Geophysical Institute of the ASCR

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.
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