Crustal structure across the Vulcan Sub-basin from seismic refraction and gravity data

Abstract

The attenuated continental crust between the Kimberley Block and the Timor Trough hosts some of the major oil and gas fields in Western Australia. The crustal structure has been interpreted from wide-angle seismic data acquired by the Australian Geological Survey Organisation during 1995/96. Ten ocean-bottom seismometers were deployed along a deep seismic reflection profile aligning NW with exploration wells Fagin and Crane. Data were recorded to offsets of 300 km. A total profile 335 km long and 48 km deep has been developed, by ray-tracing and inversion modelling of the refraction data, and verified by modelling gravity data. The model differentiates 7 crustal and 2 upper mantle layers. The crustal thickness varies between 25 to 30 km, being greatest beneath the Kimberley Block and Vulcan Sub-basin. The velocities within the upper mantle are ~8.1 km/s, while in the lower crust they vary from 5.9-6.2 km/s beneath the Kimberley Block, to 6.4-7.0 km/s elsewhere. Mid-crustal velocities are 5.5-6.2 km/s and 2.0-5.0 km/s in the overlying sedimentary section. Significant lateral variations exist in the lower crust and can be used to infer the limit of the Kimberley Block. A prominent reflector at 6s twt, which is widespread in the region, is not a significant velocity discontinuity. The Moho is not seen on the reflection records, but is a prominent feature in the refraction data. Iso-velocity lines do not necessarily coincide with reflection boundaries, possibly due to decreasing porosity with depth, especially beneath the Cartier Trough.