several operational problems prevented completion of the survey as originally planned. Shallow water and reefing prevented shooting in the north-east portion of the proposed area.

Shooting 300 per cent subsurface coverage commenced in the south-eastern part of the survey area, but, due to shallow basement at less than one second, multiple coverage was not feasible. Production shooting which gave 100 per cent subsurface coverage on the short spreads was continued throughout the remainder of the survey.

Two reflection horizons and a resultant isochron were mapped in the area. A "deep" reflector is interpreted as originating from granitic basement, while a "shallow" reflector is tentatively correlated with a horizon in the Cretaceous section.

The "shallow" horizon mapped is essentially conformable with the "deep" horizon. Both show a regional west dip with thinning of section to the east. Some north and south components of the regional west dip are established. The resultant isochron comparisons based on this work appear too limited in scope to establish any definite local structural control of deposition.