Visualization of Groundwater Flow within Touching-Vu and Matrix Porosity in an Eogenetic Karst Aquifer

摘要

In this study, we use an innovative, non-invasive technology, nuclear magnetic resonance imaging (NMRI), to visualize the direction and magnitude of groundwater flow in field samples of late Pleistocene limestone of the Biscayne aquifer. Specific goals of the first set of NMRI experiments are to map the advective velocity of water flowing at two rates of specific discharge (0.00025 and 0.00013 m/s) through a 10-cm-diameter cylindrical, epoxy-resin model. The model interior accurately reproduces a well-connected maze of ichnologically influenced, centimeter-scale, touching-vug macroporosity common within preferred flow zones in parts of the Biscayne aquifer. A second set of NMRI experiments investigates the migration of freshwater into the matrix of permeable and porous peloid-ooid grainstone initially saturated with heavy water (D2O). In the experiments on the physical model, we generate the velocity maps using phase-encoded, stimulated-echo imaging. In the experiments on the rock matrix, we visualize the progressive displacement of D2O in the rock matrix using sequential time-step images of NMRI signal strength.