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- Volume 5, Issue 8, 1987
First Break - Volume 5, Issue 8, 1987
Volume 5, Issue 8, 1987
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3D modelling of migration velocity fields and velocity error zones
Authors M.M. Loveridge, G.E. Parkes and L. HattonIn a previous paper, Parkes & Hatton (1987) described how ray-theoretical modelling could be used to investigate various migration problems, particularly those due to velocity uncertainty. This paper describes an extension of this work to three dimensions using a two-pass ray-theoretical migration technique. Firstly, a brief review of the terms and methods used by Parkes & Hatton will be given. As a two-pass ray-theoretical (as opposed to wave-theoretical) migration procedure has not, to the authors' knowledge, been described before, the second and third sections will be devoted to an overview of the approach used and to a study of the accuracy of this method. In the fourth section some examples of the application of the techniques will be presented, followed by a summary and some conclusions.
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Clay mapping using electromagnetic methods
By G.J. PalackyConductive water-saturated clays occur in weathered layers, fracture zones, and unconsolidated sediments. Generally, clays are not as conductive as massive sulphides or graphitic sediments, but their resistivity is less than that of fresh igneous and metamorphic rocks. Clay conductors can be detected by modern ground and airborne electromagnetic (EM) systems. A fully developed weathering layer consists of several horizons (from bottom to top): zone of fractured rock, saprolite, leached zone, mottled zone, and duricrust. Electrical properties of the weathered layer depend on the type of clay present and its water content. The most conductive horizon is saprolite; duricrust is the most resistive. Saprolite developed over mafic and ultramafic rocks is more conductive than that formed over felsic rocks, hence the results of resistivity or EM surveys can be used for pseudogeological mapping. Examples from the Itapicurú greenstone belt in Bahia, Brazil, a crystalline area near Nantes, France, and the Rice Lake greenstone belt in Manitoba, Canada, demonstrate that mafic and ultramafic rocks can be identified by airborne EM surveys. Often, as in the three areas shown, mafic rocks are not magnetic. In tropical areas, outcrops are scant and mapping of soil colour differences often results in some formations being missed or incorrectly outlined. In regions of temperate elimate (e.g. France), a thin cover of Quaternary sediments makes mapping of the underlying lithological units difficult. In areas covered by glacial sediments (e.g. Canada), the continuity of formations may not be obvious from existing outcrops. The extent of weathered layers in countries of temperate and cold climate is larger than previously thought. Conductive saprolite may also be preserved in a sedimentary sequence below basal sediments. Such a situation is demonstrated in borehole resistivity data from Bells Corners, Ontario, Canada. Resistivity and EM measurements can be used to map the extent and thickness of Quaternary clays. Examples of such measurements are given from the Sennecey test site, near Dijon, France.
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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)