DEEP STRUCTURAL IMAGING FROM ONSHORE RECORDING OF A MARINE AIR-GUN SOURCE IN THE GIPPSLAND AND BASS BASINS

Abstract

In 1988 and 1989, the Bureau of Mineral Resources (BMR) completed two regional seismic reflection surveys in the Gippsland and Bass Basins. Seismic arrivals from the routine air-gun shots fired during these surveys were recorded on land by BMR and the Department of Earth Sciences, Monash University. Individual analogue and digital recording stations were deployed in Victoria, Tasmania and Deal Island on the Bassian Rise. Long-offset wide-angle reflection and refraction data were obtained at these stations from traverses across both basins.

The data quality was variable, depending on local site conditions, but useful arrivals were observed over 200 km away from the source on some lines. The close shot spacing, either 37.5 or 50 m, and the large number of shots, up to 5000 per traverse, provides the opportunity for stacking and other signal enhancement techniques in areas of poor data quality.

The arrival times of the refracted events show significant delays corresponding to changes in basin sediment thickness. Preliminary results suggest no major asymmetry in the rifting process, which would require modification in the current models for rifting of the basins. Sediment and basement apparent velocities obtained from near-station records range from 4.8 to 5.1 km/s; below the deepest part of the basin, the basement apparent velocity is around 5.6 km/s. Deep crustal/upper mantle velocities of 7.2 km/s, and around 8 km/s, are also observed.

These velocities, combined with the coincident reflection data provide critical constraints on models of basement geometry. The refraction and wide-angle reflection data can be used to derive the crustal structure associated with the basins and surrounding margins. These sections will complement deep reflection profiling to test and refine tectonic models to guide further exploration.