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Geological Society, London, Special Publications; 2005; v. 240; p. 255-278;
DOI: 10.1144/GSL.SP.2005.240.01.19
© 2005 Geological Society of London

Effect of compositional and structural variations on log responses of igneous and metamorphic rocks. I: mafic rocks

A. Bartetzko1,2, H. Delius3 & R. Pechnig2

1 Present address: University of Bremen, Research Center Ocean Margins, P.O. Box 330440, 28334 Bremen, Germany bartetzko{at}uni-bremen.de
2 Applied Geophysics, RWTH Aachen University of Technology, Lochnerstrasse 4-20, 52056 Aachen, Germany
3 Department of Geology, University of Leicester, Leicester, LE1 7RH, UK

Well logging has become a standard method in the oil industry for the investigation of subsurface geology. Accordingly, interpretation techniques have been mainly developed for use in sedimentary rocks, and the log responses of sediments are well known. However, this is not the case for igneous and metamorphic rocks. We present a compilation of log responses for mafic rocks from drill-holes in oceanic and continental basement. The holes cover a variety of mafic rocks: mid-ocean ridge basalt (MORB), gabbro, basalt and andesitic basalt from back-arc basins, flood basalt from large igneous provinces (LIPs), and continental metamorphic rocks. The comparison of log responses shows that rocks from the same geological setting have similar in situ physical properties. Differences in physical properties between rocks from different geological settings are mainly related to variations in the structure of the rocks, while variations in composition have only a minor effect on the in situ physical properties. In volcanic rocks, variations in fracturing and vesicularity related to cooling of the lava strongly influence log responses. Mafic rocks from continental drill-holes were enriched in radioactive elements during regional metamorphism, resulting in higher values in the total gamma-ray compared to the oceanic rocks.





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