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- Volume 1, Issue 7, 1983
First Break - Volume 1, Issue 7, 1983
Volume 1, Issue 7, 1983
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Deep Reflection Probes in Eastern Australia Reveal Differences in the Nature of the Crust
By S.P. MathurSince 1976 the Australian Bureau of Mineral Resources (BMR) has obtained good quality deep reflection data during sedimentary basin surveys using modern digital recording and processing techniques (Mathur 1983a). Most of these recordings, made on an experimental basis, showed that good reflections could be obtained from the deep crust without extra effort other than increased recording time, and that such data over long traverses are required to study the deep crust and upper mantle in detail (Mathur 1983b). The deep reflections discussed here were generally recorded over relatively short traverses and thus do not provide sufficient quantity of data to investigate the horizontal variation of the deep crustal structure. However, the strength, coherence, continuity and spatial distribution of the reflections from the deep crust provide valuable information on the nature of the crust in several areas of eastern Australia with a resolution higher than is possible from any other geophysical method. This paper presents some of these data to show the major differences in the character of the deep reflections between the Precambrian and Phanerozoic domains of eastern Australia and discusses what these characteristics might mean in terms of the fine structure of the deep crustal rocks with reference to the gross structure obtained from the refraction studies.
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Seismic Surveying over Bunter Sandstone
By N.R. GoultyThe Bunter sandstone is a Triassic formation which attains thicknesses of hundreds of metres over many of the productive coalfields in central England. Where the Bunter outcrops, seismic reflection data using either dynamite or surface sources are generally of poor quality. This presents an awkward problem for delineation of the underlying structure in the Carboniferous. The problem is a familiar one to many exploration geophysicists who have worked in the Midlands and north of England, because the Carboniferous has potential for oil and gas, in addition to the Coal Measures. AIthough the field records and stacked sections used in this article to illustrate the variability of data quality have all been obtained in coal exploration, the comments on the acquisition parameters are equally applicable for all seismic reflection surveys in the area.
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Deep Geology from Seismic Reflection Studies in Canada
Authors A.G. Green and R.M. ClowesIn 1964, the Department of Physics at the University of Alberta initiated a programme to explore the feasibility of recording reflected seismic energy from boundaries within the lower crust and from the Mohorovičić discontinuity. This was the first attempt by a North American institute to systematically develop and apply industry-type seismic reflection techniques for investigations of the deep crust. For the first study a modification of the expanding spread technique was applied to an area of southern Alberta, where the general crustal layering was known from the results of reversed seismic refraction profiling. Kanasewich and Cumming (1965) reported on the success of this study by providing convincing evidence that reflections from the deep crust had been recorded. In particular, a strong lower crustal reflector at a two-way travel time of 11.4 sec was phase correlated over a distance of about 13 km, enabling the compilation of a T2-X2 plot and the determination of an average vertical velocity and depth to the reflector. Within experimental error, the 34 km depth to the reflector (later named the Riel discontinuity) matched the results from an earlier seismic refraction survey along the same line (Cumming and Kanasewich 1966). Encouraged by this initial success the group at the University of Alberta expanded its programme and by the late 1960s to the mid-1970s seismologists at three other institutes (University of Saskatchewan, University of Manitoba and the Earth Physics Branch of the Federal Department of Energy, Mines and Resources) were shooting short seismic reflection profiles across various parts of western Canada. It was during this period, in 1973, that Mair and Lyons (1976) recorded the first strong evidence that Vibroseis was a viabie source for probing the deep crust. They recorded a relatively continuous reflection from the Mohorovičić discontinuity along a 20 km line in the Cordilleran Orogen. In 1975, in an effort to consolidate funding and pool the limited human resources and recording/processing capabilities available in Canada, crustal seismologists from across the country joined together to form the Consortium for Crustal Reconnaissance Using Seismic Techniques (COCRUST). Since its inception most major crustal seismic surveys on the Canadian landmass have been conducted under the COCRUST umbrella. Unfortunately, due to a continued low level of funding,COCRUST has yet to capitalise fully on the earlier successes in Canadian reflection seismology. To date, most of the COCRUST funding has been directed to regional refraction surveys; accompanying short reflection profiles have been obtained only at critical locations. In this paper we review briefly, in chronological order, some of the important results of the pre-COCRUST era and then we present the conc1usions obtained from two COCRUST sponsored seismie reflection surveys and a recent government/industry sponsored intermediate depth reflection survey. Finally, we describe briefly the latest Canadian initiatives (LITHOPROBE) for mapping the third dimension of geology during the 1980s and beyond.
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Volumes & issues
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Volume 42 (2024)
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Volume 41 (2023)
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Volume 40 (2022)
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Volume 39 (2021)
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Volume 38 (2020)
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Volume 37 (2019)
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Volume 36 (2018)
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Volume 35 (2017)
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Volume 34 (2016)
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Volume 33 (2015)
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Volume 32 (2014)
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Volume 31 (2013)
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Volume 30 (2012)
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Volume 29 (2011)
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Volume 28 (2010)
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Volume 27 (2009)
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Volume 26 (2008)
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Volume 25 (2007)
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Volume 24 (2006)
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Volume 23 (2005)
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Volume 22 (2004)
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Volume 21 (2003)
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Volume 20 (2002)
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Volume 19 (2001)
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Volume 18 (2000)
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Volume 17 (1999)
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Volume 16 (1998)
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Volume 15 (1997)
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Volume 14 (1996)
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Volume 13 (1995)
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Volume 12 (1994)
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Volume 11 (1993)
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Volume 10 (1992)
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Volume 9 (1991)
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Volume 8 (1990)
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Volume 7 (1989)
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Volume 6 (1988)
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Volume 5 (1987)
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Volume 4 (1986)
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Volume 3 (1985)
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Volume 2 (1984)
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Volume 1 (1983)