Shear-wave anisotropy from dipole shear logs in oceanic crustal environments
G. J. Iturrino, D. Goldberg, H. Glassman, D. Patterson, Y.-F. Sun, G. Guerin, S. Haggas, 2005. "Shear-wave anisotropy from dipole shear logs in oceanic crustal environments", Petrophysical Properties of Crystalline Rocks, P. K. Harvey, T. S. Brewer, P. A. Pezard, V. A. Petrov
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The deployment of a down-hole dipole shear sonic tool in Hole 395A and Hole 735B marked the first two opportunities to measure high-resolution shear-wave velocity and VS anisotropy profiles in oceanic crustal rocks. In Hole 395A near the Kane Fracture Zone, dipole sonic logs were recorded from 100–600 mbsf, and allow azimuthal anisotropy to be determined as a function of depth in the crust. The magnitude of VS anisotropy varies with depth, from less than 3.2% in low-porosity flows at the bottom of the hole, to approximately 15.5% in highly fractured pillow basalts and breccias. The orientation of the fast VS direction also varies over depth, with a mean value between 75°N and 80°E, and aligns with the strike of steeply dipping structures observed by down-hole electrical and acoustic images. This fast VS angle orientation is locally oblique to the plate-spreading direction and to the Mid-Atlantic Ridge axis. In Hole 735B, drilled near the Atlantis Fracture Zone, dipole sonic logs from 23 to 596 mbsf indicate that VS anisotropy varies with depth, with averages of 5.3% in the foliated and deformed gabbros recovered at the bottom of the hole; 4.5% in undeformed olivine and oxide-rich gabbros around 300 mbsf; and 6.8% in highly deformed mylonitic zones at shallow depths. The fast VS angle also varies with depth, giving a mean orientation of approximately S45°E for well-resolved estimates in the upper interval of the hole. This direction aligns with the strike of steeply dipping fractures observed by down-hole imaging, and is locally oblique to the Southwest Indian ridge axis. Although the effects of regional stresses and local deformation of these holes may introduce anisotropy in the dipole sonic data, we conclude that crustal morphology in the vicinity of the holes contributes significantly to the magnitude and orientation of VS anisotropy.
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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.