Nuclear Magnetic Resonance Imaging of Hydrologic Phenomena In Ichnologically-Related Macropores

Nuclear magnetic resonance imaging (NMRI) was used in a series of experiments to (1) observe density-driven fluid flow through the matrix porosity of upper Pleistocene, Biscayne aquifer limestone under static conditions, (2) examine similar density-driven fluid migration from ichnologically-related macroporosity into limestone matrix under non-static conditions, and (3) quantify the three-dimensional, steady-state field of fluid velocity within the macroporosity of an epoxy-resin model. This abstract presents results from the last of these three sets of experiments and demonstrates the effectiveness of using NMRI to investigate the migration of fluids within macroporous media.

The epoxy-resin model used in this study replicates a specimen of upper Pleistocene, Biscayne aquifer limestone gathered from southeastern Florida that is riddled with cm-scale ichnogenic macroporosity primarily related to Ophiomorpha. Specific discharge through the model was controlled using a constant head reservoir and a peristaltic pump.

The region of useful, minimally distorted NMRI images measured 9 cm in length along the z-axis parallel to the predominant flow direction and 10 cm in both transverse directions. Axial and transverse resolution of the images was 0.118 cm and 0.418 cm, respectively. The time needed to collect NMRI data for all three Cartesian components of velocity was about 7.5 hours for each experiment.

NMRI measurements of velocity were collected at two steady-state values of specific discharge. Average axial components of velocity (0.016 cm/s and 0.04 cm/s) are within the range of advective flow expected in the Biscayne aquifer. In both experiments, the velocity field is nearly proportional as might be expected in a low Reynolds number, laminar-flow regime (for water with kinematic viscosity= 0.01 cm²/s moving with a velocity=0.04 cm/s in macropores with diameter=1.5 cm, Reynolds number=6). Exponential functions fit the distributions of the axial velocity. Distributions of the transverse velocities are symmetric but wider tailed than Gaussian functions.