COMPUTER-SIMULATION AND 6-DIMENSIONAL SPIN-DENSITY AND VELOCITY NMR MICROIMAGING OF LACUNAR SYSTEMS - A COMPARATIVE-ANALYSIS OF PERCOLATIONPROPERTIES

Using computer-simulated random site-percolation networks as a templat e, phantoms of lacunar systems were fabricated. The pore space was fil led with water and experimentally investigated with the aid of nuclear magnetic resonance (NMR) microimaging. A pulse sequence providing six -dimensional spin density and velocity NMR image data was employed for the combined record of the three-dimensional spin density distributio n and the three-dimensional velocity vector field of water percolating through the pore networks. From these data, three-dimensional velocit y magnitude images were derived. The exclusion of all voxels of the sp in density images with velocities below the noise level provides an ex perimental means to directly image the percolation backbone. An evalua tion procedure for the NMR image data was established that reliably re nders the pair correlation function and the mean volume-averaged poros ity as a function of the probe volume radius. Characteristic parameter s refer to the fractal dimensionality, to the correlation length, and to the short-range order. The theoretical predictions can thus be comp ared directly with experiment. For comparison, the water-filled pore s paces of other, less random lacunar objects such as glass-bead agglome rates and a natural sponge were also examined with respect to percolat ion properties.