Direct 3-D measurement of the flow velocity in porous media using magnetic resonance tomography

The pore space of a sediment column (diameter 10 cm, length 20 cm), filled with natural sediments, was imaged with magnetic resonance tomography (MRT) at a spatial resolution of 0.5×0.5×0.5 mm³. Apart from the evaluation of the resulting images, the statistical evaluation provides additional data to assess the pore size distribution. While the porosity of pores >0.2 mm can be measured directly, for smaller pores a calibration is necessary. Flow and diffusion processes were measured at a spatial resolution of 1.32×1.32×5 mm³. The time between two measurements of a cross-sectional area is <10 s. The coefficients of molecular diffusion can be measured directly and are in good agreement with data in the literature. Flow was measured in a range of 0.9-40 cm/min. A calibration within one part of the sediment column was performed. The results of the flow investigations are in good agreement with the values obtained from a tracer experiment and from a numerical model. At the given spatial resolution, flow in preferential flow paths and larger pores can be measured individually. Regions of different flow velocities can be mapped. A monitoring of flow processes is possible. Since the method is based on the movement of protons in a magnetic field, no tracer was needed to examine the dynamic processes within the column. We believe that MRT has great potential to fill the gap between labscale investigations and field tests by providing reliable data on the pore geometry and the occurring flow velocities.