The deep velocity structure beneath the Gippsland basin from long-offset seismic data

Abstract

The Bureau of Mineral Resources (BMR) undertook a regional marine seismic survey of the Gippsland Basin in 1988?89. During this survey, recording stations were deployed onshore by the BMR and the Department of Earth Sciences, Monash University, to record long-offset wide-angle reflection and refraction data from the standard marine seismic reflection air-gun shots. The purpose of recording this complementary data was to obtain deep velocities and regional structure, to map variations of crustal thickness, and to link the marine data with earlier onshore results. Arrivals from offsets up to 230 km were recorded, thus sampling the total crustal thickness. It was necessary to enhance the data at large offsets by filtering and stacking because of the relatively low power of the source and noisy conditions during the survey. The acquisition method is logistically simple, and a large amount of additional data can be gathered for negligible extra cost to a survey. Velocities between 5.3 and 5.9 km/s were obtained for the basement and mid-crust. Lower crustal velocities were around 6.3 km/s, and upper mantle velocities were between 7.8 and 8.1 km/s. Sediment velocities between 1.8 and 4.5 km/s were obtained from sonobuoys. The velocity control constrains the identification of deep reflectors and allows accurate depth conversion of interpreted seismic reflection profiles. The deep basement geometry and crustal structure were interpreted along selected traverses from the long-offset data in conjunction with sonobuoy and reflection results. The sediments in the Central Deep reach a thickness of about 12 km. The total crustal thickness thins from about 36 km in Victoria, north of the basin, to about 23 km below the deepest sediments. The boundary between the upper and lower crust, where the velocity increases from about 5.9 to 6.3 km/s, correlates to a reflector which has been previously interpreted as a major regional detachment surface associated with basin formation.