Geophysical Institute of the ASCR

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Experimental measurement of seismic velocity, porosity and permeability Crystalline rocks of metamorphic and magmatic origin are usually of very low porosity (below 3%). Porosity corresponds to grain boundaries, cleavage planes of rock forming minerals and microfractures of different shape and length that can be either sealed or open. Orientation and connectivity of microscopic pores in these rocks and its closure with respect to increasing confining pressure is of great interest as they are important parameters of dangerous waste disposal sites, yet only a limited number of methods exist of their investigation. Within this project, we examine the origin and spatial orientation of the microporosity in rocks of different microstructure and grain size by using 1) experimental measurements of elastic P-wave velocities (V P ) on spherical samples in 132 directions under various confining pressures up to 400 MPa (ultrasound pulse transmission technique, Pros et al., 1998), and processing of the measured data into DV P (Dp) differential diagrams (Fig. 8), 2) measurement of lattice preferred orientation using the electron backscatter diffraction method, and, 3) quantitative microstructural analysis of the grain boundaries orientation in thin sections parallel to the xz, xy and yz planes of the finite strain ellipsoid. Various types of monomineralic, bimineralic and polymineralic low-porosity rocks are measured. The first results show that experimental pulse transmission technique is a useful tool for the analysis and visualization of oriented microporosity in 3D (Fig. 9) and provides important basis for further study of permeability anisotropy through studied rocks. The complex research of physical properties and microstructure improves our understanding of the contribution of individual microstructures to the bulk microporosity and permeability (Špaček et al. 2004). Fig. 8. Principles of 3D acoustic measurements under varying pressure and data processing. Under low confining pressures the change in P-wave velocity is mainly due to closing of microfractures which define the porosity. Information on the orientation and amount of microfractures is obtained by comparing the velocity change under various pressures (DV P (Dp)) or through the analysis of hysteresis. 23
Fig. 9. Example showing measured P-wave velocities at atmospheric pressure, velocity difference between the atmospheric pressure and 10 MPa and hysteresis at atmospheric pressure of mylonitized limestone. Macroscopically estimated microstructure is shown (S-foliation, L-lineation). References Pros Z., Lokajíček T. and Klíma K., 1998. Laboratory approach to the study of elastic anisotropy of rock samples. Pure appl. geophys., 151, 619-629. Špaček P., Melichar R. and Ulrich S., 2005. Pore Space Geometry and Seismic Anisotropy of Rocks: 3-D Experimental Investigation. AGU fall meeting, San Francisco, USA, Petrophysical properties of upper mantle eclogites Lattice preferred orientation (LPO) of omphacite and garnet in different microstructural types of upper mantle eclogites from the Roberts Victor mine in South Africa have been studied using the electron back-scattered diffraction method. These data complete the database of lattice preferred orientations of main mantle minerals and help to understand the rheology of basaltic parts of a subducting lithosphere. In the non-foliated eclogites, the measured omphacite LPO disagrees with the cation ordering model of its development (Brenker et al., 2002). Petrological data shows that this LPO has been developed at 1000 – 1150ºC and omphacites are in the high temperature disordered structure. To further test the cation ordering model we conducted visco-plastic self-consistent simulations of the LPO development. It has been concluded that the cation ordering in omphacite cannot have a prime control on the LPO development. Finally, the LS index has been introduced as a quantitative indicator of the finite strain ellipsoid (Ulrich and Mainprice, 2005). A new type of clinopyroxene LPO has been measured in foliated eclogites and it is associated with grain growth. The new type of LPO occurs also in clinopyroxene megacrysts bands that show exsolution lamellae of garnet. Presented microstructural relationship suggests that LPO in clinopyroxenes has developed at temperatures close to melting (1450ºC). Numerical models using all known slip systems in the clinopyroxene did not help 24
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Page 45 and 46: References Vavryčuk V., 2005. Foca
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Geophysical Institute of the ASCR

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