  We have extended the utility of NMR as a technique to probe porous media
structure over length scales of ~ 100 - 2000 micron by using the spin 1/2 noble
gas 129Xe imbibed into the system's pore space. Such length scales are much
greater than can be probed with NMR diffusion studies of water-saturated porous
media. We utilized Pulsed Gradient Spin Echo NMR measurements of the
time-dependent diffusion coefficient, D(t) of the xenon gas filling the pore
space to study further the measurements of both the surface area-pore volume
ratio, S/Vp, and the tortuosity (pore connectivity) of the medium. In
uniform-size glass bead packs, we observed D(t) decreasing with increasing t,
reaching an observed asymptote of ~ 0.62 - 0.65D0, that could be measured over
diffusion distances extending over multiple bead diameters. Measurements of
D(t)/D0 at differing gas pressures showed this tortuosity limit was not
affected by changing the characteristic diffusion length of the spins during
the diffusion encoding gradient pulse. This was not the case at the short time
limit, where D(t)/D0 was noticeably affected by the gas pressure in the sample.
Increasing the gas pressure, and hence reducing D0 and the diffusion during the
gradient pulse served to reduce the previously observed deviation of D(t)/D0
from the S/Vp relation. The Pade approximation is used to interpolate between
the long and short time limits in D(t). While the short time D(t) point lay
above the interpolation line in the case of small beads, due to diffusion
during the gradient pulse on the order of the pore size, it was also noted that
the experimental D(t) data fell below the Pade line in the case of large beads,
most likely due to finite size effects.
