Deep crustal reflection recordings in Australia 1957-1973--I. Data acquisition and presentation

Abstract

The Australian Bureau of Mineral Resources (BMR) has conducted experiments since 1957 to record deep crustal reflections. These experiments were made initially during surveys conducted for other purposes. In 1968-1969, BMR conducted experiments designed specifically for deep reflection recordings in the Tertiary Murray Basin, the Precambrian Willyama Block and the Palaeozoic Tasman Geosyncline in southeastern Australia, the Amadeus Basin in central Australia, and in the Archaean in Western Australia. BMR introduced digital recording in 1976, and since then successfully recorded several long deep-crustal profiles. In this paper, we describe and discuss only those recordings made with analogue equipment prior to 1973. Apparent deep crustal reflections were recorded at most of the 46 sites investigated; at 24 of these, events occurred at times of 10-13 s. At some sites where only one or two shots were recorded, it was difficult to confirm late events as deep reflections; where investigations were more comprehensive, the events were consistent with deep reflections, although in many places they lacked continuity. The events showed peak frequencies of about 20 Hz. Recording quality was improved with the use of 8-10 Hz geophones in multiple in-line arrays of 100 m or more and filters with a cut-off about 14 Hz or less. The quality was generally better in sedimentary basins than in areas of exposed basement rocks. Where several events occurred, they generally showed a variety of dips, making identification with simplified geological structure difficult. It was not possible to interpret the reflection data alone in terms of crustal structure because of the limited data available and the large distances between most recording sites. However, the results are particularly useful in providing a preview of deep reflections that may be expected from future surveys in Australia.