The echo attenuation function EΔ(q) and the displacement probability P(Z,Δ) for
water flowing through a bed of 0.3 mm glass beads has been measured by means of Pulsed
Gradient Spin Echo (PGSE NMR). The encoding time Δ and the flow rates used
in the experiments were chosen such that the average displacement was at least one bead
diameter. The EΔ(q) shows, as expected, a diffraction peak at about the inverse of
the bead diameter. For longer times EΔ(q) shows another peak at about the inverse of
twice the bead diameter. In analogy with PGSE NMR in presence of diffusion in a closed
system, these data suggest that the pore space correlation function or features
closely related to it, can be accessed through the PGSE experiment with flow. The
conditions, which lead to this possibility, are discussed and supported with the
results of network modelling of fluid flow in a periodic structure. The true pore
space correlation function has been determined using the Patterson function approach
applied to a NMR image data set. This has been compared with the one obtained by PGSE
NMR with flow. The displacement distributions P(Z,Δ) for fluid flow/diffusion in this
system also show structural information and, it is suggested, can be regarded as a
product of the pore space correlation function with an evolving smooth distribution
which starts as an exponential and tends to a Gaussian in the longer time limit.