Deep reflection experiments in northeastern Australia, 1976–1978

Abstract

Between 1976 and 1978 the Australian Bureau of Mineral Resources (BMR) recorded deep crustal reflection data at seven sites in northeastern Australia over continuous profiles up to 15 km long by simply extending the recording time to 16 sec during normal sedimentary basin surveys. The record sections show many events with variable strength, continuity, dip, and spatial distribution. By comparing the sections from the longer and the shorter perpendicular traverses, it is possible to discriminate between primary reflections and diffractions, multiples, and other noise events. Based on their character the reflections can be grouped into zones which are interpreted in terms of the nature and structure of the crust. Most of the reflection sections show, below the sedimentary reflections, a thin (2–3 sec) reflection‐free zone underlain by a thick (9 sec or more) zone of numerous reflection segments which varies in thickness and the distribution of reflection segments. The data thus suggest that the upper crust under the sediments is similar in seismic character throughout northeastern Australia. On the other hand, the deeper crust under the Georgina and Drummond basins is significantly different in seismic signature and thickness from that under the Bowen basin and the northern margin of the Galilee basin. It is concluded that good quality deep reflections can be recorded with little extra effort during sedimentary basin surveys using modern multiple‐fold common‐depth‐point (CDP) techniques, and that the data recorded on long traverses, cross‐spreads, and expanded spreads provide information on the structure and composition of the crust and upper mantle with a resolution greater than has been possible before. Such information is valuable in studying the evolution of mineral and petroleum provinces and the lithosphere in general.