The interpretation of thermal neutron properties in ocean floor volcanics
T. S. Brewer, P. K. Harvey, S. R. Barr, S. L. Haggas, H. Delius, 2005. "The interpretation of thermal neutron properties in ocean floor volcanics", Petrophysical Properties of Crystalline Rocks, P. K. Harvey, T. S. Brewer, P. A. Pezard, V. A. Petrov
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Using near-complete chemical analyses, computed thermal absorption cross-section values (σ) are calculated for ocean-floor basalts from ODP Holes 896a and 1179D. Comparison with nuclear measurement of σ, demonstrates that this computational method is valid and clarifies the interpretation of σ in ocean-floor basalts. Boron, lithium and the rare-earth elements are important controls on the σ-values, and, of these, the distribution of the rare-earth elements is controlled by primary magmatic processes, whereas the distribution of boron and lithium is strongly influenced by secondary low-temperature alteration processes. Consequently, computed σ-values can be used to discriminate between various basalt types and to identify areas of secondary alteration.
Following calibration of the ‘fresh basalt’ signature, it is possible to interpret log derived neutron absorption measurements of a sample (Σ-values), which, when integrated with other log responses, allow the distribution of alteration within a drilled section to be mapped. Examples from ODP Hole 801C demonstrate the potential of this technique.
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Boreholes are commonly drilled into crystalline rocks to evaluate their suitability for various applications such as waste disposal (including nuclear waste), geothermal energy, hydrology, sequestration of greenhouse gases and for fault analysis. Crystalline rocks include igneous, metamorphic and even some sedimentary rocks. The quantification and understanding of individual rock masses requires extensive modelling and an analysis of various physical and chemical parameters. This volume covers the following aspects of the petrophysical properties of crystalline rocks: fracturing and deformation, oceanic basement studies, permeability and hydrology, and laboratorybased studies. With the growing demands for sustainable and environmentally effective development of the subsurface, the petrophysics of crystalline rocks is becoming an increasingly important field.